Here the authors summarize application of a spatially distributed water balance model that was tested using historical data from the U.S. portion of the Columbia River Basin in the Pacific Northwest for a very dry (1977) and very wet (1972) water year. The model adequately partitions incoming precipitation into evapotranspiration and runoff. Because precipitation in the basin is underestimated from measured data, modeled runoff is less than measured runoff from the basin during both the wet and dry years. The predicted future climate conditions significantly increase potential evapotranspiration, causing a 20% reduction in runoff relative to input precipitation, and a 58% reduction in soil moisture storage. If these changes in regional water balance are realized, the distribution and composition of forests in the Northwest would change markedly, and water resources would become more limited. (Copyright (c) Inter-Research 1993.)