HOMOGENIZATION OF MANNING FORMULA FOR MODELING SURFACE RUNOFF

Surface runoff during excessive rainfalls is one of the most important processes contributing to soil erosion. Recent physically based soil erosion models (watershed models) use a modified Manning formula descr ibing sheet flow for the calculation of the runoff. But in cropland, s urface runoff is dominated by flow in rill systems. Based on the origi nal Manning formula, we investigate the influence of rill geometry and spacing on runoff and erosion. The results differ significantly from those of sheet flow. Moreover, significant differences occur between w ide, shallow rills and v-shaped rills. The apparent overall roughness coefficient decreases with increasing rill spacing.