Operational limits to the Priestley-Taylor formula

The Penman-Monteith equation (PM) provides a direct and logical route to explaining rates of crop water consumption without the need of resorting to the artificiality of a reference crop and ill-defined crop factors. However as an operational irrigation tool, the PM is often impractical because of uncertainties about stomatal behaviour and turbulent transport. While recent progress is providing better descriptions of these processes, it is questioniable whether such detail is always warranted for practical irrigation. Another option for evaporation prediction is the Priestley and Taylor equation (PT). This approach enjoys reasonable empirical support in humid regions and our efforts here are devoted to better defining its limits of applicability. We begin by presenting previously published and new evidence in support of Priestley and Taylors' value of 1.26 for the coefficient α indicating proportionality between evaporation rate and available energy. While α may depart from this value depending on windspeed and saturation deficit, it is shown to be fairly insensitive to both over a reasonable range of conditions. A daytime mean saturation deficit of 10 gm^-3 is suggested as a likely upper limit for the range of applicability of the PT; below this limit, the extra data inputs required for the PM will not be rewarded with a better estimate of evaporation. In arid regions and under stable conditions, fluxes well beyond the PT estimate have been measured and no simple prediction alternative to the PM exists.