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).