Evaluating the Complementary Relationship for Estimating Evapotranspiration from Arid Shrublands

Given increasing demands on finite water supplies, accurate estimates of evapotranspiration (LE) from arid shrublands of the Southwest U.S. are needed to develop or refine basin water budgets. In this work, an approach to estimate the equilibrium (or wet environment) surface temperature (Te) and LE from regionally extensive phreatophyte shrublands is tested using complementary theory and micrometeorological data collected from five eddy correlation stations located in eastern Nevada. A symmetric complementary relationship between the potential LE (LEp) and actual LE is extremely attractive because it is based on general feedback mechanisms where detailed knowledge of the complex processes and interactions between soil, vegetation, and the near surface boundary layer can be avoided. Analysis of computed LEp and eddy correlation derived LE indicates that there is unequivocal evidence of a complementary relationship between LEp and LE, where the measured and normalized complementary relationship is symmetric when Te is utilized to compute the wet environment LE (LEw). Application of a modified Brutsaert and Stricker Advection-Aridity (AA) model, where Te is utilized to compute LEw as opposed to the measured air temperature, indicates an improvement in prediction accuracy over the standard AA model. Monthly and annual predictions of LE using the modified AA model are within the uncertainty of the measurement accuracy, making the application of this approach potentially useful for estimating regional LE in arid shrubland environments. Our observational evidence supports the idea of a symmetric complementary relationship.