Groundwater Storage Monitoring 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 plays a key role in connections between the global hydrological cycle and climate change and is a vital resource in many parts of the world. Groundwater storage (GWS) change is mainly controlled by a balance between discharge (including extraction by pumping for agricultural and domestic consumption) and recharge through infiltration from soil or seepage from surface reservoirs. GWS 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 GWS 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) satellites provide an alternative method for estimating GWS change. Since its launch in March 2002, GRACE has measured changes in Earth's gravity on a monthly basis for over 13years with an accuracy sufficient to detect a centimeter of water equivalent water storage change at a spatial resolution of a few 100km. This accuracy is comparable to or better than estimates from well level measurements, so GRACE provides a complementary spatial average when both measurement types are available. GRACE estimates of storage change require that other causes of gravity change be removed, usually via hydrologic and similar geophysical models. GRACE 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 United States, the North China Plain, various regions in the Middle East, and Australia's southern Murray–Darling Basin. This chapter starts with an overall introduction of global groundwater storage changes, observations and challenges (11.1), followed by a brief introduction to groundwater and its role in the global water cycle (11.2), and in situ groundwater observations and limitations (11.3). Detailed descriptions of groundwater monitoring by satellite gravimetry and related data processing methods are described in 11.4, followed by in-depth discussions of major error sources and issues in GRACE-based groundwater storage estimation and how people should address them in 11.5. 11.6 provides a comprehensive review of the progress in groundwater monitoring by GRACE satellite gravimetry using some case studies of regional long-term groundwater storage changes over the world. A summary is provided at the end in 11.7.