166 CONSERVATION OF GROUND WATER 



cultural importance are included here. 3 Detailed studies of 

 stream-flow characteristics have shown marked differences in 

 absorptive and retentive characteristics of drainage areas 

 which are superficially similar in topography and geology and 

 in which most of the surficial materials are rather poorly 

 permeable. 4 



The great majority of the experimental watersheds of Fed- 

 eral and state agricultural agencies are surfaced by materials 

 in this broad middle bracket of permeabilities, that is, fine 

 sand, sandy loam, and soils of finer texture. Studies of indi- 

 vidual plots and small watersheds have shown that the infiltra- 

 tion rate — and therefore the surface runoff — varies markedly 

 from time to time depending upon the condition of the water- 

 shed — which includes such factors as soil water and ice, tillage 

 practices, and type and density of vegetative cover. 



Detailed research into the effects of cultivation upon the 

 disposal of rainfall has generally been limited to small plots 

 or to watersheds having an areal extent up to several hundred 

 acres. Infiltration rates may vary greatly from place to place 

 even within a small area, reflecting the variations of texture 

 of the soil. As an example, 50 determinations of infiltration 

 rates within 100 acres all give different rates, and the greatest 



3 Free, G. R., G. M. Browning, and G. W. Musgrave, Relative Infiltration and 

 Related Physical Characteristics of Certain Soils, U.S. Dept. Agr. Tech. Bull. 

 729, 1940, 51 pp. As stated on pp. 3 and 4: "The 68 sites included in this study 

 were selected by soils specialists as representative of important contrasting 

 soils. . . . These 68 sites represent 39 soil series and 6 of the great soil groups, 

 gray-brown Podzolic soils, red and yellow soils, soils of the northern prairies, 

 northern Chernozan soils, soils of the southern prairies, and brown soils. They 

 also represent the nine following groups of parent material: glacial accumula- 

 tions (calcareous), glacial accumulations (slightly calcareous or noncalcareous), 

 Great Plains material, marine deposits (marl and chalk), marine deposits (sands, 

 clays, and limestones), wind-laid deposits (loess), residual accumulations, crystal- 

 line rocks, and sandstones and shales. These soils include 16 that are properly 

 classified as sandy clays and sandy loams, or sands, and 52 of finer texture, in- 

 cluding loams, silt loams, clay loams, silty clay loams, and clays. 



* Troxell, H. C, and H. M. Stafford, Natural Water Losses in Mountain 

 Drainage Areas of Southern California, Trans. Am. Geophys. Union, vol. 30. 

 pp. 752-758, 1949. 



