3 Methods
3. 1 Contribution rate of reservoir evaporation
Precipitation in the lake area is generally considered as coming from evaporation and advection water vapor, of which evaporation is an important part of the lake hydrological cycle. The value ofδ 18O and δ D in oceanic water vapor becomes poorer as the air mass moves inland, due to continuous precipitation. The equilibrium fractionation of hydrogen and oxygen stable isotopes is destroyed by the kinetic fractionation of water in the process of evaporation. There is a difference in the relationship between δ D andδ 18O in precipitation, which is called deuterium excess (Dansgaard, 1964). If there is continental water vapor input during this period, the value of deuterium excess in the water vapor will change, and if there is no continental water vapor input, the deuterium excess will remain unchanged (Craig, 1961). The deuterium excess of precipitation formed by surface water evaporation is often higher than the advection water vapor (Gat, 1994). Therefore, the contribution ratio of surface water evaporation to precipitating water vapor can be estimated according to the variation of d in precipitating water vapor (Gat and Matsui, 1991; Vallet-Coulomb et al., 2008; Ingraham and Taylor, 1991).
Assuming that the contribution rate of local evaporation isfev (0<f ev<1), the contribution rate of the water vapor input from the upwind direction to the precipitating water vapor (advected water vapor) is 1-fev . The deuterium excess value of water vapor produced by local evaporation is d ev. The deuterium excess value of the advection water vapor isd adv. The deuterium excess in precipitating water vapor (dpv ) can be expressed in the following formula (Xu et al., 2011).
d pv=d ev×f ev×d adv×(1-f ev)