The technical description is from Groundwater and Soil Moisture Conditions from GRACE Data Assimilation, one of the sets of maps available at the National Drought Mitigation Center at the University of Nebraska – Lincoln.
Launched in 2002, NASA’s Gravity Recovery and Climate Experiment satellites are unique in their ability to measure variations in water stored at all levels above and within the land surface (terrestrial water storage). However, the spatial (>150,000 km2) and temporal (monthly with a significant time lag) resolutions of the GRACE fields limit their direct applicability for drought assessment. In order to increase the resolution, eliminate the time lag, and isolate groundwater and other components from total terrestrial water storage, scientists at NASA/GSFC integrate the GRACE data with other ground- and space-based meteorological observations (precipitation, solar radiation, etc.) within the Catchment Land Surface Model, using Ensemble Kalman smoother type data assimilation (Zaitchik et al., 2008). The resulting fields of soil moisture and groundwater storage variations are then used to generate drought indicators based on the cumulative distribution function of wetness conditions during 1948-2009 simulated by the Catchment model. Houborg et al. (2011) provide complete details on GRACE-based drought indicator product generation.
Got it? This map shows groundwater storage estimated with this model, showing the extensive drought conditions in Texas as well as the wet northeast.