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Currently, observations of key components of the earth's large-scale water and energy budgets are sparse or even nonexistent. One key component, precipitation minus evapotranspiration (P - ET), remains largely unmeasured due to the absence of observations of ET. Precipitation minus evapotranspiration describes the flux of water between the atmosphere and the earth's surface, and therefore provides important information regarding the interaction of the atmosphere with the land surface. In this paper, large-scale changes in continental water storage derived from satellite gravity data from the Gravity Recovery and Climate Experiment (GRACE) project are combined with river discharge data to obtain estimates of areally averaged P - ET. After constructing an equation describing the large-scale terrestrial water balance reflecting the temporal sampling of GRACE water storage estimates, GRACE-derived P - ET estimates are compared to those obtained from a reanalysis dataset [NCEP/Department of Energy (DOE) reanalysis-2] and a land surface model driven with observation-based forcing [Global Land Data Assimilation System (GLDAS)/Noah] for two large U.S. river basins. GRACE-derived P - ET compares quite favorably with the reanalysis-2 output, while P - ET from the Noah model shows significant differences. Because the uncertainties in the GRACE results can be computed rigorously, this comparison may be considered as a validation of the models. In addition to showing how GRACE P - ET estimates may be used to validate model output, the accuracy of GRACE estimates of both the seasonal cycle and the monthly averaged rate of P - ET is examined. Finally, the potential for estimating seasonal evapotranspiration is demonstrated by combining GRACE seasonal P - ET estimates with independent estimates of the seasonal cycle of precipitation. [ABSTRACT FROM AUTHOR]

Hydrological model predictions are sensitive to model forcings, input parameters, and the parameterizations of physical processes. Analyses performed for the Variable Infiltration Capacity model show that the resulting moisture fluxes are sensitive to the time step and energy balance closure assumptions. In addition, the model results are sensitive to the method of spatial and temporal disaggregation of precipitation. For parameter estimation purposes, it is desirable to do parameter searches in water balance mode (meaning that the effective surface temperature is assumed equal to the surface air temperature; hence no iteration for energy balance closure is performed) at daily time steps. However, transferring these parameters directly to other model modes (e.g., energy balance, in which an iteration for effective surface temperature is performed, and/or different model time steps) results in changes in the simulated moisture fluxes. The simulated differences in moisture fluxes are mainly a result of the parameterization of evapotranspiration at different time steps and model modes. A simple scheme that calculates correction factors for some model parameters is developed. The scheme is used to match simulated moisture fluxes in hourly and 3-hourly energy balance mode to the daily water balance simulation results, and to match hourly energy balance runs using spatially and temporally disaggregated precipitation to 3-hourly energy balance runs using uniformly disaggregated precipitation. For both approaches, the corrected simulations match the baseline simulations quite closely, both over transects across much of the continental United States and for test applications in the Ohio and Arkansas–Red River basins. [ABSTRACT FROM AUTHOR]