Large-Scale Aspects of the United States Hydrologic Cycle

A large-scale, gridpoint, atmospheric, hydrologic climatology consisting of atmospheric precipitable water, precipitation, atmospheric moisture flux convergence, and a residual evaporation for the conterminous United States is described. A large-scale, basin, hydrologic climatology of the same atmospheric variables is also described, as well as residual surface water and streamflow divergence or runoff for various large-scale river basins terminating at the United States boundary. Climatologically, precipitation, which had a U.S. annual mean of more than 2.1 mm day−1, was largely balanced by evaporation; atmospheric moisture flux convergence was also an important contributor (~0.5 mm day−1), especially during the wintertime, and especially along the U.S. west coast. At the surface, seasonal and anomalous surface water (including snow) variations on the order of 10 cm yr1 were forced by seasonal variations of about 1 mm day−1 in atmospheric moisture flux convergence (precipitation minus evaporation) and streamflow divergence. The strongest seasonal variations were found along the West Coast. Unlike the climatological means and seasonal variations, atmospheric precipitation anomalies were best related to atmospheric moisture flux convergence anomalies and less well related to the residual evaporation anomalies. Streamflow divergence anomalies were also related to the atmospheric moisture flux convergence anomalies, especially at lags of around 15 days. A better lag relationship occurred between streamflow divergence and precipitation anomalies.