Erosivity Index Values for Northern Mississippi

Annual rainfall erosivity (R) values during 1982 to 1992 were computed from 29 standard recording rain gages in or adjacent to the 21.3 km2 Goodwin Creek Watershed near Batesville, Mississippi, using Brown-Foster, McGregor-Mutchler, Agriculture Handbook 282, and Agriculture Handbook 537 procedures. The computations were made to determine the adequacy of previously recommended erosivity values for northern Mississippi. Computed R values were substantially higher than interpolated values given in Agriculture Handbooks 282 and 537, or the Users Guide for the Revised Universal Soil Loss Equation (RUSLE). The interpolated value from the iso-erodent map used in RUSLE is only about 5790 MJmm(hah)1. The average annual R value computed with the Brown-Foster equation for the 29 rain gages was 7968 MJmm(hah)1 for all storms with no limitations on storm size or intensity parameters, 7719 MJmm(hah)1 for all storms but with a limitation of 64 mm/h on the I30 parameter of the energy-intensity (EI) product, and 7515 MJmm(hah)1 where (1) storms less than 13 mm were excluded unless the storms contained 15-min rainfall intensities of 25 mm/h or more, and (2) the maximum 30-min intensity in the EI product was limited to 64 mm/h. This latter R value was still 30% higher than the R value obtained from the Users Guide for RUSLE. Various limitations that have previously been recommended for use in calculating R values were found to affect R totals from less than one to less than 4%. This is the second study in northern Mississippi, on different intensively gaged watersheds, that has shown significantly higher measured R values than previously recommended. Results from these studies also have been supported by measurements taken at Holly Springs, Mississippi, during erosion related research during a 25-year period. These results indicate that R values in the mid-south should be recomputed using modern rainfall data to replace existing iso-erodent index maps in which estimates of erosivity often were based on regression equations rather than measured values. Preparation of a standard erosivity map for the complete eastern United States also is recommended, especially considering the availability and ease of analyzing modern rainfall computations, and the advantage of having a standard erosivity map based on measurements.