2. Study area
The Qinghai Lake Basin (36°15′-38°20′N, 97°50′-101°20′E), a closed basin with an area of 29661 km2, lies in the cold and semiarid region of China’s NE Qinghai-Tibet Plateau (Fig. 1). It also lies in a critical transitional zone where the Southeast Asian Monsoon (SEAM), the Westerly Circulation (WC) and the Qinghai-Tibet Plateau Monsoon meet (Cui & Li., 2015a; Li et al., 2018). The area and water surface altitude of Qinghai Lake were 4425 km2and 3195.82 m above sea level, respectively. The lake water has a salinity of 15.5 g/L and a pH of 9.06. Its water chemistry is characterised by ion proportions of Na+>Mg2+>K+>Ca2+and Cl->SO42->CO32->NO3-(Sun et al., 1991). Around the lake, the average annual air temperature is -0.1 ºC, and the average annual precipitation is 357 mm (Cui & Li., 2015a). There are more than 50 rivers or streams flowing into Qinghai Lake (LZBCAS, 1994). The river system is unevenly distributed, being more developed in the west and north, and less in the east and south. River runoff is mainly sourced from the Buha, Shaliu, and Haergai Rivers, accounting for more than 75% of the total runoff into the lake.
The groundwater aquifers in the regions of late Palaeozoic marine limestone and sandstones, Silurian sandstone and schist, Triassic granite, and Quaternary deposits, are primarily carbonate aquifers, clastic rock aquifers, intrusive rock aquifers, and alluvial aquifers, respectively (Fig. 2; Cui & Li., 2014). The buried depth of groundwater around Qinghai Lake usually varies between 4 and 7 m. In the sparsely populated region, the buried depth is approximately 3 or 4 m, which is shallower than the water table in the densely populated region (Xiao et al., 2012).