TIME-VARIABLE GRAVITY AND HYDROLOGY
J. Wahr, I. Velicogna, S. Swenson
CIRES and Dept. of Physics, Univ of Colorado, Boulder, CO 80309, USA

P.C.D. Milly
U. S. Geological Survey and Geophysical Fluid Dynamics Laboratory/NOAA 
Princeton, NJ 08542  USA

A.B. Shmakin
Geophysical Fluid Dynamics Laboratory/NOAA Princeton, NJ 08542  USA

Satellite laser ranging measurements have provided information about changes in 
the longest-wavelength components of the earth's gravity field.  These changes 
are likely 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 resolution of the time-variable gravity solutions has not yet 
been sufficient to separate the effects of these various processes.  This situation
 should change dramatically with the launch of CHAMP later this year and of GRACE
 in 2001.  Time-variable gravity measurements from these satellites, particularly
 from GRACE, will be useful for addressing problems in a number of disciplines. 
 Especially promising is the likelihood that GRACE will deliver estimates of 
changes in continental water storage averaged over a few hundred km on a side, 
to accuracies of better than a cm of water every few weeks.  Those measurements 
will be useful for assessing and improving global climate models, for better 
understanding large-scale hydrological processes, and for monitoring the
 distribution of land-based water for agricultural and water resource applications.
  In this talk, we describe methods of extracting the water storage signal from 
the time-variable gravity solutions, and estimate the probable accuracy of the 
results.  For our analysis we construct synthetic gravity data using various types 
of model output.  Our simulated hydrological signal is computed using output from 
a land surface model based on monthly global precipitation records.