# Groundwater Storage Changes in China from Satellite Gravity: An Overview
## 4 Discussion
### Errors in Non-Groundwater Storage Changes  
To obtain GWS changes, other water storage components, simulated by models or measured by observations, need to be removed from GRACE-derived TWS changes. Errors in these components will inevitably result in uncertainties in final GWS estimates. In situ soil moisture monitoring networks are still scarce in most regions of the world. Snow water equivalent products from passive microwave sensors also have large uncertainties [(109)]. Therefore, simulated SMS and SWES from models are generally used to isolate GWS changes from GRACE-derived TWS changes. However, there are inevitable errors in model-simulated water storage, due to uncertainties in input meteorological forcing data and imperfect simulation of models to mimic actual hydrological processes. RESS from lakes, reservoirs, and rivers can be simulated by some hydrological models or monitored by limited ground- and space-based observations [(11; 18; 19; 110)].  
We take SMS changes in the TP from GLDAS LSMs as an example to demonstrate the uncertainties in models. Figure 13 shows the comparisons of long-term SMS rates from four versions of GLDAS. Although these four GLDAS LSMs adopt the same forcing data, there are still significant discrepancies among these model outputs. The magnitudes of SMS rates from MOSAIC are largest among four models; while the estimates from CLM are far less than those from other three models (Figure 13). The significant differences among them in the TP exemplify the uncertainties in GLDAS LSMs due to different parameterization schemes of land surface. Other reasons for large uncertainty in simulated SM over the TP could be the limitation of available ground hydrometeorological observations to constrain models in this region and the complexity of hydrologic processes among surface water, soil water, glaciers, snowpack, groundwater, and permafrost [(111)]. Especially, the TP contains the largest alpine permafrost area in the world [(112)]; while permafrost-related hydrologic processes still fail to be simulated in most global models, which will attribute to large uncertainty in models over the TP. Nearly all GRACE-related mass balance studies on the TP highly reply on hydrological models, only with which other components, e.g., surface water [(113)], glaciers [(80; 114)], and groundwater [(108)], can be disaggregated from GRACE-observed total mass changes. Therefore, there would be potentially large uncertainties in these studies, due to imperfectly simulated hydrologic processes in models over the TP. It remains a challenge to quantifying GWS changes in the TP accurately. Fortunately, most of regions, where groundwater has been depleted significantly due to agriculture irrigation or other anthropogenic activities, are in arid or semi-arid plains. Therefore, non-groundwater storage changes and their errors from models are relatively small in these regions. Thus, GRACE along with ancillary models can still be used to estimate long-term GWS changes accurately in places where the contributions from other mass change components are relatively small.