A one-dimensional water balance model was developed and used to simulate water balance for the Columbia River Basin. The model was run over a 10 km x 10 km grid for the United State's portion of the basin. The regional water balance was calculated using a monthly time-step for a relatively wet year, a relatively dry year, and for a double CO2 climate scenario. Input data, spatially distributed over the grid, included precipitation, maximum soil storage retention capacity, potential evapotranspiration (ET), runoff and soil storage. Model performance was assessed by comparing modeled ET and runoff with the input precipitation data, and by comparing modeled runoff with measured runoff. The model reasonably partitions incoming precipitation to evapotranspiration and runoff. However, modeled total annual runoff was significantly less than measured runoff, primarily because precipitation is underestimated by the network of measurement stations and because of limitations associated with the interpolation procedure used to distribute the precipitation across the grid.