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A Rainfall Erosion Index for a Universal Soil-Loss Equation¹

ABSTRACT

Extensive regression analyses of basic soil-loss data were designed to determine the best indicator of the capacity of a storm to erode soil. The rainstorm characteristic found to be outstanding as such an indicator is the variable whose value is the product of the rainfall energy and maximum 30-minute intensity of the storm (designated as EI). This variable explained from 72 to 97% of the variation in individual-storm erosion from tilled continuous fallow on 6 soils. Seasonal rainfall erosion index values computed by adding the EI values of storms > 0.5 inch explained as high as 94% of the yearly deviation in total soil loss from fallow during the summer season. Tested against data from plots in continuous row crop for 10 or more years at each of four widely separated locations, the summed EI values explained from 72 to 85% of the yearly variation in soil loss within corresponding cover periods. Expected annual values of the index and seasonal distribution of the erosion potential may be readily computed from local rainfall records. This has been done for 60 locations in the 31 Eastern States.

About 8,000 plot-years of basic erosion data are being analyzed to evaluate factors for a universal soil-loss equation for which the erosion potential of expected local rainfall serves as the base. Tests show that estimates of average erosion losses computed in this manner are sufficiently accurate to serve as sound bases for conservation farm planning.

¹ Contribution of the National Runoff and Soil-Loss Data Summarization Program of the Soil and Water Conservation Research Division, ARS, USDA, in cooperation with the Purdue Agr. Exp. Sta., Lafayette, Ind. Journal Series Paper No. 1306. Presented before Div. VI, Soil Science Society of America, Aug. 7, 1958, at Lafayette, Ind.

² Analytical Statistician, Eastern Soil and Water Management Branch, SWCRD, ARS, USDA.

Received for publication September 5, 1958. Accepted for publication November 13, 1958.