GRACE Satellite-Based Monitoring of Groundwater Storage Variability

Some of the world's important aquifers are being depleted due to over-exploitation, but a lack of ground based data has impeded efforts to quantify current rates of depletion and to identify what rates of withdrawal would be sustainable.  The Gravity Recovery and Climate Experiment (GRACE) satellite mission is a valuable new tool for water resources monitoring.  GRACE launched in 2002, and provides monthly maps of variations in total terrestrial water storage (including groundwater) based on highly precise measurements of Earth's time varying gravity field.  In order to apply GRACE observations for groundwater monitoring, it is necessary to combine them with auxiliary information in order to isolate groundwater from the other components of terrestrial water storage.  A simple approach is to assume terrestrial water storage is the sum of groundwater, soil moisture, surface water, and snow water storage, obtain modeled or observation-based estimates of those components, and compute groundwater storage variations as a residual.  A more sophisticated approach is to synthesize GRACE and other observations within a physically based numerical model of land surface hydrology using data assimilation.  Results and applications of both approaches will be described.