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)