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 km(2 ) Goodwin Creek Watershed near Batesville, Mississippi, using Brown-Fo ster, McGregor-Mutchler, Agriculture Handbook 282, and Agriculture Han dbook 537 procedures. The computations were made to determine the adeq uacy of previously recommended erosivity Values for northern Mississip pi. Computed R values were substantially higher than interpolated valu es given in Agriculture Handbooks 282 and 537, or the User's Guide for the Revised Universal Soil Loss Equation (RUSLE). The interpolated va lue from the iso-erodent map used in RUSLE is only about 5790 MJ . mm .(ha . h)(-1). The average annual R value computed with the Brown-Fost er equation for the 29 rain gages was 7968 MJ . mm .(ha . h)(-1) for a ll storms with no limitations on storm size or intensity parameters, 7 719 MJ . mm .(ha . h)(-1) for all storms but with a limitation of 64 m m/h on the 130 parameter of the energy-intensity (EI) product, and 751 5 MJ . mm .(ha . h)(-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 limi ted to 64 mm/h. This latter R value was still 30% higher than the R va lue 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 t he second study in northern Mississippi, on different intensively gage d watersheds, that has shown significantly higher measured R values th an previously recommended. Results from these studies also have been s upported by measurements taken at Holly Springs, Mississippi, during e rosion related research during a 25-year period. These results indicat e that R values in the mid-south should be recomputed using modern rai nfall data to replace existing iso-erodent index maps in which estimat es of erosivity often were based on regression equations rather than m easured values. Preparation of a standard erosivity map for the comple te eastern United States also is recommended, especially considering t he availability and ease of analyzing modern rainfall computations, an d the advantage of having a standard erosivity map based on measuremen ts.