NRCS Design Storm Erosivity

Two methods are developed to estimate rainfall erosivity indices produced by four standard design storms developed by the National Resource Conservation Service (NRCS), which are used widely throughout the United States. The first method is based on regression analysis of erosivity indices calculated from the actual 24- h duration rainfall distributions and is presented in an uncomplicated form. The second method is founded on fits of an exponential distribution to the ordered rainfall intensities obtained for 6-min (0.1-h) intervals for each of the design storm types. Subsequent integration of the unit kinetic energy equation used over the storm durations, which makes use of the exponential expression for rainfall intensity, leads to an analytical approximation of the rainfall erosivity index. Although the regression equations estimate erosivity design storms accurately and are easy to apply, they are limited to durations of 24 h. The analytical solution, on the other hand, can be used to estimate erosivity indices from the four storm types for any duration of 24 h or less.