Monitoring Groundwater Storage Change From Space

Abstract

As a major component of the global water cycle, groundwater (underground water) includes water present beneath Earth's surface within pore spaces in soil and deeper sediments and in fractured formations in the saturated zone. Groundwater storage change is mainly controlled by a balance between discharge and recharge through infiltration from soil or seepage from surface reservoirs. Groundwater storage change can be monitored by water level measurements in wells in combination with geological information describing the ability of the saturated zone to store and release water. However lack of adequate in situ observations has prevented accurate estimates of groundwater storage change in most regions of the world. Only a few well-developed countries have both networks of measured wells, and sufficient knowledge of soil and rock material properties. Satellite gravity measurements from the Gravity Recovery and Climate Experiment (GRACE) and its follow-up mission GRACE Follow-On (GRACE-FO) provide an alternative method for estimating groundwater storage change. Since the launch of GRACE in March 2002, GRACE and GRACE-FO have measured changes in Earth's gravity on a monthly basis for over 20 years with an accuracy sufficient to detect a centimeter of water equivalent water storage change at a spatial resolution of a few hundred kilometers. This accuracy is comparable to or better than estimates from well level measurements, so satellite gravimetry provides a complementary spatial average when both measurement types are available. GRACE and GRACE-FO estimates of storage change require that other causes of gravity change be removed. Satellite gravity measurements have been successfully used in studies that span the globe, including Northwest India, California's Central Valley, the High Plains Aquifer in the US, the North China Plain, various regions in the Middle East, and Australia's southern Murray-Darling Basin.