Soil moisture tendencies into the next century for the conterminous UnitedStates

A monthly snowpack and soil-moisture-accounting model is formulated for app lication to each of the climate divisions of the conterminous United States for use in climate impacts assessment studies. Statistical downscaling and bias adjustment components complement the model for the assimilation of la rge-scale global climate model precipitation and temperature. The model pro duces monthly streamflow that is broadly consistent with observed streamflo w from several drainage basins in the United States for the period 1931-199 8. Simulated historical soil moisture fields reproduce several features of the available observed soil moisture in the Midwest. The simulations produc e large-scale coherent seasonal patterns of soil moisture field moments ove r the conterminous United States, with high soil moisture means over divisi ons in the Ohio Valley, the northeastern United States, and the Pacific Nor thwest, and with pronounced low means in most of the western U.S. climate d ivisions. Characteristically low field standard deviations are produced for the Ohio Valley and northeastern United States, the Pacific Northwest in w inter, and the southwestern United States in summer. Differences in extreme standardized anomalies of soil moisture over the historical record possess high values (2.5-3) in the central United States, where the available wate r capacity of the soils is high. Application of the methodology for future periods using output from the Canadian coupled global climate model (CGCM1) shows that for at least the first few decades of the 21st century, somewha t drier-than-p resent soil conditions are projected, with highest drying tr ends found in the southeastern United States. The soil moisture deficits in most areas are of the same order of magnitude as the soil moisture field s tandard deviations arising from historical natural variability. Brumbelow a nd Georgakakos [this issue] study the implications for crop yield in the Un ited States.