The maps are based on data from NASA's Gravity Recovery and Climate Experiment (GRACE) satellites, which detect small changes in the Earth's gravity field caused by the redistribution of water on and beneath the land surface. The paired satellites travel about 137 miles (220 km) apart and record small changes in the distance separating them as they encounter variations in the Earth's gravitational field.
To make the maps, scientists use a sophisticated computer model that combines measurements of water storage from GRACE with a long-term meteorological dataset to generate a continuous record of soil moisture and groundwater that stretches back to 1948. The meteorological data includes precipitation, temperature, solar radiation and other ground- and space-based measurements.
The maps are meant to depict drought associated with climatic variability, as opposed to depletion of aquifers due to groundwater withdrawals that exceed recharge. There are several aquifers in the U.S. that have been depleted in that way over the past century, such as the southern half of the High Plains aquifer in the central U.S. If the groundwater drought indicator map accounted for human-induced depletion, such regions would be red all the time, which would not be useful for evaluating current wetness conditions relative to previous conditions. On time scales of weeks to ten years, we expect that our maps will be reasonably well correlated with measured water table variations over spatial scales of 25 km (16 miles) or more. However, users should not assume a direct correspondence between these groundwater percentiles and measured groundwater levels over multiple decades.
The color-coded maps show how much water is stored now as a probability of occurrence in the record from 1948 to the present.
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. (2012) provide complete details on GRACE-based drought indicator product generation.
Houborg, R., M. Rodell, B. Li, R. Reichle, and B. Zaitchik, Drought indicators based on model assimilated GRACE terrestrial water storage observations, Wat. Resour. Res., 48, W07525, doi:10.1029/2011WR011291, 2012.
Zaitchik, B.F., M. Rodell, and R.H. Reichle, Assimilation of GRACE terrestrial water storage data into a land surface model: results for the Mississippi River Basin, J. Hydrometeor., 9 (3), 535-548, doi:10.1175/2007JHM951.1, 2008.
For further information please contact Matthew.Rodell-at-nasa.gov