A modelling approach was chosen for analyzing the effects of competition between windbreaks and crops for soil water and radiation in the Sahel. The model has a high spatial and temporal resolution to account for the heterogeneity in a windbreak-cropping system. The model was parameterised for millet (Pennisetum glaucum (L.) R. Br.), the tree species Bauhinia rufescens Lam. and soil characteristics and weather data of Sadore, Niger. Simulations were run for two wet years (1992 and 1993) and for a dry cropping season (1989). Simulations showed a strong yield reduction up to 2 m from the windbreak due to shading and/or water competition. For Sahelian sites with no access to groundwater, competition for water between trees and crops is likely to occur in the beginning of the rainy season. Water competition was surprisingly highest in the wet year 1992, intermediate in the dry year 1987 and lowest in the wet year 1993. Simulation results indicate that light and water competition depends on rainfall distribution and the period between the seasonal onset of tree and crop growth.