Extended Data Fig. 1: Illustration of atmospheric and surface constraints on ET and PET (PETe and PETa).

a, ET is constrained by the net radiation (\({R}_{n}\)) and surface moisture availability over a dry surface. b, PETa is constrained by aerodynamic conditions, assuming unlimited water and energy supply. The dry surface in a is hypothetically converted to a wet surface but remains decoupled from the atmosphere, meaning that surface temperature (\({T}_{s}\)), air temperature (\({T}_{a}\)), and vapor pressure (\({e}_{a}\)) remain unchanged from the dry surface. The only change is that the vapor pressure at the evaporative surface becomes saturated (\({e}_{s}^{* }\) replaces unsaturated \({e}_{s}\)). Consequently, sensible heat (\(H\)) over the hypothetical wet surface remains unchanged compared to the dry surface, but the latent heat (\(\lambda {PE}{T}_{a}\)) is much larger than \(\lambda {ET}\). c, PETe is constrained by the net radiation (\({R}_{n}\)), when the entire dry surface becomes saturated and fully coupled with the atmosphere. Both surface and atmospheric conditions cool down (\({T}_{s}^{{\prime} }\) and \({T}_{a}^{{\prime} }\)) and become more humid (\({e}_{s}^{* {\prime} }\) and \({e}_{a}^{{\prime} }\)) relative to the dry environment. As a result, the latent heat (\(\lambda {PE}{T}_{e}\)) increases, while the sensible heat (\({H}_{w}\)) decreases, all constrained by \({R}_{n}\).