Δ18OLW is nearly always less enriched
than Δ18Oe, meaning that leaf water is
not a well-mixed pool undergoing evaporative enrichment, so two models
were developed to explain Δ18OLW based
on its isotopic sources: the two-pool model (Eqn. 1) and the Péclet
effect model (Roden & Ehleringer 1999;
Barbour & Farquhar 2000;
Helliker & Ehleringer 2000;
Gan et al. 2003;
Barbour 2007). The Péclet effect model,
instead of the fraction of water contributed by the sites of evaporation
(α in Eqn. 3) being fixed, it was modified to be dependent on the Péclet
number (P; Eqn. 7). The Péclet number