J. Wahr (CIRES and Dept. of Physics, Univ. of Colorado, Boulder, CO 80309, USA; fax: 1.303.492.7935; e-mail: wahr at lorado.edu)

I. Velicogna (CIRES and Dept. of Physics, Univ. of Colorado, Boulder, CO 80309, USA; fax: 1.303.492.7935; e-mail: isabella at lorado.edu)

P.C.D. Milly (USGS and GFDL/NOAA, Princeton, NJ 08542 USA)

A.B. Shmakin (GFDL/NOAA, Princeton, NJ 08542 USA)

Time-variable gravity changes are caused by a combination of post-glacial-rebound, fluctuations in atmospheric mass, and the redistribution of water, snow, and ice on land and in the ocean. The spatial resolution of gravity data obtained from satellite measurements has not yet been sufficient to separate the effects of these processes. The launch of GRACE in 2001 should provide dramatically improved time-variable gravity measurements. GRACE is expected to deliver estimates of monthly changes in water storage averaged over regions of a few hundred km to accuracies of better than a cm of water. These data will be useful for assessing and improving climate models, for better understanding large-scale hydrological processes, and for monitoring the distribution of land-based water for agricultural and water resource applications. We describe methods of extracting the water storage signal from simulated hydrological gravity solutions constructed using a land surface model based on monthly global precipitation records. Spatial averaging kernels were created to isolate the gravity signal of individual drainage basins without contamination from surrounding hydrological or oceanic gravity signals. We then estimated the probable accuracy of averaging kernels for basins of varying shapes and sizes.