4.1 Isotope characteristic of precipitation
The δ18O values of precipitation ranged from -30.54 to -0.76‰ (mean -9.32‰), and the δ2H values ranged from -222.63 to -0.33‰ (mean -61.73‰) (Fig.3). The values fell within the ranges reported previously for global (-50 - +10‰ for δ18O, -350 - +50‰ for δ2H, IAEA, 2001) and China (-35.5 - +2.5‰ for δ18O, -280.0 - +24.0‰ for δ2H, Tian et al., 2001). The Local Meteoric Water Line (LMWL) was simulated by using the δ18O and δ2H content of precipitation (Fig. 3): δ2H = 7.80 δ18O+10.98 VSMOW (n = 104, R = 0.985). The slope of LMWL was similar to the slopes of meteoric water lines for Lasa (7.90; Tian et al., 2001), northwestern China (7.05; Liu et al., 2008) and western China (7.56; Ma et al., 2009). All slopes were less than 8, indicating that some non-equilibrium evaporation processes occurred as raindrops falling below the cloud base (Friedman et al., 1962; Dansgaard, 1964; Araguás-Araguás et al., 1998; Cui & Li, 2015a; Zhang &Wang, 2016). The LMWL was slightly above the GMWL due to the high d-excess value of precipitation in the Qinghai Lake Basin, suggesting that some continental moisture recycled to precipitation under low relative humidity conditions (Clark & Fritz, 1997; Pang et al., 2011; Kong et al., 2013; Pang et al., 2017).