W A13-230-l5M-L18O TEflXAS AGRICULTURAL EXPERIMENT STATION A. B. CONNER, DIRECTOR College Station, Brazos County, Texas BULLETIN NO. 411 MARCH, 19740 DIVISION OF AGRONOMY FACTORS INFLUENCING RUNOFF AND sou. EROSION AGRICULTURAL AND MECHANICAL COLLEGE OF TEXAS T. O. WALTON, President STATION STAFFf ADMINISTRATION: A B. CoNNER, M. S., Director R. E. KARPER, M. S., Vice-Director CLARIcE MIxsoN, B. A., Secretar M. P. HoLLEMAN, JR., Chief Cler J. K. FRANcKLow, Assistant Chief Clerk CHESTER HIGGS, Executive Assistant C. B. NEBLETTE, Technical Assistant CHEMISTRY: _ G. S. FRAI>s, Ph. D., Chief; State Chemist J. F. FUDGE, Ph. D., Chemist '—-i—————-, Chemist _ S. E. ASBURY, M. S., Assistant Chemist E. C. CARLYLE, B. S., Assistant Chemist WALDo H. WALKER, Assistant Chemist VELMA GRAHAM, Assistant Chemist T. L. OGIER, B. S., Assistant Chemist ATEAN J . STERGES, B. S., Assistant Chemist JEANNE M. FUEGAS, Assistant Chemist RAY TREIcnLER, M. S., Assistant Chemist J. K. FARMER, M. A., Assistant Chemist _ RALPH L. SCHWARTZ, B. S., Assistant Chemist HORTICULTURE: HAMILTON P. TRAUB. Ph. D., Chief . Horticulturist RANGE ANIMAL HUSBANDRY: J. M. JoNEs, A. M., Chief _ _ B. L. WARWIcK, Ph.D., Breeding Investigations -STANLEY P. DAvIs, Wool Grader ENTOMOLOGY: F. L. TRoMAs, Ph. D., Chief; State Entomologist H. J. REINHARD, B. S., Entomologist R. K. FLETCHER, Ph. D., Entomologist W. L. OWEN, JR., M. S., Entomologist FRANK M. HULL, M. S., Entomologist J. C. GAINEs, JR., M. S., Entomologist C. J. ToDD, B. S., Entomologist F. F. BIBBY, B. S., Entomologist CEcIL E. HEARD, B. S., Chief Inspector OTTo MAcKENsEN, B. S., Foulbrood Inspector W. R. JoRDAN, B. S., Foulbrood Inspector AGRONOMY: , . B. REYNoLDs, Ph. D., Chief E. KARPER, M. S., Agronomist C. MANGELsDoRF, Sc. D., Agronomist . T. KILLOUGH, M. S., Agronomist . E. REA, B. S., Agronomist . Agronomist _ . C. LANGLEY, B. S., Assistant in Soils PUBLICATIONS: A. D. JAcKsoN, Chief VETERINARY SCIENCE: *M. FRANcIs, D. V. M., Chief H. SCHMIDT, D. V. M., Veterinarian E. JUNGRERR, D. V. M., Veterinarian -—-——-—— , Veterinarian F. E. CARRoLL, D. V. M., Veterinarian PLANT PATHOLOGY AND PHYSIOLOGY . J. TAUBENHAUS, Ph. D., Chile! _ W. N. EzEKIEL, Ph. D., Plant athologist B: W. J. BACH, M. S., Plant Pathologist B. F. DANA, M. S., Plant Pathologist FARM AND RANCH ECONOMICS: L. P. GABBARD, M. S., Chief _ W. E. PAULsoN, Ph. D., Marketing C. A. BoNNEN, M. S., Farm Management J. F. Criswell, B. S., Assistant J N. TATE, B. S., Assistant RURAL HOME RESEARCH: _ JEssIE WIIITAcRE, Ph. D., Chief _ MARY ANNA GRIMEs, M. S., Textiles EMMA E. SUMNER, M. S., Nutrition SOIL SURVEY: **W. T. CARTER, B. S., Chie E. H. TEMPLIN, B. S., Soi Surveyor T. C. RErrcII, B. S., Soil Surveyor A. H. BEAN, B. S., Soil Surveyor BOTANY: _ V. L. CoRY, M. S., Act. Chief _ SIM0N E. WOLFF, M. S., Botanist SWINE HUSBANDRY: _ FRED HALE, M. S., Chief DAIRY HUSBANDRY: _ O. C. CoPELAND, M. S., Dairy Husbandman POULTRY HUSBANDRY: _ R. M. SHERWOOD, M. S., Chief ***AGRICULTURAL ENGINEERING: MAIN STATION FARM: . T. McNEss, Superintendent APICULTURE (San Antonio): H. B. PARKS, B. S., Chie A. H. ALEX, B. S., Queen Breeder FEED CONTROL SERVICE: F. D. FULLER, M. S., Chief S. D. PEARcE, Secretary J. H. ROGERS, Feed Inspector W. H. WooD, Feed Inspector K. L. KIRKLAND, B. S., Feed Inspector W. D. NORTHCUTI‘, JR., B. S., Feed Inspector SIDNEY D. REYNoLDs, JR., Feed Inspector P. A. MooRE, Feed Inspector SUBSTATIONS No. 1, Beeville, Bee County: R. A. HALL, B. S., Superintendent NO- 2, Troup, Smith County: P. R. JoIINsoN, M. S., Act. Superintendent N0. 3, Angleton, Brazoria County: R. H. STANsEL, M. S., Superintendent No. 4, Beaumont, Jefierson County: R. H. WYCI-IE, B. S., Superintendent NO- 5, Temple, Bell County: . HENRY DUNLAvY. M. S., Superintendent B. F. DANA, M. S., Plant Pathologist H. E. REA, B._S., Agronomist; Cotton Root Rot Investigations SIMoN E. WOLFF, M. S., Botanist; Cotton Root Rot Investigations No. 6, Denton, Denton County: P. B. DUNKLE, B. S., Superintendent No. 7, Sign, Dickens County: R. E. ICKSON, B. S., Superintendent ——---—-—. Agronomist No. 8, Lubbock, Lubbock County: D. L. JoNEs, Superintendent FRANK GAINEs, Irrigationist and Forest Nurseryman No. 9, Balmorhea, Reeves County: J. J. BAYLEs, B. S., Superintendent N0. l0, College Station, Brazos County: R. M. SHERWOOD, M. S., In charge L. J. McCALL, Farm Superintendent No. l1, Nacogdoches, Nacogdoches County: H. F. MORRIs, M. S., Superintendent **No. 12, Chillicothe, Hardeman County: J. R. QUINBY, B. S., Superintendent _ **J. C. STEPHENS, M. A., Assistant Agronomist No. 14, Sonora, Sutton-Edwards Counties: W. H. DAMERON, B. S., Superintendent E. JUNGIIERR, D. V. M., Veterinarian ———-i———, Veterinarian **O. G. BABCOCK, B. S., Entomologist O. L. CARPENTER, Shepherd No. 15, Weslaco, Hidalgo County: W. H. FRIEND, B. S., Superintendent _ SHERMAN W. CLARK, B. S., Entomologist W. J. BACH, M. S., Plant Pathologist No. 16, Iowa Park, Wichita County: E. J. WILs0N, B. S., Superintendent N0. 17, ——-——-——-i~—- ——————~——. Superintendent No. 1s, ---—-—- —————-— , Superintendent No. 19, Winterhaven, Dimmit County: E. MQRTENsEN, B. S., Superintendent —— —————-—, Horticulturist N0. 20, ————-—————— ——--0——-———, Superintendent Teachers in the School of Agriculture Carrying Cooperative Projects on the Station: W. ADRIANcE, M. S., Horticulture W. BILSING, Ph. D., Entomology V P. LEE, Ph. D., Marketing and Finance D SCOATES, A- E» Agricultural Engineering H. P. Smith, M. S., Agricultural Engineering R. H. WILLIAMS, Ph. D., Animal Husbandry *Dean School of Veterinary Medicine. **ln cooperation with U. S. A. K. MAcKEY, M. S., Animal Husbandry S. MOGFQRD, M. S., Agronomy S. JAMIsON, M. S., Horticulture R. BRIsoN, B. S., Horticulture _ . R. HoRLAcIIER, Ph. D., Genetics H. KNOX, M. S., Animal Husbandry TAs of March 1, 1930. J. F. F. W J. Department of Agriculture. ***In cooperation with the School of Agriculture. This Bulletin reports results so far available from three years’ work at Substation N0. 7, located near Spur, Dickens County, in a study of factors influencing runoff and soil erosion and the effects 0f conservation on the increase in crop yields. The results are directly applicable to an area of about 14 mil- lion acres in which Miles and Abilene soils predominate and indirectly applicable to all of Texas. The equipment used is fully described. The available rainfall data show that 85 per cent of the average annual rainfall of 22 inches, comes in this region during the’ summer months. Two well defined peaks, one in May and one in September, with a period of depression of rainfall in July, emphasizes the necessity of conserving water from the spring rains to carry crops through the nor- mally dry period in July. The possibilities of conservation are shown/by the fact that approximately 20 per cent of the total rainfall is ordinarily lost in this region as runoff. The results indicate that the intensity of the rainfall is a factor in losses of water and soil, but that other factors such as soilgmoisture content are operative to a considerable degree. The runoff losses have not been found to be in direct proportion to the steepness of the grade. The soil losses seem to be more nearly in proportion to the steepness of the grade than is the case with water losses. Tremendous quantities of water and soil have been lost from lands with even a gentle slope. Grass has been found to be a very effetive vegetative cover in conserving water. Milo was more effective than cotton, and cotton more efiective than no crop at all. The efficiency of a crop in preventing runoff seems to be due partially to its coverage and partially t_o its removal of water, which in turn affects the absorption by the soil. Tillage has been found effective in the storage of water. Contoured rows, terraces, and dikes have been found efiective in saving water and in increasing crop yields. The results indicate that in this region, it is possible by the use of contoured rows and closed terraces to conserve all the water and entirely prevent runoff. By means of diversion terraces, as much as two to four inches of water have been applied to other areas, but such applications of water have not thus far been fully satisfactory because of the difficulty of spreading and retaining the water on the land. CONTENTS PAGE Introduction Scope of work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 5 s Historical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8_f Description of area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 9 Equipment : 9 Meteorological . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 ' Control plats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Field areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 15 Analysis of rainfall data: _y a ; Rainfall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.7 - Seasonal distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 19 The mid-summer depression . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 20 Rains classified as to size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 22 ii Relation of character of rainfall to runoff . . . . . . . . . . . . . . . . . . . 22 Experimental data from control plats: Effectiveness of rainfall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 26 I Seasonal runoff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Rapidity of precipitation in relation to runoff . . . . . . . . . . . . . . . 29 Rapidity of precipitation in relation to soil erosion . . . . . . . . . .. 34 Effect of slope of land on runoff . . . . . . . . . . . . . . . . . . . . . . . . . .. 35 Effect of slope of land on soil erosion . . . . . . . . . . . . . . . . . . . . . . . 36 Relation of crops to runoff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Relation of crops to soil erosion . . . . . . . . . . . . . . . . . . . . . . . . . .. 39 ' Influence of tillage on runoff . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 40 Influence of tillage on soil erosion . . . . . . . . . . . . . . . . . . . . . . . .. 41 Experimental data from field areas: Effect of field obstructions on runoff and crop yields . . . . . . . . . . 42 Relation of terrace spacings to runoff and crop yields . . . . . . . . . l3 Relation of grade of terrace to runoff and crop yields . . . . . . . . . 44 Maximum utilization of rainfall as affecting crop yields . . . . . . . 45 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 7 BULLETIN NO. 411 MARCH, 1930 FACTORS INFLUENCING RUNOFF AND SOIL EROSION A. B. CONNER R. E. DICKSON D. SCOATES* Throughout .Texas and the Southwest, water is the chief limiting factor in crop production. Soil fertility is also a limiting factor in some ;. regions. In sections where erosion has been accelerated by methods of utilizing land, soil fertility has been depleted. ‘ é Climatic conditions in Texas are such that in nearly every part of the State crops are subject to periods of deficient soil moisture caused by <1 either a. shortage of total rainfall or a lack of proper distribution. y; Shortages of rainfall can be partially overcome by the storage of water " in the soil, thus carrying over the largest amount of soil moisture possible p for use of the crop. Unfavorable seasonal distribution of rainfall and the losses of soil and soil fertility can also be partially remedied by the i? same measures; hence, control of rainfall water and its storage in the l soil is an important problem confronting the agriculture of Texas. 3 Undoubtedly in certain seasons of low total rainfall no measure of pre- caution will fully compensate for the shortage, but in seasons of‘ fair ‘f rainfall, attended by poor distribution, it seems probable that by the Qapplication of effective methods concerned in the storage of water in the p soil, favorable growing conditions can be greatly extended and the prob- ability of good production increased. Intensity and distribution of the rainfall are important factors to be ‘Z considered. The slope of the land, the crop grown on the land, and tillage operations practiced are other important conside-rations, as they . .;__> materially affect storage of water in the soil. These same factors apply to losses of soil and soil fertility, inasmuch as water is the vehicle which i-carried away soil and soil fertility. Any means of retarding the off-flow Yoflwater also retards the carrying away of soil and soil fertility. gof experimental work conducted at the Spur Experiment Station in the ‘study of some of the principal factors concerned in losses of water by rrunoff and of soil by erosion, and the effect of these losses on the yield pf crops. Y: SCOPE OF WORK In the plan of work originally outlined, two phases of the problem to be considered: (1) a study of factors contributing to runoff _bsses_ under subhumid conditions as influenced by (a) rapidity of rain- _f all, (b) slope of the land, (c) physical condition of the land, (d) use l ‘Professor of Agricultural Engineering, School of Agriculture, A. and M. Col- l“; of Texas. The purpose of this Bulletin is to present the results so far available A 6 BULLETIN NO. 411, TEXAS AGRICULTURAL EXPERIMENT STATION of terraces and other obstructions; and (2) a study of the effects of I s preventing runoff losses 0n crop production. At the inception of this work, soil erosion was not thought to be an important consideration in the section involved (Figure 5), and hence no definite mention was made in the project of the soil-erosion problem. It was realized, however, that the set-up and equipment necessary for the measurement of water losses would suflice for measuring soil losses. In the first crop season, the soil losses were so astoundingly large that Fig. 1. Sheet erosion in a cotton field. “i? -.i Fig. 2. Sheet erosion with gullying started. FACTORS INFLUENCING RUNOFF AND SOIL EROSION 7 this phase of the Work assumed an untxpected importance in connection with the study, and measurements of soil losses have been obtained throughout in an endeavor to correlate these soil losses, as Well as the water losses, With the factors under consideration. The general method of procedure in formulating a plan of operation has been to make use of the available meteorological data and other experimental data available relating to the subject. This plan has involved establishing a set of control plats ivith equipment for obtaining Fig. 3. An example of gullving. Fig. 4. Effectiveness of~a terrace in checking the movement of water on rolling land. 8 BULLETIN NO. 411, TEXAS AGRICULTURAL EXPERIMENT STATION accurate measure of losses of water and soil under the following con" ditions: during periods of rainfall varying in size and intensity; fro‘ plats with different grades; from plats planted to different crops; w, from plats fallowed. These data obtainable from the control plats have been further supplemented by the installation of a series of field area‘. ranging from 5.5 to 11 acres in size, and variously equipped, from which, by means of weirs and water-stage recorders, an accurate measure o , water loss Was recorded, thus supplementing the small control plats by; field areas approaching closely field conditions as they exist in this regionf It is realized that, in this set-up, variations in the field areas, par 1' ticularly, will necessarily affect the records obtained, and that a period.“ of years and a duplication of some of the field areas will be required,‘ to obtain conclusive results. It is believed, however, that the use of the ;_ control plats in connection with the field areas will tend to enable veriq fication of accuracy in the field results. 1 The areas on which these tests are being conducted were put into cul-i tivation in 1909, 1910, and 1911, and were in experimental-plat work until 1924. During 1924, 1925, and 1926, there were 90 acres of cotton grown on the 100 acres in the field areas and the remaining 10 acres were i“ used for growing feed crops. The land has a gradient ranging from one- l, half of 1 per cent to 3 per cent. HISTORICAL The use of runoff water in irrigation is an old and accepted practice. - For centuries, water has been impounded by man and diverted onto fields '1 and vineyards to supplement the normal rainfall. The American Indians i of the subhumid regions were efficient in diverting water onto their fields .. during rainy periods. i These early experiences in the use of water have, perhaps, been an ,5. incentive to the more intensive and systematic studies relating to the ‘ use of water which comes as rainfall. In the development of the sub- A. humid region in the Western and Southwestern States, intensive studies , have been. made by research workers in dry-farming problems having to “ do primarily with storing and retaining moisture in the soil through cultural practices. Much work has been done relating to the percola- tion of water and on the relation of water to plants. Numerous studies have been made on the duty of water in irrigation and the discharge of water through river courses as it relates to rainfall, flood control, and silting problems. The Missouri Experiment Station initiated experiments in 1917 on soil erosion and surface runoff, and has contributed information on the subject of erosion. The Bureau of Public Roads, United States Depart- ment of Agriculture, has conducted, in co-operation with the North Caro- lina Experiment Station, experiments on soil erosion. The’ Bureau of Chemistry and Soils, United States Department of Agriculture, has con- tributed to our general knowledge of soil-erosion problems as they occur in different parts of the United States and elsewhere. The Forest Serv- a 3 .5 n3 i. 1 FACTORS INFLUENCING RUNOFF AND SOIL EROSION 9 ice, United States Department of Agriculture, has contributed to the knowledge of oiT-fiow of rain water in relation to losses on grazing and i forest areas. Agricultural Experiment Stations in various States have also contributed basic information relating to the problem. Dlrfr/buf/on of Ra/n/afl S Lex- f/ran /0 xhrfie: l0 f0 20 Nit/IA! 4- . _q, 20/aJ0/nr/m: P/ e a a J0 fa 40 inc/res Ia/ofa/l m mr/re: .6] man/b; More f/mn 40mr/reJ Fig. 5. Rainfall map of Texas, showing monthly distribution at five points. DESCRIPTION OF AREA The area to which this work applies occupies all or parts of 44 coun- ties and covers approximately 20,000,000 acres. Of this, about 14,- 000,000 acres are predominately Abilene and Miles soils, with some other soils that are closely related to them. The soils on the Station belong to the Abilene and Miles series. The Rolling Plains are generally undulating, with some rather good- sized bodies of land that are nearly fiat or gently sloping. The Abilene soils, and the closely related ‘dark soils, occupy the larger areas of nearly flat lands. The clay loam types prevail in the dark group of soils. The Miles soils are in general more rolling in many places, and the sandy types are the most extensive in this series. 1O BULLETIN NO. 411, TEXAS AGRICULTURAL EXPERIMENT STATION All of the soils of this region have a natural granular and open struc- i, ture, which allows the ready_cultivation of the land into a friable condi~ tion. This allows water to enter the soil and pass through it freely. r- It also permits easy Washing and eroding of the surface where it is at I all sloping. The subsoils, though as a rule heavier than the top soils, have an open structure favoring the absorption of water. These sub- § soils contain considerable clay and subsurface leaching of soil water does not occur to any appreciable extent, and as they consist of deep beds of g soil material on unconsolidated beds of clays, a large amount of soil water * can be held in reserve for the use of crops. The Abilene soils consist of dark-brown surface soils which in places ‘ are nearly black. The soils are 4. to 10 inches deep and rest on dark- g brown or chocolate-brown clay, which at a depth ranging from 18 to 24 f- inches merges below into lighter-colored friable calcareous clay. Nor- ' mally a layer of soft lime carbonate occurs at a depth of four or five feet. 1 In places this is a bed of soft, almost pure, lime carbonate, while in other places it is mixed with salmon-colored or yellowish, soft, friable clay. The Miles soils, where normal soil development has occurred, consist . of reddish-brown or brownish-red soils 4 to 12 inches deep underlain i by subsoils of friable clay of a red or reddish-brown color. At a depth ~ ranging from 18 to 24 inches» the subsoils merge below into lighter- ‘ colored chocolate-red friable clay, which. at two to three feet below the surface is somewhat calcareous in many places. In other places cal- i careous material is found only very slightly in the subsoil material above the lime carbonate zone, which lies four to five feet below the surface. In many places where the surface is sloping, the subsoil lies very near the surface and lime carbonate may be found in the upper soil layers and even in the surface soil. Many areas of Miles soils that are sloping and rolling have had the surface soil almost entirely removed by erosion, j even where the land has never been in cultivation. Associated with these soils are large bodies of the Vernon soils. These are the red soils of the Permian Red Beds formation. They are very easily eroded; large areas have been so badly damaged by washing and gullying that they can never be used for farming. EQUIPMENT Meteorological The meteorological instruments are adjacent to the control plats (Figures 6 and 8). They consist of maximum and minimum ther- mometers, relative atmospheric humidity thermometers, anemometer, evaporation tank, an automatic gauge for weighing and recording rain and snow, and a standard government rain and snow gauge. The auto- matic gauge (Figure 9) is particularly useful in that it records the rate of rainfall. In addition to these instruments, there are two other rain gauges placed at points on the field areas in order to check on variations in rainfall. ~ FACTORS INFLUENCING RUNOFF AND SOIL EROSION 11 Control Plats The control plats (Figure 6) are located on Miles clay loam and consist of eight plats 6 feet wide and 96.8 feet long, or 1/75 of an acre“ in size. The number of the plat, the grade, the cropping, and the treat- ment are given in Table 1. These plats are located on land that had been in cultivation for thir- teen years prior to the inception of this study. They are located adja- i I I ' C) P/a/ / P/a/Z P/afJ P/a/4 P/al J P/af 6 P/a/ 7 P/a/ 0, I . azyunxyml’ zzwwekuywazzri‘ Izymulymszyadt‘ J I) She/fer for Thermomefer: i 1 O u: A nern o mefer Co/fon (‘of/on (‘of/M Fol/aw f-al/ow Sulfa/o Milo (‘often I rvMrafl no! 5/05: i i fro/a oraf/on Tbn/r 6 I d/Omefer . i =3: q, \ \ 5. l O i Aufomafic Rain g Recording Gauge‘ I i I \ | l , 5 % ~- 6 —»- T o ‘it/of / Yo! 2 Vol J Val-l 1/471 5 V0! 6 l/af 7 Va! B Sfandard Gcvcrnmonf ‘f 3J6’, 5:3" 51,1‘ n35" '?°"" °"'°’ 500w Gougo t I I I | l l: 4"gunll'? "Work/dig Pi: v67‘ W025 v—r—l “Kora; 72/: fin, Ne. Ne. No. No. No. Ne. o, / 1 .1 4- s 6 4 7 e L \_/-J'oi/ Bo are s--\ é [DO Fig. 6. Diagram showing arrangement, size, grade, and treatment of control plats, to- gether with location of vats, working pit, and meteorological instruments. 12 BULLETIN NO. 411, TEXAS AGRICULTURAL EXPERIMENT STATION cent to the field areas as shown in Figure 10. There was a natural ..,, per cent slope, which furnished as nearly ideal conditions as could »; obtained. It was necessary to "excavate in order to form grades of than 2 per cent on two of the plats, and to make a fill on a third plat 4.; order to establish a grade of 3 per cent. These excavations and fills? have undoubtedly afiected the results on these three plats, more espe- cially during the first year. . ‘Table 1.—Control plats, number, grade, cropping, and treatment Plat Per cent grade* Cropping and treatment I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 Cotton 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Cotton 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Cotton . 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Fallow, cultivated 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Fallow, not cultivated 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Buffalo grass 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Milo 8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Cotton ‘ *Number of feet fall per 100 feet. The plats are bordered with No. ZO-gauge galvanized iron, 15 inches Wide and sunk 12 inches beneath the surface of the soil. This border is z '. g s;.,/ seal. T i? '; Q. b D b0 r053 m. A VWW/ A. a~ fir; 4 ‘gin o‘ ‘oer Z"- i. small size. This handwork is made to approximate as nearly as possible field work performed with machinery. - The cotton and milo plats are planted as near the first of May as possible. The rows are 36 inches wide and are 18 inches from the plat 14 BULLETIN NO. 411, TEXAS AGRICULTURAL EXPERIMENT STATICN Fig. 9. Recording rain gauge and charts 0n which records are made. The upper chart represents a characteristic slow rain; the lower one, a rainfall of great intensity for periods. FACTORS INFLUENCING RUNOFF AND SOIL EROSION 15 borders. Cotton plants are spaced 12 inches in the row, and milo plants 18 inches. The buffalo grass 0n plat 6 was transplanted in sods six inches square from nearby pasture lands on June 15, 1926, but it was late fall before the plat was perfectly sodded. The grass is clipped with a lawn mower, so as to maintain as nearly as possible the conditions of grass land pas- tured with cattle. Plat 5, “fallow, not cultivated,” has all vegetation removed with at weeding hoe, care being exercised not to disturb unnecessarily the sur- face of the soil. _ The suspended matter in the water reaching the tanks is allowed to settle, the water drawn off, and representative samples of water and soil taken. The residue left in the vats is removed and Weighed; samples are dried in a soils oven and the soil losses determined on a water-free basis. . Representative water and soil samples are taken following each rain and the remaining eroded soil is stored in soil boxes shown in the fore- ground of Figure 8. Field Areas There are ten field areas, each having a different treatment, located on Miles clayloam. They vary in size from 5.5 to over 11 acres. The ' size, arrangement, topography, and location of terraces are shown in Figure 10. Two of these areas, 5 and 6, were laid out and terraces constructed in the spring of 1926. The other eight were established in the spring of 1927. The field areas, with their acreages and treatments, are as follows: Area 1—Contoured rows, cropped to cotton, 11.530 acres. Area 2—Rows following the natural slope (up and down hill), cropped to cotton, 9.386 acres. Area 3—-One-foot fa1l* between level terracesj" cropped to cotton, 10.711 acres. Area 4—Three-foot fall between level terraces, cropped to cotton, 10.259 acres. Area -5—Two-f0ot fall between terraces having a fall of three inches per 100 feet along the terrace, cropped to cotton, 5.532 acres. Area 6—Two-foot fall between level terraces, cropped to cotton, 6.044 acres. Area 7—Level terraces with borders diked to hold all of the Water that falls on the area, cropped to cotton, 6.329 acres. Area 8~—Level terraces with borders diked to hold all of the Water that falls on the area, cropped to alfalfa, 7.825 acres. Area 9—Level terraces with borders diked to hold all of the water that falls on the area and the runoff water from Areas 2 and 3, planted to cotton, 9.292 acres. Area 10—Level terraces with borders diked to hold all of the water that falls on the area and the runofi" water from Areas 5 and 6, cropped to alfalfa, 8.131 acres. *“Fa1l” means a difference of elevation. One-foot fall between terraces indi- cates a difference of one foot elevation between terraces. "(Level terrace is a. terrace that is run on a contour or is level from one end to the other. 16 BULLETIN NO. 411, TEXAS AGRICULTURAL EXPERIMENT STATION we Z?» m, 8 ulna/nun”'zaz5flcrps urn” '11, '11! 'lIl In; r A rg g "_ N_o. 9,, QJZ 240/83 u U,’ 1 \\“\‘ Inuunl“ |\l l|lll|l' ' uulfl" flrca. Na 4 Z’ 4mm N° 2 a 10.2.79 Acres 9. J8 6 dares I,’ I'll‘ ,1, ‘I r‘, ‘p34 lVo. 5/ In,’ Il1’/'q7//Acye$ IAJJ 4:104 '|]IIII|‘ i llj/IIIIIIIIIIIIIIIII;|’,IH‘l’ n: "Im" I00 *'[";\ I! ’”“H“‘,,l:ulIII/, '1,’ ,,,,,,,;|;| mnnmnnun: $1 nllllhll "---..||:u|"‘ ' \L_?__’I”Z t: all" 96 ' \-—-flrea In 6E " g’ A, ' A»; m, .5 98 g ‘ - 6.044- 40,‘ A ‘a _ Acres "' ' H“ J |H|\||""lll1, l |||HI|\Iq|un|||llll\\\\flll HHH),|,4I;\II\ v _ until“. “,\“ > i” ‘ 97 "H" \\“' 96/ u ‘ .-»——-"—E"é*” L__ Tlfllunlnn“,,,||||rn""""“1 9‘ (an/IQ, %O/JJ ‘Hun-urn 017d Mofearo/aylbo/ x \ \\\|\\\u\ llllllltlllll __ f 'J /nSfg,2.un7'n/J A‘ \ I \ | 1 r . 95 94' l! 9! SCALE f - 20o , n LEGEND m” WP/F-S- ArQQ No.7 Lere/ lerroce: rv/l/r no oul/e/ IHI" TPf/OCPS. fzv wafer , [clan/ed f0 co/fon. -— P/a/ Boundaries, Areq A68 Lme/ terraces will! no ouf/e/ BOU/IdOI/PS Orv fer/coca’. for wafer ; plonlPd f0 a/fa/lb. Ardd No. / Co nrourd/Owi no fer/aces. Area No. 9 Le/e/ ferraces rwif/r no oufie/ for 111-64 No.2 Rows running nil/r $/Q’u0.9, waflrr and f0 rec/ore rm off wafer from ‘r64 No.3’ I"/"a// between lere/ lrr/uces. Pia/J Z and J,‘ phnfpd f0 QQ/fgr), flrtd Na4 Jfbl/ between lere/ ferraces. r414 fiq/o Lpye/ fpffggq; w/n, m, 1914.00. 5 J Fall pr mo’ of fer/ace ouf/ef for Wafer and 1'0 rec/era run 2 fall behave/v terraces. off from 9'01: J‘ and 6,'/u/urr/ed ‘r64 AQ- 5 {Fa/l befween level terraces. lo o/fo/la Fig. 10. .P1an of field areas showing arrangement, sizes, contours, terraces, and the location of the control plats with reference to the field areas. a FACTORS INFLUENCING RUNOFF AND SOIL EROSION‘ 17 Areas 1 to 6, inclusive, are equipped with water-measuring devices. Typical equipment is shown in Figure 11. Each of these units is equipped With a weir and an automatic time- recorder. The weir unit is built of reinforced concrete and is four feet high, 18 feet long, and six inches thick. In each unit there are two 90-degree notches with a depth of 18 inches. This design of weir is used in order to measure a very small flow as well as a large one. It records a discharge as low as .1836 cubic foot over 10-minute periods, and one as high as 8160 cubic feet in the same length of time. Fig. 11. One of the six concrete weirs with two 90-degree notches, still pond, and water-stage recorder used in measuring runoff from the field areas. ‘The still-ponds are twenty feet wide and forty feet long, with a depth of eighteen inches below the level of the notch. The purpose of the pond is to bring the water to a state of rest before passing over the weir. A concrete still-well two feet six inches square and eight feet high is placed at the edge of each still-pond. This still-well is a support for the water-stage recorder and houses the water-float, clock-weights, and counter-weights. ANALYSIS OF RAINFALL DATA Rainfall The rainfall data at Spur for a period of seventeen years show that the average annual rainfall is 22.01 inches, and that in this period vio- lent fluctuations in annual rainfall have occurred ranging from 11.09 inches in 19211 to 38.08 inches in 1;926. Table 2 shows in detail the monthly and annual rainfall for the seventeen years, and Figure 12 shows a graphic representation of the rainfall by months for the three years in which. this work has been conducted, 1926, 1927, and 1928, as % ooNN ww. Hw. . EN HmN HAHN mmH NwN 84m nmN HNoH Hm. Hm. . . . . . . . . . . . . . . . . . . . . . . . . . . 255w “H @ NHYHNm NmN 3w Hmh wN m 3m 8N HH.m 8.». Haw mNN 8a oN.N . . . . . . . . . . . . . . . . . .....owm~o>< . T . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. S No mm i. m m: m HNH m $ N Q N 3. N mm m Hm w NH H» 5 N N» N Ho m 5.53m umwHHoU Hm L . oHbm mo.” Hqofi $4.9 2e 5N $4 moN $6 9:. wNN mNN NmN . . . . . . . . . . . . . . . . . . . . . . . IBQEPH. mH W $4M. Ea mNN 5m mm N 8w oNN HN.m 09m $4» 2H 3H >H.N . . . . . . . . . . . . . . . . . . . ......:oHH6nH 2 E mo om HmN hHN mwN firm NH.N woN HN.m gm mmiN 2a m: HH. . . . . . . . . . . . . . . . . . . . . . . . ..oHHH>@@mH Nm M M H5222» 8Q .>oZ .30 flaw. .95.. D3. 25H. >32 25¢. n82 huh 48H. Hfioufi E 592w M28» "J waifi HOQHHHHHZ E @ dfifim 23 E 35oz 55o .52 Hm Hit. 5H3 Hxfiwmfioo 5mm Hm HHEQEH Hwsccm H25 DHHHHEE vmw~u><|.m uHnmrH. R U T .33 HUQ mNhH HO @235 hmmgm 622.35 J3EE 3 HQQEE/ol £8.53 E 23:05 m. HHEHHEMH M JHHQO Hvfl muéw HO 3:2: no .wH dfimsHocH dwnoHoO 0H Hi3» duEEsw E wHHHcoE N. HHEEQMH m % 89%.. mNgHfim 2.1: NH.wH wo.wm@@¢§o.HH maemfiwi 8.: 8% HwHmNmNH 8.2 H33 wwwmmfiwmoflmmmxvfi .......Hwscs< A . . . S m5 9...: mm. Nw Ea HN 8. 5 mo. mo. wm oo. 5 H o0 8. 3H hm H 2: S. .........HQQHHHQOQQ A 3w 3T2 Q H 2x Nm. NN. 8. mm N 3H 8. HH H cw. oNm S. Nw o0. 5. $6 8. . ...HQQHH.HQ>OZ X RKMN NN.? 2.. H NH.H 2.... m». ow. wm w @o.H 8. i. N w...» 8 N NH. mwN Sb mN m HAWN n: ...........HwncHuO E NHmN N92» 8 NN Hw om/m HEM co N E H 8H 3e oN N was Na. NHHN NH.H m?» HHN H Nhd Ea . . . . ......HOQHH.H0.~Q®W T HHAWN 8.9.. g m w». H56 $1. .5 3 H 09H mo. Hm w EfiN NHiH ma; HQH» wHiH 3 m. S. 3H . . . . . . . . . Q€m=< ... HmmH Sh.» mH m. DiH .5.» 3m Ho N wN. mN. o0. m“. 8n 3i hHN $1 w“. 05¢ E. mm. . . . . . . . . . . . 15E. H HNwN 5.3. 8 H S? HNH.N vPH mm 3H. RhH 8Q. 3 m. mo.“ 3H HNH. wmN Mae wNH m»; 3.», . . . . . . . . ....;.E:H. 4. $9». wHNm m». v 8. SM mHJN M3 H HQH.H 2...“ Ho. Hm w hmk HNHQN HHKH HmH HmN w... oH 3i i: . . . . . . . . . . . 15w . mom N .32. 0N S. fie 53v Hw 2:“ Eb oo. 2.. wPm Nw. hNH mmN i: 2w H R. mo; 4 0 mwoH RSH $1 8H Nm H NH. 5H Ho.H w». 3. wH. wmkm om. co. Q. oN.m mm. mNH No.H . . . . . ......£u.Hm2 N wow: 3w 3. m5 ix wH. mo. 9w. 0o. wo.H 8. HN. 3. Hm. 8. oHN mH. HH». 2H . . . . . ....?HwPEv~H N Em. 31m HNN. oH H Nb Him oo mo Hm om Hm H wN oo. NN. oo. 9.. mo. H5. oo. ..... . . . . Shmscww UH wmw$>< H55. wNmH R2 wNmH mNmH $2 mNmH NNmH HNaH oNmH 22 £2 .22 £2 22 HQHNH 22 NHaH wHHHcoH>H U B 55w Hm dZwzHonH dNmH 0H NHmH £0.35 dwHcHmh Humans HEm hHHHHcow/HILN QEPH. 18 FACTORS INFLUENCING RUNOFF AND SOIL EROSION 19 compared with the average monthly rainfall for seventeen years. Table 2 shows that most of the rainfall in this region occurs during the crop- growing season, With little or no rainfall during the Winter months. The average rainfall for a period of seven months, April to October, inclusive, is 85 per cent of the total annual rainfall. Seasonal Distribution Seasonal rainfall is a better measure of the supply of Water available to crops than total rainfall, as evidenced by the fact that localities with Kim/WI ma“; \/ J10 65. fir? May i/uo Jul p409 5921f 0d.‘ MY at Fig. 12. Rainfall by months during the three years of this work, compared with the average for 17 years. 510a 40o ,/’/'\K\\\ f‘ // N / l// \\ v x. ‘J00 rap i,‘ '1' \ \ \‘\ 1'.’ /\\ A. ‘/ § ssgmgmzf/ // ,//,>\/7 \\\ . g 2.00 k ’ ' \\‘- --r'--- ' u //’ ‘k E/ » 1",’ LOO z 0.00 v60. fié. 070/.‘ flp- My Joa Jul. Jepf Ocf Aér Lb: Fig. 13. Monthly rainfall at Substation No. 7 (Spur) compared with that at four other Stations. Average of four Stations, June to September, inclusive, 11.09 inches. Average of Spur Station, June to September, inclusive, 10.42 inches. 2O BULLETIN NO. 411, TEXAS AGRICULTURAL EXPERIMENT STATION an annual rainfall of as much as 40 inches are frequently subject to periods when the crop suffers for lack of moisture. Only a certain amount of rain is needed to produce maximum crop yields, and when an excess occurs it may introduce many complications, such as erosion, , insect depredations, deterioration of crops in the field, extra labor and costs, and numerous other harmful results. Figure 13 illustrates the characteristic seasonal distribution of rain- fall at Spur and at four other points in Texas, namely, Beeville, Denton, Temple, and College Station, where the total annual rainfall varies from 30 to 38 inches. It is noteworthy that although there is a Wide differ- ence in the total annual rainfall at Spur and the other points in com- parison, there is a marked similarity in the seasonal distribution. Dur- ing the four months critical for growing crops—June, July, August, and September—the rainfall at all these points is approximately the same. The Mid-Summer Depression This graph also shows another interesting point with reference to the period of depression of rainfall in mid-summer. Judging from the com- parisons made in this graph, many sections of Texas come into a period of deficient rainfall in mid-summer which makes expedient the storage of moisture from spring rains for the use of the crop during the fruiting period. Farm practice in the more humid sections of the State, where more or less abundant spring rainfall occurs, has led farmers there to plant crops earlier in the season for the purpose of evading insects and other troubles, without giving proper consideration to the fact that the crop would normally come into fruiting during the summer depression of rainfall. Accordingly, unless water has been stored in the soil from the spring rains, serious effects from drouth are encountered. This con- dition, perhaps, is not so nearly applicable to the subhumid section be- cause planting as a rule is deferred until May, and with the storage of the spring rainfall the immature plants can be carried through the period of depressed rainfall and come into fruiting in the more favorable mois- ture period in August and September. The average monthly rainfall for a seventeen-year period at Spur has been further analyzed by dividing the monthly rainfall into ten-day periods with the view of determining more definitely the length of the period of low summer rainfall. These results are shown in Figure 14, in which a comparison is made between the monthly average rainfall for the seventeen-year period and the average rainfall for ten-day periods during these years. . A comparison of the average rainfall for the ten-day periods with that of the average monthly rainfall shows that the period of depressed summer rainfall really begins in June and extends well into August. This is a more extended summer depression than appears to be the case when only average monthly rainfall is considered. This further em- phasizes the necessity for conserving the rather large amounts of rain- fall which occur as torrential rains in the spring and which are needed FACTORS INFLUENCING RUNOFF AND SOIL EROSION 21 to bring the plants through the period of summer depression in a healthy condition for setting a crop under favorable conditions which normally occur in August and September. Table 4.—Rainfall at Spur in inches, by 10-day periods, 1912-1928, inclusive 10-day average _ _ Monthly Monthly 10-day for 17 years Period total, average, total for calculated _to 17 years 17 years 17 years 30-day period January 1-10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.13 .37 11-20 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.77 .31 21-31 . . . . . . . . . . . . . . . . . . . . 5.35 314 1 .45 .23 February 1-10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.44 .25 ‘ 11-20 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.92 .69 21-28 . . . . . . . . . . . . . . . . . . . . 8.65 508 3.29 .72 March 1-10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. 52 .79 11.20 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.81 .84 21-31 . . . . . . . . . . . . . . . . . . .. 17.77 1 045 8.44 1.35 April 1-10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.84 1 .91 11-20 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 8.21 1.44 21.23 3.74 May 18.26 3.22 16.30 2.87 17.62 2.82 June 23.82 4.20 12.47 2.20 8.28 1 .46 July 9.33 1 . 64 5. 85 1 .03 17 . 99 2.86 August 14.89 2.62 13.87 2.44 21 .25 3.40 September 1-10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.93 2.28 11-20 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 60 3.45 21-30 . . . . . . . . . . . . . . . . . . .. 49 52 2 912 16 99 2.99 October 1-10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ._ 17 03 3.00 11-20 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 18 22 3.21 21-31 . . . . . . . . . . . . . . . . . . . . 47.39 2 787 12 14 1.94 November 1-10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 5 26 .92 11-20 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 52 .62 21-30 . . . . . . . . . . . . . . . . . . . . 13.84 814 5.06 .89 December 1-10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.59 .98 11-20 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.32 .58 21-31 . . . . . . . . . . . . . . . . . . .. 11.50 676 2 59 41 It is important to adjust the seeding date to fit the supplies of mois- ture which exist in this region. The planting of crops on the early supplies of spring moisture is likely to result in taking the crop into the period of depressed rainfall in an advanced stage, so that it suffers more heavily and fails to produce the maximum yield. On the other hand, planting can be delayed too long or until the last peak of the spring rain- fall has occurred, when the germinating seeds are subjected to drying Winds and low humidity, resulting in poor stands. The optimum plant- ing time is in May, at which time there is abundant moisture for ger- minating the seed and establishing vigorous young plants able to With- stand the subsequent dry period and utilize the rainfall at the second peak for setting fruit and maturing a crop. Perhaps the same general principles may be applied to some extent in the humid region, but it is realized that insect depredations and other factors of such character may upset the practical utility of such a practice in a humid region. ._____m____ _ 22 BULLETIN NO. 411, TEXAS AGRICULTURAL EXPERIMENT STATION Rains Classified as to Size A classification of the 861 rains during the seventeen-year period ac- cording to size is given in Table 5. The total rainfall for this period was 374.23 inches, with 170.44 inches occurring in rains of less than one inch, 203.79 inches in rains greater than one inch, and 104.40 inches in rains greater than two inches. The heavier rains occurred during the growing season. Of the 35 rains of over two inches, eight occurred in June, and seven in August, with only three in July. Over 50 per cent of the rainfall during June and August occurs in rains of over two inches. Figure 15 shows the percentage of the total water losses from runoff by months with over one-third of the total occurring in August. All the rain periods of over two inches for seventeen years, 1912 to 1928, are shown in Table 6. During this time there were three years, 1'917, 1918, and 1924, that did not have a rain period of over two inches; there were four years that had only one rain period over two inches. There have been only four rain periods in the seventeen years with a total rainfall over five inches. Heavy rain periods (two inches and more) do not occur frequently, there being an average of two and one- half such rain periods a year. Relation of Character of Rainfall to Runoff The normal seventeen-year rainfall for July and August combined is 4.89 inches. In 1926 and 1928 the rainfall for this period was 14.41 and 9.12 inches, respectively, or 2.4 times normal. Forty-four per cent of the total runoff for the three years occurred in July and August, 1926, g... a 7 Z / Z / /. \ g {m /7 %//7/ % 4% 2 W 7/ , V; / %//;/j’//f// / ._.. W / I-b 114121-91 l-u 11-0 tl-J no 11-10 2r: no 1PM am 1w u-zvlhv Ho nan/n l-la many-J! 11v li-lnrl-JI 1-10 1H0 fl-M H! ll-ntl-JI I-M 11-31211! bM/f-MZ/il Jon. F26. Mal: flaw." Mo; Jam Jul flay. Jqof fir!‘ MW 5P6- Fig. 14. The rainfall by 10-day periods showing two rain peaks in the spring, with a. rather long dry period from the last of June through July and the first part of August. The depressed rainfall during the growing season emphasizes the need of storage 0t water in the soil at the time of spring rains, as a reserve supply for crop use in mid- summer. mm» ...-»...... ~ 23 FACTORS INFLUENCING RUNOFF AND SOIL EROSION . 2. 2.2 8 .. . 8 8N N2 . . . . . . .. 2.2.8.2 2...... SN mNim . . . . . ......_.s..e 8 8H o mTmm 8N N 8N...“ 5N N2. 3N. S. 8.: ................:.£.........Q 8- 8 o 8.8 8N N 8...? 8N2 2. NNN. N2. 2.2 .... . . . . . . . .........~E...>..z NfiNN 8N2 2. 8N8 E8 2 2.8 N92 E 5.... 8 8N? ..... . . . . . . ..:.......B3o 2 NN 3.: m. N88 NNNN 2 28.2. 8N 8 82.. o2 NW2. . . . . . . . . . . ........_EBN...N 8.8 2.8 N .82.... 2.8 2 NfimN 8.2 NN 2m. 8 28.8 x: . . . . . ............SN=< .2.NN .82 N 2...... NNNH N N84... 2A3 8 82.. E S 8 f . . . . . ..:........:€ 8 N... w... NN N N2. t. 8.8.3 2 8 NN .8 2 8 2.... 2 S 3. . . . . . . . . . . . . . . :23 I 8 NNvS 8 8 8 2.NN 3 3 2. M83 82 >2. N: 2.8 82 8N m N R 8 No.2 2 NN 8 . wNeN NN $2.. Nw 8.2. 1x: .. :54 8 8H c mmAm 2N N. .88 N82 Q. 3N. Nm R2 . . . . . . . . zifia2 8 8 8 2 .3 NN; 2 N28 N...» 2. 2N. s. 8 N . . . . . . . . . . . . . . . . 22.2.1.2 8. 8. o 8. 8. 8 .882 .8... NN 3N. NN .8... . . . . ......E.:=5. uGQO 3Q —Nn-O.H. .02 QCQO 5m ~fluOvF .02 uGUO QQQ —NQOF .02 8E2: N235 .53 NEE Mo flaming 23:02 mucus“ N ~o>o m0 zfifiwfl SOC“ w QQ>O m0 Zflwiflflm JOE“ w GNHS. QUNMQ>¢< M03652 13cm. $2 a. .382 632:2: dim Ow WGZuuOUUN ©0522»? 25m QM mUOiQQ Ewfllum 03mm. 24 I BULLETIN NO. 411, TEXAS AGRICULTURAL EXPERIMENT STATION and 16 per cent in July and August, 1928, 0r a total of 6O per cent for the three years has occurred in the four months—July and August, 1926, and July and August, 1928. t0 the character of the rains. The character of the rainfall (Tables 10, 11, and 12) was not more torrential. during this period than for other periods, but the large amount of rain saturated the soil and the runoit Was undoubtedly due more to the Water content of the soil than it Was Table 6.—Rain periods at Spur of over two inches, 1912-1928, inclusive 1912 1913 June 17-18 . . . . . . . . . . . . . . . . . June 30 . . . . . . . . . . . . . . . . . . . September 2 . . . . . . . . . . . . . . . September 25 . . . . . . . . . . . . . . October 1-3 . . . . . . . . . . . . . . . . May 29-31 . . . . . . . . . . . . . . . . July 3 . . . . . . . . . . . . . . . . . . . . . August 24-30 . . . . . . . . . . . . . . October 21-25 . . . . . . . . . . . . . . March 9-11 . . . . . . . . . . . . . . . . Aplil 15-18 . . . . . . . . . . . . . . . . . June 5-7 . . . . . . . . . . . . . . . . . . . September 21-25 . . . . . . . . . . . October 12-17 October 13-15 . . . . . . . . . . . . . . March 22-24 . . . . . . . . . . . . . . . A ril 25-28 . . . . . . . . . . . . . . . . ay 7-11 . . . . . . . . . . . . . . . . . . July 20 . . . . . . . . . . . . . . . . . . . . September 14-20 . . . . . . . . . . . October 5--9 . . . . . . . . . . . . . . . . 1 920 1921 1922 1923 1924 1925 1926 NHRUW/OMIOUJUCABQMUJMIOM H . Q0 1927 1928 September 21 . . . . . . . . . . . . . . May 1-3 June 8-9 . . . . . . . . . . . . . . . . . . . October 13-15 . . . . . . . . . . . . . . April 24-27 . . . . . . . . . . . . . . . . July 28-31 . . . . . . . . . . . . . . . . . August 7-9 . . . . . . . . . . . . . . . . September 14 . . . . . . . . . . . . . . August 16-18 . . . . . . . . . . . . . . August 28-31 . . . . . . . . . . . . . . September 26-30. . . . . . . . . .. October 15-16 . . . . . . . . . . . . . . September 25-28 . . . . . . . . . . . May 13-21 . . . . . . . . . . . . . . . . . July 21-31 . . . . . . . . . . . . . . . . . Total . . . . . . . . . . . . . . . . . Awmmwpwm 0O U1 >FUJMIOMIQIO>J>MCNIOU1W>J> M3 O0 141 . Total for 17 years, 374.23. c/an. Fe b. ll/arcfi Apri/ 4/6, June c/u/y A09. Sap/l. Cerf /Vo v. De c. Fig. Per cent of rainfall coming in rain period of ovcr 2 inches, 37.81. A/0 pe/"cen/ A/o pe/"cen/ O /O 15. 2o per cen/ JO 40 .50 Percentage of total runoff from all plats occurring monthly, 1926-1928. Over one-third of the total water losses for the year occurred during the month of August, which is the normal month for crop fruiting, and the time when the demand for water is the greatest. uqvwugxw’ FACTORS INFLUENCING RUNOFF AND SOIL EROSION 25 The heavy rainfall in July and August of these years is ‘unusual. The only other rain period in the seventeen years (Table 2) that approaches the heavy rains of 1926 was in May, 1914. Runoff losses occurring in July and August, 1926 and 1928, although accounting for 60 per cent of the runoff for the three-year period, did not reflect so much in crop yields as other smaller losses have when water was more of a limiting factor. The relative small losses occurring with the torrential rains of May 18 and June 11, 1928, occurring at a time when moisture was badly needed, reflected on crop yields more than the heavier losses occurring in July and August. The loss of 7'7 per cent of the torrential rain on July 9, 1926, would have been disastrous to the crop yield of that year had it not been for the abundant rainfall that followed. The data have furnished evidence in abundance that runoff is influ- enced by the water content of the soil. On the other hand, it is a com- mon observation of soil physicists that extremely dry soil will not absorb water so readily as soil containing a fair percentage of moisture. This law cannot be verified from data during the three years of this study, nor can specific cases be given in this period, because the water content of the soil was sufficiently large at all times to allow rapid absorption. There are, however, a number of interesting cases in other years that may be cited. In 1920 torrential rains of 1.30 inches and 2.74 inches fell on April 24 and April 2'7, respectively. There had been less than two inches of rainfall for the seven-month period preceding these rains. The seed bed had been prepared for spring planting, but the soil was very dry. Following these successive rains, totaling 4.04 inches, there was insufficient moisture to germinate cotton, grain sorghum, or cow peas, on plowed land and the moisture d_id not meet under the beds of listed land. Identically the same thing happened with a rain of 1.77 inches on June 9, 1916. Observations such as these have led to the sub- stituting of a clod mulch for the dust mulch and for abandoning the drag harrow as a farm implement in subhumid regions, as this implement places the soil surface in a condition which is conducive to increasing runoff. Table 7.——Influence of July and August rainfall on runoff. July and August Rainfall, Inches Average Total annual Year annual Total Number Avera e runoff rainfall, Total torrential of rains runo from inches Total producing producing producing from eight runoff runoff runoff eight plats plats 1926 38.08 14.41 13.91 6.44 7 4.58 -6.95 1927 16.12 2.25 .92 .55 1 .03 .91 1928 19.99 9.12 6.84 4.64 8 1.64 2.59 Average rainfall July and August, 17 years, 4.89. 60.42 per cent of runoff occurrin July and August, 1926 and 1928. 43.83 per cent occurred July an August, 1926. 26 BULLETIN NO. 411, TEXAS AGRICULTURAL EXPERIMENT STATION EXPERIMENTAL DATA FROM CONTROL PLATS Efiectiveness of Rainfall l Rainfall may be classified as effective and ineffective. Effective rain- fall is that which is stored in the soil and subsequently used by plants. Ineffective rainfall is that which has no value in so far as plant growth is concerned. Rainfall may be ineffective by occurring as small infre- quent showers, which are 10st quickly through evaporation. Other rains may be rendered largely ineffective through runoff. Rainfall may occur out-of-season for the crop grown and be partially lost through evapora- tion, percolation, or transpiration in weed growth before the crop has an opportunity to use it. A11 attempt has been made to determine the moisture absorbed by the soil during the three years in which these studies have been in progress (Table 8). The rainfall for the three years averages 21.36 inches, of which 4.85 inches fell as small showers, and 3.63 inches were lost by runoff from cotton land with a 2 per cent gradient, leaving a balance available for plant growth of 12.88 inches. It is assumed that some part of this remainder was lost by percolation and evaporation and, accord- ingly, the total amount of rainfall actually used by crops was probably less than 1O inches annually. The rainfall for 1927 was six inches below normal and the ineffective light showers were numerous, so the runoff was very small. In 1926 the runoff was heavy and the ineffective show- ers were few. These two factors seem to have a tendency to balance each other, and it is probable that the amount of soil moisture available for plant use does not vary as widely from year to year as does the rainfall. Table 8.—Effectivcness of annual rainfall. Lost as runoff _ Rainfall, Ineffective from 2 per cent **E_ffective Year inches small showers grade planted rainfall, inches to cotton inches 1926* . . . . . . . . . . . . . . . . . . .. 27.99 2.50 7.13 18.36 1927 . . . . . . . . . . . . . . . . . . . .. 16.12 6.59 .57 8.96 1928 . . . . . . . . . . . . . . . . . . . .. 19.99 5.47 3.19 11.33 Average . . . . . . . . . . . . . . 21.36 4.85 3.63 12.88 *From June 18 to December 31, 1926. _ _ "The amount lost through evaporation and percolation is not accounted for in these figures . Seasonal Runofl’ Attention has been called to the seasonal distribution of rainfall and its importance to crop production. The total monthly loss in water by runoff from each of the eight control plats for each of the three years in which this work has been in progress, together with the average monthly loss, is shown in Table 9. Figure 15 is a graphic representa- tion of these losses expressed in percentages, and indicates the seasonal periods in which losses occur. It is significant that July and August 27 FACTOR-S INFLUENCING RUNOFF AND SOIL EROSION QC. Q Fa. QC. C C .. . ....... . . . . . . . IQUGHUV< 2x c 8. oo. 8. ww.. o9 o9 9x o9 o o wmmw . . . . . 133m 223m 8. o S. o0. 8. 8. 8. 0o. co. 8. o o E2 . . . . . . . . . . . . . . 522m 238m 8 o mm. S. Kw 3 w s. oo 8 oo o o $2 . . . . . . . . . . . . . . .395 223m dwzfim E3 :5 m|w wfinw @ @@. @@. @@. fiAw. ¢%. Aw a . . . . . . . . . . . . . ... . . . . ...UwNHQ>< oo. o we. o0. i. .w 8Q hw. E m w o9 mo. o o Q2: . . . . . . . .w.8~z:=v 2E 32am 8. o 3. 8. S. mm. mm. w oo. w; . ma. o o S2 . . . . . . . 629516 2E 22am 3. o was 8. E é 8 N 3w oo oo 8. o o m5: . . . . . . . .w....§>.:=@ S: 222m 3.8m Emu :5 mvlm, wmwm @ aw C . . . . . . . . . . . . . . . . . . . . ..®@Nh0>< 2x o 8. oo. 8. 2 . w 3. ma” o9 8. o o ME: . . . . . . . . . . . e~§>£=Q asim oo. o hm. ma. 8. o9 EH 8. 8. S. o o .82 . . . . . . . . . . . eufi>£=o asim Ma. o ma.» .3. 2.. w» m mm o0 8 oo. o o £2 . . . . . . . . . . ._.8~>s_=Q 22am dwmhm wcoo 3a N|w~ wmwnw Aw Aw G . . . . . . . . . . . . . . . ..QwGHQ\/< oo. o 0o. oo. 5. bu. w .3. mm. 8. oo. ¢ c $2 . . . . . . . . . . . . . . . . . . . . dofioU 8. o o0. o0. 8. 8. B. S. o0. 8. o o R2 . . . . . . . . . . . . . . . . . . . . A5300 mo. o om. w mm. mm. m 2w. w B. 0o. oo. 8. o o $2 . . . . . . . . . . . . . . . . . . . . £0300 dwzfim wave .35 Nlm wmwnw Q C aw . . . . . . . . . . . . . . . . . . . . ..®mNhQ>< oo. o oo. oo. wh. ma. mv. on. oo. oo. o o wmww . . . . . . . . . . . . . . . . iidowwcU oo. o oo. oo. 8. oo. .5. oo. oo. 8. o o S2 . . . . . . . . . . . . . . . . . . . icofioU ww. o ww.w mm. S. 3a mm. o0. oo. 8. o o $2 . . . . . . . . . . . . . . . . . . . icofioU .355 wave Ea wulfl wfinw c aw @ . . . . . . . . . . . . . . . . . . . . ..QWN.~U>< co. o o0. oo. mm. mw. ww. mw. oo. oo. o o wmmw . . . . . . . . . . . . . . . . . . . icofioU oo. o oo. oo. co. oo. wm. oo. oo. oo. o o nmmw . . . . . . . . . . . . . . . . . . . icofioU mo. o mm. mo. “w. no. we. oo. oo. . oo. a o wmww . . . . . . . . . . . . . . . . . . . 2.5500 ._...>Q1_Iw 3E dwnw .>oZ JOQ JQQW .m=< 32w. 25w. >32 :54 v32 Awoh dmw. .w~2-@~2 .555. “on 3:2: E dwmwa wobcoo Eob toasmwlkm 03mm. 28 BULLETIN NO. 411, TEXAS AGRICULTURAL EXPERIMENT STATION m: . ~ 0 3.2 B6 2.3 @990. wwfi 2 b i. om. 0 o . . . . .. . . Jrficofi wcwinouo toanu 12:25 omwkoiw Mo omficookom a. 0 mm. mo. I; om. U». M2. oo. oo. o o ................omm._u>< 8. o 0o. 8. obm t: 8.. m... 8. oo. o o . . . . . . . . . . . . . . . . . . . . . cofioU oom o oo. com oo. oom m9. om. oo. oo. o o . . . . . . . . . . . . . . . . . . . . c0300 mo o .5. m; E w .3 w mm oo. oo. oo. o o wmm" . . . . . . . . . . . . . . . . . . . . e826 .222» E3 SQ mlw .35 oo. o mm. mo. mw. 8.. w“ S. 8. co. o o . . . . . . . . . . . . . . . . . . . . . émw$>< oo. o 0o. oo. Nvm mo. 2. S. oo. o0. o o . . . . . . . . . . . . . . . . . . . . . Z052 00. 0 00. 00% 00. 00@ M: 00.. 00@ 00. 0 0 . . . . . . . . . . . . . . . . . . . .. . .012 5. o 09H no wo m hm ~ ww oo oo 0o. o o wwmfi . . . . . . . . . . . . . . . . . . 2.3012 . .255 Eva 5Q Nllb XE doQ .>oZ 4B0 43m .w=< 33. 05:. 2x2 05¢ v82 but dam .UUSCSGOU J:COE LOG manic Cw éfinnisccu 80C bocsfllkm Emma. FACTORS INFLUENCING RUNOFF AND SOIL EROSION 29 are the two months in which the heaviest losses occur. The losses during these two months amount to 60.42 per cent 0f the total losses for the year, During the month of August 33.83 per cent of the total water loss occurred. The fact that these heavy losses occur during the period when the growing crop is making its heaviest demands for moisture probably ex- plains the marked increases that have been secured in crop yields through checking the runoff. It is rather surprising that the month of July, which is the period of low rainfall between the early June and late Au- gust peaks, should show such a heavy loss. This may be due in part to the abnormally heavy rainfall in July in 1926, and it is probable that when records for a longer period are available the average losses for July will not be so large. It is realized that a three-year average is a short period from which to draw conclusions, but the presentation of the data for this short period of time will at least indicate the trend of seasonal loss in this region. Rapidity of Precipitation in Relation t0 Runolf An attempt has been made to study runoff as influenced by the in- tensity of the rainfall. The self-recording rain gauge has made it pos- sible to measure the rainfall occurring at any given period, so that the fall can be classified as to intensity. On the other hand, the runofi oc- curring was obtained only for the total rainfall periods, and cannot be classified for the different periods of intensity. Nevertheless, a classi- fication of the rainfall as to its intensity has been made as follows: (1) Torrential rainfall, including all rainfall which occurs at a rate greater t'han .75 inches per hour; (2) Medium rainfall, including all rainfall which occurs at a rate between .40 and .75 inches per hour; , (3) Slow rainfall, including all rainfall which occurs at a rate of less than .40 inches per hour. The classification of rainfall for each of the rain periods producing runoff during the three years, together with the percentage of runoff for the period for each of the plats, is shown in Tables, '10, 11, and 12. Inasmuch as the intensity of the rain as it varied in any rain period can only be considered in connection with the total runofi for the period, it is well to point out some individual rains that may indicate the rela- tionship between intensity of the rain and runoff. The rainfall on July 9, 1926, shown in Table 10, totaling 1,63 inches, 94 per cent 0f which has been classified as torrential, had an average loss from the eight plats of 77.07 per cent. This period was largely a period of torrential rain- fall and with a heavy runoff. On the other hand, it is significant that this rainfall followed the day after a rainfall of .92 inches, which prob- ably left the soil fairly well supplied with moisture and contributed to the runoff. 30 BULLETIN NO. 411, TEXAS AGRICULTURAL EXPERIMENT STATION doBc=->o ud~%c . . . . . . 3.2 §.- 3.5 E5 $.43. 3% 5a 21 . 3.5 3a 3.3 =28» 3a wmfio>< Q33 swam» . . . . . . I .. . . .. ........om.m~ mmd $6 ......w.~..w .........1so.m. 2S 2x S? mm. 8. 3Q $43. 3.: . £4 $4 8. $4 c 8. e 8. 23m m3; $3 S42 QTN 3S 3%; $5 . Q3 NQN 2: NQN c 8. o 8. 2.3 3S 2% $.45; $3 22K 3% 5.3 _ 5.» 2E fi 8. c 2.. E. ow. 3am 3E £3 _ 3% 3.2 QWE 2Y3 ma? __ 3w m: w” o». o 8. g 5A mma 8N E3 2.. _ 8. 3.»: £45 flaw. ., $2 22 fi S.“ a w». w. 2. 3.3 2m 2w 8.2 $.23 2.3. 3.3. 31$: w; 2. o 8._ o; 8. 2: 2. 2.3 S; $2. $5. 8.2 5.3 2. 2. Maia. 8.. 3d 3 2., m» m? S. 3A :3.” 33 $3. 3cm M53 “is. £2 3.2.. N: $4 2 3A c 8. S 3. 22a $2 +98 mi: 8.: 3w... 35m 3.9M. 3...» m2 q 2. w” 8. w“ w”; 8.3 o3; $3“ $25 3.3 SAN 2Q £2 , $4. m: Q 8. n» 3N mm m: 36w .22 $3 3A. 2&3 m .2 5.5 3.3 m: EA o 8. S“ 3. m. 3A 8.: Qwms.“ ES. 8. mm 3.. a 34$ 3. 3 flaw. .. 3A m. 3. .. 8. a $4 $6 9N2 ma.“ _ .5 E...“ 5.3 3N... 8. 8. 2. S ma. E 5 o 2.. :2 $2 5mm 8...; 3.3 3.; 34$ “ma... Ma.» $4 S 3. 2w 3.. o 8. $2M. .55 éiéa 33 3% “=3 8% E3 8.? 22a .25 2% Q0300 0:8 aims» 3n 80:8 c028 note» notoo monoi. 5m -5 3m ém Sm ém w 59a 65g .32.» ammo. ps3 i=5 Bo=am 28v ~95 “=3 “E3 4E $2 w Eob w no :5 m :5 m .55 m mam... 5a m hon N En m Sm o -53 n?“ mo 2.2m QSQB i2 =8 w 22m m. S m w Rim .8.“ w m. ism m “Em m ixm m aim 38m. 82m EEUQE iflumrflcm. E3 .5235 Rama? m 22m Sm E5 ma hfiougw mom? ma i2 ~85» mo i6... ham $2 5% inane» 89a ~32» moi: 8 nosfifi a fififi z hiadmli 02$ v 31 FACTORS INFLUENCING RUNOFF AND SOIL EROSION cmc . . . . . . .. $6 vcA c ccdm cwcm mad 21m cw; . cccm “ma: 3cm 3.82am wuw~w>< ... 3cm cmcmw . . . . .. . Iiitmccm ......~.c.m 3m . . . . I-ewlw 3.2 8N 2am cc. cc. c 21:. 3a cc. cc. cc. a. x“ cm. cc ma. c cc. cm . N. 2a. $3 cc. cc. c 3% a2 cc. cc.. cc. § . m 8 cc. I mm. a. $4 3w cc mg cc. cc. c mi: cc. cc. cc.. cc. £4 a“ 3.. c cc. S. cc. S. m cc 5% cc. cc. c S5 8. cc. cc.. cc. 8. 9 cc. S. 3. $ 3. 3.2 a: 22 2.2 cc. c 3.3 31$“ 8.8 m2 w: ma. c cc. c cc. 2: w? i; c» SS 2:. cc. c wccn cc. cc. S. cc. w”. 2: ma. c cc. c cc. w: ca. 5a SA. cc. c 2.2“ m; m; $4 B Sc c» ml 2 n“. S mm. 3.5 m m5 £3 3% fifi c 2.9m 33 83 3% 5.2 GA 2 S. c cc. cw 3A m3. 2.. :2 cc. cc. c 3cm cc. cc. cc. cc. n». c cc. c cc. 2: 2... 2a :2 $8 cc. cc. c m2; 8. cc. cc. cc. 3 I 2. c cc. cw Q. @350 .315 is» m2? Ema $50 25o m2? 22g 2% imam aofico 2am aims: “o: 30:3 note» E58 cote» @212: 8m ém 5m ém 5m ém w So: ..$¢_m £22m 26¢ E3 Qnwe 30:3 i=3 33 £59; “:3 £8 “m; w E9: w no Sm m :5 m .59 m fizz. 3a m 8n m .3 m :5 c -53 5E co 22G Si? $2: =$ m. 3mm N. mfim c ism 8n m v Ham m 5mm w JEm m fifim 13cm. 32w Esnzz mfifipthom. E8 Hawk? .623 m. tam Em mi: co hwfiwkflzO @255 ms $2 .632» “C “Z50 3m 3cm fifim Eshoo Sch hwcw? moE: 2 news?" E. micfin“ .3 aficmmnmnhmm wfimm. N m T A T S T N E M T1 R E P X E L A R w L U C I m. A S A X E T l, 1 4 0. N m T E L L U B 32 S? . . . . . . . . . . . . . . I oono ooo a. 3.5 £2 £2 3.8 21. . . . . . . . . . . . . .. $43 . . . . .. oo.; Noon .22 3a Quasi .. non» wwnovnnt .. . . . . . . . .. .. noon 22$; . . . . 222w ....»o.n ........_ooom. 3Q on oolwfi no. no. 8H 2.2 no. no. no. no. Q. . . . . .. . . . . . . . . . . .. 2: no. o H8O N: on No.53 no. no. no. s? 2mm no. no. no. n». . . . . .. o». . .......wo.§< w: m: ooii no. 8H no. 32 no. no. no. no. no. . . . . .. . $2. 2. $3.. 3% £2 33mm 82 £2 on.» oo.; 3E oo.: “mo... 3.2 5A :2: 3A 1w .95. $2 2a nnnoo moon no no 3.9. 3 m. 3.3. 2.x“ no. on. . . . .. n2 on. . . . . . 21;... 33. 3.2 3w 2.8% 3m. an 8; oomn» noon 3.2. noon an no. .2: mo. . Zion b3. fin 8 onowo no. no.. no. 32 5.... no. no. no. on. .. . .. . .§ 33. no.8 £2 3E2. $5 no. no. So» no.3. an? 3% on.” £4 .. non» n: .. Inn b3. 2.5 ion owonnw no.2 no. no no.3. 3% 8Q E5 no. n». o2 w». . . .25 b3. 3:. 93S nononw Mano 32 no. 2 a noon no.3 no.2. 5.2 no. . o2 no. 3.1.3 oo3. non 2 noon: no. no. no. noo oo. no. no. no. 2.. . . o2 o... . noon 3» 3.2mm 3o... no.3 no. 3E 3% 3.9. £2 n: on. 3 on. . 84o 5o minnow 23m non no. 8.5 no? :2 oo.»n $2 no. . n2 no. .. So s. nonw» no. no. no. mg... 5i non no. no. 5. o2 »m. . .. N2 n2 oiwwoq no. no. no. 5.5 2x no. no. no. £4 . o. o 22o no. no. no. o. no no. no. no. on. . .. a; 8 No.9: no. no. oo. owfi no. no. no. no. on. .2: on. . E3 .53 . nooo mono ocoo mono 25o mono 22o 2% flow oofioo 01.6 onousn ooq Bozé moron qonnoo oooooo monofi 8m .3 8m -3 pom -3 » 89a an“? .22.“ E3 ps3 G3 Bonfi oooo You 28o ps3 .28 o8- w 89a x so Sm n 5o n 8m m £80 .62 m 8m a 3o 3 3a o éio =12? Bonm ofiok 32 =8 w 32m » “Em n 3E H3 u w 3o n “so w Em n Em n25. 52w unison nnESBo. E3 oooo? “woo? . m “Em . Em , £2 no oonofiooo oooi 3 $2 ooonk no 33 Em noon .22“. moonooo Bob 6GB moi: 8 oomofloo E noonfioo no nonnnmfimuknm oSom. FACTORS INFLUENCING RUNOFF AND SOIL EROSION 33 On September 29, 1927, shown in Table 11, a rainfall period of 2.24 inches occurred, of which 62 per cent was classified as torrential rain. This rain came following a dry period of some days, the previous rain occurring on September 6. This rather heavy rain period, with more than half the rain classified as torrential, showed an average runoff loss of only 7.39 per cent for all the plats, and here again it is obvious that the moisture condition of the soil was an influencing factor. This same condition seems to have obtained for the rain period of September 6, 1927, 46 per cent of which was classified as torrential, and with a loss from only one of the eight plats. Still another rain period, June 1, 1927, totaling 1.41 inches, 88 per cent of which was classified as tor- rential rain, lost an average of 21.36 per cent. This relatively small loss, from a rain period essentially torrential in character, followed a period during the month of May when there was practically no effective rainfall, indicating again that the water content of the soil is a material factor in the amount of runoff resulting from torrential rains. On August 4, 1928, a rainfall of 1.21 inches occurred, all of which was classified as torrential, and with an average runoff loss of 52.85 per cent from the eight plats. This rainfall occurred four days after a rain period and the heavy losses can be credited in part at least to the fact that the soil was fairly well filled with moisture. Torrential rains occurred on May 18, May 20, June 11, June 17, July 23, July 30, and August 4 in the year 1928, and, in all these cases, rather heavy runoff losses were recorded. It is significant that during this period there were frequent effective rains. Table 13.—Relation of percentage of runoff to days elapsed since preceding effective rain. Rank, Per cent runoff according to lowest Treatment 0-10 days 11-20 days 21-54 days per cent between between between runoff rains rains rains Plat 1 Level——cotton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. 6 1 . 5 4. 6 Plat 6 2 per cent grade—b1_1ffal0 grass . . . . . . . . . . . . . . 11.4 7~.2 0.0 Plat 7 2 per cent grade-—-m1l0 . . . . . . . . . . . . . . . . . . . . . . 18.1 4.5 4.0 Plat 8 3 per cent grade—cotton . . . . . . . . . . . . . . . . . . . . 23.7 7.0 8.6 Plat 2 1 per cent grade——cotton . . . . . . . . . . . . . . . . . . . . 25.2 10.5 8.9 Plat 3 2 per cent grade—cotton. . . . ._ . . . . . . . . . . . . . . . 27.9 11.9 7.4 Plat 4 2 per cent grade——fallow, cultivated . . . . . . . . . . 44. 8 22.8 19. 1 Plat 5 2 per cent grade—fallow, not cultivated . . . . . . . 53.1 28.2 30.6 Since the moisture condition of the soil has obviously affected the, runoff, the different rains from which runoff occurred during the three- year period have been classified as to the number of days which had elapsed since the previous effective rain. On the basis of these classi- fications, the runoff in gallons for each of the eight control plats has been recorded, and the runoff expressed in percentages with results shown in Table 13. The results are graphically shown in Figure 16. It is obvious that in all of the plats the moisture condition of the soil as indicated by the number of days since the preceding effective rain has 34 BULLETIN NO. 411, TEXAS AGRICULTURAL EXPERIMENT STATION been a material factor in the losses. The graph also shows that the c1- on the land, as it has undoubtedly influenced the moisture content y the soil, has been an influencing factor. This emphasizes the fact t - the moisture content of the soil is an important factor in determin” the runoff from any given rain or rain period, and it further emphasiz the necessity of measuring the inflow of Water for a given rainy peri in order to have comparable data as t0 the influence of intensity of thi rainfall on runoff. l Rapidity of Precipitation in Relation to Soil Erosion In the study of intensity of rainfall in relation to soil erosion, the; same difficulties arise as apply to a study of intensity of rainfall in‘ relation to runoff. The records classifying the rainfall in given periods; as to intensity and a lack of a corresponding period classification of erosion taking place under these different intensities of rainfall, make-T it impossible to arrive at any definite conclusion. Per- rem/- Run Off‘ 60 a \ \ <30 \\‘ \0~/00o \‘\\ \ \\ ~\\¢ \ g Z0 ‘\§\D\TI3 ’ \ <>\\u~ 4 . ____ —~~\ \ ’” \’-»~*"-*~—~~:.\_ I % ~\§_ —”” \\ *1 \\ . ‘\_ ‘)\\ 0 K m "s ‘m\ § u fi “ ‘o x as wg Eg e5 l? s“ \§ “g \5\» \\ \\ \ \ s “Q bx \ “x “s. “x \"-\ s3 \\ N\ N Q N Q N q N Q N \ ‘\°\' u Q M3 wk “m \\> "an w? Ncfi wk» Fig. 16. Relation of time of occurrence of preceding rain to runoff. ., ig-l-qvwswwrv w; v -. .. FACTORS INFLUENCING RUNOFF AND SOIL EROSION 35 However, by comparing the data presented in Table 14 for each of the three ‘years, and assuming that the erosiveness was equally great for all the other rainfall as for torrential, it can be estimate-d that in 1926 approximately 14,000 pounds of soil per acre were lost from 8.78 inches of torrential rain; whereas in 1928 approximately the same amount of torrential rain, 8.47 inches, resulted in a loss of approximately 8,000 pounds to the acre; whereas in 1927, when the torrential rain represented about one-third of the total rainfall, a loss of only 1,000 pounds to the acre is chargeable to torre-ntial rainfall. The fact that, in general, 20 per cent of the total rainfall comes in ineffective showers and causes no runoff, makes actually much wider differences than these estimated differences, and altogether the results " indicate that the losses from torrential rains vary widely in different seasons and perhaps are influenced to a large degree by other factors than the intensity of the fall. Table 14.—E1Tect of torrential rains on erosion Total Torrential rainfall Erosion from Plat 3, Year rainfall, cotton on 2 per cent grade, inches Inches Per cent ‘ pounds of soil per acre 1926 . . . . . . . . . . . . . . . . . . . . . . . 38.08 8. 78 23.06 55 ,896 1927 . . . . . . . . . . . . . . . . . . . . . .. 16.12 5.80 35.98 3,185 1928 . . . . . . . . . . . . . . . . . . . . . .. 19.99 8.47 42.37 17,105 Effect of Slope 0f Land on Runoff Four plats are included in the series of control plats having grades of 0, 1, 2, and 3 per cent. The data secured are shown in Table 15, in which the runoff is expressed in inches per plat and in p-ercentage. In general, the plats with 1, 2, and 3 per cent grades have lost from five to six times more water than the level plat. The 2 per cent grade lost slightly more water than the 1 per cent grade, but the difference is not comparable to the difference between the runoff from the 1 per cent grade and that from the level plat. The 3 per cent grade in every case lost less water than the 2 per cent grade. This is probably due to the filling in of soil to establish the grade, which process left this plat in condition to absorb more water than a natural 3 per cent grade would have absorbed. Some slight filling was necessary to establish the 2 per cent grade and this may account for no greater difference in loss be- tween the 2 per cent grade and the 1 per cent grade. Considering the fact that the soil on these grades has been in place for three years and apparently has been well settled by the repeated frequent and heavy falls of rain, it is surprising that the losses are not more nearly in proportion to the grade——in fact, these results indicate that,the losses are not in direct proportion to the steepness of the grade, and that large water " losses occur on areas with very little slope. 36 BULLETIN NO. 411, TEXAS AGRICULTURAL EXPERIMENT STATION Table 15.——Influence of slope of land on runoff from plats planted to cotton, 1926-1928. Runoff in inches* Runoff in percentage Slope 1926 1927 1928 Av. 1926 1927 1928 Av. Plat 1, Level . . . . . . . . . . . . . . . . . .. 1.03 .21 .59 .61 4.08 1.89 4.69 3.55 Plat 2, l per cent grade . . . . . . . . .. 6.91 .37 2.63 3.30 27.31 3.41 20.91 17 21 Plat 3, 2 per cent grade . . . . . . . . .. 7.13 .57 3.19 3.63 28.20 5.25 25.31 19 58 Plat 8, 3 per cent grade . . . . . . . . .. 5.68 .56 2.78 3.01 22.47 5.12 22.09 16 56 *The total amount of rainfall from which runoff occurred in 1926, 1927 and 1928 was 25.30, 10.92, and 12.60 inches, respectively, wlth an annual average of 16.27 inches. Table 16.—Influence of slope of land on soil erosion from plats planted to cotton, 1926-1928. Surface soil eroded Loss per acre (pounds) (Inches) Slope 1926 1927 1928 Av. 1926 1927 1928 Av. Plat 1, Level . . . . . . . . . . . . . . . 22,449 1,760 1,541 8,583 .077 .006 .005 .029 Plat 2, 1 per cent grade . . . . . . 41,303 3,234 11 ,336 18,624 . 142 .011 .039 .064 Plat 3, 2 per cent grade . . . . . . 55,896 3,185 17,105 25 ,395 .190 .010 .058 .086 Plat 8, 3 per cent grade . . . . . . 57 ,342 4,588 14,788 25,572 .197 .015 .050 .087 Effect of Slope of Land on Soil Erosion A record of the soil losses obtained from each of the control plats, established on different control gradients and all of which was cropped to cotton, is pre-sented in Table 16. It is observed that from the average of the three years, the soil loss is twice as great from the plat with 1 per cent grade as it is from the level plat, and approximately three times as great from the plat with 3 per cent as from the plat With level grade. - There is an indication that the losses of soil are more directly in proportion to the steepness of the grade than are the water losses. These results show rather conclusively that cultivated lands with as little slope as 1 per cent are in danger of being impoverished by losses of soil and soil fertility much more rapidly than level land. Relation of Crops to Runoff It has long been recognized that vegetative cover is effective in re- tarding the oif-flow of water from the land. Vegetative cover in the form of forests, grass lands, and even cultivated crops forms a natural obstruction to the movement of water. Not only is the live vegetative cover effective, but the little cover from plants further safeguards against the rapid movement of water and gives it more time to penetrate the soil. Four of the control plats, each with a 2 per cent grade, one planted to cotton, one milo, one buffalo grass, and one fallowed, furnish some data as to the effectiveness of crops over fallowing. The results obtained for the three years in water losses from these plats are shown in Table 17. FACTORS INFLUENCING RUNOFF AND SOIL EROSION 37 The fallowed plat shows an average annual loss of 6.08 inches, the cotton plat an average loss of 3.63 inches, the milo plat a loss of 2.22 inches, and the grass plat a loss of 1.53 inches. The heaviest loss from both the grass plat and the milo plat occurred during the first year, the losse-s from these two plats being relatively small in the two succeeding years. The grass plat lost most heavily during the first year before the surface had become completely sodded. It is obvious that grass is very effective in preventing runoff. a The relatively low water loss from the milo plat is attributed to the fact that milo is a heavy user of water, and also produced a large plant growth and crop residue that retards the fall and surface movement of water. The leaf sheaths of milo form a receptacle for water and retain a portion of the rainfall which is evaporated before it reaches the soil. Milo also draws heavily upon the water supply at the time of year when the greatest amount of runoff ordinarily occurs. Table 17.—Influence of crops on runoff from plats with 2 per cent grade, 1926-1928. Runoff in inches* Runoff in percentage Crop 1920 1927 1928 Av. 1920 1927 1928 Av. Plat 3, Cotton“... . . . . . . . . . . . . .. 7.13 .57 3.19 3.03 28.20 5.25 25.31 19.58 P131 4, Fallow . . . . . . . . . . . . . . . . .. 12.33 1.84 4.07 0.08 48.74 10.88 32.34 32.05 Plat 11131111810 Grass . . . . . . . . . . .. 4.47 0.00 .11 1.53 17.05 0.00 .87 0.17 Plat 7, Milo . . . . . . . . . . . . . . . . . . .. 5.03 .18 .87 2.22 22.24 1.54 5.90 10.20 *The total amount of rainfall from which runoff occurred in 1926, 1927, and 1928 was 25.30, 10.92, and 12.60 inches. respectively, with an annual average of 16.27 inches. Table 18.——Influence of crops on erosion from plats with 2 per cent grade, 1926-1928. Surface soil eroded Loss per acre (pounds) (Inches) Crop 1926 1927 1928 Av. 1926 1927 1928 Av. Plat 6, Buffalo Grass. . . .. . . . 22,640 O 171 7 ,603 .077 .000 .005 .026 Plat 7, Milo . . . . . . . . . . . . . . . . 31 ,733 887 1 ,872 11 ,497 .109 .003 .006 .039 Plat 3, Cotton . . . . . . . . . . . . .. 55,986 3,185 17,105 25,395 .190 .010 .058 .086 Plat 4, Fallow (cultivated)... 81 ,459 6,922 23,437 37,272 .280 023 .080 .127 It is interesting to note in Table 9 the effect the condition of the growing crop has on runoff. In August, 1926, the runoff from the milo plat was 2.08 inches, and from the corresponding cotton plat, 3.22 inches. During this period the milo plant was making its greatest demand on the soil moisture. The milo plant matured and became dormant in September, while the cotton plant continued vigorous growth into Oc- tober. The runoff from the milo plat in October was 1.60 inches, and the cotton, 1.20 inches. Buffalo grass has been very effective as a cover and has practi- 38 BULLETIN NO. 411, TEXAS AGRICULTURAL EXPERIMENT STATION cally eliminated the loss through runoff during the years 1927 and 1928.. It emphasizes the importance of maintaining 0n grazing lands a good’ grass cover and indicates that heavy losses can occur on grazing lands; when these are overstocked and the grass cover partially destroyed. it The fallow plat furnishes a splendid example of What is taking place’ on orchard lands. The cultivation and covering on orchard land, with? the exception of the tree cover, is very much the same as on the fallow ‘ plat. It is obvious that the terracing of orchard lands will facilitatef , the absorption of Water and greatly increase the chances of profitable f A production in farm orchards in this section. " 1.926 - 1527 [:1 1.92s 7% 63/270? 0/00’ 419/71‘ Grade f/oa/ce/zf Grade Z/oa/ ce/r/ érade J/oe/ can)‘ Pun off Inc/res o d‘ I0 If 30 if Fig. 17. Influence of slope on runoff from land planted to cotton. A926 - 1927 [I] 192a V/fi Grade 0 per aw!‘ érda/e / per can)‘ é/ad? Z per (ew/ Grad! J/oe/ can)“ -. g zlnéla..l'.i?»alflumi\invlzmmiiw _l_. fray/on ~Towpe/4c/e a 10 2o J0 4a J0 ca m Fig. 18. Influence of slope on soil erosion from land planted to cotton. FACTORS INFLUENCING RUNOFF AND SOIL EROSION 39 Relation 0f Crops to Soil Erosion Vegetation is a contributing factor in controlling erosion. The grass, when a solid turf has been established 0n the plat, has been very effective in preventing soil wastage. The plat planted to milo was second in effectiveness. It should be stated here that only the heads were harvested from the milo plat, the stalks, leaves, and roots being left to be incorpo- rated in the soil. i The average of the three ‘years (Table 18 and Figure 20) probably does not show the relative effectiveness of crops in checking erosion so well as the average of the last two years, since the plats during the first year were lacking in plant residues and the grass plat was not completely sodded. The average soil loss in tons per acre during 1927 and 1928 Bz/ffa/a Grass s] A926 M/Yo §UI 6b //0/7 Faflau/ 6.///////a/‘ed Puma/Y‘ //7 Cfiifs‘ 0 a“ 1p 1; 30 3.;- Fig. 19. Influence of crcp on runoff from land with a 2 per cent grade. Baf/a/a 6/45: A926 - 13.27 C] M970 ' l_ 495a W/fl (of/‘o/r Faflan/ fia/f/ra/‘ed fma/on-E/is/e/Jcre a 1a e0 ea w w w 1v Fig. 20. Influence of crop on soil erosion from land with a 2 per cent grade. 4O BULLETIN NO. 411, TEXAS AGRICULTURAL EXPERIMENT STATION was: Grass plat, .04; milo, .69 ; cotton, 5.07 ; and fallow, 7.59. These data are in agreement with the general opinion that constant cropping to cotton is hard on the land and indicate that the ill effects are due largely to erosional losses as Well as to plant food removed with the harvested crop. Influence of Tillage on Runoff Two control plats, both with a 2 per cent grade, were fallowed, one cultivated and one not cultivated. Both plats, however, were kept free from weeds. The soil of the cultivated plat was spaded when the other plats in the series were prepared. Subsequently cultivation was given from time to time in the same manner as the plats on which crops were grown. The water losses recorded from each of these plats are shown in Table 19. During the first year there was little difference in the runofl’ from the two plats, as both were spaded in June in establishing the grade, but in the next two years the difference in loss from the cultivated plat and from the plat not cultivate-d was between onelialf and two-thirds. Such differences mean the difference in storage of a F 1.926 - /927 [I] 1.928 '////, aw” ("u/f/i/a/ea’ Faflo/x/ A/affu/f/ra/‘ea’ V , /% Puma/f fnrfie: a .5 1a u" e0 5.; Fig. 21. Influence of tillage on runoff from land with a 2 per cent grade. 192a - ma? [I] 492a 7/}, fiZV/a/x/ fa/f/x/a/‘aa’ i /_0//0/V 4/02‘ fa/fix/afea’ V / w f-ms/o/r-Jémr per/fare 0 10 20 so 4a 6'0 a0 7v Fig. 22. Influence of tillage on soil erosion from land with a 2 per cent grade. .2 m... w”, ‘H, ,_ . "mw-"me-w v-Ivwm<—' FACTORS INFLUENCING RUNOFF AND SOIL EROSION 41 considerable amount of Water and indicate the importance of preparing land and thus putting it in the best condition to ab-sorb rainfall. Figure 21 shows graphically the results secured in each of the three years in runoff from the two plats. Table 19.——Influence of tillage on runoff from plats with 2 per cent grade, 1926-1928. Runofl‘ in inches* Runoff in percentage Treatment 1926 1927 1928 Av. 1926 1927 1928 Av. Fallow, Cultivated . . . . _ . . . . _ . . .. 12.33 1.84 4.07 6.08 48.74 16.88 32.34 32.65 Fallow, not cultivated . . . _ . . . . . .. 12.09 3.57 6.48 7.38 47.78 32.69 51.45 43.90 *The total amount of rainfall from which runoff occurred in 1926, 1927, and 1928 was 25.30, 10.92, and 12.60 inches, respectively, with an annual average of 16.27 inches. Table 20.——Influence of tillage on erosion from plats with 2 per cent grade, 1926-1928. 1 ‘gift-rpm . . Surface soil eroded Loss per acre (pounds) (Inches) Treatment 1926 1927 1928 Av. 1926 1927 1928 Av. Fallow, cultivated . . . . . . . . . .. 81,459 6,922 23,437 37,272 .280 .023 .080 .127 Fallow, not cultivated . . . . . . .. 77,301 13,664 39,743 40,269 .256 .047 .136 .146 Influence of Tillage on So-il Erosion The effect of cultivation on soil losses, as determined on tWo fallow plats (Table 20 and Figure 22), one being spaded in the spring and cultivated and the other receiving no treatment at all, With the exception of having Weeds pulled up or clipped With a hoe, is not in accord with general opinion or findings in other erosion studie-s. The soil losses from the fallow plat not cultivated have bee-n greater than on plats Where cultivation was given. The average loss of soil from the fallow plat not cultivated for the three years, 1926-1928, has been over 20 tons of soil annually, and Where cultivation was given, 18 tons annually. The erosional losses in 192'?’ and 1928 on the cultivated plat have been 57 per cent of those from the plat receiving no cultivation. EXPERIMENTAL DATA FROM FIELD AREAS Work with the field areas, which varied in size approximately from 5% to 11 acres (Figure 10), was included to secure information as to the effect of preventing runoff on crop yields. The study is classified under four headings: Effect of field obstructions on runoff and crop yields; Relation of terrace spacings to runoff and crop yields; Relation of grade of terrace to runoff and crop yields; Maximum utilization of rainfall as affecting crop yields. 995°?‘ 42 BULLETIN NO. 411, TEXAS AGRICULTURAL EXPERIMENT STATION Effect of Field Obstructions on Runofl‘ and Crop Yields The influence of obstructions 0n runoff is shown by comparing the results from Area 2, which had rows running with the slope; Area 1, which had contour rows; Area 6, Which had two-foot fall between level terraces; and Area '7, which was dike-d so that no water could run off. The results secured in measuring the water losses from these- areas are- shown in Table 21. The water losses in 1927 were low on all areas, and the crop yields as shown in Table 22 are in accord with these losses, though the diiierences are not very great. The relative losses in 1928, however, are not consistent either with our knowledge of the effectiveness of obstructions in retarding water, or with the crop yields for 1928 shown in Table 22. This inconsistency, which occurs particularly in comparing Areas 1 and 2, can be explained by the fact that the long rows running with the slope on Area 2 permitted the concentration of water on the lower part of the area. This movement of water left the land at the upper end of the area in a smooth condition and with a large proportion of sand particles on the surface, a condition favorable to destructive wind effects. On June 9, a sand storm occurred, resulting in heavy damage to the young cotton plants on the upper part of this area, not correspondingly heavy on other areas. The diked border at the lower end of Area 2 held much of the water that fell on this area, resulting in an exceedingly good crop on that part of the area to balance the damaged crop on the upper part of the area and introducing a source of error which affects these results. This same condition did not occur on Area 1, where the rows were contoured and where the water was evenly distributed. Table 21.—Run0ff in inches and percentagges from field areas, as influenced by obstructions, 1 . 27-1928 Runoff, percentage Runoff, acre-inches Area Treatment 1927 1928 Average 1927 1928 Average 2 Rows running with slope. . . 6.75 38.22 22.48 .3822 2.88? 1.632 1 Contoured rows . . . . . . . . . . . 2.59 49.74 26.17 .1469 3.727 1.936 6 2-ft. fall between level terraces . . . . . . . . . . . . . . 1 . 99 11 .47 6. 73 . 1128 .860 .486 7 Diked . . . . . . . . . . . . . . . . . . . . 0.00 00.00 0. 00 .0000 .000 .000 Table 22.—Influence of obstructions to runoff on crop yields from field areas, 1927-1928. Yield, pounds of seed cotton Area Treatment 1927 1928 Average 2 Rows running with slope . . . . . . . . . . . . . . . . . . . . .. 646 156 i 401 1 Contoured rows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 648 300 474 6 2-ft. fall between level terraces . . . . . . . . . . . . . . . .. 611 466 538.5 7 Diked . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 753 586 669.5 FACTORS INFLUENCING RUNOFF AND SOIL EROSION 43 The acre yields from the different areas shown in Table 22 indicate in a general way the relative values of the different kinds of obstructions. The yield of 156 pounds of seed cotton in 1928 on Area 2 is low, due to the damage resulting from the sand storm. The trend of these results indicates the effectiveness of obstructions in preventing runoff and the resultant increase in crop yield due to the saving of water. Two other areas, each containing 4.58 acres, one level-terraced and with contoured rows and the other without terraces and with rows run- ning with the slope, showed a.n average acre yield of 952 pounds and 669 pounds of seed cotton, respectively. These areas, however, Were not equipped with measuring devices, and it was impossible to secure a record of the water lost. " Area '7, Which was diked to hold all water, has been exceptionally interesting. The months of June and July, 1927, and May, July, and August, 1928, had abnormal rainfall. The soil on this area absorbed the water from these heavy-rain periods without apparent damage to the growing crop. Soils and subsoils having an open structure, such as these soils have, allow a rapid infiltration and can store large amounts of water for subsequent use by plants. Relation of Terrace Spacings to Runoff and Crop Yields Field Areas 3, 6, and 4, containing approximately 11, 6, and 10 acres, ' respectively, were all terraced; the terraces were run on contour lines but were given a fall of one, two, and three feet, respectively, between the terraces. The results for the two-year period are shown in Table 23. The cotton yields from these same plats are shown in Table 24. These results are directly contrary to what would be expected, and the yields secured from each of the three areas seem to conform closely to the amount of water saved. Table 23.——Runoff in inches and percentagleéasgérzigagield areas, as influenced by terrace spacing, Runoff, percentage Runoff, acre-inches Area Treatment 1927 1928 Average 1927 1928 Average 3 l-ft. fall between level terraces . . . . . . . . . . . . . . 6.49 35.90 21.19 .3675 2.692 1.529 6 2-ft. fall between level terraces . . . . . . . . . . . _ . . 1.99 11.47 6.73 . 1128 .860 .486 4 3-ft. fall between level ' terraces . . . . . . . . . . . . . . 0.00 19.60 9.80 .0000 1.432 .716 These three areas are not uniform with reference to the natural slope of the land and soil. By referring to the map of the field areas (Figure 10), it is noted that Areas 3 and 6 are on the crest of the field and that Area 4 is much better adapted with reference to soil and topography. 44 BULLETIN NO. 411, TEXAS AGRICULTURAL EXPERIMENT STATION Accordingly, observations of the movement of water over these areas during the two years lead to the conclusion that the areas arenot com- parable. The experiment will have to be revised to eliminate these sources of error. It is contemplated to duplicate these areas in another part of the field and to add a number of other areas for the purpose of checking the reliability of the results accruing from the series involved. Table 24.——Influence of terrace spacing on crop yields from field areas, 1927-1928. Yield, pounds of seed cotton Area Treatment 1927 1928 Average 3 1-ft. fall between level terraces . . . . . . . . . . . . . . . . . . 590 297 443.5 6 2-ft. fall between level terraces . . . . . . . . . . . . . . . . . . 611 _ 466 538. 5 4 3-ft. fall between level terraces . . . . . . . . . . . . . . . . . . 602 356 479 Relation of Grade of Terrace to Runolf and Crop Yields Areas 6 and 5 furnish a comparison of terraces that have a slope of three inches in 10-0 feet with terraces built on a level. These two areas» are each approximately six acres in size and are located on an‘ almost ideal slope for such comparative work. Table 25 shows the amounts of water lost from each of these areas for each of the two years. It is observed that approximately four times more water was lost on the te-rraces that had a. fall than on the terraces that were constructed on the level. The average runoff from Area 6 in 1927 and 1928 was .4861 inches and from Area 5, 1.815 inches; that is, the level terraces held 1.329 inches more rain water than did the terraces with a slope. The differences in cotton yields are significantly in favor of the level terraces (Table 26). Table 25.-—-Runofi water in inches and percentages from field areas, as influenced by terraces with and without fall, 1927-1928. Runoff, percentage Runofi‘, acre-inches Area Treatment 1927 1928 Average 1927 1928 Average 2-ft. fall between levcl terraces . . . . . . . . . . . . . . 1 .99 l 11 .47 6. 73 . 1128 .860 .486- 5 2-ft. fall between terraces with 3-in. fall . . . . . . . . . 9.90 40.94 25.42 .5602 3.070 1.815 The terraces were built on Areas 5 and 6 in 1926, but the equipment for measuring runoff was not installed until 1927. The acre yield of seed cotton in 1926 on Area 6, having level terraces, was 683 pounds; and Area 5, having a slope of three inches in 100 feet along the terrace, produce-d 609 pounds of seed cotton to the acre. The increased yield from the area having level terraces over the area having terraces with a FACTORS INFLUENCING RUNOFF AND SOIL EROSION 45 slope of three inches in 100 feet on the terrace, in 1926, 1927, and 1928, was 74 pounds, 79 pounds, and 173 pounds of see-d cotton per acre, respectively, with an average for the three years of 109 pounds of seed cotton per acre in favor of the level terraces. This average yield is not surprising in view of the additional water saved. It is commonly known that a small. rain at the critical time often materially increases the yield. Table 26.—Inf1uence of terraces with and without fall on crop yields from field areas, 1927-1928- ' Yield, pounds of seed cotton Area Treatment 1926 1927 1928 Average 6 2~ft. fall between level terraces . . . . . . . 683 611 466 587 5 2-ft. fall between terraces with 3-in. ' fall . . . . . . . . . . . . . . . . . . . . . . . . . . . 609 532 293 478 Maximum Utilization of Rainfall as Affecting Crop Yields Since the conservation of rainfall is desirable in this region, it seemed well to include some field areas on which all of the rainfall was retained for comparison With some other field areas which received not only the rainfall but had definite amounts of water diverted from other areas to supplement the rainfall. Accordingly, a group of four field areas Was established. These in- cluded Field Areas 7 and 8, “planted to cotton and alfalfa, respectively, . both equipped with closed terraces on contours, and Field Areas 9 and 10, planted to cotton and alfalfa, respectively, both equipped with borders and a system of diversion terraces designed to save not only the rainfall but to take care of additional measured water diverted into these areas (Figure 10). It was realized that such a practice might require considerable alter- ations in these diversion terraces. and obstructions before these areas could be expected to handle most efficiently the rainfall and additional measured water diverted onto them. However, both of the crops used, cotton and alfalfa, are heavy users of water. This is especially true of alfalfa, which is an ideal crop to utilize water diverted from other areas, not only because it uses water practically throughout the year, but because alfalfa. is usually planted on areas where runoff water is avail- able. The four field areas, 7, 8, 9, and 10, were established in 1927, 7 and 9 putinto operation in 1927, and 8 and 10 in 1928. Records were secured as to the effectiveness of closed terraces built on contours from Areas 7 and 8 in both 1927 and 1928, with the result that no losses of water from rainfall occurred on either of these areas, either in 1927 or 1928, showing " that under such conditions as obtained in those year's the use of contoured . terraces on such slopes can be made effective in preserving all the rain- ;‘ fall. Records of crop yields for 1927 and 1928 were secured from Area 7 46 BULLETIN NO. 411, TEXAS AGRICULTURAL EXPERIMENT STATION planted to- cotton with yields amounting to 753.04 and 1586.66 pounds of seed cotton to the acre, for the two years, respectively. » The effectiveness of closed terraces built on contours in terms of cro yields is shown by the fact that in 1927 the yield of cotton from Area 7, so equipped, was 169 pounds of seed cotton per acre greater than the average yield of cotton on the four field areas equipped with open ter- races, and 233 pounds greater than the average of the four same field areas for 1928. Field Area 9, planted to cotton and equipped with borders and a system of diversion terraces, was designed to prevent any water loss from runoff and during the year 1928 received a measured amount of 4.99 acre-inches of water per acre from other areas. This amount of water was sufficient to cause breakage in the borders and terraces, and a loss of part of the diverted water during 1928. This breakage caused inaccuracies in both the rainfall saved and the additional water supplied. This area produced a yield of 441 pounds of seed cotton to the acre in 1928, or 88 pounds more than the average yield of the four plats equipped with open terraces. Such differences as these indicate the possibility of the use of closed terraces built on contour lines in subhumid regions. Field Areas 8 and 10 were planted to alfalfa in September, 1928, but notes on yields were not secured although there was an observable difier- en-cein the crops on the two areas in favor of Area 10, which received 2.73 acre-inches per acre diverted runoff from other areas. This area produced four cuttings as against three cuttings from Area 8, which received only the rainfall, and there was a marked difference in the Fig. 23. A broad-base terrace under construction. This terrace is 24 feet wide at the- base and is two feet high. Crops can be grown on top of this terrace arid it can be planted and cultivated with two-row implements. h. t. m‘ ._ ....z.-i.zm...,sijq.uemm.m.éézsalafiét¢isi ‘mi-mad FACTORS INFLUENCING RUNOFF AND SOIL EROSION 47 growth on the two areas for each of the cuttings, again indicating the feasibility of using diversion water in growing alfalfa. Field Area 10, like the corresponding area, 9, planted to cotton, developed some breaks in the border terraces, and lost a considerable amount of water, nullifying the records as to the amounts of water actually retained on the land. The system of diversion terraces on both Areas 9 and 10 is imperfect in the retention of rainfall and equal dis- tribution of water brought "in, and accordingly, the perfection of these diversion terraces is one of the problems to be studied and improved through experience. Observations made thus far on these areas, both as to the amount of water retained and the resultant increased crop growth, indicate the importance of developing this series in order to get definite and reliable information as to the feasibility and profitable- ness of impounding runoff for use in crop growth. Fig. 24. A broad-base terrace in a cotton field. This terrace was constructed in March, 1928, and the picture was taken in Decembenr The best cotton in the field was made on top of the terrace and on the rows just above the terrace. The solution of the problem lies in devising some system to bring the water onto the land with reduced velocity, and even“ distribution of this water over the area, thus allowing the maximum absorption and reducing the chance of breakage in the diked borders and the consequent loss of water. The construction thus far has been inadequate to accomplish these ends, and it is felt that such a system can be worked out only by means of the trial-and-error method preparatory to studies of the effect of application of such additional wateron crop yields. 48 BULLETIN NO. 411, TEXAS AGRICULTURAL EXPERIMENT STATION ACKNOWLEDGMENTS The authors wish to make acknowle-dgment to W. E. Flint, Agrono- mist, stationed at Spur Station, for valuable assistance rendered in the collection and preparation of data; to M. B. Bentley, Agricultural Engi- neer, Extension Service, A. & M. College; John T. Egan, former District Agent, Extension Service, A. & M. College; and county agents of Texas, for valuable suggestions in perfecting plans and equipment; to R. G. Hemphill, Associate Irrigation Engineer, Bureau of Public Roads, United States Department of Agriculture, for assistance in engineering problems; to W. T. Carter, Bureau of Chemistry and Soils, United States Department of Agriculture, for advice and counsel as to soil differences and soil problems; and to B. E. Karper, Vice-Director, Texas Agricultural Experiment Station, for assistance and suggestions in the preparation of the manuscript. SUMMARY Since Water is the chief limiting factor in crop production in the subhumid region of Texas, the study of the rainfall, its character,.seasonal distribution, and means of increasing the amount absorbed by the land, and measuring the effects of’ such moisture conservation on crop yields, is of the greatest practical importance to the region. The plan of the work reported in this publication relates to the study of factors contributing to losses of rainfall water and soil and the effects of conservation of rainfall water and soil on crop production. The results reported were obtained at Substation No. 7, near Spur, Dickens County, Texas, located in the Bolling Plains region of North- west Texas. The soil on which this Work was done- is Miles clay loam and the results are considered as directly applicable to parts of 44 countie-s and approximately 14,000,000 acres of land of the Miles and Abilene and related series and indirectly applicable to all of the sub- humid portion of Texas. The equipment used in this Work consists of a complete set of meteoro- logical instruments, a series of control plats equipped with tanks to catch the runoff water and eroded soil, and a series of field areas 5 to 11 acres in size, six of which are equipped With still ponds, weirs, and water-stage recorders for measuring the losses of water. The average rainfall at Spur for a period of 1'7 years shows that 85 per cent of the total rainfall comes during the growing season for summer crops. The monthly distribution of rainfall shows two rain peaks, one in May and one in September, with a period of depression in July. A comparison of the distribution of the rainfall at Spur and at four other stations in Texas shows a similarity in rain peaks and in the summer period of depressed rainfall. A A study of the distributionof the rainfall at Spur by 10-day periods FACTORS INFLUENCING RUNOFF AND SOIL EROSION 49 shows that the period of depressed rainfall begins in June and extends to August. A classification of the rains for 1'7 years as to size shows that 45 per cent of the total rainfall occurred in rains less than one inch in size and that the remaining 55 per cent was approximately equally divided be- tween rains of one to two inches and rains of over two inches. Approximately 20 per cent of the total rainfall has been ineffective because it has occurred in small showers, another 20 per cent was 10st as runoff, leaving approximately 60 per cent of the total rainfall to be absorbe-d by the soil. Measurements of the water lost from the eight control plats for the three-year period show that 60 per cent of the total water losses occurred during the months of July and August and in August alone 33 per cent of the losses occurred. Studies of the influence of the intensity of rainfall on runoff indicate that other factors such as moisture content of the soil at the time of rainfall exert considerable influence on the losses, requiring additional refinements in methods and procedure for conclusive results. The intensity of the rainfall seems to have the same relation to erosion that it has to the losses of water by runoff and requires the same methods for further study. The results indicate that runoff losses are not in direct proportion to the steepness of the grade, but that tremendous water losses occur on areas with very little slope. The results indicate that losses of soil by erosion are more directly in proportion to the steepness of the grade than is the case with water losses and that slopes with as little as 1 per cent grade are in danger of being impoverished rapidly by soil erosion. Grass was found to be an effective vegetative cover in retarding the off-flow of water. Milo is more effective than cotton and cotton more effective than fallow. The efficiency of a crop is partially due to its coverage and partially due to its removal of water from the soil, which in turn affects the absorption of water. The results indicate that vegetative cover is a contributing factor in controlling erosion, the losses of soil being in proportion to the effective- ness of the crops as a vegetative cover and its use of water. The soil losses were in direct proportion to the water losses. Tillage of land as compared with untilled fallow land in conserving water shows that the losses from the tilled plats were from one-half to two- thirds as much as from the plats not cultivated and that the soil losses as influenced by tillage were approximately in the same proportion. Results from field areas as to the effectiveness of obstructions in pre- venting runoff and 1n increasing crop yields were not consistent, but in general indicate that considerable amounts of water can be saved by the use of contoured rows, level closed terraces, and dikes, and that the crop yield is in proportion to the amount of water saved. 5O BULLETIN NO. 411, TEXAS AGRICULTURAL EXPERIMENT STATION Results as to the influence of spacing between terraces on runoff and crop yields are inconclusive. The results where level terraces were compared with terraces built withi a fall of three inches in 100 feet have consistently shown that level ter-L races are much more effective in saving water and the level-terraced area" has shown an average annual yield of 109 pounds of seed cotton to the ‘l acre more than where the terrace had a fall of three inches in 100 feet. Studies relating to utilization of rainfall and its effects on crop yields indicate that under the conditions in this region all of the rainfall can i? be retained on land planted to cotton and alfalfa by the use of closed i level terraces. The work also indicates that in special cases from two 1 to four inches of additional water can be applied to the land by means f of diversion terraces, but this requires the perfection of some system to "l spread and retain the water on the land. Preliminary observations made as to the results in crop yields indicate that alfalfa, and perhaps cotton, can utilize additional water advantageously.