Surface runoff scale effects in West African watersheds: modeling and management options

Measurements of surface runoff from uniform slopes of different lengths in West Africa have shown that longer slopes tend to have less runoff per unit of length than short slopes. The main reason for this scale effect is that once the rain stops, water on long slopes has more opportunity time to infiltrate than water on short slopes. A validated simple model is put forward that quantifies the scale effects and predicts under which circumstances they can be expected to be most significant. An overview is given of management options that are available to farmers to reduce runoff at the field and slope level. The potential effects of these management options on the hydraulic conductivity, Ksat, and Manning's roughness coefficient, n, are qualified. The model is subsequently used to calculate the effect of changes in Ksat and n on the runoff from slopes under rainfall conditions found in West Africa. Finally, two design examples are given, namely maximum field lengths and dimensioning of interceptor drains.