Surface water balance of the continental United States, 1963-1995: Regional evaluation of a terrestrial biosphere model and the NCEP/NCAR reanalysis

The land surface water balance of the continental United States is analyzed from 1963 to 1995 using a terrestrial biosphere model (IBIS), reanalysis d ata from NCEP/NCAR, a hydrologic routing model (HYDRA), and numerous observ ational datasets. Emphasis is placed on evaluating the performance of IBIS and the reanalysis, particularly over the central United States. IBIS is fo rced with daily climatic inputs from NCEP; an additional simulation is perf ormed using observed precipitation. The NCEP reanalysis is found to have ex cessive precipitation and evapotranspiration over the central United States (particularly in the summertime), an exaggerated seasonal cycle of runoff, and low snow depths. The net surface water balance exhibits a dry bias tha t is corrected by nudging soil moisture toward climatology. Unfortunately, this correction term is large and appears to have a detrimental impact on o ther water balance components (particularly runoff). Fields that are reason ably well simulated in the reanalysis include fall and winter precipitation over the central United States, soil moisture in Illinois, and interannual variations in runoff. Results from the IBIS simulations show generally bet ter agreement with observations than the NCEP reanalysis but continue to ha ve nontrivial errors in certain fields. Over the central United States, the se discrepancies include high winter/spring evapotranspiration (1 mm d(-1) too high), low snow depth, and weak spring runoff (30-50% too low). The err ors are at least partially caused by underestimated cloud cover and early s pring green-up. A spatial analysis of the U.S. water balance reveals that s ome of the strongest seasonal and interannual variations in precipitation, evapotranspiration, and soil moisture occur over the central United States.