# Groundwater Storage Changes in China from Satellite Gravity: An Overview
## 3 Results
### North China Plain  
The NCP, also known as the Huang-Huai-Hai Plain, is the largest alluvial plain and the most important agriculture base in China (Figure 1). The NCP covers an area of 320,000 km\({}^{2}\) with a population of about 300 million. The narrow definition of the NCP, which is more widely used, refer to the plain area between the Taihang Mountains in the northwest and the Yellow River in the southeast, with an area of 140,000 km\({}^{2}\) (Figure 3a). Hereinafter, we will adopt this narrow definition of the NCP. From a hydrogeological perspective, the NCP can be divided into three distinct hydrogeological zones: i.e., the piedmont region of the Taihang Mountains, central plain, and coastal plain [82; 83], which include all the plains in Beijing and Tianjin municipalities, Hebei province, and northern parts of plains in Henan province and Shandong province. Due to long-term warm and dry climate and intensive anthropogenic activities (agriculture, industry, and urbanization), the groundwater in the NCP has been over-exploited since the 1970s [24]. More than 70% of groundwater exploitation is used for irrigation in the NCP [84; 85]. Based on groundwater well observations, some studies have reported long-term groundwater level declines and GWS depletion in some parts of the NCP [24; 83; 86]. Figure 2: GWS rates over China from (**a**) GRACE; (**b**) WGHM; and (**c**) PCR-GLOBWB during 2002–2014. Blue lines highlight the three main aquifer basins in China. [PERSON] et al. [25] found the large-scale mass loss in the NCP using GRACE satellites, with a rate of 2.4 cm/yr from 2003 to 2007. Similar mass loss signal in this region was also revealed by other studies [35; 36]. Further, [PERSON] et al. [22] concluded that the majority of the mass loss is mainly caused by groundwater depletion in the NCP and found a GWS depletion rate of 8.3 \(\pm\) 1.1 km\({}^{3}\)/yr (2.2 \(\pm\) 0.3 cm/yr) from 2003 to 2010. Since the GWS depletion rate in shallow aquifers from the government reports is only \(\sim\)2.5 km\({}^{3}\)/yr in the same time period, far less than the GRACE estimate, [PERSON] et al. [22] concluded that the difference between the estimates from GRACE and official reports indicate the great contribution of GWS depletion from deep aquifers in the NCP. Further study by [PERSON] et al. [43] revealed the heterogeneous GWS change features in the NCP, i.e., faster depletion in the piedmont region of the NCP. As shown in Figure 3b,d, GRACE-derived GWS rates show the GWS loss in the NCP with relatively coarse spatial resolution and damped magnitude compared to the results from WGHM and PCR-GLOBWB. At a spatial resolution of 0.5 degrees, GWS rates from WGHM and PCR-GLOBWB  
Figure 3: (**a**) Topography of the North China. The black boundary outlines the NCP. The red boundary shows the study region including the whole NCP and mountains in Hebei province and Shanxi province. The white line shows the boundary of the North China Aquifer System, same as Figure 1. The blue line shows the Yellow River. GWS rates from (**b**) GRACE; (**c**) WGHM; and (**d**) PCR-GLOBWB in the North China during 2002–2014. further highlight the significant GWS depletion in the NCP, consistent with some regional groundwater models [87; 88]. Figure 4a,b show groundwater level rates from 2005 to 2013 based on ground monitoring wells in the NCP from unconfined and confined aquifers, respectively. Significant groundwater declines in shallow unconfined aquifers are located in the piedmont region of the Taihang Mountains (Figure 4a); while the groundwater depletion in confined aquifers occurs over the whole plain (Figure 4b). For shallow unconfined aquifers, GWS changes can be estimated by multiplying phreatic groundwater levels with specific yields. Based on the mean groundwater level changes in the NCP from MWR and the mean specific yield of 0.06, the GWS rate in the NCP is \(-\)1.5 km\({}^{3}\)/yr during 2002-2014. For deep confined aquifers, however, GWS changes contain elastic and inelastic components [89].