# Groundwater Storage Changes in China from Satellite Gravity: An Overview
## 5 Conclusions  
GRACE satellites have revolutionized our understanding of the Earth's gravity field changes, and the inferred mass transport and redistribution. From a hydrological perspective, GRACE provides a novel measure of water storage/flux changes from space, and brings new insights into global/regional water cycling and climate change research. As an important component of terrestrial water, groundwater is poorly monitored by ground observations at regional and global scales. By inferring column-integrated water storage changes from time-variable gravity fields observed by twin satellites, GRACE mission provides a new dimension for measuring GWS changes from space. Among three main aquifer systems in China, GRACE detects significant GWS depletion in the NCP, with a rate of 7.2 \(\pm\) 1.1 km\({}^{3}\)/yr over the period 2003-2014, consistent with the estimate from ground well observations. Compared to GRACE and ground observations, two global HMs (WGHM and PCR-GLOWB) also captured the GWS depletion signal in this region, but with overestimated rates. Long-term groundwater decline in the NCP can be attributed to a decade-long drought period and intensive groundwater-based irrigation in this region. Ground well observations also highlight the heterogeneous contributions of GWS depletion from different aquifers. In the piedmont region of the Taihang Mountains, groundwater depletion mainly occurred in unconfined aquifers; while groundwater from confined aquifers was depleted over the whole NCP. For the LRB, significant interannual fluctuations in GWS were detected by GRACE, consistent with annual precipitation anomalies in the region. GRACE-derived GWS in the LRB declined at a rate of 5.0 \(\pm\) 1.2 km\({}^{3}\)/yr from 2005 to 2009, and recovered slowly since 2010, agreeing remarkably with ground well observations. However, GWS decline rates simulated from WGHM and PCR-GLOWB are underestimated over the period 2005-2009. Ground well observations highlight that the depletion mainly occurred in the western part of LRB. For the Tarim Basin, in the spatial domain, there is no significant GWS depletion signal within the basin based on GRACE. In addition, there exists significant long-term non-groundwater mass loss/gain signal on the north/south of the basin. Limited ground well  
Figure 13: SMS rates from GLDAS LSMs during 2002–2014 with the versions of (**a**) NOAH, (**b**) VIC, (**c**) MOSAIC, and (**d**) CLM. observations show groundwater level declines on the north of the basin, consistent with slowly declining time series in GWS from GRACE. GRACE-based GWS change estimation still contains large uncertainties due to the errors in both GRACE-derived TWS changes and non-groundwater storage changes from models. Any uncertainty in model-simulated storage changes, especially in soil moisture, will result in uncertainties in final GWS estimates. Additionally, potential significant contributions from surface water and glaciers, which are not considered in most LSMs, might cause biases in final GWS estimates too. Another challenge is the errors in GRACE-derived TWS changes. At present, the original GRACE Level-2 products are still dominated by correlated errors; thus, post-processing is needed to suppress the noises, which in turn has resulted in signal attenuation. Current spatial resolution of GRACE is limited to \(\sim\)300 km or longer, due primarily to intrinsic measurement errors in satellite payloads and the orbit configuration of satellites. Coarse resolution of GRACE also limits its applications in groundwater hydrology [115]. GRACE has ended science operations in September 2017, after more than 15 productive years in orbit [116]. Its successor mission, GRACE Follow-On (GFO), is scheduled to be launched in May 2018 to continue GRACE's climate data record. A new payload, i.e., the laser-ranging interferometer (LRI), will be on the GFO satellites as an experimental instrument, and on the future Next Generation Gravity Mission (NGGM) satellites.