SEPTEMBER 1959 S 0 me Princip lei" and Practices in i!» Irrzgation of T exai" Soil; OAtLAM Swcamm unusrcno oowv EE LlPSCOMB new MOORE ROBERTS HEMPHILL sore ouaum POYTER y“ an ‘WHEELER fi/ AR l“? >4 DEAF smml RANDAL DONLEY ICOLLWGS’ ‘c us won ‘ PARMER castno SWISHE amsooE‘. HALL , CWLD- I RESS ‘_¥Ll HARDEMAN ' a BAILEY LAMB HALE no noun _com_E \"'\»1wu_sa M FOARD l G5 R‘ ‘l’ - R WlCHl A . l M, . D KENS KlN KNOX BQYLOR CHER . COCHRAN, HOCKL v 8800* CROSBY . , \ " L JP + ~\~" p 4e a v I - Y" CK‘ ' ACK E -FOE - l‘ ‘ | S RZA “my I STONE ' 545K513“ _ YOUNG J . . YOA nv urm )~ WALL Mo ON H i c _ _ _ . ‘ ' ' ' ML‘ alnsi _ wanton . " wooo , . - SHACK ‘ STEPHENS PALO ARK sounnv ' FISHER _ JONES ' F0 _ _ Pmm MONTA o 0 o t o ~41! p” 21 ' 0 Z Q U! c 3 l’. Z i _--—' ‘VI x» 1 z (Y ‘ E f] z '\ r- , > Z P z an U‘ ___. FRANK if . a: a. O Z Q HARRlSON GAINES DAWSO RDEN . um 5f, _uoo0 _ o 5O - Ems "'1. _ ' PANOEA Auanzws MARTIN nowan ‘MITCHE - N A" .7 ‘CM-L A" “D 5mm ygjfg RUSK‘ ~“ K _ \»< NAV .f ' I . LL . ' - L?" - TER- - J31‘ RSO - - E- “$5 LOVlNG ‘ WlNKLER ECTOR Mloumo '3“ S 0K5 . °°"“ "'/ ~ BOSQUE /\_ L>--‘ -=[. CL N ~ v- a - K W“? RUNN s- LEM . an - . NE 9;,‘ “- ' . 1 _ "Wdarwmon ’ K LVHE~ 5 . . z i, ' ——' ' H ' - LENN - / - <1 : ---- EH ii ‘ ". 'F \ @\3 ‘ ‘ . M LL CORYELL - ' LETN CRANE UPTON REAGAN m,“ \ c _ H . lRION ' ULLO u_ \ ' H u» _ REEVES GREEN S , _\ - / - e ' seen L MM" ROBE. / ' '* . | A / ~ ~L~L~ 1 - J e ,l S i '_ . l ‘if . ‘ URNET .>*- - w“ \H“R 5J5 n - g z HUQ$PETH CULBERSON wma l _‘ /' N ' ' SCllLElCl-(ER _ MEN“ _ , Km ' P5505 CROCKE" - M‘5°" - LMNO ' WlLLl MSON /I R OS I "‘ Jg; DAV|5 V _ ’ - l I ' - g '\___._Q\;i_k.w. - ‘ ’ _ _ . \ a. a SON ‘ - . RDIN ' . suttou - KIMBL - - _. . . I 1 _ w‘ BL > E M y l TGOMERY.) _ 0;“. _| _ . - -—"-'1 . _ l ' YQ . - ' . ' .., U85?“ ‘Jgrrgn . TERRELL - HAYS Asfgop Y _ _ KERR - . ,~ A “‘ mam ' ~ KENDALL)’ - , YE E _ m- X c CBLL ~& “ ' FORT ' f _ __- - ‘ . awn ' - I "W4 PE / l D . . C\_ , EX ' o ALES/KLAVA I \_ ,-__- r n ALDE MEomA /- _ ARTON anazom WIL N \ - w / * - - \. ""‘__——-' - JACKSONY '\. _ MATAGORDA uvwA U? BREWS ~ R i VAL VERDE I EDWRROS . ‘ OMAL REAL ' . . ' . BANOERA ’ i MGR o. m o ATASCOSA VKARNES‘ ZAVALA 1 . . I ‘NORM A 60L: - i-i-i-iii- LAND RESOURCE AREAS w, ~ 2,:z\. / East Texas Timberlands H NOYlh Central Prairies ' “yam” '7 wEae ouvAL nuscEs - gaeazna I Rio Grande Plain _[_l-_ _ , Blackland Prairies Edwards Plateau ma. , East Cross Timbers Rolling Plains i ‘ Grand Prairie HiQh Plains /» West Cross Timbers ' ~ (Bottomlands not shown due to limitation of scale) MAVERICK JlM WELL$ mTlrflDOmp- ZZI-x<-— ' CAMERON TEXAS AGRICULTURAL EXPERIMENT STATION R- D- LEWIS. DIRECTOR. COLLEGE STATION. TEXAS [Blank Page in Original Bulletin] FOREWORD publication is designed to present some basic principles and practices of irrigation that will be lto the irrigation farmer. It should be used as a guide and will require revision as dictated by l, 'ence and as additional field data become available. 4 A en a farmer changes from dryland to irrigation fanning, he often overlooks the importance of the nships which exist among soil, water and plants. Good irrigation practices also must take into ac- - such important factors as soil fertility. amount and quality of water. adapted crops and varieties, land _l-- ation for irrigation, water distribution over the land, irrigation timing to suit crop needs, rate of j application for a particular soil type. capacity of soils to store and release water. rates of seeding and development and distribution within the soil storage reservoir for obtaining both water and plant nu- y The development and maintenance of a good soil and water management program require that A factors be understood by the individual irrigation farmer so that maximum efficiency can be ob- l - from the use of irrigation. “production. Much more efficient use of the water and soil resources for crop production will be re- 1 - if the dwindling water supplies are to be conserved and used most effectively. Since the competi- lamong agriculture. industry and municipalities for water will become greater in the future. it will be nary for each segment to make the most efficient use of its share of water if the public interests are i served in the most effective manner. The variable distribution of rainfall continues to be the most important factor affecting the yield and ty of crops in Texas. As a result. the irrigated acreage has grown from approximately l million acres _' 9 to about 7 million acres in 1959. This expansion has come about largely by individual develop- of ground-water resources. ‘ln many cases. special problems will exist on individual farms. Where they occur, it will be neces- _ to consider them on a local basis. If a farmer cannot cope successfully with a particular problem con- J g soils and irrigation, he should consult a local representative of the state or federal agency con- l d for additional technical help. iThe irrigation farmer is urged to contact and work closely with personnel of these agencies. Their f: ' g an irrigation system and in pump-power requirements. Since soil and climatic conditions vary greatly over Texas. it is impossible to give a set of rules for )- 'on which can be applied to every problem area. However. by studying and considering all aspects ¢ interrelated soil-plant-water system, the irrigation farmer should be in a much better position to 1 out a suitable soil and water management program for his farm. It is of maior importance that the - or be more concerned about what is taking place below the soil surface than he is about the con- 1-: above it. ‘ “i t . The great demand for irrigated water makes soil and water management increasingly important in l and experience often may save him many hours of work and sizable sums of money, especially in _ CONTENTS Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . j: Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I . . . . . . . . . . . . . . . . Rainfall Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . .; Soil and Water Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - . . . . .5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - . . . . i‘ . . . - . . - . . . . . . . . . . i . - . . . . - . . . . - - . - . - . , . . . . . . . . . . . . . . . . . . - . - . - - - Q Q . Q - . Q ¢ | - - | Q - Q Q Q ¢ Q - - - - - - . . . . - . - . . . . . . . . . - . . . - . . . - . . . . . . . . . - - - - Q . Q ¢ . Q ¢ - u - - - - ¢ ¢ - - Q Q . ¢ - - - . - - - - - - Q u ‘ . - . - - . ¢ - - - - - ¢ . - - . - . - | . ¢ - - - - - - . - - - - - -‘ - ~ . - . - - - - - - - . . . - . - - . . - ¢ . - . . - . - - - - . . - . . . - - ‘ - . - . - . . - . . - - - - . . - - . . . . . - - - - .- .-......-...-.-.~.-..--...-..........-...-.--.......-............i l’ . . . . . . . . . . . . - . - . . . . . . - - - - Q - - . Q - - - Q - - Q . - . - Q - - - - ~ . - - - - - - - - . . i . o Q - . I - - - - - - . | - . - . - - - - . . . - . . . l I . - - . - . - . . - - - . . ~ . . - - - - - - - - . . . . . . , . . - - - . - - - . . - - - - . - - . . - . - - . - - - . - - - 1 Methods of Conveying Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preparation of Land For Irrigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Common Methods of Surface Irrigation Furrow Irrigation Border Irrigation . . - - - . - - - . - . - . - - - . . . . - - . - - - - - - . i . - - - - . - - - - . . - - . - . Q . - . - - - - i - ll . . . . V . . - - - . - - - . . . . . . - - . - - - - - . Q - . ¢ - - - - - . - - - . - . . . . . . . - . - . . . - . l - . . . - - . . . . . . . - . . . - . - - . . . . . - - - - - - - . . . . ¢ . - - - . - o Q . ¢ ¢ ¢ - . - Q ¢ u ¢ - ~ - - - ¢ - - - - . . . - - ¢ - - a a - - - - . . - . . . . . . - . . - . - . . - . . Y ..............-.-ii.-.-A...-..-.--..-....-..-.--.-..-.--...--.-.--.-......... Sprinkler Irrigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..i Frequency oflrrigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . When to Irrigate . . _ . . . . . . . . . . . . . . . . . . . - . - . . . - . - . - . . - . . . - . - . - . . . . . . . . . - - .- . . . - - - . - . . - . - . . - - - . - . - - .- . . . . . - - - . - . . . . . - ¢ . . - - . - Q Q Q n Q ¢ ¢ Q - - - - - - - - - . . , , — - - . - . - - - . - - - - . - . . . . . - - - - . - . . - . . . - . ' . - . . . - - . . - . - - - . - - . . - . . - - . Q - - . - . - . . - I ‘ O IIIIIOIIIIIIIIIIIOOIIIIUOII Irrigation Frequency Guides Soil Moisture Indicating nevigggfifIfjfffffflIfiIIiIIifIffjfIIIIIIYIIIIIIIIIIIIIIII} Amount of Irrigation Water to Apply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aclmowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - . . . - - . - . - ~ - - . . - . . . . . . . . . Calculating the Amount to Apply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Calculating Irrigation Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . q . . . . . . . . . . . . . . . . . . . . Water Use Problems . . . . . . . . - . . - . . - - » - ~ - ¢ - ¢ . - n ¢ Q ¢ - - - . . . - - . - . . . - . . - . - . . . . . - . - ll - . . . . . - . - . . . . . . - ll Formulae and Data for the Irrigator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Definition of Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - . - . - - . . - . - - - . - - - . - - . - ' - - . - . - - . . I ATER IS ONE of the most important factors controlling crop yields in Texas. Even the presentirrigated acreage is estimated ear 7 million acres, the water supplies have A xed t0 the limit in some of the irrigated and nearly exhausted in other parts of the " Although more available acreage is well w to this type of intensive agriculture, ansion of irrigation to other areas will endent largely on water resources as yet ‘loped. pplemental irrigation is essentially a meth- providing water for plant use when the g: rainfall is inadequate. To a lesser ex- rrigation water also may be an aid in con- soil and air temperatures, reducing wind - by making vegetation possible and to te the leaching of excessive soluble salts accumulate in the root zone. 1 important objective of irrigation is to ute water uniformly over the field so that pil storage reservoir can be filled with a um of losses by surface run-off, deep per- a below the root zone and evaporation. In ng, each plant has a better opportunity in its share of water and nutrients from il reservoir, thereby making the most effi- use of all water applied. Irrigation must i ied on to conform with certain basic ag- 'c and engineering principles if these ob- tiare to be accomplished economically and _n y. .~ use of water for irrigation during the 9 years has grown tremendously in the hu- nd sub-humid regions of Texas. At pres- .. e interest in supplemental irrigation is y rapidly eastward to the humid region of iTexas where the average annual rainfall e than 40 inches. Many farmers in the é areas have found that supplemental irri- is necessary almost every year to obtain ’ ble yields and quality of crops. Although u; cannot be prevented, their severity can ened by making the most efficient use of ptl and water which may be available for 10H. Fce supplemental irrigation is instituted into ing enterprise, it becomes an integral part __ farming operation and should not be used i; y as “stand-by insurance.” When used j. the latter way, the application of irrigation f is likely to be done too late to insure maxi- ireturns. A farmer cannot afford to rely 'n predictions before starting to irrigate. though rainfall usually provides a good mc Principles and Practiccs in t/zc Jrriyaticn cf Zfci/as Sci/s MORRIS E. BlOODWORTH Associate Professor. Department of Agronomy portion of the needed water for crop production each season, its frequency is not dependable enough to eliminate the need for advance planning of each irrigation in the water management program. Irrigation agriculture today includes all phases of production from a fundamental know- ledge of soils and crops to harvesting and mar- keting. This fact is made more impressive when one considers that supplemental irrigation in some sections of Texas makes it possible to grow as many as three crops in one year on the same land. Such an intensified agriculture requires much thought and foresight by the farmer. Irri- gation also requires sound planning; knowing how, when and how much fertilizer to apply; initiation and maintenance of a well coordinated soil and water program to suit crop needs; and" above all, initiative by the farmer to study and improve his own farming practices from day to day and season to season. Experience has shown that water, if controlled and applied wisely in the required amounts and at proper stages of plant growth, will conserve soil productivity; increase yields through more efficient use of wa- ter and fertilizer; make more crops adaptable to a given locality; give better control of crop qual- ity and maturity; offset serious drouth hazards; help to control wind erosion; and usually return greater profits. Although irrigation farming has many ad- vantages, it also has its disadvantages. Perhaps the greatest problem facing all irrigated areas is an ample supply of water of suitable quality. Many of the surface streams ceased flowing during the recent drouth and the ground wa- ter supplies diminished at an alarming rate. With the increasing demand and competition for water, the need for better physical control of both rainfall and irrigation water on farms be- comes more apparent each year. Only through a coordination of good physical control of irri- gation water over the land and the best soil management practices can the farmer hope to obtain maximum production per unit of water used. Irrigation farming also requires more capital for initiating and maintaining farm operations. This presents a drawback to many farmers be- cause of the initial investment in developing a water supply and acquiring the additional equip- ment and power necessary. Problems of addi- tional labor, increased fertilizer requirements, insect, disease and weed control, tillage methods and many other production factors are multiplied 5 Figure 1. Rainfall belts in Texas. Values shown are annual rainfall averages. Texas Agricultural Exten- sion Service Leaflet Z32. and intensified with the inauguration of an irri- gation program. It is essential, therefore, that the potential irrigator consider all aspects of irrigation farming before definitely deciding his course. Irrigation involves much more than the application of water to the soil. No two soils are exactly alike in their chem- ical or physical properties, or in their behavior when subjected to irrigation practices. Irriga- tion on shallow, sloping soil is dangerous from the standpoint of erosion, but deep soils permit mechanical treatment which will afford better water distribution and control. Unless properly Figure 2. The average normal Texas warm season rain- fall, April through August. Texas Agricultural Exten- sion Service Leaflet 244. 6 controlled, irrigation water may become a h 7' to productivity by wasting water through face run-off and deep percolation below the p, zone; by leaching soluble plant nutrients ‘l the zone of maximum root concentration; by making conditions more favorable for ei starting or aggravating an existing draia (salinity) problem. This latter condition is q desirable because in manywcases the land is i for agricultural use unless reclamation meas’ are instituted. Such measures often are expensive and require several years before c can be grown profitably again. ' Even though the potential irrigation fa may have an ample quantity of both Water " land, their wise use and management, along r other contributing factors, often dictate whe irrigation will become a permanent and p A able part of his farming operations. RAINFALL DISTRIBUTION The average annual rainfall in Texas v: from about 6 inches at El Paso to more t 50 inches in a portion of Southeast Texas, shown in Figure 1. A complete summa , weather records for Texas through 1953 is ;'-_ in Texas Agric. Expt. Stat. Bulletin 787, f April through August precipitation, or W season rainfall, is shown in Figure 2. ' These maps show a wide variation in r fall over the State; a large portion of it is, received during the summer growing seasonf pecially in South and East Texas. As a re during critical periods of plant growth (bl fruiting-maturing stages) in which moistur, needed by plants, a deficiency of rainfall quently exists. .- Eighteen to 24 inches of annual rainfall ’ result in relatively high yields of most c y provided it falls during the critical stages:- moisture demand by crops. However, muc . the rainfall throughout Texas occurs from t I derstorm activity which causes precipitating fall at high intensity rates, and the soil is not 1f to absorb water fast enough to prevent la »‘ scale run-off. Almost complete loss of rainw occurs in some cases; consequently, one ca depend on average annual rainfall values. G f ers should be more concerned about the sea‘ rainfall frequency distribution pattern; tha, the periods when rainfall is most likely to u and the amounts that may be expected du any given period. Such information is valuj in planning water management programs for irrigation of crops. » son. AND WATER MANAGEMENT Soil and water management have long i’ recognized as important factors affecting . yield and quality of crops throughout Texas. important fact is well illustrated when one ~=_ the interrelationships which exist within amic system composed of soil-water- vrertility-climate and how a variation in r can change the overall equilibrium of em. farmer works with a three-phase phys- 5: em composed of solids (soil particles), f (water and soil solution) and gases (oxy- rogen, carbon dioxide and others). To maximum yield and return from his in- “t, the irrigation farmer is required to 'n the proper balance among these phases g the plant will have the best possible en- ent in which to grow. approximate quantity of solids, liquids ‘yes (air) contained in a cubic foot of fer- } in good physical condition is illustrated e 3. - percentage of each constituent will vary ‘Ail to soil and often from day to day. For le, following heavy rainfall or an irriga- ‘II percentage of water in the pore spaces Il particles will be greater than that of e opposite is true during dry periods i-soil moisture is low. The amount of air also can be changed by tillage operations. 1:. usually increases the air capacity when re relatively dry and decreases it if the “Ire tilled while too wet. haps the most important single factor for ining the best irrigation practices to be i; the physical properties of the soil. The p’ ent and retention of Water and air in the Y»: are important factors affecting plant V?» . The rate of movement and availability e soil components for plant use depend y on the physical and chemical character- of the soil. I? planning soil and Water management prac- the farmer should analyze the fundamental ‘concerning the physical, chemical and bio- _, characteristics of his soil and how each modified with the proposed practices to tituted. The depth of soil available for owth, moisture retaining and release char- tics of the soil throughout the rooting available water storage capacity, nutrient ing ability of the soil and physical struc- haracteristics are important factors which y affect and often control plant growth. 5| agricultural‘ soil may be considered as a for “plant food” storage. It is an anchor- w plants, contains bacteria and other micro- j isms and serves as a reservoir for the stor- f water needed for plant growth and de- ment. Generally, soil texture, structure and 'ty can change or modify the effectiveness factors in crop production. e question arises as to what is soil texture, ure and porosity, and how do they affect Water management practices under irri- ' conditions. souos PORE SPACE I-—— 50% —~—*I<—— 50% —-—-*| Minerals I Water I 48% I 25% I I I I I I I I I L _ _ - __ _ _ _I | \ | \ I \ \ Airzso I \ \ I \ '° I \ I I I I I I k \ k \ \ \ \ \ Figure 3. A generalized block diagram showing the approximate composition oi a cubic ioot oi fertile soil in good physical condition. From “The Story of the Soil." An agricultural soil consists of a mass of solid particles with spaces between them. The solids may vary from rocks and stones several inches in diameter to a size so small that a high-power microscope must be used to detect them. This latter size range is oftenreferred to as “colloidal” and is exemplified by muddy water, which is 9O 3O \\7‘°'\‘T “x/ v v“ ”*7\/\ So 550 ° u :9 ‘ ‘..\ 0o o ‘b o ‘b PERCENT SAND Figure 4. The percentages of sand (0.05 to 2.0 mm). silt (0.002 to 0.05 mm) and clay (below 0.002 mm) in the basic textural classes. Modified from USDA Handbook 18. To use the chart: (a) determine percentage compo- sition of sand, silt and clay in the sample of soil: (b) locate individual percentages along each side of the triangle; (c) extend a line from each soil fraction until the three intersect: (d) read textural classification di- rectly from the chart. nothing more than small clay particles in sus- pension. By grouping the different particle size ranges, it is possible to divide the groups or fractions into gravel, sand, silt and clay. These are termed “soil separates.” Then, the combina- tion and mixture of these size ranges in the soil is referred to as “soil texture.” To be classed by a textural name, such as sandy loam, silt loam, clay and others, the soil must contain the approximate percentage range of soil separates as shown in Figure 4. Soils in which the sand fractions predominate usually are referred to as “coarse textured.” Those containing a large portion of clay are called “fine textured” soils. Those soils containing size fractions in between, such as a loam or silt loam, usually are termed “medium textured.” Most well managed agricultural soils have a tendency for the individual soil particles to go to- gether (aggregate) to form relatively stable clus- ters. The size, shape, stability and spacial ar- rangement of these naturally occurring aggregate clusters is called “soil structure.” Some of the clusters may be soft and crumble readily, others may be more stable when wet and resist consider- able pressure. Soil structure is considered to be good when the sand, silt and clay particles are as- sociated in water stable aggregates which in turn are formed into larger clusters. A soil lacks structure when the aggregate clusters disinte- grate into their component particle size when wet by water. Good soil structure is important in agricultural soils because of its favorable in- fluence on water and air movement within the root zone. Soil porosity (pore space) is the amount of space not occupied by solid soil particles. In considering porosity of soils as related to struc- ture, the total porosity is not always as important “as the relative size distribution and shape of the soil pores. "As an example, clays tend to have higher total porosities than sands; however, the pores are relatively small and contribute to high PORElAlRl SPACES FILLED WITH WATER A B C sou. SATURATED WITH eooo sou. WATER - AIR AVAILABLE son. WATER WATER CONDITIONS REMOVED TOO WET ABOUT RIGHT l9_O__DR_Y Figure 5. Soil-water-air relations in an agricultural soil iollowing an irrigation. Plenty of available water and air. is not available to plants. . textured sands will release water to plants ¢_ SOIL PARTICLES YSOIL PARTICLES (c) Plenty oi air space—only water films around the SOil particles. but this w i water-holding capacities, slow water infilt I and internal percolation rates through the, profile and, in many cases, to insufficient r tion. Sands have fewer pores per unit vol but they are larger. This characteristic us accounts for better aeration and more rapi filtration and internal transmission of Y within the soil profile; however, the amou available water that can be?» supplied by I sands usually is less than clays. The uw more readily than the clay soils. . The clay fraction is the active part of‘ soil, both chemically and physically. The f and silt fractions are much less active an some cases, are almost inert. A high clay tent is more desirable from a fertility (chem standpoint, but undesirable in most cases I‘ considering physical characteristics. Ther i‘ for a good irrigated soil, one wants a soil '_ the proper portion of sand and clay which ably would be of medium texture; open stru for deep penetration of roots, water and air possess good water storage capacity along i acceptable surface and internal drainage acteristics. Although such a soil is rarely f in irrigated areas, it is to be desired. j These soil physical properties in irrig agriculture are important because a phy change in the soil can modify greatly the ment in which roots grow. Should the soil o ' become filled with water for long periods, A‘ respiration and growth will be retarded =f because of a lack of oxygen. » Figure 5 shows a simplified sketch of, relations which exist among soil aggrel water and air in the soil mass. Immedi following an irrigation, the soil pores are g with water, as indicated by (a). After a dad so, the excess water drains downward th a the soil profile unless a soil physical con_ exists which retards the percolation of W Following the percolation of gravitational SOIL PARTICLES PORE (AIR) SPACES (a) Too wet—poor aeration. (_ _ soil depths, only thick water films re- sorbed to the soil particles. This condi- mits air to fill the pore spaces formerly j7~ by water, as illustrated by part (b) of 5. As water continues to be removed n; soil by plants and evaporation, the water ome reduced in thickness which makes ter available for plant use. This condi- shown in part (c) of Figure 5. Although ‘isture films remain around the soil par- he water is retained with such high ten- l at the roots are unable to extract it suf- faslt to offset wilting and loss of turgor l ce s. ,, e the drier soil particles have much thin- s of water surrounding them, there is ncy for water to be drawn away from ad- ‘f»80il particles which have thicker moisture That is, water will be moved from zones of tension (wet) to those of higher tension This action is called “capillary attraction’? ‘pillary conductivity.” Ifexample of this mechanism of water move- occurs when the furrow method of irri- j is used to “sub” water to the top of the }bed from the lister furrow. Such a con- exists when seed are “dry-planted” and g-rigated. ance oi Soil Structure irhaps the most basic and far-reaching m confronting agricultural crop produc- i). Texas is poor soil structure. il structure is dynamic and can change as lt of time and management, or both, and tremendous effect on plant root environ- Clay plays an important role in structure ten is a controlling factor. Other factors may influence soil structure are tillage in and equipment, growing plants (type Aunt and distribution of the root system), “cal activity, organic matter content and fte (wetting-drying and freezing-thawing). dification in any of these conditions or ply others often is reflected in structure es within the soil. ater serves as a prime agent in breaking , soil aggregates by causing a swelling ac- which separates them into smaller aggre- i or eventually to the individual soil sep- Another action of water is to disperse Lil by falling raindrops. Through continued g by tillage and application of irrigation i without regard to the necessity of main- i): g good soil arnanagement practices, serious i ure deterioration also may occur. In many the dispersed soil particles are in the col- l range and are carried into the soil pores eposited as solid material. A continuation 's cyclic process eventually causes the pores ome filled with the solid material, which es internal water and air movement, and Figure 6. Poor management oi irrigated soils soon leads to unfavorable soil structure. The soils become cloddy and hard. which contribute to poor water-air relations. This is a Willacy loam soil. contributes to an unfavorable soil physical con- dition. Since the dispersing action usually occurs during each 1rr1gat1on or ra1n of moderate to high intensity, aggregation in the surface layer" eventually becomes so broken down and puddled that crust formation occurs at the surface, which in turn may restrict the diffusion of oxygen and infiltration of water into the soil. An example of such dispersing and puddling action is shown in Figure 6. The loam soil has been managed poorly under irrigated conditions and does not afford a favorable environment for plants to grow. Etiect oi Soil Compaction and Impervious Zones The movement and transmission of water in some soils of the irrigated regions of Texas are retarded greatly by the presence of a “hardpan” or “impervious zone.” This zone usually is con- fined to the top foot of cultivated soil and varies in thickness from 4 to 9 inches. These values i 0 Y’ l‘ v u ' ‘K 25,. 1 v§ COMPACTION ZONE _g________.._. ‘liar-nun ...,.._.__. -—:*;=:a-:< _4i.»=¢_. * " -:\:._.'-- _ " - '-'*‘ \\ ' ‘ \ ] \_ \\\ . - \. \ \ “_ \ |'.\ '.\~.’_.-‘. '. ‘_\~ .'\' ’ ._ ‘__ . Q \\ \, ‘2n\ :- ~- ‘Z ' '7- ‘ FREE PERCOLATION \ \ ' .1 \ \ v ' \- _ . \ _ ‘ \ »_ _ \ .. \ ’ ' \ ~ ‘u a |\ \ w i‘ l‘ _ _ \ ‘ I \\ \ *- \ x’ s ‘ _\ _ Q ‘y, '1‘ r.’ ‘ | ‘ ' "‘ .- (" \ I‘ \‘ “ "- ~ ' 1' 1' x .'> (l- ‘t T‘ -' ..£’ \ |'\' \\ \. \ " -‘ ' " . , ' * ' ' \1 \ __ .-.~ \ l . ~ | \ *-, , ‘ _\ \ \ r - ‘v . , l l ; ‘. _ I‘ __\ d '_- -.. _\ -\“. '\\ t\, ‘ ;"“\-\ __, * \\ _ x _' ‘ \~ ~ ‘ l o o‘ x .. * '.'\- 1 '- 5 ~ 0.. \ .- -“ ‘.. ' Figure 7. Influence of compaction zones on water per- colation and root growth. will change for different locations and soil types throughout the State. However, a soil (physical) condition of this kind greatly retards the perco- lation of water, restricts the plant’s»root system to a shallow depth, reduces available water stor- age capacity, lowers the oxygen diffusion rate to depths below the impervious zone and ad- versely affects fertilizer placement and utilization by plants. Studies conducted at the Lower Rio Grande Valley Experiment Station at Weslaco show that the rate of water transmission through the compacted zone in a fine sandy loam soil has been as much as forty times slower than in the soil profile above or below the compacted zone. The existence of such an unfavorable soil condi- tion is detrimental to good water and plant relations. The influence of compaction zones upon wa- ter movement and root development in the soil profile is illustrated in Figure 7. If high intensity rainfall occurs, or if irri- gation Water is applied at a faster rate than will percolate through the compacted zone, water will run off on sloping land, or may accumulate above the zone and cause a saturated condition on lands which are relatively level and do not have ade- quate drainage facilities. The large amount of soil reservoir storage below the compacted zone will not receive sufficient moisture unless water is applied to the surface for long periods. Ex- cessive irrigation water near the surface for long periods will affect plant growth adversely and possibly result in the accumulation of salt and eventually a salinity condition. Plants grown on soil in which a compaction zone is present often will show signs of nutrient defi- ciencies and moisture stress more readily than plants grown on soils where the soil-water-air relations are more favorable. The prevention of soil compaction zones has no exact practical solution at the present time. Certain tillage practices and cropping systems, however, tend to accelerate the formation of a compacted zone or so-called hardpan. The THE PLANT "PUM P" up“ <-—- 477496. , FIELD l .:,_-§e<:;; CAPACITY AVA|LAB_l_E WATER - a. w|LT|NG UNAVAILABLE PERCENTAGE WATER l A / SANDY LOAM SOIL l.0 — l.5 Inches/Ft. CLAY SOIL 2.0 — 2.5 Inches/Ft. Figure 8. A simple illustration of the eflect of environ- mental control on water use by plants. 1O formation of such a zone in some soils is a" ated closely with the type of clay mineral p ‘ in the soil, while its formation in others tributed to the loss of organic matter and ce chemical constituents needed in the soil to 1g tain a desirable structure. Two approaches are being made to all the detrimental effects of compacted zones p nomic and deep tillage practicfes (subsoilic chiseling). The agronomic phase consists 0 stuting and maintaining a well planned soil- agement program in which grasses and 5f rooted legumes are included to improve soil x ical properties. a Chiseling or subsoiling is used in many as a remedial measure but is considered a. porary relief practice. This treatment pro cannot be recommended in many cases of the heavy power equipment required an, expense of maintaining such an operation. some soils, the benefits from chiseling are s, lived because the soil tends to run back tog again. However, by combining the two pro l a more feasible method of helping to allevia 1 formation of the compacted zone may be po for future consideration. 5 Research has shown that it is possible f’ crease the effective rooting depth of cotton? other crops by the use of grasses and alfa 5 a rotation. The extensive cotton root system, make more efficient use of water stored inI soil reservoir at the deeper depths. This is portant where irrigation is practiced because extension of many additional roots to depths of the soil profile often may save on more irrigations during a crop season. _ saving also reduces the cost of labor and pol Soil Physical Properties Affecting Water Availability The significant factor in soil moisture a ability to plants is that the plants should be to withdraw Water from the soil fast enoug offset losses by transpiration and that use plant growth. The exact criterion for w moisture depends greatly on the depth and tensiveness of the root system, stage of ¢ growth, water storage properties of the soil, at which" the soil will release water to roo given tensions, transpiration characteristi f individual crops and weather factors which? fluence evapo-transpiration. , Water can be lost from the root zone (0 foot depth) in three ways: water can be rem from the shallow surface layers of the i through evaporation; water can percola ’ depths below the root zone when the soil is y irrigated or during periods in which excec amounts of rainfall occur; and water can‘ “pumped” from the soil by plants and trans to the air as Water vapor (transpiration). " agricultural crops are growing actively, the gr, ter loss from the soil usually can be at- FINE w to transpiration of the plants. This is l-OAMY SANDY CLAY lly true when plants reach the stage of WCHES OF WATER I. f: where they afford shade for the soil sur- APPLE” T° lllllllllllllllllllllwIlllllllllllllIlllllllllllllllIlllllllllllllllllllllllllllllIlllllllllllllllllllllllllllllllllllllllllllllllllllllllllll l " SOIL SURFACE ' lllCh in turn makes conditions fOI‘ evap- less favorable. a m“lllllllllllllllllllllllllllllllllllllllllllllllllllllllllWM% Y, _ . . . wu_|_ yet THESE "DRY i =i -=_re 8 illustrates the influence of chmatlc ifiiLsnel-cilisppgfgmflet‘m. ii 11 environmental factors on a plant when E" THE SHADED AREAS i ' as a “pump” for removing water from the rage reservoir. “Plant pump” control is ,, ,. 2Ft it about by climate and the available water will iii $5111 _ soil. A comparison of the two water reser- fiifiLggifi 332i» Mmsw" reveals that clay soil contains a larger 5H 'e 0f water than sandy loam at any given _ ing head” or tension. As the water lift moisture tension) becomes greater, more APPRQX|MATE QEPTHQF4Ft s is required to remove water from the soil “T” "E"“"‘“'°" ‘FEET’ 3'0 "8 "6 "3 "' oir because it is held more tightly by the SZQLQ,%TEO‘§“QZ.T ‘WES 1.0 L6 1.8 2.2 2.1 rticles and becomes less available to the '_ Although the Volume of Water contained Figure 9. Ranges in available soil water per ioot oi e soils is greater, its rate af release to depth in Texas soils oi diiierent texture. _ng plants may be much slower than the 1e rate normally found in Sandy Soils. soil texture, structure and other soil character- istics. The amount of water retained in the soil i: percentage,” both of which are illustrated gure 8. Field capacity is the quantity of l, retained in the soil after gravitational wa- s drained away following an irrigation or iusually 1 to 3 days later. The permanent _g percentage is the quantity of water re- ng in the soil after plants have withdrawn "ey can and permanent wilting occurs. Per- wilting does not necessarily mark any ite limit in the movement of water from the l: the plant. It indicates that the soil mois- content available for plant use is so low absorption becomes too slow to replace water by transpiration; consequently, plant cells Where salinity is a problem, the effect of sol- uble salts in the soil becomes important in water availability and must be taken into account. The use of saline waters for irrigation is not recom- mended. However, if their use is necessary, ad- ditional water should be applied at each irriga- tion to dilute the salt concentration in the root zone and to furnish extra water for leaching the soluble salts downward in the soil profile. In some cases, the amount of additional water needed for leaching may be as much as 15 to 75 percent, but usually is 10 to 20 percent for non- saline soils. iturgor, the plant wilts and eventually dies. The fact that different textured soils retain g capacity and permanent wilting percentage and supply different quantities of water is shown 0t constant points for any given soil type in Figure 9 and Table 1. Figure 9 indicates that ries, but are ranges and can change with a 3-inch irrigation (net water—exclusive of LE l. PHYSICAL CHARACTERISTICS OF SOME TEXAS SOILS AFFECTING WATER AVAILABILITY TO PLANTS A Capacity. Quantity of water retained in the soil after gravitational water has drained downward following an irriga- or period ot considerable rainfall (1 to 3 days later). Usually expressed in percentage. nent wilting percentage. Quantity of water remaining in the soil after plants have withdrawn all they can and wilt ently. Usually" expressed as a percentage. able water. The quantity of water retained in the soil between the limits oi field capacity and the permanent wilting l‘ tage is termed “available water" for plant use. It usually is expressed in inches per toot 0t soil depth. h. density of soil (sometimes called volume weight). Mass ot soil per unit volume of soil that has been oven-dried to out weight at 105°C. ’ Oven-dry weight of given volume of soil density = ’ Volume occupied by the soil mass ll Watereetaining properties of Soils are between these limits is termed “available water” mary concern to the irrigation farmer— for plan? 11.56‘ Available Water: usually is ex"..- "rms “field eepaeity” and the “permanent pressed 1n inches per foot of SO11 depth. Field capacity,‘ Approx. permanent Available water? Bulkflensity Available I percent wilting percentage” percent incvggéjgeot .°..’;'§,',“ii..... 1Z2}? 3.33 $1? 3132 32? i133 ijiilil 17-20 7.5- 9.5 9.5-lU.5 1.53 1.7-1.9 J '°‘"'“ i323? 132315113 131321313 i112 iiiiii? serves as a handle. losses) will wet the different textured soils to approximately the indicated depths, assuming that no barriers, such as compacted zones or clay barriers, interfere with water percolation through the soil profile. The values of available water per foot of depth are aproximates in both the figure and the table and should be used only as a guide. The depth of water penetration shown in Figure 9 stresses the need for studying and ob- serving the root distribution patterns of plants and the soil moisture conditions at different stages of plant growth. It emphasizes the fact that consideration should be given to the type of soil and its physical characteristics before ap- plying irrigation water. For example, if the maximum root concentration was in the 0-2-foot V zone (Figure 9) and sufficient moisture was be- low this depth, the 3-inch irrigation would have been too much water for the loamy sand and insufficient for the clay loam and clay soils. Therefore, a portion of the water applied to the sandy soil would have been lost by deep percola- tion below the root zone. The reverse would have occurred in the clay loam and loam soils because a part of the 0-2-foot zone would not have been wet sufficiently, A simple way to check water penetration following an irrigation is to dig a hole with a post-hole digger, sharpshooter shovel or soil auger within the root zone a day or so following the irrigation. Generally, the separation between moist and dry soil can be detected easily by the color and the “feel” of the soil. If one wishes to check penetration during an irrigation, a simple soil probe can be used by the irrigator. The probe usually is constructed from a 1/2 inch diameter steel rod, 3 to 4 feet in length. A 5%,- inch or 1-inch diameter pipe welded on the rod The handle length varies between 18 to 24 inches. The ease with which the probe moves into the soil gives an indication of the depth to which water has penetrated. As the probe moves into dry soil, more pressure is required to move it deeper. Any obstruction in the soil, such as impervious zones or clay pans, will affect the use of a probe. Such a method is not exact, but it indicates to the irrigator whether the irrigation water is penetrating to the predetermined depth within the soil profile. Soil moisture is not equally available to plants between field capacity and the permanent wilting percentage. The ability of plants to absorb water decreases as the soil moisture tension increases. Plants often can withdraw water down to and sometimes below the permanent wilting percent- age of the soil on which they are growing. How- ever, their growth will be decreased before the soil moisture in the zone of maximum root concentra- tion is depleted to such an extent that signs of se- vere wilting are shown by the plants, especially in the mid-morning when hot, dry weather condi- tions prevail. As illustrated in Figure 9 and Table 1, variations in the ability of different 12 soils, or even a soil within a single field, tot + fer and release water make it difficult to f' definite point at which plants will need irrigaw» However, to keep plants growing and frui properly requires that the available soil mois be maintained in ample quantities throug the season, especially during the bloom, frui and maturing stages of growth. Plants she not be “dried out” to develop Zor “force” a de and more extensive root system. In most such a practice only retards plant growth is detrimental to quality and yield. ' Too much water applied during the - stages of plant growth often retards growthi creating an unfavorable condition in the p environment. Such practices tend to keep _ soil too wet in the root zone of young pla: which results in a reduction of oxygen, ca . the soil to remain cold during the spring a, makes the young plants more susceptible to s ling diseases. Excessive water during the latter stages». maturity for many agricultural crops may -; in too much vegetative growth. It is essen therefore, that the “proper amount” of soil l. ture be readily available at all times in the I of maximum root concentration. The “pro amount” of soil moisture may be compared ~- that quantity of moisture needed to germi and sprout cotton or corn seed at planting t' a That is, if the grower is sure that the soil in.» root zone contains sufficient moisture for . seed germination and growth, then more t, likely the soil still contains sufficient water , continuous root and top growth. Otherwise, ‘ soil should be kept moist, but not wet or satura for prolonged periods. . IRRIGATION WATER SOURCES Wells Many supplemental irrigation systems de on water being supplied from shallow and d wells. Water from underground reservoirs used largely because of a lack of dependable f face water supplies for furnishing dome, municipal, industrial and agricultural needs. ‘ fact, the underground water supplies for u.‘ areas of the State are so heavily taxed that 1 water level is lowering rapidly. The pump has reached such a level in some cases that A economics of such water use for agricult purposes is questionable. Several questions should be considered b ; proceeding with plans for drilling a well l irrigation purposes. They are: (1) Can be obtained at a reasonable depth to supply needs for irrigation purposes? (2) If suffic water can be obtained, is its quality permiss. for use on the soils and crops to be irriga How much salt does the water contain and w are the concentrations of the principal const' ents (cations and anions)? Water suitable = ng is not necessarily acceptable for wide- f- use in irrigation. (3) Are legal require- involved, such as drilling permits or a _ ion between underground flow and per- t» Waters? Other problems and questions if) ise for any particular area of Texas, but éjust listed should be foremost in the farm- "nd. ells usually are expensive from the stand- f». of drilling and placing into production. f er, they do have an advantage in that Water _ilable for irrigation when needed. For pe irrigation, it is desirable that the well ce at least 450 gallons per minute (1 cubic second) for about every 60 acres to be ted. Wells of this capacity should not be ded on to irrigate more than 80 acres. Some tion wells are in use which produce as little ' to 100 gallons per minute, but such cases vii too common except where small acreages volved. e pump capacity needed depends greatly amount of water available, acreage to be ted, type and diversity of crops to be grown, i. of year in which the water demand will ‘e greatest, length of growing season and gtal water deficiency that will need to be up by the use of supplemental irrigation. f these factors are important when consid- the type and size of Well pump to buy. If ems are encountered which the farmer does eel he is capable of handling, he should con- the local county agricultural agent or op- ‘,0 personnel of such an agency as the Soil rvation Service. Generally, most of the Jercial organizations that sell and service i; pumps also have competent personnel avail- 1for consultation with farmers on such prob- ' as wells and well drilling. Servicing of pips and motors is important and should be “dered when purchasing Well pumping equip- "iischarge curves and water application times different quantities of irrigation Water are in Figures A, B and C of the Appendix. ace Reservoirs arm reservoirs can be used for storing water V I g periods of heavy rainfall. Such Water ‘be used on crops or pastures during a drouth. 2i reservoirs should be designed and con- 'ted by competent personnel. Such factors ji site, size of watershed, type of soil, amount ater storage capacity needed for irrigation {domestic use and the size of the spillway for s“ water discharge should be considered , u y. , nless a farm water reservoir has a constant 5i e of water supply, such as a spring or ar- »: well, the water storage capacity should '3 bout twice the amount needed for irrigation. added capacity is to allow for losses due ‘epage, evaporation, livestock-domestic uses and any small conveyance losses which may occur 1n transit from the reservoir to the field. The storage capacity of a farm reservoir can be estimated roughly by multiplying the surface area (acres) by its average depth in feet. As an example, assume that a farmer is interested in irrigating 10 acres of tomatoes and plans to construct a reservoir of sufficient size that five irrigations of 2 inches each can be applied. Then, 5 >< 2 I 10 inches of water needed for each acre (10 acre-inches), or 10 >< 10 acres I 100 acre- inches (8.3 acre-feet) of water will be needed for the 10-acre field. At least 200 acre- inches (16.6 acre-feet) of Water should be avail- able from the proposed reservoir (to take care of the losses and supply the amount of water needed for irrigation. To construct a reservoir to ful- fill these requirements would necessitate it cov- ering 2 acres with an average depth of 8.3 feet; or, any similar multiples of 16.6 acre-feet which would best suit the individual farmer’s set-up and reservoir location. The exposed surface area of water should be as small as possible to reduce evaporation losses. This usually necessitates a deeper reservoir. Rivers and Small Streams Water for supplemental irrigation is supplied in some areas of Texas by rivers and small streams. In many cases, this source of water has been adequate for the farmer’s use, but in others the quantity of water has not been sufficient to meet the requirements of the crops being irri- gated. This is especially true during peak de- 5000 73 —Legend- 4000=parts per million (p.p.m.) 6.3=(ECx|O3) millimhos/cm. Salt Concentration of Water -p.p.m. and Millimhos/cm.(ECxl03) G7 OI llllllllllllllllllllllllll‘llll|lllllllll|IIIIIIIIIITYIYI iiiliiiiliiiiliiiiliiiiIniilniillluliiiililiilliiilnlfl 0 I 2 3 4 5_ Tons of Salt Per Acre—Foot of Water Applied Figure l0. Effect of irrigation water quality on salt deposition in’ the soil. As shown above. water con- taining 2.500 parts per million will deposit 3.4 tons of salt per acre-foot o! water applied. * ~ 1s mand periods when water is needed most by the crop. Therefore, it is essential that the potential supplying power of the river or stream be con- sidered to prevent the possibility of “getting caught” during the growing season with some expensive irrigation equipment and no water available for irrigation. There are certain legal restrictions on the use of water which flows in rivers and streams within the State. The potential water user should con- tact the State Board of Water Engineers in Aus- tin before proceeding with plans for using water from these sources for irrigation purposes. IRRIGATION WATER QUALITY A-ll waters used for irrigation contain various amounts of different chemicals which are re- ferred to as soluble salts or dissolved salts. Water in the soil contains soluble salts and is referred to commonly as the soil solution. Concentration No"' Co**+Mg++ Meq._/I. Meq/l. 2o o -o.25 0.50 -o.1s |.o :5 no s no 5 |s o 2o A i B Figure ll. How to determine the sodium-adsorption- ratio (SAR) value of irrigation water. Example: The chemical analysis of water to be used for irrigation showed the following concentrations (milliequivalents per liter): calcium (Ca), 3.22; magnesium (Mg). 1.99: and sodium (Na), 5.12. Calcium plus magnesium : 5.21 meq/1. By locating the sodium and calcium plus mag- nesium concentrations on each of the two vertical lines (A and B). and then connecting the two points. the SAR value is shown to be about 3.2 on line C. The same result can be obtained by solving the equation for de- termining the SAR value. Further explanations and definitions can be found in the appendix. Modified from USDA Handbook 60. 14 of the soil solution depends largely on the qu of irrigation water being used, moisture con of the soil and the physical-chemical chara istics of the soil. Generally, water in the top. inches of soil will be two to four times the i centration of irrigation water; however, w, at depths of 4 to 8 feet may be three to I times more concentrated than the irriga water. Such an increase ‘inkconcentration ally is brought about by the leaching of sol salts downward in the soil profile, mostly result of water evaporation from the upper . levels. The effect of salt concentration in i’ gation waters on salt deposition in the soi illustrated in Figure 10. Some of the individual constituents of the v solved salts may be beneficial to plant grol and soil physical properties, but others are ha a ful. In most cases, the total concentration; dissolved solids for most irrigation waters acceptable quality will range between 100 i‘ 2,000 parts per million (p.p.m.). However; exact limit can be set for the maximum pe H sible salt content of irrigation water since s limitation depends greatly on the kind of c: in solution and the type of the soil, subsoil dr age characteristics, tolerance to salt of the to be grown and water management practices , the farmer. On deep, coarse-textured, permeable and w ‘ drained soils, which contain plenty of free é cium carbonate (CaCO3), the use of water w. taining high salt concentrations may not be g detrimental if proper precautions are tak Only water containing relatively low concen _ tions of salt should be used on fine-textured (c loams and clays) soils because large amounts salt remain within the root zone due to the l soil permeability and poor internal drain characteristics. An analysis of the total soluble salts or i: salts does not tell the entire story. One to know the concentration of the individual c stituents to evaluate properly the desirability"; a water for irrigation. In general, howev water quality may be determined by the I concentration of soluble salts, relative propo of sodium to other cations, boron concentrati- and the bicarbonate concentration as related that of calcium and magnesium. The soluble salts are formed by combin’ the positively charged (cations) and negativ charged (anions) parts. The more impo salt constituents in irrigation water are sodi I calcium and magnesium, which are known, cations, and "chloride, bicarbonate and sulf” which are called anions. Other constituei which generally are present in low concentratii ’ are potassium, carbonate, nitrate, silica and , ron. Other chemical "substances may occur‘ irrigation waters, but they are unimportant most cases and are seldom reported in analyses. i 4 convenience, it is customary to report ations of cations and anions in water es as though the actual salts existed in the A Some of the most common salts in irri- 4 waters are: ‘ ON FORMULA 1o E CATION ANION (SALTS) i Calcium Bicarbonate Ca (HCOS) 2 1 m Calcium Sulfate CaSO4 Calcium Chloride CaClz Magnesium Bicarbonate Mg (HCOs) 2 u salts Magnesium Sulfate MgSOi ¢ salt Sodium Chloride NaCl , rs salt Sodium Sulfate NazSOr f ~ a Sodium Carbonate NazCOa pg soda Sodium Bicarbonate NaHCO3 1:: lcium in the form of calcium sulfate (gyp- plis probably the most beneficial of the ele- ~= added to the soil through irrigation water. ‘sium also is present in the form of mag- 7;: sulfate which, in combination with cal- " helps to prevent sodium from damaging the cal structure of the soil. An ample supply I ium tends to cause flocculation of the soil * les and keeps the soil from becoming hard ‘O a permeable. dium is troublesome in many irrigated soils = se it tends to cause puddling or dispersion e soil particles, which in turn causes a seal- j ffect and lowers water infiltration and per- ility characteristics. Such an occurrence ecially prevalent in soils low in active cal- Sodium occurs in solution as the chloride, te and bicarbonate, all of which are unde- le from the standpoint of soil physical prop- _.= and plant growth. amage by sodium is determined largely by relationship between sodium and calcium- esium. A given amount of sodium may not rmful if the water or soil, or both, contain ively large quantities of calcium and mag- m, or if the soil contains relatively large ' nts of limestone or gypsum. Harmful ef- . of sodium sometimes may be decreased by v ing gypsum to soils which do not contain a ient calcium. High concentrations of both or m and bicarbonate in irrigation waters are ii ially undesirable. t is difficult to set precise limits for the per- ‘ble sodium percentage because various types oil usually react differently to water con- ng a given amount of sodium. As long as * sodium percentage remains below a range ll to 60 percent, trouble not likely will be en- tered unless {the same water contains large tities of bicarbonates. xcessive chlorides and sulfates often will y tip burning o:t' the leaves and, in some cases, cause defoliation. Irrigation waters which ‘in relatively high concentrations of chlorides ially 250 to 400 p.p.m.) sometimes are con- red undesirable for use with salt-sensitive TABLE 2. PERMISSIBLE LIMITS OF BORON FOR SEVERAL CLASSES OF IRRIGATION WATERS‘ Sensitive Semi-tolerant Tolerant Boron class crops. crops. crops. p.p.m. p.p.m. p.p.m. 1 0.33 0.67 1.00 2 0.33 to 0.67 0.67 to 1.33 1.00 to 2.00 3 0.67 to 1.00 1.33 to 2.00 2.00 to 3.00 4 1.00 to 1.25 2.00 to 2.50 3.00 to 3.75 5 1.25 2.50 3.75 ‘From USDA Handbook 60. plants. Chloride salts are soluble and can be leached from the root zone, provided the soil is permeable and has good sub-surface drainage. Boron is found in most irrigation waters. Its concentration may vary from a trace to several parts per million. Although boron is essential for plant growth, the requirements are very small and the element may become toxic to many plants when concentrations above 1 or 2 parts per mil- lion are reached. Unlike chloride and sulfate salts, boron compounds (borax) are not easily leached. A general classification of irrigation waters according to boron concentration is shown" in Table 2. The relative tolerance of some agri- cultural plants to boron are presented in Table 3. Salinity The classification of irrigation waters accord- ing to quality requires a chemical analysis to determine the total concentration of the dissolved constituents, the percentage of sodium and usu- ally the amount of boron. The total concentration of salt may be expressed in terms of electrical conductivity, equivalents per million (e.p.m.) of cations and anions, milli-equivalents per liter or the dissolved solids '(see page 53 of Appendix.) Generally, the electrical conductivity is measured because such a determination may be made rap- idly and precisely. By using electrical conduc- tivity measurements, the U. S. Salinity Labora- TABLE 3. RELATIVE TOLERANCE OF SOME AGRICUL- TURAL PLANTS TO BORON‘ Tolerant Semi-tolerant Sensitive Athel Sunflower (native) Pecan Date palm Acala cotton American elm Palm Potato Navy bean Sugar beet Pima cotton Plum Garden beet Tomato Pear Alfalfa Sweetpea Apple Gladiolus Radish Grape (Sultanina Onion Field pea and Malaga) Turnip Barley Kadota fig Cabbage Wheat Persimmon Lettuce Corn Peach Carrot Grain sorghum Orange Oat Avocado Zinnia Grapefruit Pumpkin Lemon Bell pepper Sweet potato Lima bean ‘From USDA Handbook 60. 15 TABLE 4. CLASSIFICATION OF IRRIGATION WATERS ON THE BASIS OF ELECTRICAL CONDUCTIVITY‘ 3 Conductivity range, Parts Class micromhos/cm.’ per million 1 100-250 70- 175 2 251-750 176- 525 3 751-2250 526-1575 4 Above 2250 1576 and up ‘For further explanation, see page 53 of the Appendix. ‘To convert to mhos/cm" divide these values by 1,000. “Classed according to U.S. Salinity Laboratory. Riverside. California. tory at Riverside, California, has divided irri- gation waters into four classes with respect to conductivity. These classes are listed in Table 4. The classification of irrigation waters is based on the assumption that most waters will be used under average conditions with respect to soil texture, permeability, drainage characteristics, amount of water used, local climatic conditions (temperature and rainfall) and salt tolerance of the crops grown. Therefore, where conditions deviate widely from average conditions in the area, the water should be used with caution. _Class 1 (C-1 )-L0w Salinity Water. water is safe to use on practically all crops soils with little chance of saline conditions veloping. Some leaching is desirable to ‘a soluble salts moving downward. Soils of T tremely low permeability may not get suffic leaching under ordinary practices and shoul checked occasionally. ‘a Class 2 (OW-Medium Salinity Water. ‘t ters in this class can be used to irrigate ' which are relatively permeable and plants w have a medium salt tolerance. Generally, -; practices for salinity control will not be requi unless insufficient leaching occurs. Class 3 (C-3)—High Salinity Water. class of water should not be used on soils w have inadequate subsurface drainage (low I meability). Salt-tolerant plants should be " sidered and special soil-water management p _ tices should be maintained in cases where water in this class is used. Adequate sub-surf drainage facilities should be installed if sa spots occur in the field and cannot be allevia by leaching. TABLE 5. THE RELATIVE TOLERANCE OF SOME CROP PLANTS TO SALINITY‘ Salt tolerance High Medium Low FRUIT CROPS Salt tolerance High Medium Low F ORAGE CROPS EC X 103:18 EC X 103: 12 EC X 103:4 Date palm Pomegranate Pear Bermudagrass White sweetclover White clover Fig Apple Rhodesgrass Yellow sweetclover Red clover Olive Orange Rescuegrass Perennial ryegrass Ladino clover Grape Grapefruit Barley Dallisgrass Peach Birdsfoot trefoil Sudangrass Strawberry Hubam clover Lemon Alfalfa (California Avocado common) Tall fescue Oats (hay) Orchardgrass Smooth brome Sourclover EC X 103:12 EC X 103:4 EC X 103:2 VEGETABLE CROPS FIELD CROPS EC X l03:l2 EC X 103: 10 EC X 103:4 EC X 103:16 EC X 103: 10 EC X 103:4 Garden beets Tomato Radish Barley (grain) Rye (grain) Field beans Kale y Broccoli Celery Sugar beets Wheat (grain) Spinach A Cabbage -Green beans Cotton Oats (grain) Bell pepper Rice Cauliflower Sorghum (grain) Lettuce Corn Sweetcorn Flax Irish potatoes Sunflower Carrots Castorbeans Onions Peas Squash Cucumber Cantaloupe ECX l03:10 ECX 103:4 ECX 103:3 ECX103:l0 ECX 103:6 ‘Modified from USDA Handbook 60. Numbers following EC X 103 are the electrical conductivity values of the saturation’ tract in millimhos/cm. at 25° C. that are associated with a 50% ing salt tolerance within each group. However, a difference of several places in a column may not be of too great tance. l6 decrease in yield. The crops are lisited in order of deer: is l (C-l)-—Ve1"y High Salinity Water. Y which fall in this class should not be used , : tion under ordinary circumstances, but, pecial conditions, occasional use would be ible. Such conditions would entail per- soil, adequate sub-surface drainage and lication of excessive irrigation water to adequate leaching to prevent a build-up of in the root zone. Only crops which will i‘ léigh concentrations of salt should be re . f~ y plant species may differ markedly in lerance to saline waters and salty soil Tons. The effect of salt often is very pro- “=4 in the growth and appearance of some lspecies, whereas the adverse effects are gradual and not as quickly recognized in jpecies. Most crops can be appraised for lerance by their ability to survive under " conditions or by the effect of salinity on ields. The relative salt tolerance of some in Texas is shown in Table 5. - reaction of plants to salinity may be vced greatly by local climatic conditions. ' ore, suitable salt-tolerant strains and va- . should be selected for local conditions and g based on conditions which may be far- from the local environment. 'gation water quality also may be classed ing to the extent and rate at which the .'ll absorb sodium from water that contains n amounts of this chemical constituent. wrke such classifications, a method called the m-adsorption-ratio” (SAR) was developed e U. S. Salinity Laboratory. This method }:- on the adsorption of sodium by the soil g be used as an index of the sodium hazard iter. The defining equation for the sodium- I tion-ratio is: SAR I Na* V Ca++ _+_ Mg++ 2 Se Na‘, Ca“ and Mg“ are concentrations J in the chemical Water analysis as milli- alents per liter of the respective ions. A ,1 chart (Figure 11) was developed to de- " e directly the SAR value for a given water “ut having to use the formula to make cal- IOIIS. rigation water classification with respect SAR values also was divided into four ps by the S. Salinity Laboratory: low p), medium "(S-2), high (S-3) and very high j sodium water. These classes are based arily on the effect of sodium on soil physical itions. It was found, however, that plants itive to sodium may be injured because of _ cumulation of this constituent within plant Se, even though the concentration of sodium in the Water is not sufficient to cause harmful effects on soil physical properties. These four classes are: Class 1 ( S-1 )—L0w Sodium Water. This wa- ter can be used on most soils with little danger of the accumulation of harmful concentrations of exchangeable sodium. There may be a possi- bility, however, of sodium-sensitive tree crops, such as avocados and stone-fruit trees, accumu- lating enough sodium to become injured. Class 2 (S-2)—Medium Sodium Water. There is a good possibilitythat this water could present a sodium hazard on fine-textured soils which have relatively high cation exchange capacities and poor sub-surface drainage characteristics. The presence of liberal quantities of gypsum in the soil may offset some of the ill effects of this water when used on the fine-textured soils. On coarse-textured soils, which contain permeable subsoils, S-2 Waters can be used with little likeli- hood of adverse effects on the soil or plants, provided good water management practices are followed. Class 3 (S-3)—High Sodium Water. The use of this water may produce harmful levels of ex- changeable sodium in many soils. Such water also will require the initiation and maintenance of special soil management practices such as good surface and sub-surface drainage, frequent leach- 1oo 2 3 4 5 e 7891000 2 3 4 sooo >- | I I 1 | 1 1 1| | | | 55 _ >i ‘r ao- \\ _ _, 2e c2-s4 — <5 10 I 24“ c3-s4 “ g E 22 ' c4-s4 ' <1: < c1-s3 Q 2O _ - <1 g a I § 18 - »- 1 g g g cz-ss i 5 cu E |6— " SE ‘é m O 2 q) l4’- "" Q i? l2 c1-s2 C34” y c1 z — - 8\\ g c2 s2 8 10m l c4-s5 - ca-sz \ c4-s2 _ c1-s1 c2-s1 \ cs-s1 _ LOW I 0mm III 2_. __ _ c4-s1 o 1 1 1 1 1 1 111 1 1 1 10o 25o 15o 2250 C2488 cououcruvmr - nvucnomuos/cm (ECxIOG) AT 25° c. 1 2 3 4 1.ow msmum men veav 1-1161-1 SALINITY HAZARD Figure 12. How to classify irrigation water on the basis of the salinity and sodium hazards. Adapted from USDA Handbook 60. 17 TEXAS AGRICULTURAL STATION College Station, Texas State Chemist APPLICATION FOR ANALYSIS OF WMER . A sample or water to be used for irrigation or for livestock will be analyzed for inorganic salts for a tee of $3.00, provided the sample is sent in accordance with the instructions given below. The fee covers only part of the cost of analysis. we do not examine the water for bacteria or for possible medicinal uses. Directions for sampling: Rinse a clean guart bottle or fruit Jar with the water three times and then fill it. Be sure to place your name and address on the outside o! the package. Ship prepaid to the State Chemist, College Station, Texas. Nail this tom and either a Postal Money Order or Bank Cashier's Check (a personal check is not accept- able) tor $3.00, made payable to the State Chemist, to the same address, or attach letter to package containing water sample. l. Name and Post Office address of the person to when the analysis is to be sent. q] 2. Distance and direction oi’ the source o! the water tress the nearest town. _ ‘>4 M11» 5- W rr<== in_fi..._}iali=mw- (direction) 3. This water is {ran a wall v/ strean_____ionfi 0!‘ 1AM 1h I1’ I "all. slva depth and capwiw 5- For what sumo" is the "W" w l” 6. State fully any trouble or difficulty which you have had frun using the water. 7. Does the soil to be irrigated contain free lime tone or gypsum 14.0 T Does it have a tiQrt subsoil ‘I Is the top soil heavy 14,4 loam "m? 4--—1 l 6. Give all other possible information that relates to the water or the soil on which the water is to be used for irrigation. cjfladwn. Ll/WVO-WQO/wlvflzul-(JG/l-(J/j fiaauwvw,aio7i/ula-Jw-uialfesi Figure 13. Application for an analysis of water to be made by the State Chemist's Laboratory, College Sta- tion. Texas. ings and the addition of organic matter for im- proving the physical condition of the soil. Soils which contain large amounts of free calcium or gypsum may not develop high levels of exchange- TKJMS AGRICULTURAL EXPERIMENT STATION State Qzenist Department College Station, Texas Report of Water Analysis nation; ‘y! m1’. J-VK Char...» Received: JZJ/J‘) County: 7/5,»! Collected by: 334i” Joe Sourcfl tutu. Rewrwfi 1/41/47 '72 u s e J Depth: 4;.‘ Mabanl/ Texas ‘ Laboratory No. L07 Chemical Analysis Cations: P,p.m. B.p.n. Anions: lhp-m. l.p.n. Total Solids . a! Tons per lhlcium lg. Carbonate g acre-foot Magnesium 2g J. Bicarbonate 1.1 s Total Solids!‘ 0 PPM Sodium ‘[2 d "g2 Sulfate _ ti‘ Total Hsrdnessjizo Grains Total g l‘ 11mg‘ Chloride‘? u a as CaCO; per gallon Total 24$ Domestic Use: 'l'l'he United States Public Health Service standards do not recommend for dosiestic use any water which contains total solids in excess of 1,000 parts per million or chlorides in excess of 250 p. p. m. Hypothetical Combinations (Tons Er acre-foot! Calcium bicarbonate . Z f llagnesiuzs bicarbonate - 0Q Sodium bicarbonate Calcium sulfate Magnesium sulfate Sodium sulfate Calcium chloride Magnesium: chloride . a3 Sodium chloride mZa Sodium carbonate Remarks: The following general renarks are based principally upon intonation in U. S. D. A. Handbook 6O and T. A. B. S. Miscellaneous mblication lll. We recommend tint you consult your county agricultural extension agent or StS technician for further inforlntion concerning the use and nnnagement oi’ this water on your specific soils and crops. should be used wisely l. The salt content of this water is relatively high and the water/oaQoG-ho-usad. on soils which have restricted drainage. Even with adequate drainage, special mnagement for salinity control will in most cases be required and plants with good salt tolerance should be selected. 2, The water is relatively low in sodium in relation to calcium and magnesium and an be used for irrigation on almost all soils with little danger of the development of harmful levels of exchangeable sodium. l / l/a7nxal .- a 3 .2. Jrillu/vn) 7151714405 / Figure 14. Results of on analysis of water mode by the State Chemist's Laboratory. l8 able sodium if managed properly. In some a chemical amendments may be required to repl the exchangeable sodium; however, the use such amendments may not be feasible where ter is used which contains high salt concent tions. i Class 4 (S-4)—Very High Sodium Wat This water generally is uqnsfitisfactory for i ’ gation purposes unless it is"low to medium salinity, and where the use of gypsum or amendments can make the water usable. =1 may be sufficient soluble calcium in some ' careous soils to decrease the sodium hazard j preciably, and this factor should be consid in the use of water in this category. In so which have high pH values or soils which .. non-calcareous, the sodium status of wa classed as C-1 and C-2, may be improved adding gypsum. These irrigation water classifications on t basis of the salinity and sodium hazards are h; nearly so simple as they may appear. They = general and should be used as guides with classification diagram of Figure 12. Most pr lems concerning water quality must be solved , an individual basis with full consideration of r" factors involved. * Interpretation oi Water Analyses The interpretation of a water analysis 6 ally is based on the assumption that the Wa ' will be used under average farming conditio and that a knowledge of the soil physical char teristics and salt tolerance of crops to be gro are understood by the farmer. " As indicated previously, greater empha should be placed on the concentrations of é individual chemical constituents in the wa . rather than the overall concentration or total s uble salts. To illustrate this point, a. sample ' terpretation of a water analysis made by t State Chemist at College Station is presented. . The original application, as shown in Figu 13, was obtained by Mr. Doe from his local coun agricultural agent, completed and forward with the water sample to the State Chemi laboratory. The water sample was analyzed a a report of the chemical analysis, along wi recommendations as to the restrictions to be i posed, were returned to Mr. Doe. The repo by the State Chemist is shown in Figure 14. ' The procedures and calculations used in : riving at the interpretations given in the wa ‘ analysis report were: ~ 1. Total solids (p.p.m.) = 236 + 714 = 9 p.p.m. . 2. Total solids (tons per acre-foot) = (total p.p.m.) (000136) I (950 (000136) =1. - Parts per million to equivalent parts per '1 .._ j; 79 ("to e.p.m. I ---——’ I 3.95 for calcium 20.04 ' 98 . I ——— I 8.04 for magnesium 12.2 59 I ——— I 2.57 for sodium 23 256 I ———- I 4.20 for bicarbonate 61 _ 348 I I 7.26 for sulfate 48 110 I ——— I 3.10 for chloride 35.5 Although the electrical conductivity is not , in the analysis report, it can be calculated the parts per million are known. ctivity (micromhos/cm.-EC X 10°) I parts per million 0.64 erefore, conductivity I IImicromhos/cm. (EC >< 10°). A e conductivity also can be determined ically from Figure 15, as indicated by the lines. The results are shown to be about . micromhos/cm. Sodium-adsoprtion-ratio (SAR) I l Na* \/ Ca++ + Mg++ 2 en, SAR (values obtained from step 3) I 2.57 I 2.57 I 1.05 a V3.95 + 8.04 \/ 599 . 2 I om Figure 11, the SAR value obtained hically is equal to 1.0. 7v 1,, solutions, equivalents per million (e.p.m.) and milli- valents per liter (meq./l) are numerically identical ipractical purposes in these calculations. ‘ valent weight of calcium I 20.04. For equivalent hts of the other cations and anions, see page 53 of Appendix. (2.57) (100) Therefore, for all practical purposes, the SAR value calculated and obtained graphically are the same. Na* in e.p.m. >< 100 Percent sodium I I Total cations in e.p.m. I 17.6 % 14.56 6. After completing the previous five steps, a classification of this water according to the sodium and salinity hazards can be done by using Figure 12. To use this figure, proceed as follows: The conductivity value of 1,475 or 1,485 micromhos per cm. (EC >< 10°) from step 4 is located on the bottom scale, and the SAR value of 1.0 from step 5 is located on the left vertical scale. By extending lines from these points, as shown in Figure 12, they intersect in the area C-3, S-1, which means that the water is classed as having a high salinity (C-3) and low sodium (S-1) hazard. Thesame classification also is indicated on the State, Chemist’s analysis, as shown in Figure 14. Interpretation of other water analyses can be made by following the” same procedure. The classification of irrigation waters as to suitability for crops is general and requires sound judgment on the part of both the interpreter and irrigator. The grower may use water of ques- tionable quality to good advantage by maintain- ing a well-balanced soil and water management program in which the harmful effects of the water can be minimized. By knowing the capa- bilities and limitations of his soil and water re- sources, the conscientious farmer is less likely to “salt out” his land. 5000 [——- 3000 —- 2000 v rr-luul I000 l l l l l l 700 500 400 300 Concentration —- P.P.M. "l ' l l I‘l""""'l 200 I50 I00 80 6O f‘ I | |T1v|||| 80 I00 200 400 600 I000 2000 Conductivity - Micromhos/ Cm. (ECKIOG) at 25's. 4000 6000 Figure 15. Concentration of irrigation water containing soluble salts in parts per million (p.p.m.) as related to electrical conductivity. Modified from USDA Handbook 6U. l9 Figure 16. Unlined and poorly managed irrigation ditches waste valuable water, which seeps through the sides and overflows the top. Such conditions are con- ducive to salt and weed control problems. Note the wasted water around this distribution ditch. Courtesy Portland Cement Association. METHODS OF CONVEYING WATER Assuming that sufficient good quality water is available for irrigation, the next consideration is transporting and distributing it over the farm. Probably the most common method of water distribution is the open ditch. This type of water conveyance is temporary and is constructed by hand or with farm equipment. Such a method is economical in cost of construction, but often results in large seepage losses on sandy and some loam soils. These losses may cause a water- Figure l7. Lining a farm irrigation distribution lateral to prevent seepage and to give better control oi irriga- tion water. Such canals are located where the installa- tion will be a permanent part of the irrigation system. Courtesy Portland Cement Association. 20 Q logged area or salinity problem and almost alwa contribute to a weed problem that is diffici to control. Consequently, if open ditches t used, either as temporary or permenant fi laterals, the farmer should watch the seepa hazard closely. Many unlined ditches in the older irriga areas of Texas have been replaced recently i lined canals. Concrete used ‘as a lining preven seepage losses and gives better control of i Q gation water. By using the new techniques installing concrete, as shown in Figure 17, t“ cost of construction has been reduced to ma this method competitive with underground en crete pipe and portable surface pipe. Water c be removed from the canals by use of siphons, shown in Figure 18, or by special turn-out ga fig located in the side and near the bottom of a, canal. Such a system affords excellent wa -; control at all times and reduces labor and ca ‘ maintenance costs. Other materials used on a limited scale g canal liners are polyethylene plastic, butyl ru ber and finely ground clay. The first two - synthetic materials. Although such material still are in the experimental stage, they off possibilities for use by the irrigation farmer canal liners for temporary and semi-permanen farm irrigation laterals. i Underground concrete pipe for the conveyan p of irrigation water has been used for many year s This method has been accepted widely becau it eliminates seepage and gives the operator a; solute control of his irrigation water supply. ‘ i shown in Figure 19, concrete pipe is installe below the ground surface to average depths 0 18 inches to 4 feet, depending on the size of pi and the topography. This type of installatio permits productive use of the soil above th pipe. Concrete pipe is sealed at each joint whic permits it to be installed to grade up and down hill. This feature makes possible the conveyanc of irrigation water to land on rolling topography which normally could not be served efficientlyé by the use of surface ditches. Valves at the? soil surface give the irrigator a method of rogue lation and control of the irrigation water through-i out the system. Two points should be considered when using concrete pipe for irrigation water distribution, systems. First, many of the medium and fine- textured soils (loams and clays) swell when wet and shrink upon drying. Such conditions may? cause cracking of the sealed mortar joints be- tween pipes unless rubber gaskets are used be-i tween the joints. Second, cold weather may cause? cracking at the joints if cold air or extremely- cold water are permitted to enter the lines when the pipes and surrounding soil are considerably; warmer. Concrete pipe installations are perma- nent and costly mistakes can be eliminated or* minimized by discussing the related problems with specialized technicians. i Figure 18 Irrigation water is controlled better and ._"labor costs are reduced greatly by using ‘lined canals. The siphon tubes or gates installed near the bottom oi the canal allow the irrigator to adjust the water flow as desired. Courtesy Portland Cement v Association. Figure l9 Laying 12-inch concrete pipe ior the dis- tribution of irrigation water. Turn-out valves will be placed on risers to bring the water to the surface of the field. Such valves also make it possible for the irri- gator to control the volume oi water be- ing applied. Courtesy Portland Cement ‘- Association. Figure 2U f Light weight portable aluminum pipe can ’ be used to convey water from under- ; ground pipelines to surface or sprinkler ’ irrigation systems. It also can be used to control the water flow to individual furrows. Courtesy. Portland Cement As- sociation. Probably the most popular method of con- veying irrigation Water Where new supplemental irrigation systems are being installed is the use of portable irrigation pipe. The new light weight aluminum pipe with quick couplings makes it possible for one or two men to set up an irriga- tion distribution system in a relatively short time. The two types of portable irrigation pipe gen- erally used for Water conveyance are gated and the regular low or high pressure sprinkler pipe. Portable pipe used in this manner practically eliminates the loss of Water in transit and makes Water control on rough and rolling land much more efficient. Although considered as a tem- porary method for conveying water, portable pipe Will last many. years if handled and stored properly. Depth - Feet 980th - Feet (D@\|O$Ul#UJN— Figure 21. Above-Uneven distribution of irrigation water in the soil results in waste and affects plant growth and yields adversely. Below—Uniform distribution of irrigation water gives all plants an equal opportunity to obtain PREPARATIDN OF LAND FOB IRRIGATION The main objective of irrigation is to tribute water uniformly over the land so the soil storage reservoir can be filled minimum of losses from evaporation, surface off and deep percolation. Each plant will have an equal opportunity. to obtain Water nutrients from the soil reservoir, thereby the most efficient use of all Water applied. In preparing land for irrigation, im factors to be considered are the location of Water supply, amount of water available, type, topographic features, crops to be under irrigation, and Whether the Water a tion is to be by a sprinkler or surface system. irrigation Heod Ditch Heod Ditch water. This results in more uniform growth, higher yields and a saving of water. 22 The need for good irrigation water distribu- g is illustrated in Figure 21. These simplified hes emphasize that the irrigator must have lplete control of the irrigation water from p ‘a time it enters the field distribution system a 'l it is applied to the soil and reaches its pre- strmined depth within the root zone for plant l. Only through such control can the grower y; assured that his irrigation water will be used ectively. A well planned and engineered irri- Von system is designed to fit the soil and to- graphic conditions, water supply and crops to grown. s f, Many irrigation farmers now know that Ythods and technical “know how” are available ich will enable them to apply water uniformly d economically over almost any type of topog- Yhy being used for growing field and vegetable fps, pastures and orchards. Q The application of irrigation water uniformly th surface methods requires that the topog- iphy be relatively level or the top soil suffi- ntly deep so that “mechanical leveling” can done. The leveling process removes “humps” if fills the low areas and makes it possible for application of irrigation water uniformly over ye surface. Most lands prepared for “level” rface irrigation methods purposely have a very ght grade for better water movement over the rface and to provide some drainage in periods excessive rainfall. However, in some of the ‘lore arid areas of Texas, irrigation runs are nstructed to be almost level from one end to other and crosswise. ~ The term “leveling” means land shaping, land rming or grading to a relatively flat surface. e high spots are cut away and the soil is re- oved to fill the low areas in a field- The more desirable length of irrigation runs nerally are 400 to 600 feet for sandy soils and jl to 900 feet for clay loams and clay soils. ; ese distances will vary from farm to farm ause of the differences in soil texture, rate of *ater infiltration, water quantity and availability, ’ e of crop being irrigated and the amount of xbor available. With the relatively flat irrigated areas, pro- fions should be made for surface drainage fa- ' ities to be used in connection with the irriga- 'on enterprise. This is especially important in l ens where high intensity rainfall is likely to i fiur during seasons of the year when supple- ental irrigation water also is being applied to be land. . Land that has been leveled usually will re- gquire additional “maintenance” for a year or so, especially where déep cuts and large fills are imade. There always is a tendency for the cuts to swell and the fills to settle following the applica- Ytion of water. By adding these final touches éafter the land has been cropped several times, the maintenance problem on such land usually is minimized. Figure 22. Heavy equipment can be used effectively where the amount of soil to be moved is too great for available farm equipment. Courtesy Soil Conservation Service. Although the initial installation of a well planned and properly designed irrigation system often is costly, the increase in yield and quality of crops, saving in labor and conservation of water usually repays the initial outlay in a short time. Sometimes yields are increased several- fold as a result of leveling and better water management. Additional benefits also are de- rived from more effective use of fertilizers. Experienced personnel familiar with the area and its problems should be consulted concerning the design and installation of an irrigation sys- tem. A well-engineered plan which is managed and maintained properly will save money in the long run. COMMON METHODS OF SURFACE IRRIGATIQN a Water can be applied to the soil by sev- eral methods. Each affords acceptable water control if used under the conditions for which Figure 23. Next in importance to land preparation for uniform distribution of irrigation water is the mainte- nance of the irrigation system. Unless properly main- tained by smoothing the small ridges and low spots resulting from normal tillage operations, irrigation sys- tems will not function efficiently. Courtesy Be-Ge Man- ufacturfng Company. 23 Figure§25 The border system of irrigation gives the farmer control of the irrigation water. A more uniform distribution of water usually is obtained. Figure 26 The contour bench system also is adapted for row and tree fruit crops. Irrigation water is controlled and distributed easily and usually is conveyed to the individual border intervals by underground concrete or portable pipe with a valve opening in each border interval. The borders are permanent and can be crossed easily by farm equipment unless deep cuts and large fills were made. Courtesy, Soil Con- servation Service. Figure Z7 The broad bench system has the advan- tage of a, “level" irrigation system for good water control and distribution. Gen- erally. each large bench is treated as a field in land leveling and irrigation man- agement. Courtesy Soil Conservation Service. designed. The method of irrigation se- d should be governed by such factors as the and its physical characteristics, topography he land, source and amount of available water, ity of water, kinds of crops to be grown, I "lability and cost of labor, type of farming Tpment and others which apply directly to an yfvidual farm. A certain method should not jselected just because a neighbor is using it. A ow Irrigation fProbably the oldest and most widely used hod for applying irrigation water to field and vegetables is the furrow method. As i wn in Figure 24, this method uses the furrow g the control of water after it leaves the head- ‘h or portable gated pipe. On land that has been leveled or graded, relatively smaller of water should be used in the furrow to g ent erosion and loss of “tail water” at the ggom of the slope. Where the land has been led and well prepared, larger streams can be y without fear of erosion or loss of water at end of the rows. If the individual furrow are too small, too much time will be re- q- ed to reach the end of the furrow and the near the ditch or outlet will be over-irrigated ore the lower end has been wet sufficiently. § urrow stream of 20 to 30 gallons per minute .well defined, clean furrows usually will afford i equal distribution of water on leveled land for the lengths of runs previously mentioned. er certain conditions of tillage where the ter flow velocity in the furrow is retarded by diness or vegetation, larger amounts of water ' f p, furrow can be used without damage to the if» or cause excessive erosion in the furrow. - der Irrigation Border irrigation consists of dividing the d into borders by using low dikes or ridges. ter turned into the bordered area moves to- " d the far end in a sheet and is confined by 2- ridges on either side. In areas where the Figure 24. A pre-planting irrigation {or cotton with water applied by the furrow method. Water is taken from the topography, depth of soil and the quantity of water will permit its use, the border method is the most efficient method now available. It is an easy and economical way to irrigate and re- quires a relatively small amount of labor. Probably the greatest obstacles to the use of this method is that relatively large heads of water are required and the soil has to be suffi- ciently deep so that leveling within the borders can be done. Cross slopes within borders should be elimi- nated by grading and smoothing to prevent water from concentrating along the low side of the border. The fallior grade down slope will de- pend largely on the head of available water, soil type, texture and infiltration characteristics, and the kind of crops to be grown on the area. Smaller, temporary borders may vary in height from 6 to 12 inches (after settling) with a base width of about 2 to 3 feet. The more permanent type of borders usually has a settled height of about 1 to 2 feet and a base width of 8 to 12 feet. Such ridge heights will handle volumes of water rang- ing from about 1 cubic foot per second to as much as 8 cubic feet per second, respectively. Gen- erally, the smaller borders are constructed by use of farm bordering equipment, whereas soil for the larger and more permanent borders often is placed by earth-moving equipment at the time of the leveling operation. A variation of the border method of irriga- tion, as described above and illustrated in Figure 25, consists of constructing parallel borders on the contour or near contour, as shown in Figure 26. The width between borders is the same from one end to the other which greatly simplifies farming operations in that no point rows are necessary. The system is adapted to row and broadcast or tree fruit crops. Such a system lends itself nicely to rolling topography provided the soil depth is sufficient to permit leveling between the borders to afford acceptable water control and distribution. ffhead-ditch by siphon tubes. Courtesy Soil Conservation Service. 25 Figure 28. Sprinkler irrigation permits the control of irrigation water on sloping land and on soils underlain with highly permeable subsoils. Courtesy Allis-Chalmers Manufacturing Company. From the standpoint of irrigation design, wa- ter management and methods of cropping, about the same problems are encountered with the con- tour bench system as with the border method previously described. However, on deep, coarse- textured soils, the possibility of water seepage from higher to lower benches should not be over- looked. Such a condition, if allowed to exist, ulti- mately could cause a salinity problem. Broad Bench System In many cases the depth of cut required for a single field may be excessive. When such con- ditions occur, it becomes necessary to divide the field into two or more larger benches so that the cut and fill in.any one location of the field can be kept to a minimum. In considering the engi- neering aspects of design and management of the .50 4° — FIRST OPEN BOLLS ' BOOTAHEAD .30.. / \MIL GRAIN 71/ \ - SORGHUM / ‘\~\goucn .2o_ , A ‘a // 83a‘? AZ' \ ‘\\R",EN|NG DEFOLIATED _ $9119" X/ )- umves\\\ /\/ A \\ .IO _ / / new /’ J FORMATION / ' / Z LETTUCE Q/InlllllllllnlllllllnllllLlLnj o no 2o so 4o so so 1o a0 so |oo no I20 13o 14o I50 GROWING SEASON - APPROXIMATE DAYS FROM PLANTING .50 E .40 .- LF 2 " ‘ A ALFA“! Xi hi GRAINS FIRIHCORNI \ READING (WHEAT) / MOUGH \ 001' \_/\\ nnrsmus (vmsn) EVAPOT RANSPIRATION - Inches / Day .30 _. cons 1/ ,*’ no -—————1—1§ , sums uomrnae 2o / ' ‘ ,LZrAssz-:|.s 7f’ \ A ' /"‘/ \‘*x .|o_ / o llllllljllflljllfllllllllllll IO 2O >30 4O 50 50 70 8O 90 I00 IIO I20 I30 I40 I50 __I_/ 20 4O 60 80 I00 I20 I40 I60 I00 200 220 240 Z60 2B0 300 3/ IRRIGATED WINTER WHEAT}! O O GROWING SEASON - APPROXIMATE DAYS FROM PLANTING Figure 29. Plants use small quantities of water during the early stages of growth. Daily water use increases during the fruiting and maturing stages, then decreases following these periods. 26 irrigation system, each bench is treated as separate field. This method has the operation advantages of regular field leveling, but do have the disadvantages of abrupt changes of e1 vation between benches and the possibility of ‘ seepage hazard for lower benches, especially I. deep, permeable, coarse-textured soils unless ter is applied judiciously. SPRINKLER IRRIGATION Sprinkler irrigation often makes it possib to irrigate land that otherwise might be no, irrigable because of limited water supplies topographic features. Rough and rolling topo raphy, coarse-textured, highly permeable or ve _1 slowly permeable soils, shallow surface soils, o combinations of these conditions, often dicta the use of the sprinkler method for applying i gation water. In addition, sprinkler systems us i, ally are much more efficient in using small volumes of water than are required by most s face methods. Land preparation generally costs less Whe at sprinkler systems are used because of the reduc cost of leveling, and an investment in a permanm water delivery system is unnecessary. In t; cases, irrigation water can be controlled to per p application at the location and time when need The rates of water application also can be co trolled to correspond with the infiltration rate I; the soil. Such control reduces water losses d to deep percolation or run-off. The disadvantages of sprinkler systems f that the purchase of this equipment requires large initial investment and annual depreciati may be expensive. Generally, the power cost - f labor needed for operating sprinkler systems ceeds that required for well designed and adapt surface methods. The distribution of irrigati water applied by sprinklers also may be affec :5 by relatively strong winds which are common some sections of Texas. Sprinkler irrigation systems generally classified as portable, semi-portable or stationa i; For most agricultural purposes, the first classifications are more commonly used. Spri location are fixed permanently. igure 30. Available soil moisture can be estimated by obtaining soil samples from diiferent depths within the zone maximum root concentration, usually 0 to Z feet. Leit—soil samples can be taken with a shovel; or right-a sim- i1 e soil sampling probe. a irrigation systems usually are composed of plight line pipe (perforated) or revolving head ‘it klers, and may be “high pressure” (30 dsper square inch or more) or “low pres- i” systems (20 pounds per square inch and Q). As a rule, the perforated pipe systems op- a on the lower pressures, but the revolving i sprinklers operate in both ranges, depending ye type of rotary head used. Pipe for both is made from galvanized iron or alumi- in section lengths of 10 to 40 feet, with 20 probably being the most common length in g Pipe diameters range from about 2 to 7 depending on the amount of water to be i arged by the system. All of the portable i’ ems make use of “quick couplings” which are ' 1 ned to permit coupling and uncoupling as and rapidly as possible, yet maintain tically a water-tight seal between joints. ijlPortable irrigation systems usually are con- ed those in which the pump and lateral line moved together, or where the line is portable ‘i the pump remains at a central location of ~r supply for the field. The pump is mounted uch a manner that it can be moved conven- ly from one location to another. Lin the semi-portable system, the main line l. be a quick-coupling line which is seldom Ted from one location, or a permanent line fed below the soil surface. The motor-pump bination can be portable or mounted perma- , ly in one location. Stationary or fixed systems are in limited use, , usually are confined to small acreages, such egetable crops, vineyards and nurseries. The ration of this typepf irrigation system is con- d to a single area in which the pump and Smaller kl eter pipe generally is used and is supported ave ground at heights of 6 to 10 feet. The fles are spaced about 3 feet apart on the line. so is possible to use perforated pipe of smaller i eter with this type of system. In either case, the pipe is coupled so that the system can be oscillated mechanically to obtain water distribu- tion over the land. Line spacings usually are 30 to 60 feet apart with operating pressures about 50 pounds per square inch. Another type of sprinkler system, which can be a permanent or portable unit, consists of large sprinklers which operate at a pressure of about 100 pounds per square inch and discharge 200 to 650 gallons of water per minute. Such units often cover an acre or more at a single setting and are adapted especially to minimize the labor required for moving portable laterals. An ample water supply and increased power for pumping are required for this type of system. Like the surface methods of irrigation, sprinkler systems should be designed to meet the Panhandle-High Plains ll; m _. fm-VQ Pecos Valley-Trans-Pecos ' i i I ‘*7 West Central Texas l Rio Grande Plain ---j_»:-,__Y§--_l--»- Gulf Coast Prairie '"-" A/Mm w" East Texas Humid Region Central Blackland - Grand Prairie 8 Rolling Plains-West Cross Timbers Figure 31. Roughly-drawn irrigated areas oi Texas. Exact Texas land resource areas are shown on the cover page. NUDUI-fiblh)“ 27 WATER DEMAND PERIODS, APPROXIMATE PEAK DAILY WATER USE AND WATER REQUIRE If TABLE 6. CRITICAL FOR SOME CROPS GROWN IN TEXAS Approximate Water Crop‘ Critical water demand periods” water use, requirement. inches per days inches‘ - Cotton Bloom stage until about 1/2 to 3/4 of bolls are mature 0,25 1Q 0,40 15-26 , Corn Tasseling-silk stage until grain becomes 0.20 to 0.28 8-12 (humid) firm 0.28 to 0.38 15-22 (semi- Grain sorghum Boot, bloom and milk-dough stage 0.25 to 0.40 16-20 ‘ Wheat-spring Boot, bloom and early head stages 0.22 to 0,35 i} 14-22 Tomatoes Bloom and fruit enlargement period 0,25 to 0.35 f» 12-18 Lettuce and cabbage Head formation until they become firm 0.17 to 0.20 7-12 Irish potatoes Tuber swelling period 0.16 to 0.20 12-15 Carrots When root enlargement and swelling starts 0.15 to 0.19 8-10 . Onions Bulb formation to maturity 0.15 to 0.20 10-12 ‘ Alfalfa Bloom and seed forming 0.27 to 0.40 30-50 t Soybeans Bloom and seed formation 0.25 to 0.35 22-26 _,» Castor beans Bloom stage to physiological maturity 0.33 to 0.40 25-30 ‘ Sesame Bloom stage to physiological maturity 0.26 to 0.35 25-30 Peanuts Prior to and during time nuts start to form 0.22 to 0.25 6-12 Perennial grasses Seed head formation 0.25 to 0.35 25-35 Legumes—summer Bloom and seed forming 0.22 to 0.33 15-24 A —winter Bloom and seed forming 0.13 to 0.19 12-16 Rice Head formation and filling 0.25 to 0.30 35-45 Peaches Fruit setting and fruit swelling 0.22 to 0.26 22-30 Oranges and Grapefruit Fruit setting and enlargement stages 0.25 to 0.35 30-40 ‘The crops listed are only a cross-section of the different ones grown throughout the State. zPeriods based on the stage of growth in which a lack of soil moisture probably would be most detrimental to yield or qu ity. or both. “Daily evapo-transpiration rate (approximate). growth. These are expected average maximum daily water use values for the in vidual crops. Generally, the periods of maximum water use for most crops will be during the bloom and fruiting stages In some cases, it may extend about half through the maturing stage. soils. crops. varieties and planting dates throughout Texas. these values are somewhat general. the individual irrigated areas can be found in the irrigation frequency guides which follow in this section. ‘The water requirement values shown are usually the extremes which can be expected. The smaller values apply to more humid regions. while the larger values are to be applied to the semi-arid to arid regions of Texas. Where saline so’ and waters are encountered, the requirement will be increased 25 to 50 percent and even more in such areas as the Tr _ Because of the wide variation in clim v . More precise values Pecos. TABLE 7. ESTIMATING SOIL MOISTURE BY USING THE “FEEL" AND “APPEARANCE" OF THE SOIL Soil texture Moisture Percentage of content available water Coarse Medium Fine (sands) (loams) (clays) y. Dry Below the wilting Dry. loose. flows DrY find POWdQIY t0 Dry. hard and often she v percentage through fingers. crumbly. Crumbs and signs of severe cracking ‘ crusts can be broken the surface. Crumbs ~- down easily to powdered hard and difficult to bre condition. down. Low 25 to wilting Crumbly. Soil somewhat crumbly. Soil still somewhat har percentage will not hold together or Darker color than above. but can be shaped parti ball when pressed in hand. but difficult to make hold or formed when press Soil appears to be dry. together when pressed in firmly in palm of ----l palm of hand. May be crumbly. Fair 25 to 50 Soil appears to have more Soil will hold together Soil pliable and can A m moisture than indicated and can be formed when formed into a ball wh good above since it can be Pfessed- SOme Soils Will pressed. Some clays formed slightly when remain crumbly Gt this be ribboned at this mo’ pressed. Still remains moisture content. ture content. Soil ceases I somewhat crumbly but can crumble freely as moist be formed into a ball if approaches 50%. pressed firmly when soil , moisture approaches 50%. Good 50 to 75 Soil can be shaped and Soil becomes somewhat Soil can be formed into formed when pressed in plastic.- can be formed ball and can be ribbon palm of hand. (Soil breaks readily and may become ' easily between thumb an easily. but will not stick. slightly slick-looking. forefinger. 1 Excellent 75 to field Soil can be shaped easily Soil can be formed Soil feels slick; can v capacity and will form into a ball. readily into a ball and balled easily and ribbo' Usually does not stick. ribboned if sufficient clay freely. May become stic Soil and water sheen on is present. Also. may if high in clay. Soil she palm of hand after press- stick if high in clay. Soil may be left on hand ing. Not sticky. sheen on palm after pressing, if moisture pressing. May be sticky. °,,. i Too wet Above field Free water sheen can be Free water sheen can be Free water sheen on so (saturated capacity seen on soil and hand observed on soil and in and hand. Clay soil pu soil) when pressed and balled. palm of hand after dles freely, is soft and sticf squeezing. Water and soil can be squeezed be- tween fingers. when the ball is pressed firmlv in the hand. to the hand when presse fof each specific area to be irrigated. Capa- 'rsonnel should be consulted on the design, tion and maintenance requirements of ., ler units before making an investment. t. FREQUENCY OF IRRIGATION j e problem of determining when to irrigate cific crop is one of long standing. The . ency of irrigation is controlled by the “ nt of available water contained in the root of the soil and the rate in which water is gpired by the crop and evaporated from the surface (evapo-transpiration). The many i ent kinds of agricultural plants grown l ghout Texas under different climatic con- ,ns and on different soil types prevents the opment of exact rules for determining n” to irrigate each individual crop under and all conditions. 4 igation research in Texas indicates that the Y? g of irrigation water applications to coincide ‘y critical stages of plant growth appears to f- much greater importance than the total ‘r supplied. The grower too often becomes APPROXIMATE DAYS BETWEEN IRRIGATIONS F. Loamy Slcllrldy Clay ‘and Loam Loam Loam Clay Q e C‘? e c; -25 45- 55- ’ g0? —4O -50 160 - ‘ us 35_ s 45* 55‘ "a -: :'_‘ o 35 r40 _5o _6o ‘_ 25:1 I: 45- 55g IO“ ' ‘~30 -40 -9 12o 1 '50 le- - 8- |7_"|B 25_: 35—_ 45.. -7 -I6 — _ '5‘ _' ‘~30 I“ _ Pivot Line 6- "'4 _g0 I- f w I3‘ [9- -' 35__ —I2 “'8 25- _' “L5 “A l?“ r24 ,_ ® -|6 23- "do -|o ns- "22 29- \\ 9_ "l4 -29 27- @ \\\\ l3- l9- 25:26 \\_\8\\_l "8 -24 ‘%|T< \ z3_ l u- _|6 -22 7 ' 2|- I5- "° ~20 -|4 l9- 9" - u,” I8 l7- __8 -|2 ~16 1r ||- l5- l -|4 — IO |3-- ure 32. Irrigation frequency guide Ior the Panhandle- » Plains of Texas. area 1. concerned as to whether his crop will require 15 or 2O inches of water for the season. Instead, he should be more concerned about scheduling his irrigations to meet the requirements of the crop during the critical stages of plant growth. Research data, such as shown in Table 6, make it possible to determine the critical stages of water demand for a number of crops grown throughout the State. Such data also make it possible to give generalized irrigation schedules from which the individual farmer can make the necessary adjustments to meet his particular needs for the crop being irrigated. Irrigation frequencies should be based primarily on the physical characteristics of the soil and the need of the specific crop being grown. Local weather conditions play an important role in that they influence both the evaporation of water from the soil and its transpiration by the plants. A practical guide of suggestions for determining irrigation frequency follows: (1) Ample soil moisture should be available at planting time to soil depths of about 5 to 6 feet, depending on the soiltype and crop to be APPROXIMATE DAYS BETWEEN IRRIGATIONS Fine goomy Sandy Clay and Loam Loam Loam Clay e2 e c; c; e 25E: -40 4s- 155 I 3-20 35130 -40 50- -60 l5-:M -3 35- -45 55- ‘5-42 252- _-so 40- ~50 H- -[ -' . 4s- —lO l9- 2° 25-" ‘-35 9- -|e - - _-4o E a nq-‘IG I 3°: " Q '5' “~20 '- ss- m .45 1- -|4 ,9- _ _ 1&1 . . l3- -|e _-25 _~ 9o PIVOI Lme -5 42 |7_ _ do Q (D -|s - - 5 435 5 ||-' |5_, 2 ' ___ - ' 0- - z -|o -|4 — .30 @ -|9 25- ' |3- ,8_ ~24 g .25 (l) 9' 23.. _ -|2 —|7 p; 20 © @ l6‘ ~22 E "B u- 2|- ? as -|s ~20 EJO 7__ -IO '4_ I9" ' -|e .05 g 9- l“ a l7- lz- -|s ~a _H l5- -|4 7d“ IO l3- -e -|2 a” ll- l", Figure 33. Irrigation frequency guide for the Pecos Valley and Trans-Pecos area, area 2 29 grown. In some soils, this depth of water pene- tration cannot be obtained because of shallow surface soils which overlay subsoils of very slowly permeable clay or limestone. Unless the necessary soil moisture has been supplied by rainfall, a preplanting irrigation should be con- sidered a must for those crops which are not dry-planted. Many vegetables are seeded in dry soil and then irrigated. (2) Irrigation should start when about 50 percent and not over 60 percent of the available moisture has been used from the zone in which most of the roots areconcentrated. The zone of root concentration will depend largely on the type and age of plant as well as the depth of soil for root development and distribution. The root zone should be kept moist, but not wet. (3) Crops often are over-irrigated while young and under-irrigated during the early fruiting and maturing stages of growth. Young plants use small quantities of water (.05 to 0.15 inch per day), as shown in Figure 29. The ma- jor soil water loss during the early stages of growth can be attributed mostly to evaporation APPROXIMATE DAYS BETWEEN IRRIGATIONS - Fine Loamy Sandy Clay Sand Loam Loam Loam Clay _- i? ._ q- 952 25-" —' " ‘_ '- _2o ' _ 50- ‘ '8' 1e 30-1 4o a5 '45 5o p |4_ 1-25 3O ' 40- ' -12 1 _- ~55 "45 2°"- -' 40- 5 10- - _-25 a -9 -_ __ 30:’ _35 l! 45 -|5 - _ if 8' 14- 2°“ ‘ 25 m .40 Pivot Line _7 _|3 l8_"'9 .. 3o_ £55 (D 12- -17 _' _' g 6“ -11 16- 2o - _' 3o ‘l5 ' -25 ' @ 10- W -19 24_ 5.25 @ 1e- -23 l; (D ‘9 "3 ~11 22- E” (5 -21 a IZ- I6- 2“ a‘ 15 2o- _| | _ gm 14 1e l9 g2‘ .05 l“? |Q.. 5 -15 -l7 h 1s- 9 12- -15 -11 8" 14- 10- -13 _..9 l2‘ -1_| l Figure 34. Irrigation frequency guide for West Central Texas. area 3. 30 of water from the soil surface. When most pla start t0 bloom and set fruit, their daily wa use rate increases rapidly (0.15 to 0.20 inch day) and continues until most of the fruit; matured (0.20 to 0.40 inch per day or more du ' certain periods of hot, dry weather). Du ' these stages of growth, most of the soil water» used or transpired directly b_ the plant, or bo which makes water evaporxagion losses from v soil of secondary importancef ' (4) Irrigate only when the soil will hold g irrigation without having deep percolation los or water-logging of the soil. " a; g’- ~ (5) Irrigate on an economically feasi schedule by regulating the cropping system Q the water supply. Such coordination will ass that all crops can be irrigated before the pla i become severely stressed from a lack of s moisture. If the irrigated acreage is large, ' gation should start soon enough so that 11, areas receiving water last will not become _ dry before it can be applied. Plants stres severely for long periods may never fully recov APPROXIMATE DAYS BETWEEN IRRIGATI Fine 1 Loamy Sandy Clay ~- Sand Loam Loam Loam Clay ‘ (D Q2) Q @ _ a _ - 2 r w I 25 - _ _ _ a 19f‘? - 145 55‘ ‘_ ,4_ _-3o 40- '50 _ I _ -: _ 45- - 12 25} :_35 JO _5o 1 IO- _- 30-: " -9 -2o -_ - 35-_ 45— , > e- - 125 I- _-40 i. g _7 1 I _—3O _- ; c: 45 15- ‘_ _' 35- LU . . s- -'4 20- - _ s) A0 P1vot L1ne L,” _ 25- _- - ‘i, (D _|2 " _,_ ~30,» 0.35 " — — “L E "' - - - ' | .30 ® F'o _|5 —20 il- " Z l9- o .25 © 14- 25 F24 . E @ 9* ,3 A8 25- ‘ %.2Q Q5) l7-< r22,’ 2.15 -8 12- -l6 21- , <1: _. gm ~11 '5“ l9 F20 i __ T 2.05 7 "'4 § 10- H8 IJJ 13- |7_ ‘g “l2 -|6v 11 '5' 8_- ~14 _, -1o A 13- 9-1. 4 "2 . 11-- 1 Figure 35. Irrigation frequency guide for the Rio Gran, Plain. area 4. foregoing suggestions indicate that many f rollable factors are present within the soil- Water system which greatly influence water '_ ility and its use by plants. The highly fx interrelationships require that an irri- farmer know his soil from the standpoint ’ Jability to store and supply both water and 1 nutrients, as well as to determine the h and rooting characteristics of the crop grown under irrigated conditions. an illustration, suppose that a shallow- 3 crop, such as lettuce or cabbage, is being ,4 on ‘a well-drained, fine sandy loam soil. ially, about 85 percent or more of the total ystem of these plants will be confined to 3 12 to 18 inches of soil. With such a dis- ‘on of roots, one would be wasting irriga- _ater, time and possibly soluble plant nu- if water were applied in such quantities ten enough to maintain high levels of soil lure much below the zone of maximum root ‘tration. This does not mean that the = will not use moisture below a depth of a 18 inches. Instead, it means that the rate APPROXIMATE DAYS BETWEEN IRRIGATIONS Fine Sandy Loam Loam Clay Loam Cloy Q Q9 T “' T W -40 ~ _" 35_;3o ‘-40 -so _" 255: 35- 45' '-2O _:30 _|0 __|6 “22 -. Pivot Line |5- ® "l4 |9_ 25_“ s3- "8 _|o -|4 9- ,l i. 36. (Irrigation frequency guide for the Coast i‘ , area 5. of water withdrawal below this zone will be much slower because of less root activity at the lower depths. Therefore, to saturate the soil profile down to 3 and 4 feet would be a needless waste of water and cause excessive leaching of plant nutrients. If the irrigator is preparing to apply water to alfalfa, Hubam clover or a perennial_grass which has reached maturity, he needs to be much more concerned about the lower soil depths, pos- sibly down to 5 and 6 feet, depending on the soil type and depth to which roots have developed in sufficient quantities to affect appreciably the water withdrawal rates. All three of these crops have relatively deep root systems, which means that perhaps 80 to 85 percent of the water used may come from the 0 to 3-foot depth instead of the top 18 to 24 inches, as indicated in the case of the more shallow-rooted lettuce and cab- bage plants. Therefore, because of their more extensive root systems at the lower depths, the deeper-rooted crops are able to use a larger amount of the moisture available in the soil reser- voir. This physiological characteristic of these APPROXIMATE DAYS BETWEEN IRRIGATIONS Fine Loamy Sandy Clay Sand Loam Loam Loam Clay Q ,1 i? _ T IBI-zo _3Q ‘l; 1 _ ls-Zlfi 4° -5° - 131m 253- 35- 45- _ "310 @1320 f3‘) ~40 -5o 9- 11-"8 _' 35a 4s- ‘B _|6 25‘- _ _ |5- -_ _ _ >_ 7 -a4 - _-so 4° 4 _6 l3- '9_-20 _ Q -|2 _|8 “ 35* m .45 _ H_ _ 25_ ~- u 5 l? _ a. 90 Puvot Lune _|o l5 '46 -_ 13o a @ L4 9- - - 5 .35 -l4 _2o . g 3o ® -8 13- |9- 25_ I __|2 —l8 -24 z _ _ 9 25 @ _,_ H n? g3 22 v- @ ‘ ‘ -|s ~ é '20 © l5 2|- “ -|0 “ g) J5 ,_6 “Z0 i -|4 m- a: .|o 9- 1- | |3_ “l6 8 .05 l < 5_ I7- u): f8 -l2 -|e L ll“ |5.. 7- -|o "4 |3— 9- -l2 l, ll- --|o Figure 37. Irrigation frequency guide for the East Tex- as Timberlands-Humid Regiom-area 6 31 plants permits a reduced frequency 0f irrigation as compared with the irrigation frequency of the more shallow-rooted crops. Another factor which greatly affects the fre- quency of irrigation is soil salinity. Where sa- linity is a problem, soluble salts affect water availability to plants. The presence of salt in the soil makes water less available to plants and may cause them to appear as if the available soil moisture had been almost exhausted. Changes in soil moisture stress of saline soils are somewhat more gradual and, although plants may be sub- jected to high stress conditions, there is no abrupt transition in the turgor condition of the plants. On non-saline soils, there usually is a rela- tively abrupt transition from low-moisture stress to high-moisture stress conditions and the plant wilt systems are readily apparent. In selecting crops for saline soils (Table 5), particular attention should be given to the salt tolerance of the crop during the germination and fruit setting periods. The use of saline waters for irrigation is not recommended. Because of prolonged drouth con- APPROXIMATE DAYS BETWEEN IRRIGATION Fine Sandy Cloy Loom Loom Loom Clay @ Q <2 Q 3O 25 45_I— 55_ ‘I, I5 35‘ 45' ~- ‘3 I; 2:50 35:40 _5o I I 25-: _" 45.. IO -_ -- 9 ' 2o ‘p30 ‘:40 >_ 3 I9: -_ "_ g 7 '7_-I8 25__ 35- m .45 -I6 '_ _- u] _ _ 6 I5- _ _-3O 1A0 PIVOI LIne 7 H4 -2Q _ 8 (D B‘ '9- I8 _ I .35 5 H2 — 25; 2 I7- -24 | .30 (g; II- —l6 g3- -22 I5- 5 .25 ® -IO 2|- l- - —20 q @ I4 E .20 9_ l9_ f}; I3- 5 J5 -I8 _ -l2 g .lO 8 |7- g ‘()5 II- -I6 > 7-- LLI |5_ -IO —|4 9- l3- -I2 —-e II- --IO Figure 38. Irrigation frequency guide {or Central Black- land and Grand Prairies regions. area 7 32 ditions and an urgent need for water, this Q tice is followed often in many of the irrigat areas of Texas. Using saline waters for irrig tion requires more frequent irrigations a necessitates the application of about 15 to i percent additional water at each irrigati to dilute the salt concentration in the r0 zone. The additional water“ also aids in leac ing the soluble salts downward in the s profile. (See Texas Station Bulletin 876, Salini Control in Irrigation Agriculture, for more deta’ concerning the management of saline soils.) a" WHEN TO IRRIGATE i No “foolproof” method or instrument is ava' able to indicate when irrigation water should applied to a farmer’s field. There are, howev several methods which may be used with reaso able accuracy under field conditions, provided t“ user understands thoroughly the mechanism t‘ operation as well as limitations of the meth being used. y APPROXIMATE DAYS BETWEEN IRRIGATI gmeu SVgrydFine 8P“ Cl Ofl n O O g Loony LoonIl Looym LooIn Clay (D _; : Q _- i~ 35 4o _ I 30 ~40 I ' - 25 35_ 45 Mo _50 _ . 2o :30 45 F 25-: 35:; -4o -5° ; _< I5 4 ' -: 3-30 “f” I - 25- _ 5 II "9 j I3‘) Q I7“ - 35-; m .45 IO -Is _- _ y w _ _ '5* -2o ' * ‘ E 4o PIVOf LIne 9 I-|4 |9_ a 251.24 13o g (D a I3~ -I 23- r ' n: 55 H2 |7- -22 t o I6 3'- l E 7 ' -2o ' , .30 (g) II- |_-,_ l9 25- 24 5 25 <3 -Io -I4 -Ia 25I I" _ 22 fr! 2o ® @ 9_ l3- I7 it 6.’ -Is 2' 1 Q .I5 ~12 ~20 4 I5- T‘ -a I9— i? .lO H- l4 -.- - . -I8 . g .05 I -I6 ' 9_ -I2 I I5- II- f __8 “I4 _ -Io I I5~ 9d d2‘? II- --s g_ Lu Figure 39. Irrigation frequency guide {or the Rolli Plains and West Cross Timbers. area 8. . if Plant Observation Perhaps the oldest and most widely used method of estimating “when” to irrigate is plant observation. Essentially, such a method depends on observing plant color, rate of growth of the terminal buds and wilt systems (the rolling or curling of leaves) . The disadvantage of this method is that by the time wilt and color symptoms appear, the plants often have been stressed too much and irrigation water has been withheld too long. Most plants turn dark bluish-green and lose turgidity when subjected to serious stress for water. Using“ the plant wilt symptoms as an indi- cator of water availability is not always reliable. TABLE 8. EVAPO-TRANSPIRATION VALUES TO BE USED WITH THE PANHANDLE-HIGH PLAINS IRRIGATION FRE- QUENCY GUIDE; HIGHEST AVERAGE DAILY WATER USE EXPECTED Average peak period Crop Pizlxcfllrsie water use. inches per day’ Cotton Iuly, August 0.25 to 0.35 Corn (field) Iune. Iuly 0.25 to 0.30 Corn (sweet) Iune, Iuly 0.18 to 0.22 Grain sorghum Iuly, August 0.30 to 0.40 Small grains April, May 0.14 to 0.17 Alfalfa 6. biennial Iune, Iuly, August 0.27 to 0.32 sweetclover“ Soybeans Iuly, August. Sept. 0.25 to 0.33 Legumes—Annual Summer Iune. Iuly, August 0.15 to 0.20 Winter April, May 0.12 to 0.15 Wheat—(winter-spring) April, May 0.22 to 0.35 Oil seed crops sesame Iuly, August 0.24 to 0.33 castor beans Iuly, August 0.33 to 0.49 Grasses-Perennial warm season Perennial cool season Close grow- ing pasture _ grasses Vegetables-shallow rooted‘ medium to deep-rooteds lune. Iuly, August Iune, Iuly, August 0.25 to 0.30 April, May 0.20 to 0.25 0.15 to 0.20 0.20 to 0.22 0.17 to 0.20 Iuly, August I une, I uly. August 112Because of the wide variety of crops grown, their differ- ent uses and different planting dates over the area, the peak use months will vary. Also, local weather condi- tions; will greatly affect the daily water use rates throughout the area. Peak period water use usually oc- curs during fruiting and maturing stages of growth. There may be times during this period in which water use may be greater than the indicated values. Irrigation frequency for these crops on deep, well-drained soils will sometimes be extended for these crops because of their ability to utilizerdeeper moisture than other field crops. " Shallow rooted-moisture control zone—0 to 1 foot. Fre- quency of irrigation will be approximately half the num- ber of days indicated on the frequency irrigation guide. Medium to deep rooted-moisture control zone—0 to 2 feet. Frequency of irrigation will be about three-fourths the number of days indicated on the frequency irrigation guide. U, I5 U\ Many plant species have a natural tendency to Wilt at mid-day and in the afternoon if hot, dry Weather prevails. Wilting usually is caused by periods of high plant transpiration rates during which the roots system is not able to supply water fast enough to offset the water deficit, even though the soil may contain plently of available moisture. When such a condition exists, plants show a loss of turgor in the leaves and appear as though a severe soil moisture deficit is present. Probably the best wilt symptoms of soil mois- ture deficit are when plants show evidence of se- vere wilting in the early morning, 9 :00 to 10:00, during the warm to hot part of the growing seasons. Signs of wilting at this time of day indicate that irrigation water has been withheld too long and that the plants are in a highly stressed condition because of a lack of water. Feel and Appearance of the Soil Another practical method used widely to indi- cate the irrigation needs of plants is to estimate soil moisture by the feel and appearance of the soil. Even though it has some shortcomings, this method has merit because of its simplicity and widespread application on many different soils. Its use requires experience and a knowledge of the soil on which observations are to be made. TABLE 9. EVAPO-TRANSPIRATION VALUES TO BE USED WITH PECOS VALLEY 6. TRANS-PECOS IRRIGATION FRE- QUENCY GUIDE: HIGHEST AVERAGE DAILY WATER USE EXPECTED Average peak period Crop Pzlcglfflhlgle water use. inches per day’ Iune. Iuly. August 0.25 to 0.35 Iune. Iuly 0.25 to 0.33 Iune. Iuly, August 0.27 to 0.32 Cotton Grain sorghum Alfalfa 6- sweetclover“ Legumes (Annual) Warm season- clean tilled (rows) Iune. Iuly, August 0 l8 to 0.23 close growing 015 to 0.18 Cool season-— clean tilled (rows) March. April 0.15 to 0.17 close growing 0.13 to 0.15 Pasture grasses Perennial warm season Iune, Iuly, August 0.28 to 0.32 Perennial cool season April, May 0.21 to 0.25 ‘The peak use months will vary within the area because of planting dates. crop use and weather conditions. Early plantings will have the peak use periods different from those plantings made during thelatter part of the planting season. Because of saline waters, frequency may be more often in some cases than indicated on the guide. “Daily water use rates will vary within the area throughout the growing season because of changing weather condi- tions. Moisture control zone considered to be 0 to 2 feet. for all crops listed except alfalfa, sweetclover and peren- nial grasses. “Moisture control zone for these crops will be deeper (about 0 to 3 feet) because of the more extensive root systems on the deep, permeable soils. Irrigation interval for shallow phase and rapidly permeable soils will be less than for the deeper, fine-textured soils. 33 Estimation of available soil moisture by this method is made by digging in the plant root zone (0 to 30 inches or deeper) with a shovel, soil auger or post-hole digger. Several handsful of soil are taken in each 6-inch increment of depth down to at least 2 feet. The soil is then pressed firmly several times within the palm of the hand and compared for feel and appearance by using the information given in Table 7. Several mois- ture observations should be made at different locations over the field to obtain a representative sample of the soil moisture conditions which exist in the zone of maximum root concentration. Soil moisture observations should be made at least weekly throughout the growing season and pos- sibly more often during the critical moisture de- mand periods. Irrigation Frequency Guides A third method, which was developed for practical application in estimating the irrigation ‘frequency of various crops, is based on the irri- gation frequency guides shown in Figures 31 to 39, and Tables 8 to 15. The division of Texas into eight irrigated areas was based largely on soil characteristics, climatic conditions and crops common to each area. These guides are based on three primary fac- tors which control irrigation frequency-the TABLE 10. EVAPO-TRANSPIRATION VALUES TO BE USED WITH WEST CENTRAL TEXAS IRRIGATION FREQUENCY GUIDE," HIGHEST AVERAGE DAILY WATER USE EXPECTED Averaged peak perio Crop Pzigttllse water use. inches per day‘ Cotton Iune, Iuly. August 0.21 to 0.27 Grain sorghum Iuly. August 0.21 to 0.25 Small grain April, May 0.15 to 0.17 Grasses Warm season Iune, Iuly 0.15 to 0.17 (clean tilled) ‘ Cool season April, May 0.13 to 0.16 (clean tilled) Summer perennial Iuly, August 0.25 to 0.30 Winter perennial May 0.18 to 0.20 Legumes-—Annual Summer (clean filled) Iuly, August 0.22 to 0.25 Winter (clean tilled) March, April 0.13 to 0.17 Alfalfa-sweetclovers Iuly, August 0.22 to 0.30 Pecans’ Iuly, August 0.24 to 0.27 Vegetables— September/October 0.15 to 0.18 October. November 0.12 to 0.15 Warm season“ Cool season’ ‘Water use rates will vary with the area throughout the growing season, depending on crop use. planting dates and prevailing weather conditions. Consider soil depth- shallow soils require less water per irrigation with more frequent irrigations. Usually shallow soils require about half the amount of water per irrigation as needed by deep- er soils. Deeper soil moisture used by alfalfa, pecans. perennial grasses and legumes. Consequently, irrigations will be less frequent than other crops because of deeper and more extensive root systems: however, more water will be required at each irrigation. “Moisture control zone—0 to 4 feet. “Moisture control zone—-0 to 11/2 feet medium to deep-rooted vegetables. 34 amount of available water in the soil, the d evapo-transpiration rate and the type of cr to be irrigated. ~ The similar terms “evapo-transpiratio “daily water use rate” and “consumptive u are defined as the amount of water used ' plants and evaporated from a cropped ar Therefore, by knowing thej-amount of availa water in the soil (usuallyexpressed as inches o» foot of depth) and the daily evapo-transpirati rates for a particular crop grown in one of t eight irrigated areas, the irrigation frequen can be estimated reasonably well by dividingt amount of available water (inches) in thé =5 fective root zone by the daily evapo-transpirati rate (inch per day). Q The approximate number of days betw irrigations (frequency), as presented in the i p gation guides, can be extended by allowing t available moisture in the root zone to go belo the suggested level of 50 percent before irri, tion is started. However, it is not recommend that the soil moisture in the zone of maxim _ TABLE 11. EVAPO-TRANSPIRATION VALUES TO BE U 5 WITH RIO GRANDE PLAIN IRRIGA'I'ION FREQ =1 '9_ GUIDE: HIGHEST AVERAGE DAILY WATER USE EXPEC 1 Azeragq. pea pen ’~ Crap Pgglznfilsse water us A . inches » per day’ 3 Cqtton Iune, Iuly 0.25 to »" Grain sorghum- fall October. November 0.18 to 0.231 spying‘ May, Iune 0.25 to 0.30 Corn (field) May, Iune 0.22 to 0. Corn (sweet) May 0.15 to 0.10 Legumes—annual April, May 0.15 to 0.1 Alfalfa Iune, Iuly August 0.27 to 0. z Pasture grasses Iune. Iuly, August 0.25 to Small grain February. March. ; April 0.14 to 0.18’ Vegetables“ _ Shallow-rooted April, May, Nov-. “ Dec. 0.15 to 0.13‘ Medium to deep- May. Iune. Och. [gated NOV. 0.14 ‘l0 0.18‘ Citrus‘ Iune, Iuly, August 0.25 to 0. l‘; ‘Peal: use months for most crops in this area may be dill ent within the area because of the wide variation in ~. mate and crops grown. such as found in the Lower 1% Grande Valley. 2Water use may vary over a wider range than in the ~01 areas because of the wide range of crops and differ weather conditions within the area. “Moisture control zone for shallow-rooted vegetables—0 1 foot. Cabbage, lettuce, onions. radishes. carrots. broccoli. cauliflower and Irish potatoes. Moisture con zone for medium to deep-rooted vegetables—0 to 2 fe Tomatoes. beans (lima and snap), turnips. peppers, ~‘ cumbers, cantaloupes and watermelons use about one- - , to a half the frequency, as indicated on the guide for s low-rooted vegetables, and about two-thirds the Ireque indicated for the medium to deep-rooted vegetables. - ‘Most effective moisture. control zone——0 to 4 feet. Int . planted cover crops or grass sod will require some -»-, tional moisture. Increase citrus irrigation frequency ~ about one-third the number of days indicated by the gation guide. k concentration go below 35 to 40 percent 'ability for long periods because of the re- ] ation of plant growth and development. simple illustration in the use of the irri- '_ ion frequency guides follows. Area 1 guide i; used, but the same procedure will apply for "des in each of the other seven areas. y. Example: If cotton is growing on a clay loam y; and is beginning to bloom and fruit heavily ‘ 'ng the middle of July, the following pro- ure should be followed. From the daily evapo- i-nspiration values for cotton during this peak ter use period, as shown in Table 8, the daily ter use for “dry weather” conditions will vary tween 0.25 and 0.35 inch per day. Therefore, at ‘s stage of growth, assume the daily water loss 0.25 inch per day. I To find the approximate number of days be- een irrigations, proceed as follows: (1) Locate the circled number under the soil ‘w on which the crop (cotton) is grown. The cled number is 4—clay loam soil. ?l,_(2) Then, locate the corresponding circled _ ber on the short, vertical line at the left. is called the pivot line. _LE 12. EVAPO-TRANSPIRATION VALUES TO BE USED " GULF COAST PRAIRIE IRRIGATION FREQUENCY (3) Place a pencil point on the pivot line cor- responding to the circled number l». (4) With the use of a straight edge, draw a line from the 0.25 inch per day evapo-transpira- tion value through the pivot point 4, and extend it to intersect the vertical line with the corres- ponding circled number 4-day loam. (5) The extended line intersects near 17 days. (6) Therefore, the irrigation interval will be approximately 17 days. Each of these general guides should be used with full consideration of the type and age of crop being irrigated, plant root distribution and zone of maximum moisture use, soil profile char- acteristics and prevailing weather conditions. All have an important bearing on water avail- ability and use by growing plants. TABLE 13. EVAPO-TRANSPIRATION VALUES TO BE USED WITH EAST TEXAS-HUMID REGION IRRIGATION FRE- QUENCY GUIDE: HIGHEST AVERAGE DAILY WATER USE 1"" HIGHEST AVERAGE DAILY WATER USE EXPECTED Average Peak use peak period monthsl water use, inches per-day’ A -*, on Iune, Iuly 0.25 to 0.32 - May, Iune 0.23 to 0.27 gain sorghum May, Iune 0.25 to 0.28 grain April, May 0.13 to 0.17 ' ” mes f ual April, May 0.13 to 0.17 erennial May. Iune, Iuly 0.28 to 0.35 1 59S ual May, Iune 0.27 to 0.32 ' gfenniql Iune, Iuly, August 0.23 to 0.25 k 9 pqgtures MCIY. Iune, Iuly 0.25 l0 0.35 ’ .25 to .30 etables‘ ‘ Shallow-rooted Seasonal 016 to 020 Medium to deep- i-rooted Seasonal 0.14 to 0.17 use months will vary because of different planting tes, crop use and prevailing weather conditions in this 16¢»? ange of water use values in this area is more widespread (cause of the variable weather conditions along the Gulf a ast. g -- more details, see Texas Agric. Ext. Serv. Bulletin 872. 'ce—A Big Business on, the Gulf Coast Prairie." (‘getables grown in thisg“, area are seasonal, mostly for lo- consumption. Smaller water use values are for cool - on vegetables and larger values to be used with vege- les grown during the warm season. Shallow-rooted etables—most effective moisture control zone—0 to 1 ‘t. Medium to deep-rooted vegetables—most effective listure control zone—0 to 2 feet. For a distinction in allow and medium-rooted vegetables, refer to irrigation "de for Rio Grande Plain—Area 4. EXPECTED Avkerage pea period Crop 123211111159 water use. inches per day’ Cotton Iune, Iuly, August 0.20 to 0.25 Corn (field) Iune, Iuly, August 0.18 to 0.22 Corn (sweet) Iune 0.17 to 0.20 Sweet sorghum Iune, Iuly 0.20 to 0.23 Peanuts Iune, Iuly 0.21 l0 0.25 Legumes Annual-Summer Iune, Iuly, August 018 to 0.22 Winter April, May 014 to 0.17 Perennial Iune, Iuly. August 022 to 0.27 Pasture grasses Warm season lune. Iuly, August 0.23 to 0.27 Cool season February, March, April 0.12 to 0.15 Roses Iune, Iuly 0.20 l0 0.25 Vegetables“ Shallow-rooted May, Iune, Iuly 0 22 to 0.24 Medium to deep- 0.20 to 0.23 rooted Blackberries MGY. lune 0-18 t0 0-22 Deciduous fruits‘ May, Iune, Iuly 0.22 to 0.26 Peaches Plums ‘Peak use months will vary for most crops in this area. Some crops planted earlier in the southern part of the re- gion also depends on crop use. zLower water use values to be used with most eastern sec- tion and the larger values are to be used for sections near the western part of Area 6. aShallow-rooted vegetable crops considered to be cabbage, onions, beets. spinach, collards. squash and Irish potatoes. Moisture control zone—0 to 1 foot. Use about half the irri- gation frequency for these vegetables as indicated on the irrigation guide (for soils with shallow top-soil). Medium to deep-rooted vegetables considered to be tomatoes, beans (lima and snap). watermelons, cantaloupes, cucumbers and turnips. Moisture control zone—0 to 2 feet. Use about two- thirds of the irrigation frequency for the crops as indicated on the irrigation guide. ‘Most effective moisture control zone for these fruit tree crops considered to be 0-3 feet. Where cover crops are planted or grass-weed cover is grown, the daily water use rate will be increased some, usually 0.04 to 0.05 inch per day during peak water use period. 35 Soil Moisture Indicating Devices Modern instrumentation, which may be used by farmers as a guide for indicating “when” to irrigate may be divided into moisture tension, electrical resistance, thermal conductivity and neutron scattering. Moisture tension instruments measure the force with which water is held by the soil par- ticles. This force is referred to as soil moisture tension and controls greatly the moisture avail- ability to plants. Such moisture-indicating de- vices are called tensiometers. They consist of a porous cup, a vacuum gauge or mercury column type of indicator and a water-filled connecting tube between the cup and indicator. When the cup is placed in the root zone of the soil, water is free to move through the porous wall and come to equilibrium with the soil water as illustrated in Figure 40-left. As the soil dries, water moves from the porous cup and causes a vacuum to be indicated on the gauge; therefore, the drier the soil, the higher the gauge reading. When irri- gation water is applied or rainfall occurs, water returns through the porous cup and releases the vacuum which, in turn, is indicated on the gauge as a lower reading. Generally, tensiometers are more sensitive in the higher soil moisture ranges and less sensitive in indicating the lower ranges. In coarse-tex- tured soils (sands), about three-fourths, in some TABLE 14. EVAPO-TRANSPIRATION VALUES TO BE USED WITH THE CENTRAL BLACKLAND AND GRAND PRAIRIE IRRIGATION FREQUENCY GUIDE; HIGHEST AVERAGE DAILY WATER USE EXPECTED Average peak period Crop Izggfithzfe water use. inches per day’ Cotton Iune. Iuly, August 0.21 to 0.26 Corn Iune, Iuly, August 0.20 to 0.25 Grain sorghum May. Iune 0.19 to 0.24 Small grain“ April, May 0.13 to 0.15 Legumes—Annual Spring-early summer May, Iune 0.14 to 0.20 Fall-winter March. April, May 0.11 to 0.14 Alialia‘ Iune. Iuly, August 0.24 to 0.33 Pasture grasses‘ Warm season Iune, Iuly, August 0.24 to 0.32 Cool season March, April, May 0.12 to 0.15 ‘Peak use months will vary because oi the length oi plant- ing season in Area 7 as well as the variation in climate irom north to south and east to west. "Lower water use rates are expected ior the southern and eastern sections oi Area 7. Larger water use values are to be used ior the northern and western sections oi Area 7. Also, water use rates will vary within each section because oi local weather conditions, type oi soil and stage oi plant growth. aThe irrigation frequency ior small grains, vetch and peas growing on loam and clay soils usually will be about two- thirds the number oi days as shown on the irrigation guide. ‘Irrigation trequency ior alialia and perennial grasses grow- ing on deep. medium to tine-textured soils can be increased by as much as one-third as obtained irom the guide with- out doing harm to crop growth. 36 case more, of the available moisture range t‘ be covered by the tensiometer units. In 10a and clays, the available moisture range cove accurately by tensiometers usually varies fr one-fourth to one-half. Therefore, this type unit is used mostly in connection with cr' grown on coarse to medium-textured soils. ’_ shown in 40-center, several units usually are. at different depths in thezrioot zone at each cation in the field to follow the rate and zone' moisture use by the crop. i Electrical resistance moisture units indic soil moisture indirectly by measuring the w’ trical resistance of a porous cell or block bufi in the soil within the root zone (40-right). ter the porous cells come to equilibrium wi the surrounding soil moisture, their resistan can be measured by a resistance meter and c0 verted to moisture values (percentage of m0 _ ture in the soil) from calibration charts whi sometimes are supplied by the manufacturer. most cases, it is desirable to calibrate the uni in soil on which the crops are to be grown. S0 -_ meters simply indicate “wet,” “medium” or “d v ranges. Several different types of resistan units are in use and available commercially. The electrical resistance method of measuring soil moisture is less sensitive than tenslomete TABLE 15. EVAPO-TRANSPIRATION VALUES TO BE US WITH THE ROLLING PLAINS AND WEST CROSS TIMB IRRIGATION FREQUENCY GUIDE; HIGHEST AVERA DAILY WATER USE EXPECTED Averagef peak peri. Crop iiggthlgfe water use inches l per-day’ * Cotton Iune, Iuly, August 0.25 to 0.30? Corn Iune, Iuly 0.22 to 0.27, Grain sorghum Iuly. August 0.25 to 0.30; Small grains” April, May 0.16 to 0.13; Legumes-Summer Iuly, August 0.23 to 0.26 Winter April, May 0.13 to 0.17» Alialia Iune. Iuly, August 0.30 to 0.35 ’ Grasses—Warm season Iune, Iuly, August 0.25 to 0.30 c Cool season April, May 0.17 to 0.22 f Vegetables” ,_. Shallow-rooted May. Iune, Iuly 0.18 to 0.22 i Medium to deep- I rooted 0.15 to 0.20 . Peanuts Iuly, August 0.20 to 0.257 Deciduous tree fruits‘ May, Iune, Iuly 0.24 to 0.27} ‘Peak use months also will vary in this area because 7 planting dates, varieties, crop use and weather conditio “The smaller daily water use rates may be expected in q eastern and southern part oi Area 8, while the larger valu can be expected to occur in the northern and tar weste part oi this area. ; SThe irrigation frequency ior small grains and shallow-root vegetables should be about a halt to two-thirds the ir quency indicated by the frequency irrigation guides. M. eiiective moisture control zone ior shallow-rooted veget bles—0 to 1 toot, and ior medium to deep-rooted vegetabl —0 to 2 ieet. 1 ‘Most eiiective moisture control zone ior the fruits grown Area 8 considered to be 0 to 3 ieet. Cover crops or gra weed vegetative covers will increase the daily water by approximately 0.05 to 0.07 inch per day. ~16. APPROXIMATE AVAILABLE WATER PER FOOT X f OF SOIL Available water. inure inches per foot oi soil sands 0.8 to 1.1 loams 1.2 to 1.7 1.7 to 1.9 ams 1.9 to 2.3 2.1 to 2.7 her soil moisture contents, but generally j; a wider range, especially in fine-textured Where moderate amounts of soluble salts in the irrigation water or soil, or both, re- _ce readings of the units buried in the soil are affected. Therefore, if resistance blocks for irrigation control, the concentration uble salts in the soil should not be high to affect adversely the resistance read- her methods for measuring soil moisture, w are gaining prominence and offer future gilities, are (1) those based on thermal prop- of the soil or porous media in moisture tbrium with the soil Water and (2) neutron ring. The latter method makes use of ra- ive materials such as radium-226 and ium. e thermal conductivity units are not af- if. by relatively high salt concentrations and qde an indirect measure of soil moisture ghout the available water range. There- their usefulness appears to be much greater of the irrigated areas than the resistance- jmoisture indicating devices. Calibration and i, 'ng of daily or weekly readings are similar ose used with the resistance blocks. idvantages of the neutron method include q ing moisture determinations with a mini- i, disturbance of the soil under study, and a tube tilled with rotor which connects with ceramic cup. urtesy, Caliiornia Agricultural Experiment Station. ure 40. Leit—Simpliiied diagram illustrating the operating parts oi a tensiometer when installed in the soil. Center—Vacuum gauge tensiometers installed at depths of 9. and 30 inches for irrigation control of cotton growing in a fine sandy loam soil. Right-Cone-shaped stake with gypsum resistance cells. Tapered hole for the stake is drilled with a specially designed auger shown at the Resistance measurements are taken with a portable. battery-operated meter. Other resistance-type blocks be installed and readings taken in a similar manner. Courtesy oi Rayturn Machinery Corporation. good sensitivity over the entire available soil moisture range. Metal access tubes (about 2 inches in diameter) are placed in the soil to the required depths. The open, metal tube permits the probe (neutron source and detector) to be lowered in the soil profile for moisture measure- ments at different increments of depth. From calibration charts, which are provided by the manufacturer, the amount of available Water can be determined on a volume basis. Such an ar- rangement also makes it possible to obtain nu- merous moisture measurements at one location without further disturbance of the soil following the initial installation. The moisture readings are plotted similarly to those outlined for the other moisture indicating devices. ' Equipment for practical field application of both methods is now available commercially. The question may be asked, “What type of results can be obtained with the moisture indi- cating devices?” A typical set of daily tensiometer gauge read- ings for one cotton season in the Lower Rio Grande Valley is shown in Figure 41. Tensio- meters were set at 9, 18 and 30 inches at six locations in the cotton field to determine the rate of moisture use by the cotton plants and the depths from which the water was extracted. By plotting daily or weekly readings, it is possible to approximate how often irrigation is needed and the depths to which water should penetrate to refill the zone from which it has been removed. Figure 41-top shows that too much irrigation Water (I) was applied because the root zone was kept too wet during most of the growing season. Rainfall (R) added to this adverse growing con- dition, especially during the early part of the season. 37 IRRIGATED COTTON 0- EH i FIRST OPEN HULL "mvisr FERlOD '°— _ ' " ' \ i ZO- 30_ 4O .- § a LEGEND c 5°__ #- 9 DtI-‘PTH g 50- TO.- QO __ I l 1| I I} w- m l l 1 I 1 1 l 1 c * " 1 l l 1 l l l. i ' 1 l . l1 l 1 . n 5° lo 2° 5| l0 2° 3Q l0 Z0 1| IO 2° RPRIL MAY JUNE JULY AUGUST O WET IRRIGATED COTTON FIRST OPE“ BCLL |°- mvssr PERIOD ZO._ 3° _. g 4O _ u T LEGEND m 5° —- ._. s‘ nzwm u !-~~-i 13- . § - e—- so‘ i’ 5O ._ 70._ , 1 1 l 1 F l l l‘ CO _.. \\\‘ l- T l l 9°»- EY i 1 g 1 l r1 n a n i l i l i l , | l 1 l l . 1 .1 1 I 1 . 1 I 1 1 1 1 1 so 1 zo a1 1o zo so 1o z s1 1o 20 ‘PHIL MAY JUNE JULY AUGUST o ET NON - IRRIGATED COTTON IO FIRST OPEN BQLL 2O _ umvsst vzmon 30_ /F\/°*"~ _ 7X g 40... n < ... u l: 5Q _ teams g _ s‘ oa- "r11 a °ri°lU 0 60._ l- TO_. U°_ w ~ 2R1 n n 15 a l 1 i1 i l 1 1 1 1 I 1 1 3° |° 2° 3| ID 2O l0 Z0 3| l0 Z0 APRIL _ MAY JUNE JULY AUGUST Figure 41. Daily plots oi tensiometer readings through- out a cotton growing season for three soil moisture levels—wet, medium and dry. The “I" indicates the date oi irrigation and “R" represents the days in which rain fell. Soil type, Willacy fine sandy loam. Top- Too wet: high soil moisture level maintained during the season. Middle-Moisture about right. soil mois- ture kept above 5U percent in the U-Z-ioot zone through- out the season. Bottom—Too dry: no irrigation water was applied during the growing season. .. The opposite occurred in cotton which not irrigated, as shown in 41-bottom. This ton was too dry which resulted in reduced yi By proper timing of irrigations (I) with fall (R), as presented in 41-middle, the c0 plants were kept in a more vigorous growing fruiting condition. If electrical resistance 1a ture units had been used~,_‘~1;he meter read’ would have been plotted in a like manner ‘ the resulting curves being similar in shape. Experience shows that two to three p should be set at each location at depths of '1 9, 18 and 30 inches, depending largely on" type of crop grown. v For shallow-rooted c ' the 9-inch depth is used often as the irriga, control depth, whereas the 18-inch depth w be used for irrigation control with the de’ rooted crops such as cotton, legumes and pe, nial grasses. Moisture readings with these instrum represent a small soil area that surrounds INCHES OF WATER TO APPLY Fine Loamy Sandy lClay Sand Loam Loam 61am 4 Q -- :-l,7 r _ v.35 -2.6 ' -2.8 ‘"14 p3 8 _3.4 ~45 ' l5. ' _ - . | -5.1 4-3 -6.0 _ 6'8 _6'4 —s.2 5D Pivot Line ' 7 7 _ -5-7 1Q G) £85 —7.7 - u-l ' l‘; 4.0 -99 A G ® .2 ' ' ‘ — 0.3 Z 3D @ / i‘? r I "85 _a 6 h _ . _, -11.s 8 g’ @ * -1o.o ' u. 2 ° --12.s Q - -1o.3 E -1 1.4 3. 1.0 o —|2.0 -l2.8 L“ 3 -13'r Note: Values given are amounts of water to be applied when soil is completely dry. If only 507, of the available soil moisture has been used, then use I/2 the amount 45-4 indicated by the, guide, etc. Irrigation efficiency calculated to be 70°/.. --I7.l Figure 42. Water application guide tor Panhandle- ' Plains of Texas, area 1. r block; consequently, sufficient locations be established over the field so that a entative observation of moisture for each w can be obtained. Moisture observation fl should be established near the beginning, middle and near the end of an irrigation or sprinkler line so that differences in water p; 'bution over the land can be detected. ; important problemencountered with mois- measuring instruments when installed in tmorillonite-type clay soils is the severe a 'ng which occurs. As the soil dries, it which causes contact between the soil athe moisture indicating device to become i’ When this occurs, the instrument is T red ineffective. It seems, therefore, that ‘installation and use of such devices in clay _; which are subject to rather severe cracking i be questionable in some cases. iThe future use of moisture-indicating instru- tt ts appears to be promising, but, the irrigator understand their limitations and capabili- INCHES OF WATER TO APPLY Fine Loamy Sandy Clay Sand Loam Loam Loam Clay 2-2 g2 o _-|.4 ' . '_2|| -2.4 ' .. - -2.6 T23 _34 -3.3 -3.6 _ ' -4l ~43 _45 -3.s -s.o P - _ _ -s."r -s.7 '49 ,:_ Pgot Line _6_4 _ r51 h $2 E. --7.2 -6-8 * ' -65 -e.s : "81) :_ C?) I "9-2 -?.7 "8-3 f - ~82 f. @ "I03 5 <53 —9o F J-I m : - _99 —|o.3 E -|o.3 L _| I 6 -i L6 ' ~|2.4 a —-l2.9 f —|3.| Note Values given are amounts of water to H66 be applied when the soil is completely dry. If ' only 50% of the available soil moisture has been used, then use I/2 of the amount “'47 * indicated by the guide, eta Jijrigation efficiency calculated to be{_-70%. Use of fie? saline water will necessitate the addition ' of larger amounts for leaching. ——I6.4 -—2ai ‘Figure 43. Water application guide for the Pecos Val- ley and Trans-Pecos area. area 2. ties thoroughly in order to use them effectively for irrigation control. AMOUNT OF IRRIGATION WATER TO APPLY Water storage capacity of the soil, infiltra- tion rates and the amount of water in the soil at the time of irrigation are principal factors which govern the quantity of water that should be applied at each irrigation. The availability of water for crop use between irrigations or periods of rainfall depends largely on the water holding and release characteristics of the soil per foot of depth and the depth of'soil through which the root system extends. To consider the potential water supplying power for plant growth of each soil would be difficult; therefore, Table 16 was prepared as a general guide for estimating the amount of water available from different textured soils. INCHES OF WATER TO APPLY am n a“ u, Eilii a Sand y Loanil Loam kgaym Clay . 4 5 -i_- 1r v T 1- T -|.s - _ — 2.8 —2.3 _ D 3 7 I25 1 4'6 -3.4 , ' 3-3 , ' - 3.8 —5.6 ,_ r- -4.e 3'9 -e.s _ _ 74 _ 5 7 / -4.9 5 0 PIVQI Line -—8_4 _ /B 5'2 ' - - 5.8 G c) --9.3 76f - F {f / ' _6 4 -ee I 40 / _gQ \- © ui 3 / -9 I -7,7 -7.7 S 3 0 -a.2 =1 / C‘? -|o.3 9) / © > —9D u. 2-0 --||_4 o #99 '95 I . E -|a3 a l.O -| 1.5 -I L6 a| |_5 ——l2.8 -|3.| —l3.5 ~l4.8 —|5.5 ‘#65 J-|7.2 Figure 44. Water application guide for West Central Texas, area 3 39 Table 16 shows that sandy soils are able to retain and supply considerably less water than the clays, which means that water will have to be applied more frequently to sands. A given quantity of water will wet sands to a greater depth than clays. Therefore, Where the soil has become so dry that plants show systems of medium to severe moisture stress, the quantity of Water needed is approximately equal to the amount that the soil is capable of holding within the depth depleted by the plant roots. A simple field method of estimating the amount of water to be applied by irrigation is the “feel method” of moisture determination (Table 7) in conjunction with the available water storage characteristic of the soils, as given in Table 16. For example, if it is found that the zone of maximum root concentration of cotton (0 to 30 inches) growing on a fine sandy soil contains about 50 percent moisture, the amount of water needed to bring the depleted moisture zone to field capacity can be estimated as follows: INCHES OF WATER TO APPLY Fne Loomy Siandy Clay Sand Loam Loam Loam Clay £2 Q9 Q9 .5. . F F -l.4 ’ 12A -2.4 :2 a +2.7 .. ' -35 > -3.2 :3.6 _3.7 -4.3 _ 4 8 -4_Q —5.0 . P —5.r -s.o '4'? 5.0 Line L6.4 _ "5.4 F. - ‘ . __ -7.3 1E1 7.2 * _ -s3 I 4.0 -3 5 6'6 {j ® g —9 7 _ 8 I -7.4 9 30 @ ' -1.e _i 5 @ g —ll.O m @ . LL —95 Q 2 ° —-|2 | I P9.4 w 3, -|o.e Q |.0 —ll.O "'22 —l|.| —-l3.6 -l2.6 Note: Values given are amounts of water #30 to be added when soil is completely dry. ' If only 50% of the available soil moisture has been used, then use V2 of the amount “'43 indicated by the guide, etc. Irrigation efficiency calculated to be 70%. 49 -| . ~ -i~|5.7 --|6.7 Figure 45. Water application guide for the Rio Grande Plain. area 4. 40 »- Amount of available water per foot I a inches (from Table 16). ‘ Amount of water available in the 30-i‘ depth I (1.5) (2.5 feet) I 3.7 inches. If only 50 percent is needed in the root i; then (3.7) (0.50) I 1.8 inches required. Assuming 70 percent irrigation efficiency, 1.8 inches _ total water to be applied I ———-—— I* 0.70 inches (approximately). A second method developed for estima the amount of irrigation water to apply is sh in Figures 42 through 49. These water ap cations guides cover the eight irrigated a of Texas, as outlined in Figure 31. A typical problem that often confronts H irrigation farmer in determining the amount’ INCHES OF WATER TO APP f Fine Sandy Clay i Loam Loam 6am Cl l2.4 ~ -_3_4 ~21; F45 —3.4 _5_7 ~45 T63 -s.7 ~52 -8.0 _ . . 79-2 ~12 5n Pivot Line 405 _ -5_a l; (D - u; --|l.4 '35 r U. | 4.0 “8.6 *- I l" ® _ 3 Q —ll.4 _ ® i _i g (4) -|2.a ' -l2.0 $29 ——l4.3 E fi -l3.7 c: l.O -I5.4 a l- Note: Values given are amounts of water to be added when soil is completely dry. lf only 50% of the available soil moisture has been used, then use ‘l2 of the amount indicated by the guide, etc. Irrigation efficiency calculated to be 70%. Figure 46. Water application guide for the C‘ Prairie, area 5. 1, s, to apply to a specific crop and soil type A tomato grower in East Texas has a sprinkler unit that Will cover an area of ‘t wide and 400 feet long at one setting. rinklers are spaced 40 feet apart on the ‘VA calibration of the pump and sprinkler indicated the unit would apply about 150 V; per minute to the soil. The tomatoes are g1; on a fine sandy loam soil, and irriga- ater is to be applied when approximately rcent of the available water has been used f; top 2 feet of soil. His problem is to de- i e the length of timerequired for a sprink- A ting so that the top 2 feet can be brought ‘capacity. , m Figure 4-7, “Water Application Guide Texas Timberlands--Humid Region,” p t ount of water required for the top 2 feet soil is shown to be about 3.7 inches; how- ilonly half of this quantity will be needed, proximately 1.9 inches. ‘i amount of water applied in inches per can be found in Figure H of the Appen- .. INCHES OF WATER TO APPLY Fine L S d Clo Sgglgw L33"? Loam Loaxn Clay g2 <12 a -|.3 '-i.9 "-9 » 25 , -2.4 ' -2.a lss ‘as ' ‘3-4 _-3.9 d] -3.6 * -4.5 ,1 -4.3 Q; -52 _@5 - _Pivot Line '_ 5,, _ 4-8 _ _5_2 (D L 5 4 - 5 6 i _ , . s i -6.| ‘s’? g -6.4 ® b 6.8 -?.4 _ ' i @ 1.5 4.2 @ -s.s ' © - —e.5 -L9.3 *5 _8.6 ~97 -l0.0 no.9 —lO.3 i121 -ll.4 i — l2.0 ‘Note. Values given are amounts of water f be applied when soil is completely dry. _ it _lf only 50%" of the available soil moisture —l2-8 lhos been used, then use ‘r42 of, the amount indicated by the guide, eta’ Irrigation _|37 efficiency calculated to be 70%. ' i’ 'L|4.3 -l-i5.4 e 47. Water application guide for the East Texas jv- erlands-Humid Region-area 6. .. dix. With the 10 sprinklers and a Water appli- cation of 150 gallons per minute, each sprinkler head will discharge 15 gallons per minute. The Water application rate of the sprinkler unit in inches per hour is 0.60. Since the amount of water needed is 1.9 inches and the rate of application is 0.60 inch per hour, the time required will be 1.9 -I— 0.60 I 3.2'hours (approximately) . Adjusting the Water application rate to the infiltration or absorption rate of the soil is im- portant. ‘If the infiltration rate is about 1 inch per hour, the application rate of 0.60 inch per hour is not excessive. However, on ‘medium and fine-textured soils, the Water intake rates are often 0.50 inch or less per hour. Under such conditions, the sprinkler rate of 0.60 inch per hour Would be excessive and would result in wa- ter ponding or possibly run-off. Therefore, when INCHES OF WATER TO APPLY Fine Sandy Clay Loam Loam boom Clay Q @ Q2 @P I i l -2.2 - _ -32 —2.4 :-4.3 l3 6 4'2 -4.o -55 ' - ls_4 _ - —4.s "4"? -7.5 _ - - - -e.o _ p '85 -s.0 F59 __ Plqlgof Line "E95 _ ‘5-3 t3 g - 10.7 -1.3 - u 5 ' -7e '8") ‘4-0? —e.s ' o g ® -9.1 _9_4 i-3-°:- . -|oo _l : _ - s5) : @ :09 _ I -ll.0 i529? -l-l2.l I -l2.0 I ; 5: I 2s _ _| . B’ |.0:— ' 1 -|4.0 —l4.3 “'5'? -|s.o Note: Values given are amounts of water _ to be applied when soil is completely dry. If only 50% of the available soil moisture 4&0 has been used, then use ‘l2 the amount indicated by the guide, etc. Irrigation _ efficiency calculated to be 70%. +200 Figure 48. Water application guide for the Central Blackland and Grand Prairies regions. area 7. 41 using sprinkler systems, the application rate should not be greater than the soil intake rate. To determine when irrigation water has reached the pre-calculated depth, the irrigator can use a soil probe, a sharp-shooter shovel, post hole digger or soil auger for obtaining soil samples from depths below the surface. The transition zone between wet and dry soil usually 1s a relatively sharp break. Where tensiometers or resistance blocks are used as moisture indi- cators, their readings will be lowered when irri- gation water reaches the depth at which the porous cups or blocks are buried; therefore, these units can be used to indicate depths of water penetration. A third method for calculating the amount of water to apply during an irrigation is dis- cussed in detail on page 45 of the Appendix. ACKNOWLEDGMENTS The author thanks members of the Depart- ment of Agronomy, substations of the Texas Ag- INCHES OF WATER T0 APPLY Fine Very Fine Silty Sandy Sandy Clay Clay am Loam Loam Loam Clay Q G2 GP lg}; . L i L33 -2.6 " _ 45 ' 4'8 .- 5"? _3 8 -3.2 _ - _ ~34 T”. -5.| ‘4-3 -a.o . bag _64 ~47 5D Pivot Line {J05 _ "5-7 F (D _ v5.2 m --| L4 '77 ‘if .3 ‘l 4.0 _9_0 hzz g ® i_|o 3 —6.8 ' ~85 ,9 3.0 @ _ _-,_8 =' @ I -| L6 o ‘f’ 2o -|o.0 u- ' --|2.9 ° -94 "86 E —||.4 S |.0 o -I |.0 -|2.e _|o_3 --|4.3 -|2.s Note; Values given are amounts of water H2O to be added when soil is completely dry. If only 507. of the available soil moisture -|4.3 has been used, then use l/2 of the amount indicated by the guide, etc. Irrigation efficiency calculated to be 70%. "3-7 --|5.7 —-l5.4 Figure 49. Water application guide for the Rolling Plains and West Cross Timbers, area 8. 42 ricultural Experiment Station, Extension Se and cooperating ARS-USDA (Soil and Wa personnel for their help and many suggesti during the preparation of the manuscript. Thanks also are due to W. F. Hughes, (US l, PERB) Agricultural Research Service, for , contribution in land economics (irrigation), i; to Dean J. B. Page of A&M’s Graduate Sc and Director R. D. Lewis of the Texas Agri tural Experiment Station for their suggesti and review of the manuscript. . Appreciation is expressed also to the i; contributors of photographs and other illustrit material. , REFERENCES 1. Baver, L. D., Soil Physics. John Wiley and Sons, i New York, N. Y. 2. Bloodgood, D. W., Patterson, R. E. and Smith, R. f Water Evaporation Studies in Texas. Texas A Exp. Sta. Bul. 787. 1954. = 3. Bloodworth, M. E., Burleson, C. A. and Cowley, W. Effect of Irrigation Differentials and Planting Da on the Growth, Yield and Fiber Characteristics Cotton in the Lower Rio Grande Valley. Texas A 1 i, Exp. Sta. Progress Report 1866. 1956. 4. Bonnen, C. A., McArthur, W. C., Magee, A. C. ~ Hughes, W. F., Use of Irrigation Water on the Hi Plains. Texas Agric. Exp. Sta. Bul. 756. 1952. 5. Christensen, P. D. and Lyerly, P. J., Water Qu ' _ as It Influences Irrigation Practices and Crop ' 1 duction. Texas Agric. Exp. Sta. Cir. 132. 1952. ‘y 6. Crafts, A. S., Currier, H. B. and Stocking, C. f’ Water in the Physiology of Plants. Chronica B anica Co. Waltham, Mass. 1949. 7. Eaton, F. M., Formulas for Estimating Leaching =i Gypsum Requiements of Irrigation Waters. Te "V Agric. Exp. Sta. Misc. Pub. 111. 1954. t 8. Fisher, C. E. and Burnett, E., Conservation and. R ization of Soil Moisture. Texas Agric. Exp. Sta. B 767. 1953. ' 9. Gausman, H. W., Salt Tolerances of Five Grass Texas Agric. Exp. Sta. Progress Report 1620. 19 10. Gerard, C. J., Bloodworth, M. E., Burleson, C. A. a Cowley, W. R., Cotton Irrigation in the Lower '. Grande Valley. Texas Agric. Exp. Sta. Bul. 916. 19v 11. Hayward, H. E. (Editor). The Salt Problem in I I gated Agriculture. USDA Misc. Pub. 607. 1946. ‘ 12. Hughes, W. F. and Motheral, J. R., Irrigated A n’ culture in Texas. Texas Agric. Exp. Sta. Misc. ' = 59. 1950. - 1.3. Hughes, W. F. and Magee, A. C., Changes in Inve ment and Irrigation Water Costs, Texas High Plai, 1950-54. Texas Agric. Exp. Sta. Bul. 828. 1956. . 14. Israelsen, O. W., Irrigation Principles and Practicl i John Wiley and Sons, Inc. New York, N. Y. 19 i1 15. Jensen, M. E., Research Shows When to Irriga Winter Wheat. Soil and Water. 1957. . 16. Lyerly, P. J . and Longenecker, D. E., Salinity Con i in Irrigation Agriculture. Texas Agric. Exp. Sta. B _ » 876. 1957. " 17. Magee, A. C., Bonnen, C. A., McArthur, W. C. J Hughes, W. F., Production Practices for Irriga =1 Crops on the High Plains. Texas Agric. Exp. S j Bul. 763. 1953. » 18. Magistad, O. C. and Christiansen, J. E., Saline Soi I Their Nature and Management. USDA Cir. It 1944. olds, E. B., Research on Rice Production in Tex- Texas Agric. Exp. Sta. Bul. 775. 1954. Yards, L. A. (Editor). Diagnosis and Improve- _t1g§4Saline and Alkali Soils. USDA Handbook i I s, P. E. and Swanson, N. P., Level Irrigation. J our. t’? & Water Conserv. 12:209-213. 1957. sell, E. J., Soil Conditions and Plant Growth. gmans, Green and Co. New York, N. Y. 1950. w, B. T. (Editor), So-il Physical Conditions and t Growth. Academic Press, New York, 1952'. ferud, A. (Editor), Water. USDA Yearbook of * ‘culture. 1955. - i en, R. 1)., Alfalfa Production in Texas. * 'c. Exp. Sta. Bul. 955. 1957. anson, N. P. and "Thaxton, E. L., Irrigation Re- ements for Grain Sorghum Production on the ’ ;_ Plains. Texas Agric. Exp. Sta. Bul. 846. 1957. _@-= l ton, E. L. and Swanson, N, P., Guides in Cotton p. 'gation on the High Plains. Texas Agric. Exp. . Bul. s38. 1956. p-omas, G. W. and Young, V. A., Relation of Soils, Nfinfall and Grazing Management to Vegetation, . estern Edwards Plateau of Texas. Texas Agric. p. Sta. Bul. 786. 1954. Texas 29. 30. 31. 32. 33. 34. 35. 36. . Water For Texas. Thorne, D. W. and Peterson, H. B., Irrigated Soils- Their Fertility and Management. The Blaikston Com- pany. Philadelphia. 1949. Thornton, M. K. and Templin, E. H., The Soils of Texas. Texas Agric. Extension Service Leaflet 74. Thurmond, R. V., Box J. and F. C. Elliott, Texas Guide for Growing Irrigated Cotton. Texas Agri. Extension Service, Bulletin 896. Trew, E. M. and Hoveland, C. S., Irrigated Pastures for South Texas. Texas Agric. Extension Service and Texas Agric. Exp. Sta. Bul. 819. 1955. Truog, E. (Editor), Mineral Nutrition of Plants. University of Wisconsin Press, Madison, -Wisconsin. 1951. Texas Education Agency. Water. Bul. 578. 1956. UNESCO. Reviews. of Research on‘ Problems of Utilization of Saline Water. Columbia University Press. New York 27, N. Y. 1954. ' Water For Texas. Proceedings of the Second Annual Conference on Water for Texas. The A. & M. College of Texas, College Station, Texas. Water Research and Information Center. 1956. Proceedings of the Third Annual The A. & M. College Water Research Conference on Water for Texas. of Texas, College Station, Texas. and Information Center. 1957. 43 APPENDIX Use oi Discharge Curves EXAMPLE A-1: An irrigation pump is dis- charging water through a short outlet pipe that has 8 inches inside diameter (Figure A), but the quantity of water being discharged is not known. The discharge pipe is full, and the problem is to estimate the quantity of water flowing from the pipe. (1) The first step is to determine the value of “Y,” as shown in Figure B, for a value of H : 12 inches as indicated. This value can be found by measuring with a rule or tape. (2) Assume that “Y” was found to be 18 inches when the 8-inch (I.D.) pipe was full. This is sketched on Figure A. (3) After locating the va1ue—-18 inches as the “Y” projection—proceed across until the line marked 8-inch inside diameter of pipe is inter- sected. Then, project a vertical line upward until the line, “Discharge in Gallons Per Minute,” is reached. (4) Read the discharge in gallons per min (g.p.m.) directly from the chart—-about ', g.p.m. for this example. ' EXAMPLE A-2: AnB-“inch (I.D.) discha pipe from a pump lacks 2 inches of flowing f (see detail drawing Figure B). The “Y” jection was measured and found to be 18 inc with H I 12 inches (always the same with ‘ curves). The problem is to determine the n} proximate amount of water being discharj from the pipe. ‘ (1) On the bottom of the sheet in Figure locate the pipe size (8-inch I.D.). ' (2) Project a vertical line until it inters the curve which shows a “freeboard” of 2 inch (3) Then, extend the line until it interse l the “Y” projection line of 18 inches. 1 (4) From this point, continue the line ve tically until it intersects the margin on which a j DISCHARGE m GALLONS PER MINUTE o 0 0' o O o o O o o 8 8 8 8 8 34 9 8 § 9 § g g 8 § é 2 3 '2 g 5 <2 a 9 <2 g K1 % g g 34 a2 = = pa. ‘I 3o JIL e 28 / 3 l3- 26 .25. I 24 _2§_ ‘ f2 2a _22_ O E 2o 1L I E I8 '8 Z i O p _._ Q ‘a’ o I4 I-i E ‘L I2 __ f>- - IO __ DISCHARGE CURVE 6 FOR PIPES FLOWING FULL -- TRAJECTORY METHOD 4 H=|2" -— 2 o I I I I I I I I I I I I I I I I I I l I I I Figure A. 44 'ted the values of “discharge in gallons per 40% of the total soil moisture used is obtained ” from the 0-1 ft. depth. l) For this example, the discharge would be 30% of the total soil moisture used is obtained _~,750 g.p.m. . from the 1-2 ft. depth. l} _ 20% of the total soil moisture used is obtained lculatmg the Amount to Apply from the 2-3 ft. depth. l‘ e amount of available Water in the Soil for Of thG total SOY-ll moisture USQd lS obtained Iuse is determined greatly by soil character- frern the 3-5 ft- depth- fl Many crops grown in irrigated sections _ _ _ _, r to have similar soil moisture extraction 100% tetal readlly aVaIlable Ineletnre- s. Although a soil may contain a given int of available Water per increment of depth s), a portion of the water may become _ ble so slowly for plant utilization that h will be retarded. Therefore, the term l ily available moisture” is used often to indi- These values will change with different soils, location within the State, crops, methods of til- lage and irrigation practices. However, these percentages give a base from which to work in calculating irrigation needs for a particular soil fthat portion of the available water which and crop’ the 11Sed readily Der lnerernent ef Sell depth The values in the accompanying table are "'11 tlle reet Zene Witllellt affeetlng’ adversely general for Texas soils. Such values often vary ate ef Plant greWtn and reprednetien- Fer for the same soil within a given locality because 5 irrigated crops, the following soil moisture of different inherent soil characteristics and til- ‘a tion pattern occurs: lage and cropping practices imposed on the land. DISCHARGE IN GALLONS PER MINUTE . N\\\ \\\ X \ \ \- __\ l \ \ l/W Y/ H, .\qqqQq§\\ \\A\§.\\v@ %// \‘\\\\\\\\\ \I\ _ \ \ \ / / / \\ss>>xx \\ \j\\ \\\\\p v / » \\\\.\\.\\\_\\\\\\\\\\\\ . \ l\\ <~\\ \\ \ \ W? \ \Q§§\\. \\\\\\\ \\ \§\ K \\\\ I ;/§( /I / §§g§\\\\l\\\\‘\\xxézé\s ‘\\\\1\\. l l / I DQQ:’\\\\ 4 »/\ g< ,, -,§g§§§q§\s§\§g .... K W X \ .\ \ \ \ \ \ \ \ 6‘ ‘\;s§s< 0.60 I 5.7~ moisture). 40% of the available soil moisture has b’ used in the 1-2 ft. depth I 3.8% (9.5 >< 0.40 A ' 3.8% moisture). no Examination of the soil for moisture at lo depth shows that about 20% of the available ter in the 2-3-foot zone has been used (9.5 X 0, I 1.9% to be replaced) and approximately 10 of the available water in the 3-4-foot zone r been consumed (9.5 >< 0.10 I 0.95 or 1% to, replaced). Therefore, the inches of Water quired to be replaced in the 0-4-foot soil profile PDBd 100 0-1 ft. zone—(5.70) (1.57) (12") I 1.07 g 1-2 ft. zone—(3.80) (1.57) (12”) I 0.71 g 2-3 ft. zone—(1.90) (1.57) (12”) I 0.36 v 3-4 ft. zone——(1.00) (1.57) (12”) I 0.18 2.32 With an irrigation efficiency of 70%, total amount of water required so that the inches can be replaced in the soil profile will‘, 2.32 approximately —— I 3.3 inches. 0 Where irrigation Water does not contain r tively large quantities of soluble salts, it is l sirable to add at least 0.50 inch, or in some up to 20% additional water, to the calcul value. With high salt concentrations, such . those found in the Trans-Pecos area, as much i 50% additional water sometimes is necess a The purpose of the extra Water is for leac , to prevent the upward movement of soluble w and to prevent a zone of salt accumulation in- root zone. a p. alculating Irrigation Efficiency ‘gation efficiency usually is defined as the - "(percentage or inches basis) of irriga- "ter delivered to the farm or field that is le in the soil for consumptive use by crops. ,. often called “field irrigation efficiency” ‘j easured in the field or plot area. ‘calculate the irrigation efficiency, it is ry to make the following measurements p: each irrigation: rea of land being irrigated—acres. iqil moisture content before irrigation— ‘ually expressed as a percentage. if.» moisture content after irrigation—usu- ly expressed as a percentage. (Sample .1 tions should be chosen to make sampling w day or so after irrigation as easy as ssible because of the muddy soil.) eed bulk density for each foot increment , depth to about 5 feet. llepth of water penetration in inches. ize of irrigation stream—gallons per min- i e or cubic feet per second. ime required to irrigate—hours. urface run-off, if any. a.‘ AMPLE: Following an irrigation of cot- " e amount of water applied was calculated 4.0 inches. The land had been leveled for ion, so no run-off occurred. Pre-irriga- _ poisture samples had been taken to a depth . et, and the average bulk density (DB) was é) to be 1.50. The average moisture content soil before irrigation at the 0-48-inch depth 3.1%, and after irrigation 17.0%, or an T. in soil moisture of 3.9%. Therefore, the k in soil moisture in inches following the ion can be calculated in the following man- . PDBd lume increase of water I 1 100 (3.9) (1.50) (48) -—-——-——— I 2.80 inches i 100 j igation efficiency I crease in soil moisture) (100) A_ depth of water applied ; 7 .80) (100) 4.0 Water Use Problems Any pump manufacturers and irrigation dis- " have charts and graphs for quick compu- tation of the more common problems concerning water volumes pumped, such as the amount of water applied per unit of time. Since these sim- plified methods are not available to most irriga- tion farmers, a simplified formula, along with some example problems, is given for use in work- ing out some of the more common water use problems which confront the irrigator: (Q) (T) I (I) (A) where Q I amount of water in second feet (cubic feet per second-c.f.s.). (Water in gallons per minute) Q = a 452.6 Q I c.f.s. or acre-inches per hour. T I time required to apply water to a given area—hours. I I amount of water applied—inches. = I area of land irrigated—acres. EXAMPLE 1: An irrigation farmer wishes to apply 4 inches of water to a 40-acre field. His water is pumped from a well which has been metered at 950 gallons per minute (g.p.m.) . Ap- proximately how long will be required to make this application of water? Assume irrigating 12 hours a day: 950 g.p.m. Q I ———-—— I 2.1 c.f.s. or acre-inches 452.6 per hour. _ <1) (A) _ Q (4 inches) (40 acres) (hour) I 76.2 hours. 2.1 acre-inches Irrigating 12 hours. per day will require (76.2 hr.) (day) _--_- = 6.4 days. (12 hr.) EXAMPLE 2: Same problem as Example 1 except Figure C is used. With a pump discharge capacity of 950 g.p.m., a 4-inch application to 1 acre will require 1.9 hours. Therefore, for 40 acres, the amount of time required would be (1.9) (40) I 76 hr. or 6.3 days (12 hours op- eration per day). EXAMPLE 3: An irrigation pump has deliv- ered water to a 60-acre field for 96 hours at an average rate of 1,020 g.p.m. How many inches of water were applied to the field? _ (o) (T) A I 47 1020 g.p.m. 452.6 Q (c.f.s.) I I 2.2 c.f.s. or acre- inches per hour (2.2 acre-inches) (96 hr.) Then I I ll 9° an Hr. (60 acres) inches of water applied. EXA-MPLE 4: An irrigator Wants to apply 4 inches of water to a 120-acre field in 180 hours (15 days by irrigating 12 hours per day). He has a Parshall flume installed in his delivery ca- nal and both will handle Water quantities up to 5 c.f.s. How much water will be needed (c.f.s. and g.p.m.) during each 12-hour run so that the fiieldvcan be irrigated in the allotted time of 15 ays . (Q) (T) = (T) (A) __ (I) (A) _ (4 inches) (120 acres) _ 180 hours (Q) Q I 2.7 c.f.s. Q I (2.7 c.f.s.) (452.6) I 1222 g.p.m. EXAMPLE 5: An irrigation Water supply of 600 g.p.m. is used to irrigate 40 rows spaced 38 inches apart and 400 feet long. (1) What is the size of the area being it gated? (2) How many inches of water will be app in 1 hour? 7 (3) How many hours will it take to ge. inches of water into the root zone, assumin 70% irrigation efficiency? 1 s‘. (1) Area irrigated I p (40 rows) (3.17 ft. wide) (400 ft. long) (ac 43,560 sq. ft. I 1.16 or 1.2 acres. l it 600 g.p.m. 452.6 acre (2) I 1.3 acre-inches per hour 1.3 acre-inches . Then I 1.12 inches per h: hour (1.16 acres) being applied to the 40-row area. (3) Time required to get 3 inches of water l: the root zone (70% efficiency) I ‘ 3.0 inches (hr.) —----——— I 3.8 hours. 1.12 inches (.70) § Example: A well producing 620 gallons per minute (gpm) will pump 4 inches of water per acre in a.’ 2.9 hours as shown by the arrow line below. This assumes no losses due to seeprgge, 3 evaporation, and other wastes. With a 70% application efficiency, then =4.l hours é would be required to pump so that 4 inches of water would be supplied to the soil storage 3 reservoir after losses were considered. ' Note: All values based on water delivered at the pump or discharge outlet. No losses considered. ‘s’ g 2000 o Q o l; ,—_' IOOO :- 8 aoo - 2): 600 - : E soo — ° 40o - é 0 300 — a. E a. 200 - |oo l I lllllLl lll I lll .l .2 .3 .4 .5 .6 7.8.9l 2 3 4 56789l0 2O 3O 4O 6O B0 WATER APPLICATION TIME — Hours (Approximate) Figure C. 48 _- : example 5 but using charts in Appendix. i irrigated: Figure D-1.2 acres. A ount of water applied per hour: Figure _A 1.3 acre-inches per hour per acre g ber of hours required to get 3 inches of ter into the root zone (assuming 70% ef- giency): Figure C—2.3 hours to apply 3 it hes to 1 acre. (1.2 acres) 2.3 hours i‘ en, I 3.8 hours. (acre) (.70) MPLE 6: An irrigation water supply of .p.m. is being used to irrigate 20 rows, _~lf1ave a spacing of 40 inches and a length eet. i) How many acres are being irrigated at _. 0-row setting? ) How many inches of water are being ap- Qper hour to the 20-row area? ) How long will be required to apply 3 of water? a g om Figure E, it is found that 20 rows, (ving a row spacing of 40 inches and a Jngth of 750 feet, contain 1.1 acres. A or ROWS numeen or ROWS a _ FEET AREA - ACRES stones-s aoo $5--545 4l.2-—43.6 25° sos--321 2sa--212 20%--2L8 l§'s“l?'§ i4§--isz '5° I23--BO |o3~-ms e2--e1 12--11 s2--as 4s—-52 4J—-44 14--as Z6-“ZS 200 IOO 90 70 /36" Row Spacing 60 38" Row Spacing Z0--Z2 LT--LB 50 ‘T3~ .5 \ L0--M as-wm ea--12 ass-ea ‘\ w ha--5| 4i+-44 54-~ss .21--29 20 24--25 204-22 17-1-18 l5 i4-—is JO'FM -, ..os-- or IO 40 / ? 1 Use nversion hart: Place straight edge across chart from of Rows" to the "Length of Rows" being irrigated, and read directly ' number of acres on "Area" line which corresponds to row spacing. ‘(A = 25 rows - 800 feet long equals l.5 acres (approx.) for 38" spacing or l.3 acres (approx.) for 36" row spacing. Figure D. (2) From Figure G, a water supply of 600 g.p.m. will produce about 1.3 acre-inches of Water per hour. It was found from Figure E that the area covered was 1.1 acres. 1.3 acre-inches Therefore, ----i—-—-— I 1.18 or (hour) (1.1 acres) about 1.2 in. per. hr. (3) The time required to apply 3 inches of water to the surface (assuming no losses) will be 3 in. (hr.) -—-———— I. 2.5 hrs. 1.2 in. Water Application with Sprinklers The purchaser of a sprinkler irrigation system should get one designed specifically for the needs of his soil and crops. The unit should be able to apply water at a rate consistent with the avail- able Water supply, absorption ability of the soil and topography of the land. For example, the more porous soils (coarse-textured) may take Water at the rate of several inches per hour, whereas, some of the “tight” clay soils (fine-tex- tured) may have infiltration rates below 0.5 LENGTH OF ROWS NUMBER OF ROWS FEET sooo ee.1-- 12.2 30o 573-- $0.2 35°° 45c --4e.| 401 --4z.i 34.3 ~- as: 2°°° 28.6-—30.l 20° 22.9 —-24.| ziggn§g ‘500 l6:O—- i618 '50 i3] -- |4.4 HA--MO 9]--95 E0-—8A &B——Z2 5]--60 45--48 40--kl 3§——3B 2B—-23 23--24 lB‘"20 L5——I5 4°° i.l--l.2 ‘ ‘m .95--l.0 , _zz--.ee-- 300 _____ —-~"' .se--.72 3o .s|-»-.e4 .53——.56 A5--48 . a--.4o . 4--36 200 .30--52 20 26--.2a 22--24 J9--20 n50 l5 J5--J6 AREA - ACRES 250 IOOO 800 700 ssssg 600 42" ‘Row Spacing 500 40" Row Spacing Jl--J2 p00 .O7--.O8 IO How to Use Conversion Chart: Place straight edge across chart from "Number of Rows" to the "Length of Rows" being irrigated, and read directly the number of acres on "Area" line which corresponds to row spacing. Example: 36 rows — 280 feet long equals .77 acres (opprox.) for 40" row spacing or .80 acres (approx) tor 42" row spacing. Figure E. I 49 inch per hour. Therefore, the design and layout of a sprinkler system will involve considerably more than just computing the engineering data necessary to get the irrigation water distributed over the soil surface. The following examples are presented only to familiarize the farmer with some of the more simple computations used in sprinkler design. If he is convinced that a sprinkler will be the most efficient and economical Way to apply Water on his farm, then competent personnel should be con- sulted on the design, installation and maintenance of a sprinkler unit for his particular needs. EXAMPLE 1: Water required—g.p.m. Formula to use: (A) (I) (27,154) (T) (D) (60) Where: A I acres to be irrigated g.p.m. 7-: I I inches of water to be applied 27,154 I gallons in 1 acre-inch of Water T I hours of operation per day D I days required to cover the area 60 I minutes in one hour Assume that 2 inches of Water are to be applied to a 60-acre field in 10 days with a daily oper- ating period of 12 hours. How much (g.p.m.) will be needed to meet these req ments? The field is 1,320 feet wide by 1,980 long. » g.p.m. I (27, 154 gal.) (2 in.) (60acres) (hour) (da (12 hours) (10 days) minutes) (acre-i p I 452.5 g.p.m. l This will be the amount Water required to b». livered onto the field. Therefore, a greater i?!» tity of water should be made available for pu ing to account for any unavoidable losses in t i sit. EXAMPLE 2: Number of sprinklers -! for a given area. ~ Factors to consider in determining the n A ber of sprinklers are: location of Water sourc field, size of field and topography and shape? field. i Assume an 80-acre field with the length ‘ the sprinkler line containing the sprinkler h 1 to be 1,320 feet. Sprinkler irrigation lines ar be ‘60 feet apart with sprinkler spacings to 40 along the line. Therefore, the number of tsp ' lers needed will be 1320 + 40 I 33 sprinkl Available Water (from Example 1) supply to line is set at about 452.5 gallons per min i . c‘ 2400 — ‘é o, e s q, s 3 2200 - o, ,0 $19: Q0“ yo‘? gcPQ‘ yo)?‘ $03‘ ‘puff’ gout?’ o - w § 5% o i‘ 1 e \\ \‘L (D g Q Q Q‘ Q7 I " Q l‘ Q‘ 9) m _ o: J: Iv; v g I800 — 8 “’ g - f Q I600 — A O I400 _ EXAMPLE: A pump is supplying water at _ E » rate of lOOOgpm. In lO hours about ; p’ I200 - 22 acre inches would be pumped as i1 8 ' by arrow lines. With an application IOOO - - " m efficiency of 70% there would be app o 30o (22)x(.70)=l5.4 acre inches of water L" - replaced in the root zone. ' g 500 EXAMPLE 2: lf 7 acres were irrigated g 400 ' during the IO hr. period, the approximate 1 Q amount of water that was stored g Q- in the soil would be =2_2'ns_' 5 20o y , 7 Acres l i, n‘ . .iii.i.l.i.i.i.i.I.i.i.i.i.l.i.i.i.i.l.iii.i.i.l.i.i.i.iil.i.i.i.iil.i.i.i.i.l.i.i.i.i.l.iii.i.i.l.i.i.i.i.l.i.i.i.i.l.i.i.i.i.l O 5 IO l5 2O 25 3O v ACRE INCHES OF WATER (NO LOSSES INCLUDED) NOTE: To determine the amount of water pumped for periods longer than I2 hours, find the quantity pumped for I hour v (from above chart) and multiply by the total number of hours of pumping time. EXAMPLE: IOOO g.p.m. for I hour = 2.2 Ac. ins; For 3O hrs. (2.2)(30) = 66 Acre inches. With a field efficiency of 70%, then (66)(TO)=46.2 Acre inches for storage in soil reservoir. lf IO Acres were irrigated, there would have been = 4.6 inches of water stored in the soil. IO Acres Figure F. 50 3s 4o 4s so s5 so 65 n, the approximate amount of water avail- vie per sprinkler will be 452.5 + 33 I 13.7 llons per minute per sprinkler. The nozzle .. ‘i and pressures required to give this quantity water per sprinkler head can be obtained from q- sprinkler irrigation dealer or manufacturer. l EXA-MPLE 3A: Rate of water application i’ "i; length of time per set. The rate of water application by) the sprink- l; : can be determined by the following formula: Rate of water application I (96.3) (gallons per minute) q (S) (D) 5,. Where g.p.m. I sprinkler discharge ‘ S I spacing between lateral lines d I spacing between sprinklers on the line Rate of water application I (96.3) (13.7 g.p.m.) I 0.55 inch per hour ; (40 feet) (e0 feet) 0m Example 1, the amount of Water to be ap- was 2.0 inches. Therefore, the time re- ired for the sprinklers to discharge 2 inches of ter per setting will be: F Time per sent I 2.0 inches (hour) + .55 inch 3.6 hours (approximately). using a 3.6-hour-set for each 2.0 inch appli- tion of Water, the irrigator can make about » ee line sets per day because some time will be , uired in moving the portable line from one to the next. j EXAMPLE 3B: Rate of Water application J length of time per set. (Same as 3A except ; ure H is used to eliminate most of the calcu- tions.) .. From Figure H, locate the individual sprinkler tput of 13.7 g.p.m. Project a straight line a Lross it until it intersects the line which indi- tes a sprinkler spacing of 40 >< 60 feet. Then, end the projection downward until it inter- '- ts the “Rate of Water Application” line at 55 inch per hour. . v 2.0 inches (hour) 1. Time per set I III-I I 3.6 hours ' 0.55 inch ater application rates from different line and ‘rinkler spacings cannbe determined from the phs in Figure H by using the same procedure. » To complete the design of a sprinkler irriga- q system would require the design of main and A ral pipe sizes and the amount of horsepower ded to apply the 452.5 g.p.m. to the soil at a . te of .55 inch per hour. The final design of 3 is an engineering problem and will not be covered in this publication. The potential irri- gator should contact competent personnel con- cerning pipe and pump design in order that the most efficient and economical unit can be pur- chased. Formulae and Data ior the Irrigator WATER MEASUREMENTS 1 acre-ft. I 43,560 cubic feet or 325,850 gallons 1 acre-inch I 27,154 gallons. (amount of water required to cover 1 acre to a depth of 1 inch). 1 gallon I 8.34 pounds I 0.1337 cubic foot of water ' 1 cubic ft. I 7.48 gallons I 1,728 cubic inches I 62.37 pounds 1 cubic ft. per second I 448.8 g.p.m. 450 g.p.m. I 1 acre-inch per hour (approximate) G.p.m. >< 0.002228 I cubic feet per second Pump discharge in g.p.m. 450 (approximate) I acre-inches per hour Total water pumped in acre inches Number of acres over which water was applied Average depth of water applied-inches Acres irrigated I (number of rows) (row width in feet) (length of rows in feet) + 43,560 CONVERSION GRAPH OF GALLONS PER MINUTE TO ACRE INCHES PER HOUR 0,2400- 532200 rMu III 32000 — I800 — 6 o o l r 3 o o I 55 oo oo l'l'l 800 600 400 Amount of Water Applied — Gallons 200 '|'I -qi-q-—-<--<-— o |||l1|1|||1||l|||||| ||l1|||||1||l||||||||1l||nl l.0 2.0 3.0 4.0 5.0 Acre Inches 0f Water (N0 Losses Included) Figure G. 51 TABLE ‘OF MEASUREMENTS Mile Chain ~ Rods 1/8 10 40 1/4, 20 80 1/2 40 160 1 80 320 12 inches I 1 foot 36 inches I 3 feet I 1 yard 10 millimeters I 1 centimeter 100 centimeters I 1 meter I 3.28 feet 2.54 centimeters I 1 inch 1 rod I 161/3 feet 1 chain I 4 rods I 66 feet 5,280 feet I 1 mile I 1.60 kilometers LAND MEASUREMENTS Feet 660 1320 2640 5280 1 acre I 43,560 square feet I 10 square chains l-acre I 208.7 feet by 208.7 feet PRESSURE HEAD MEASUREMENT‘ 2.31 feet head 0f Water I 1 pound per €_ inch (1 lb./sq.in.) a 1 foot of Water head I 0.433 pound per sq ‘a inch (lb./sq.in.) i 1 atmosphere I 14.7 pounds per square inc _. 76.39 cm. of mercury . 15 atmosphere I 220.5 polfnds per square in€ 1 bar I 0.9869 atmosphere I 14.5 pounds» square inch. i 1 millibar I one-thousandth of a bar. g.- 1 H. P. I 550 foot pounds per second I? Watts or .746 kilowatts PULLEY SPEEDS Revolutions of two pulleys over which a i? is run vary inversely as their diameters. Ot i Wise, as the diameter of a pulley becomes lar its speed becomes less. _ nd I ND g 40 acres (Square) _ L320 ft‘ by L320 feet Where n I revolutions per minute of dri 640 acres I 1 square mile pulley 1 s40? . R '- ‘i? loo 6 0P O L ~* \'\' Q \+ \ \ -- - o 5 o * ,9 4P Q \ L- b @ @ 6'1" ‘+6 E 357 bi .50 ‘1~ 3 i‘ Z r 5 .- g: I‘ 3:130} 9 1i —| 25f- at? if z <0 L Q l— :. vé E20}- q, S r S o Z " p5 P J l5;- x _ . E T u: f"“_* 0- I‘ m IO:- -l l‘ I 5 z 20*” Q ; > L. 5 5; g L / 71minnmllnnlinnlinlilinilnnnlnnililnlmil|1|||l||nllnllllllllillnliliiInn]nmlilinlliiiliiniln t 0 .20 .40 .60 .80 I .00 l.20 I .40 l.60 I .80 2.00 2. A RATES OF WATER APPLICATION — INCHES PER HOUR (Per Sprinkler) Example: A sprinkler discharging I2 gallons per minute with a line spacing of 40ft xGOft. will disch" water oi the rote of approx. .48 inch per hour. Figure H. 52 diameter of driven pulley revolutions per minute of driver pulley P;- diameter of driver pulley OIL MOISTURE PERCENTAGE percentage is based on oven-dry weight w '1 dried in oven at 105°C until weight is t. Drying usually requires about 24 iisture percent I v i of wet soil — weight of dry soil) (100) Weight of dry soil OTHER CONSTANTS AND CONVERSION FACTORS ‘afoot of soil weighs about 4,000,000 pounds foot of water weighs 2,718,144 pounds ‘lper million (p.p.m.) I grains per U. S. llon multiplied by 17.1 in salt per acre-foot of water I I rts per million) (0.00136) fcal conductivity (EC >< 10“ @ 25°C.) di- ded by 100 is approximately equal to total ions or cations in equivalents per million .p.m.). of sodium of a water is determined ‘ taking that part which is sodium (e.p.m.) j d dividing it by the sum of the quantities » j calcium, magnesium, sodium and potas- 'um, and the result expressed as a percen- ige of the total cations. ctivity to parts per million ' .p.m. I (0.64) (EC >< 10“) to be used with ater in the range between 100 and 5,000 I icromhos / cm. per million to equivalents per million. ivide concentration of each constituent in a rts per million by its equivalent weight. Muivalents per liter (from chemical anal- is) to parts per million: a ultiply meq.,/l. for each ion by its equiva- nt Weight and obtain the sum. dard atmosphere I 14.7 lb./square inch I 76.39 cm. of mercury (@ 20°C.) I 1,036 cm. of water column (@ 20°C.) I 34.01 ft. of water col- umn (@ 20°C.) ‘yr I 1,023 cm. of water column or 0.9869 j_ tmosphere r I 14.504 lb./square inch libar I one-thousandth of a bar CHEMICAL SYMBOLS, EQUIVALENT WEIGHTS AND COMMON NAMES Equivalent Chemical Common name weight symbol (grams) ions Calcium 20.04 Ca Magnesium 12.16 Mg Sodium 23.00 Na Potassium 39.10 K Chloride 35.46 Cl Sulfate 48.03 SO, Carbonate 30.00 CO3 Bicarbonate ion 61.01 HCO3 Salts: Calcium chloride 55.50 CaClz Calcium sulfate 68.07 CaSO, Gypsum 86.09 CaSO4 . 2H2O Calcium carbonate 50.04 CaCOa Magnesium chloride 47.62 MgClz Magnesium carbonate 42.16 MgSO4 Sodium chloride 58.45 NaCl Sodium sulfate 71.03 Na2SO4 Sodium carbonate 53.00 NazCOs Potassium chloride 74.56 KCl Potassium sulfate 87.13 K2804 WEIGHT MEASUREMENTS 1 milligram I 0.001 gram 1000 milligrams I 1 gram 1 gram I 0.001 kilogram 1000 grams I 1 kilogram 453.6 grams I 1 pound Definition oi Terms‘ Aeration (soiU-The process by which air and other gases in the soil are renewed. The rate of soil aeration depends largely on the size and number of soil pores and on the amount of water clogging the pores. A soil with many large pores open to permit rapid aeration is said to be well aerated, while a poorly aerated soil has few large pores or has most of those present blocked by water. Aggregate—A group of soil Particles cohering so as to behave mechanically as a unit. Available soil moistarca-Usually considered as the quantity of soil water available for plant use. (Quantity of water retained in the soil be- tween field capacity and permanent wilting per- centage and expressed as a percentage or inches per increment of soil depth—inches per foot.) Bulk density-Mass per unit bulk volume of soil that has been dried to constant weight at 105°C. Symbol-DB. Cation exchange-Interchange between a cat- ion in solution and another cation on the surface of a colloidal or other surface-active material. ‘Most terms relating to soil physical properties were taken from the Report of Definitions as published and approved by the Soil Science Society of America. Soil Sci. Soc. Amer. Proc. 20:430-440. 1956. 53 Cation exchange capacitg—The sum total ex- changeable cations adsorbed by a soil, expressed in milliequivalents per 100 grams of soil. Meas- ured values of cation-exchange capacity depend somewhat on the method used for the determina- tions. Cultivation-A mechanical stirring of the soil in place as in seedbed preparation or weed con- trol. Electrical conductivity (EC)—A water con- taining dissolved salts will conduct an electric current. The amount of current conducted will depend on the kinds and number of dissolved salts. Such an electrical property is used as a reliable measure of the salt content of water. The reciprocal of the electrical resistivity. Re- sistivity is the resistance in ohms of a conductor, metallic or electrolytic, which is 1 cm. long and a cross-sectional area of one square centimeter. This term EC usually is expressed in reciprocal ohms per centimeter, or mhos per centimeter. EC >< 10“ I millimhos per centimeter. EC >< 10‘ == micro-mhos per centimeter. Electrical con- ductivity (EC >< 10‘ @ 25°C.) divided by 100 is approximately equal to the total anions or cations in equivalent parts per million (e.p.m.). Elec- trical conductivity multiplied by 0.7 is approxi- mately equal to the total dissolved solids in parts per million (p.p.m.). Evaporation-Process by which the precipi- tation reaching the earth’s surface is returned to the atmosphere as vapor. It also refers to the net rate at which liquid water is transferred to the atmosphere. Evapo-transpiration-The sum of water re- moved by vegetation and that lost by evaporation for a particular area during a specified time. Usually expressed as inch per day. Field capacity—Field moisture capacity. A-mount of water remaining in a well-drained soil when the velocity of downward flow into un- saturated soil has become small. It is expressed as a percentage of weight of oven-dry soil.’ l Impeded drainage—-Condition in which the downward movement of gravitational water is hindered. impervious zone—A zone located in the soil profile which may vary in thickness and which is resistant to penetration by fluid or roots. Infiltration—The downward entry of water into soil. Infiltration rate—The maximum rate at which a soil, in a given condition at a given time, can absorb water. Also, the maximum rate at which a soil will absorb water impounded on the surface at a shallow depth when adequate pre- cautions are taken regarding border or fringe effects. Defined as the volume of water passing into the soil per unit of area per unit of time. Irrigation—-Artificial application of water to the soil for supplying crops. 54 .with the water in the soil. Irrigation efficiency—The ratio of the consumed by crops of an irrigated farm or p f to the water diverted from a river or other ural water source into the farm or projec nals. When concerned with one field, it us is termed field irrigation efficiency. Irrigation requirement-"i-The amount of f ter, exclusive of precipitation, which is req for crop production. Includes surface eva tion and other economically unavoidable r and usually is expressed in inches or feet f given period of time. ‘ Moisture percentage—The percentage of ‘j ture in the soil, based on the weight of oven soil (105°C.) ‘ Moisture stress—The sum of the soil moist, tension and the osmotic pressure of the soil :5 tion. ‘ Moisture tension—-The equivalent neg gauge pressure, or suction, in the soil mois Soil moisture tension is equal to the equiv negative or gauge pressure to which water f, be subjected to be in hydraulic equilib ' through a porous permeable wall or memb Mulch—A loose covering on the surfa the soil. Usually consists of organic residues". may be loose soil produced by cultivation or o inorganic materials. a Osmotic pressure--The equivalent neg I pressure that influences the rate of diffusio water through a semi-permeable membrane. , Oven-dry soil-A soil dried at 105° C, 110°C. until it is in moisture equilibrium at temperature. The usual drying time is 48 hours. * Pans-Horizons or layers in soils that strongly compacted, indurated or very hig clay content. 1 Parts per million (p.p.m.)—-1 p.p.m. part, by weight, of constituents per 1 mi parts by weight of water or soil. a Permanent wilting percentage—Amountf water remaining in the soil when plants l, removed all they can and wilt permanently. I pH—The negative logarithm of the hydr i ion activity of a soil. Expression of the inte of the acid or alkali in a water or other solum The pH scale extends from strongly aci through neutral--7, and to strongly alkalin ';_ Most irrigation waters fall in the 7.0 to 8.5 of pH. Permeability-—(Soil) Readiness with w; air, water or plant roots penetrate into or i through its pores. Permeability- (qualitative) The quality, I state of a porous medium relating to the ;_ ness with which it conducts or transmits fl Permeability-- (quantitative) The Q property designating the rate or readiness g porous medium transmits fluids under id conditions. The equation used for ex- the flow will take into account the prop- _f the fluid so that proper measurements 'ven medium will give the same perme- y-value for all fluids which do not alter the f: Permeability unit will usually be ex- ; as a velocity or rate such as centimeters r. ’e space (porositgp-The part of the soil fwhich is not occupied by soil particles. ‘ io of the sum of the volumes of the liquid V‘ = phases to the sum of the solid, liquid and wses of the soil. q ne soil-Soil that contains sufficient sol- Alt to interfere with the growth of most Q nts. For the purpose of definition, it is or which the conductivity of the saturation cis 4 or more millimhos per centimeter ine-sodic s0il—A soil containing both suf- g soluble salt and exchangeable sodium to re with the growth of most plants. Quan- ',ly defined, it is a soil containing an ex- fwble sodium percentage of 15 or above with ation extract conductivity of 4 millimhos ._.timeter or more @ 25°C. ‘ii-affected s0il-—A soil that contains either ‘ent soluble salt or exchangeable sodium, or o interfere with the growth of most plants. ‘i0 s0il-A soil containing sufficient ex- lwble sodium to interfere with the growth .; crop plants. Quantitatively defined as a q an exchangeable sodium percentage f}, than 15, and either with or without ap- fble amlounts of soluble salt. Also known as 'li soi . Soil ai¢—The combination of gases occurring 1n the gaseous phase in soil. Soil pores-Interstices between soil particles (voids). Soil salinity-The amount of soluble salts in a soil, expressed in terms of percentage, parts per million or other convenient unit. i Soil striicturw-The size, shape, stability and spacial arrangement of naturally occurring ag- gregated clusters. Soil texture-The relative proportions of the various soil separates (sand, silt, clay) in a soil material. ' Surface sealing—The orientation and packing of dispersed soil particles in the immediate sur- face layer of soil whereby it becomes almost im- permeable to water. Tilth—The physical conditions of soil relative to its response to tillage machinery and its me- chanical impedance to root penetration. Transpiratiowr—Loss of water from living plants. Usually expressed as inch per day. Water 'requii'ement—The amount of Water re- quired by a crop in a given period of time for its normal growth under field conditions. It in- cludes surface evaporation and other economically unavoidable losses and usually is expressed in inches or feet per unit of area. Water utilization efficiency-Amount of crop yield produced per inch of water used (evapo- transpiration). Usually expressed as pounds or tons per inch of water. Wateiloggeck-State of being saturated with water. Texas. MORE DETAILS AVAILABLE Detailed information concerning the irriga- tion and water requirements of 17 agronomic and horticultural crops throughout Texas is available in a separate report. the report may be obtained from the Agri- cultural Information Office, College Station, Copies of 55 Location oi field research units oi the Texas Agricultural Experiment Station and cooperating agencies ORGANIZATION OPERATION Research results are carried to Texas farmers, ranchmen and homemakers by county agents and specialists of the Texas Agricultural Ex- tension Service joclay ,5 WQJQCIPCA 5 jO/YLOIWOLU ,5 Mayra/id State-wide Research The Texas Agricultural Experiment Statiofi, is the public agricultural research agencyl. of the State oi Texas, and is one of ten, parts of the Texas A&M College System‘? IN THE MAIN STATION, with headquarters at College Station, are 16 sub matter departments, 2 service departments, 3 regulatory services and, administrative staff. Located out in the major agricultural areas of Texas, 21 substations and 9 field laboratories. In addition, there are 14- cooper stations owned by other agencies. Cooperating agencies include the T Forest Service, Came and Fish Commission of Texas, Texas Prison U. S. Department of Agriculture, University of Texas, Texas Technoloy" College, Texas College of Arts and Industries and the King Ranch. ~ experiments are conducted on farms and ranches and in rural homes. l THE TEXAS STATION is conducting about 400 active research projects, gro in 25 programs, which include all phases of agriculture in Texas. t. v these are: Conservation and improvement of soil Beef cattle Conservation and use of water Dairy cattle Grasses and legumes Sheep and goats Grain crops Swine Cotton and other fiber crops Chickens and turkeys Vegetable crops Animal diseases and parasites ‘ Citrus and other subtropical fruits Fish and game Fruits and nuts Farm and ranch engineering Oil seed crops Farm and ranch business ~ Ornamental plants Marketing agricultural produc - Brush and weeds Rural home economics Insects Rural agricultural economics Plant diseases Two additional programs are maintenance and upkeep, and central servi AGRICULTURAL RESEARCH seeks the WHATS. the WHYS. the WHENS. the WHERES and the HOWS of hundreds of problems which confront operators of farms and ranches, and the many industries depending on or serving agriculture. Workers oi the Main Station and the field units of the Texas Agricultural Experiment Station seek diligently to find solutions to these problems.