Explaining spatial variability in mean annual runoff in the conterminous United States

The hydrologic concepts needed in a water-balance model to estimate the spa tial variation in mean annual runoff for the 344 climate divisions in the c onterminous United States (U.S.) were determined. The concepts that were ev aluated were the climatic supply of water (precipitation), climatic demand for water (potential evapotranspiration), seasonality in supply and demand, and soil-moisture-storage capacity. Most (91%) of the spatial variability in mean annual runoff for the climate divisions in the conterminous U.S, wa s explained by the spatial variability of mean annual precipitation minus m ean annual potential evapotranspiration. When soil-moisture-storage capacit y and seasonality in supply and demand were added to the water balance, the explained variance in mean annual runoff increased slightly, and the error in estimated mean annual runoff decreased significantly. Adding soil-moist ure-storage capacity and seasonality in supply and demand provided the most improvement in areas where seasonal supply and demand are out of phase.