An accurate assessment of the moisture content of surface soils in arid and semiarid systems is necessary to predict the productivities of shallow-rooted plants and decomposition rates of surface and near-surface litter. Soil water loss has often been assumed to proceed in a stepwise manner, beginning at the surface and proceeding to deeper layers as the profile dries. However, recent studies suggest that plants draw water simultaneously from many or all soil layers where roots are present. We examined the potential significance of this phenomenon in a semiarid ecosystem utilizing two simulation models, one integrating soil water loss throughout the profile and the other sequentially removing water from soil horizons. Field observations indicate greater moisture retention in shallow soils than could be explained by a stepwise removal of water; a better correlation was achieved by simultaneously drawing water from all horizons.