Abstract
The Gravity Recovery and Climate Experiment (GRACE) satellite mission, which was in operation from March 2002 to June 2017, was the first remote sensing mission to provide temporal variations of Terrestrial Water Storage (TWS), which is the sum of the water masses that were contained in the soil column (i.e., snow, surface water, soil moisture, and groundwater), at a spatial resolution of a few hundred kilometers. As in situ level measurements are generally not sufficiently available for monitoring groundwater changes at the regional-scale, this unique dataset, combined with external information, is widely used to quantify the interannual variations of groundwater storage in the world’s major aquifers. GRACE-based groundwater changes revealed significant aquifer depletion over large regions, such as the Middle East, the northwest India aquifer, the North China Plain aquifer, the Murray-Darling Basin in Australia, the High Plains, and the California Central Valley aquifers in the United States of America (USA), but were also used to estimate groundwater-related parameters such as the specific yield, which relates groundwater level to storage, or to define the indices of groundwater depletion and stress. In this review, the approaches used for estimating groundwater storage variations are presented along with the main applications of GRACE data for groundwater monitoring. Issues that were related to the use of GRACE-based TWS are also addressed.
Highlights
Water volume stored in the aquifers is estimated to be about 23.4 × 106 km3, i.e., around half of the fresh water that is only about 2.5% of the total water present on Earth [1]
If a dense soil moisture network exists in areas that are larger than several hundred square kilometers as Illinois or the region of the High Plains aquifer in the United States of America (USA), providing information on the water contained in the soil at several depths, groundwater storage changes can be inferred removing the anomaly of soil water content to the anomaly of Terrestrial Water Storage (TWS) that was derived from Gravity Recovery and Climate Experiment (GRACE) [49,50,51]
GW storage that was obtained by the difference between GRACE-based TWS average of Center for Space Research (CSR), GFZ, and Jet Propulsion Laboratory (JPL), SW storage that was derived from the multi-satellite observations [101], and soil moisture (SM) from the average of the outputs from Land Dynamics (LaD) [102] and WaterGAP Global Hydrology Model (WGHM) between January 2003 and December 2004, were compared to in situ wells levels and surface water levels derived from radar altimetry
Summary
Water volume stored in the aquifers is estimated to be about 23.4 × 106 km, i.e., around half of the fresh water that is only about 2.5% of the total water present on Earth [1]. A global groundwater depletion of 4500 km (or 41.4 km3·year−1) was estimated between 1900 and 2008 [11], which represents a major threat to global water security, potentially causing a decline in agricultural productivity and energy production It might result in conflicts around the world [12]. Numerical simulations that were based on climate change scenarios, and taking population density into account, revealed that between 15% and 20% of the world population will be affected by groundwater decreases by the year 2050 [16] In this context, long-term reliable, compact and frequent observations of the aquifer levels are necessary to monitor groundwater storage and its changes over time. The discussion section will present the current limitations of the GRACE-derived GW changes, and which improvements can be expected with the future launches of the GRACE Follow-On (GRACE FO) and the Surface Water and Ocean Topography (SWOT) missions in 2018 and 2021, respectively
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