ANALYSES OF SLOPE AND RUNOFF FACTORS BASED ON THE WEPP EROSION MODEL

Under process-based erosion model development, the source of sediment transported off a field is separated into that from interrill and rill areas, and separate detachment equations are developed. Limitation of this spatial separation is that, in many conditions, rill and interrill areas are not clearly defined a priori. We interpreted the erosion and deposition equations used in the current Water Erosion Prediction Project (WEPP) in an alternative fashion such that spatial separation of rill and interrill areas is no longer required. Analytic solutions were derived for the WEPP erosion and deposition equations under general conditions. Simplified solutions for a specific case, uniform rain on uniform slope, were examined closely for slope and runoff effects on sediment delivery. Under both erosion and deposition conditions, analytic solutions show a linear relationship between sediment yield, q(s) and slope, S. The dependency of q(s) on runoff, q(w), is either linear or quadratic depending on whether the system is dominated by an erosion or deposition regime. These analytic findings explain results obtained from laboratory studies in which sediment yield was collected under variable slope and rain intensities.