A Penman for All Seasons

Abstract

Ten forms of the Penman combination evapotranspiration equation are reviewed and compared with lysimeter estimates at three locations. Aerodynamic and canopy resistance forms by Monteith and Thom and Oliver and an empirical form by Wright best predict daily lysimeter measurements at Kimberly, Idaho, and Coshocton, Ohio. Canopy resistances of 40–80 s/m and momentum roughness heights of 15 mm for clipped grass and 45–70 mm for tall grass and alfalfa result in best estimates by the Monteith and Thom‐Oliver methods. The original Penman and Priestley‐Taylor versions underestimated evapotranspiration in the arid Kimberly environment. Average standard errors of estimate of the best equations average about 0.8 mm/day over growing seasons.