# Groundwater Storage Changes in China from Satellite Gravity: An Overview
## 3 Results
### North China Plain
Elastic GWS changes can be estimated by multiplying water levels in confined aquifers with elastic storage coefficients. The mean elastic storage coefficient of deep confined aquifers in the NCP is 0.00125, which is about 2% of the mean specific yield of shallow unconfined aquifers [22; 87]. Thus, the elastic GWS changes in the NCP is negligible. The other GWS change component, i.e., inelastic GWS changes, represents the irreversible water storage released due to inelastic compaction of the aquifer system [89]. Thus, land subsidence volume represents the change in water storage in the aquifer system [89; 90]. Based on the GPS observation network in the NCP, the surface vertical deformation in volume is \(-\)6.3 km\({}^{3}\)/yr from 2002 to 2014 [91]. In Figure 5a, the time series of GWS changes in the NCP from in situ observations represent a sum of GWS changes in unconfined aquifers from monitoring wells and the long-term trend in GWS in confined aquifers from GPS observations. As shown in Figure 5a, GWS changes in the NCP during 2002-2014 from WGHM and PCR-GLOBWB mainly show a linear decline, at a rate of 12.8 \(\pm\) 0.2 km\({}^{3}\)/yr and 9.7 \(\pm\) 0.2 km\({}^{3}\)/yr, respectively. GWS depletion rate in the NCP from GRACE is 7.2 \(\pm\) 1.1 km\({}^{3}\)/yr for the same time period, which agrees with the result from in situ observations (i.e., 7.8 km\({}^{3}\)/yr) better than those from WGHM and PCR-GLOBWB. Good agreement between space- and ground-based observations indicates that global hydrological models prone to overestimate the GWS depletion signal in the NCP. For example, [PERSON] et al. [11] also found that WGHM 2.2a overestimates the GWS depletion rate in the NCP in the 2000s (i.e., \(\sim\)20 km\({}^{3}\)/yr) by approximately a factor of 4, compared with the independent estimates from GRACE and from regional groundwater models. [PERSON] et al. [11] attribute this overestimation of  
Figure 4: Groundwater level rates based on ground monitoring wells from (**a**) unconfined and (**b**) confined aquifers in the North China during 2005–2013. GWS depletion rate to the strong underestimation of diffuse groundwater recharge in the NCP from WGHM. The updated WGHM 2.2b model, however, improves the simulation of GWS depletion signal in the NCP significantly, although the overestimation still exists. In addition to linear negative trend signal, GWS changes from in situ observations show significant seasonal variability and interannual fluctuations. GRACE-derived GWS changes agree well with those from in situ observations at seasonal and interannual timescales. Both of them reach maxima in early February. Interannual fluctuations in GWS in the NCP from GRACE and in situ observations show favorable agreement with annual precipitation anomalies. As shown in Figure 5, GRACE-derived GWS recovery for 2003-2004 and 2012-2013 is consistent with positive precipitation anomalies in these years. In contrast, accelerated GWS declines in 2002, 2006 and 2014 inferred from in situ observations and from GRACE can be attributed to the strong thoughts in these years (Figure 5). The values of PDSI in the NCP are generally negative since 2002, which indicate a long-term drying climate in the past decade (Figure 5b). This continuous drought condition in the NCP agrees well with the long-term GWS depletion signal inferred from GRACE, ground observations, and hydrological models. Temporal variability in PDSI, similar to annual precipitation anomalies, is also consistent with interannual GWS changes from GRACE and ground observations (Figure 5).