Water at the sites of evaporation is assumed to follow the Craig-Gordon model, so Δ18Oe can be derived from equilibrium fractionation (ε+), the ambient to intercellular air vapor mole fraction (e a/e i), the kinetic fractionation (εk), and air vapor enrichment (Δ18Ov; Eqn. 4) (Craig & Gordon 1965; Dongmann et al. 1974; Flanagan et al. 1991).