Effect of Rainfall Measurement Interval on EI Calculation

Abstract

ABSTRACT THE most widely used soil erosion prediction tool is the Universal Soil Loss Equation (USLE). With the USLE, estimated soil loss is computed as a product of six factors including the rainfall erosivity factor, R. R is defined as the average annual value of the rainfall erosion index, EI. By definition, values of EI and R should be computed directly from long-term rainfall records measured at short time intervals during which the intensity is essentially constant (i.e., using so-called breakpoint data). However, because of the scarcity of breakpoint rainfall data, most of the published values of R for the western United States were obtained from an estimating procedure. The objective of this study was to determine if EI values could be calculated using hourly rainfall data. These data, although more widely available than breakpoint data, have not been used to calculate EI and R. We conducted a detailed statistical analysis of rainfall data, collected on a 15-minute measurement interval, from three sites in western Oregon. The energy and intensity components of EI were calculated for individual rainstorms using 15 and 60-min measurement intervals. Regression equations were developed for EI calculated on the basis of 15-min data as a function of EI calculated on the basis of 60-min data. Values of r^ exceeded 0.98 indicating that hourly rainfall data can be used to estimate EI for use in the USLE. The method shows promise for use in other areas where hourly, but not breakpoint, rainfall data are available.