# Groundwater Storage Changes in China from Satellite Gravity: An Overview
## 3 Results
### Tarm Basin  
Based on the WHYMAP delineation of world's largest aquifer systems, the Tarim Basin aquifer is defined as the plain area of Tarim River Basin (TRB), with an area of 450,000 km\({}^{2}\) (Figure 1). In addition to the plain area, the entire TRB also includes the southern foot of the Tien Shan Mountains, eastern foot of the Pamir Mountains, and northern foot of the Kunlun Mountains (Figure 9), where melting glaciers and snowpack are primary contributors to streamflow and groundwater recharge over the TRB [96; 97]. As the largest endorheic basin in China, the TRB is also known as the driest region on the Eurasian landmass with the Taklamakan Desert in its center occupying 33% of the TRB's area. The average annual precipitation in the TRB is about 120 mm [98]; while the average annual potential evapotranspiration reaches 1000 mm [99]. Due to increasing temperature in the TRB since  
Figure 8: Groundwater level rates based on ground monitoring wells in the LRB and its surroundings during 2005–2013. the 1970s [98; 99], glacier retreat and enhanced snowmelt in the mountain regions of TRB, especially in the Tien Shan Mountains, lead to significant seasonal changes of glacier-fed runoff in the TRB [96]. Although the TRB is extremely arid, GRACE satellites still provide useful information on TWS changes in the basin [34; 100; 101]. Based on GRACE data, [PERSON] et al. [101] found an increasing TWS rate of 1.6 \(\pm\) 0.6 mm/yr (1.7 \(\pm\) 0.6 km\({}^{3}\)/yr) over the TRB during 2003-2011. However, [PERSON] et al. [34] and [PERSON] et al. [100] showed a negative GRACE-derived TWS rate of \(-\)1.6 \(\pm\) 1.1 mm/yr and \(-\)1.4 \(\pm\) 0.5 mm/yr during 2002-2015, respectively. The discrepancy between the results in the first study and the following two studies can be attributed to the different time spans used in these studies, considering the significant interannual variability in TWS from 2002 to 2015 in the TRB [34; 100]. It should be noted that only gridded Level-3 GRACE products were used in these studies, without considering the leakage errors in the products. Although gridded scaling factors were used in their studies to correct leakage errors and signal attenuation in estimating TWS changes, there are large uncertainties in these scaling factors [53; 102]. Because the scaling factors were calculated based on hydrological models, which generally fail to simulate actual TWS changes accurately especially in arid and semiarid regions [102]. Take the TRB as an example, the glacial meltwater from surrounding mountains has a decisive effect on river runoff and water availability over the basin [96]. However, glacier dynamics cannot be included in most models. It will result in large uncertainties in scaling factor estimates, which are calculated based on these models. As shown in Figure 9, there is no significant or undetectable GWS rate in the TRB simulated by WGHM or PCR-GLOBWB. However, GRACE detects a significant mass increase in the Kunlun Mountains and mass loss in the Tien Shan Mountains. It is worth noting that only changes in soil moisture and snow were simulated from models and subtracted from GRACE-derived TWS changes  
Figure 9: (**a**) Topography of the TRB and its surroundings. The red boundary outlines the plain region of TRB. The blue line shows the boundary of the TRB, same as Figure 1. Locations of two cities where well observations are available, i.e., Korla and Kashgar, are also shown. GWS rates from (**b**) GRACE, (**c**) WGHM and (**d**) PCR-GLOBWB in the TRB and its surroundings during 2002–2014. to obtain GWS changes. Thus, trend signals from other TWS compartments, e.g., glacier and surface water, might be non-negligible and included in GWS rates from GRACE. For the TBR, the mass loss to the north of TBR shown in Figure 9b is confirmed to be attributed to glacier mass loss in the Tien Shan Mountains [78; 79]; while the mass increase to the south of TBR can be explained by lake mass increase in the TP [103].