The factors and processes that may explain the observed coexistence of trees and grasses in savannas are not well understood. Here we propose a new hypothesis that addresses this issue. We hypothesize that variations in elevation at relatively short horizontal scales of ~1 km force similar variations in soil moisture and thus create significantly different hydrologic niches within any large area. Under water-limited conditions the relatively wet valleys favor trees, while the relatively dry hills favor grasses. This coexistence of trees and grasses is only possible for a window of climatic conditions that are characteristic of savannas. To test this hypothesis, numerical simulations are performed for the region of West Africa using a model that simulates vegetation dynamics, the Integrated Biosphere Simulator (IBIS), and a distributed hydrologic model, Systeme Hydrologique Europeen (SHE). IBIS is modified to include the groundwater table (GWT) as a lower boundary. The spatial distribution of GWT is simulated by SHE. At 9¿N the model simulates trees even when the GWT is assumed to be infinitely deep; at 13¿N the model simulates grasses even when the capillary fringe of the GWT reaches the surface. However, for the transitional climate, at 11¿N, trees are simulated when the GWT is at ~2.5 m from the surface, but grasses are simulated when the GWT is deeper than 2.5 m. These results suggest that the variability of soil moisture forced by topography can be a determinant factor of vegetation distribution within savannas. Furthermore, they confirm that this role of topography can be significant only in a certain climatic window characteristic of savannas.