# Groundwater Storage Changes in China from Satellite Gravity: An Overview
## 1 Introduction
GRACE-derived TWS changes can be further used to infer GWS changes, if other water storage components from soil moisture, snow/ice, and surface water can be accurately estimated from models or observations and removed from TWS changes [14]. Thus, GRACE represents a powerful tool to estimate GWS changes, especially in regions where groundwater depletion is significant but groundwater levels are unmonitored or poorly monitored. Many studies have successfully used GRACE data to estimate GWS changes especially its depletion in many regions, e.g., in India [15; 16; 17], California's Central Valley, USA [18; 19], High Plains, USA [20; 21] and in the North China Plain (NCP) [22]. As the largest developing country, China has been increasingly experiencing severe water scarcity with rapid population growth and socio-economic development. Especially in the North China, due to drying climate and intensive human activities in the past four decades, groundwater is used and even over-exploited to meet ever-expanding water demand [23; 24]. GRACE data have been widely used to estimate TWS/GWS changes over China. [PERSON] et al. [25] was one of the first to assess long-term TWS changes over the whole China using GRACE. Based on GRACE observations, they found TWS declines in the NCP and the southern Tibetan Plateau (TP) and TWS increase in the Three Gorges Reservoir (TGR) and the Inner TP (ITP) during 2003-2007 [25]. With the increase of GRACE observations over time, seasonal, interannual, and long-term TWS variations in China at basin scales were further analyzed using GRACE [26; 27]. Later, numerous case studies have shown that GRACE can be used to detect spatio-temporal TWS/GWS changes and anomalies in many regions of China, which are related to climate extremes and human activities. For example, GRACE successfully detectssignificant TWS deficits in the Southwestern China during the severe droughts in 2010 and 2011 [28; 29]. Similar water storage deficits during droughts are also captured by GRACE in the Yangtze River Basin (YRB) [30; 31; 32], the Northwest China [33; 34], and the North China [35; 36]. In addition, considering the water fluxes from a hydrological point of view, GRACE is also used to estimate river discharge in the YRB and human-induced evapotranspiration (ET) change in the NCP [37; 38]. By integrating GRACE data into hydrological models, [PERSON] et al. [39] found human-induced TWS increase in the middle and lower reaches of YRB due to intensive irrigation and reservoir operation. Based on GRACE data, some other studies also found human-induced increment in TWS due to water impoundment of TGR in the middle of the Yangtze River [27; 40]. In addition to surface water regulation, another potentially significant anthropogenic impact on TWS changes can be groundwater pumping especially in the arid/semiarid North China. Extensive groundwater pumping could lead to irreversible groundwater depletion, which might be detectable for GRACE. In the NCP, [PERSON] et al. [22] found a GWS depletion rate of 8.3 \(\pm\) 1.1 km\({}^{3}\)/yr from 2003 to 2010 using GRACE, which can be attributed to intensive groundwater-based irrigation. Since then, several studies reported GRACE-based GWS changes in some regions of China [41; 42; 43; 44; 45; 46]. According to the World-wide Hydrogeological Mapping and Assessment Programme (WHYMAP), China has three of the world's thirty-seven large aquifer systems [47], i.e., the North China Aquifer System, the Song-Liao Basin, and the Tarim Basin (Figure 1). The diversity in climate conditions and human activity intensity in these three aquifer systems will result in different GWS changes. Although prior studies have reported GWS changes in some of these aquifers, earlier results did not provide a thorough or comprehensive characterization, nor an elaborate error assessment on the contemporary, i.e., last decade and a half, GWS changes in these aquifers within China.