The Gravity Recovery and Climate Experiment (GRACE) mission has demonstrated the ability to synoptically monitor the temporal and spatial variations in total water content over continental areas, providing a key measurement which can improve our knowledge and understanding of the water cycle. Changes in terrestrial water storage especially with regard to ground water, are poorly known and sparsely sampled. These essential components of continental hydrology have been inaccessible to any form of synoptic remote sensing until recently. The GRACE mission provides the means to remotely sense gravity changes at regional scales. Through a unique and well tested solution formulation, our group proposes to provide sub-monthly measures of the change in continental water storage based on the GRACE intersatellite range-rate measurements. These anomalies will have a resolution of 4ºx4º and will be provided every 10 days over the period of the GRACE mission.
To date, GRACE has principally been used improve both stationary and monthly-resolved spherical harmonic models of the gravity field. Monthly harmonic models have been used as a proof of concept to demonstrate the resolving power of GRACE to monitor mass flux globally [Wahr et al., 2004; Tapley et al., 2004]. Although monthly gravity models have produced intriguing results, information at submonthly time scales is lost and spatial and temporal aliasing through the estimation of static global parameters has been a major stumbling block for the exploitation of the GRACE data. However, by using solutions restricted to local regions as an alternative and through the application of accurate forward models (to eliminate tidal and atmospheric pressure mass flux signals), we have developed a highly accurate technique for monitoring continental water mass storage. Our mass concentration (mascon) representation largely mitigates the spatial and temporal aliasing problems encountered with monthly GRACE solutions using Stokes coefficients.
We have produced 10-day estimates of 4ºx4º mass flux anomalies (Rowlands et al., 2005). Upon eliminating atmospheric and tidal mass variations, detailed understanding of terrestrial water storage at scales comparable to medium to large aquifers, can be directly monitored. These estimates of terrestrial water storage are valuable for predicting biological and agricultural productivity, flooding, and the level of sustainability or depletion of ground water systems. These mass flux estimates will fill a critical gap in achieving a more complete understanding of the Earth's hydrological system. These mass anomalies will be provided along with comparable hydrological estimates (from Global Land Data Assimilation System, Rodell et al., 2004).