Bulletin No. 326 - Rainfall and Irrigation in Relation to Soil Erosion

This paper attempts to point out to farmers, agricultural specialists, and others who may be interested, pertinent facts regarding rainfall intensity and other meteorological data that have to do with the wearing down of range and agricultural soils, and to present by means of diagrams illustrations of general relationships between the rate of wearing down of irrigated soils, the size of the irrigation stream and the slope of the eroding surface. Large streams running down steep slopes, whether from rainfall or from irrigation, constitute a destructive process that cannot be completely controlled. Fortunately, vegetation protects the soil against the destructive effect of rain and irrigation water, and, conversely, water promotes the growth of vegetation. A most constructive step, therefore, would be a carefully considered plan for grazing and for maintaining a vegetative mantle on the soil surface wherever and whenever this can be done without hindering farming operations. Under normal conditions when the surface soil is in good health, rain and irrigation water may penetrate better than when it is puddled and compacted. Obviously runoff occurs when rainfall intensity exceeds the infiltration capacity of the soil. The amount of erosion depends upon the size of the stream and the slope of the eroding surface as well as upon the character of the surface soil. Even though the rainfall is comparatively light, if it exceeds the rate at which it may be absorbed by the soil, the size and velocity of the resulting stream must increase as it moves down the slope