A Graphical Interpretation of the Rescaled Complementary Relationship for Evapotranspiration

Wet-surface evaporation equations related to the Penman equation can be represented graphically on vapor pressure (e) versus temperature (T) graphs (Qualls & Crago, 2020, ). Here, actual regional evaporation is represented graphically on (e, T) graphs using the Complementary Relationship (CR) between actual and apparent potential evaporation. The CR proposed by the authors can be represented in a simple and intuitive geometric form, in which lines representing the regional latent heat flux, LE, and the wet surface (Priestley & Taylor, 1972, ) evaporation rate, LEPT, intersect at e = 0. This approach allows a graphical estimate of LE (or the corresponding mathematical formulation), provided available energy, wind speed, air temperature and humidity, and roughness lengths for momentum and sensible heat are available. The wet surface temperature is needed, and a calculation method for it is provided. The formulation works well using monthly data from seven sites in Australia, even when the same value of the Priestley & Taylor parameter alpha is used for all sites. Overall, compared to eddy covariance measurements, root mean square difference averaged 19 W m(-2); this compares favorably with the CR formulation proposed by Brutsaert (2015, ).