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 describing sheet flow for the calculation of the runoff. But in cropland, surface runoff is dominated by flow in rill systems. Based on the original 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 wide, shallow rills and v-shaped rills. The apparent overall roughness coefficient decreases with increasing rill spacing.¿ 1997 American Geophysical Union