400 brown. EFFECTS OF SOIL CONSERVATION [Ch. 22 



These research data give a basis for estimating the potential re- 

 duction in sheet erosion that might be achieved by modification of the 

 causal factors. Although rainfall intensity and frequency cannot in 

 themselves be changed, analysis of their monthly expectancy in any 

 area gives a basis for planning maximum protection of the soil sur- 

 face by cover crops, mulches, etc., in those months when the erosion 

 potential from rainfall is greatest. The effects of length and degree 

 of slope indicate the relative need for measures such as terraces, diver- 

 sion ditches, strip crops, and benching to reduce the effective slope and 

 break it into shorter lengths. Inherent differences in soil erodibility 

 suggest adjustment of land use to afford greater vegetal protection of 

 more erodible soils and more use of less erodible soils for row crops. 

 It indicates also possibilities for reducing the inherent erodibility by 

 changing the soil structure through tillage practices and by plowing 

 under cover crops, mulches, and manure. Lastly, differences in effects 

 of vegetal cover emphasize the need for rotation systems of farming, 

 use of cover crops, and conversion of some row-crop land to permanent 

 cover. 



Experiment station data have shown (Brown, 1948) that a good 3- 

 or 4-year rotation will reduce sheet-erosion loss to 14 to 45 percent of 

 that occurring under one-crop system of cultivation. Contour farm- 

 ing, strip cropping, and terracing, under various soils, slopes, and rain- 

 fall, effect reductions ranging from 10 to more than 90 percent, averag- 

 ing generally about 50 to 75 percent (Stallings, 1945a, b, c). 



These experimental results have led conservationists to the conclu- 

 sion that sheet erosion on agricultural land can be reduced 50 to 75 

 percent or more while agricultural production is sustained and im- 

 proved by using every acre in accordance with its capabilities and 

 treating every acre in accordance with its needs. 



Other types of erosion may require special forms of treatment. 

 Stabilization of major gullies, valley trenches, or stream banks, for 

 example, may be accomplished in some places by planting of trees and 

 vines; or, if particularly unstable, they may require small dams, revet- 

 ments, jetties, or other control structures. It is physically possible in 

 areas of more than 20 inches of rainfall to stabilize completely most 

 gullies and virtually eliminate them as a source of sediment. The less 

 erosion can be controlled by vegetation, the more costly the control 

 becomes, and the more likely that it must be accomplished through 

 some federal program such as that contemplated in the Flood Control 

 Act of 1936 (U. S. Congress, 1936) . In areas of less than 20 inches of 

 rainfall, more reliance must be placed on structural measures such as 

 revetments, check dams and debris basins. Economic rather than phy- 



