Converting Reference Evapotranspiration into Turf Water Use

Introduction: Accurate estimates of turf water use are required to effectively manage a turf irrigation system. In Volume I of this series entitled “Basics of Evaporation and Evapotranspiration (ET),” we indicated that actual turf water use (ETt) is rarely measured in the real world. Instead, we use meteorological data and a mathematical model known as the Penman-Monteith Equation to estimate reference evapotranspiration (ETos) — the ET from a tall, cool-season grass that is supplied with adequate water. In the lower elevations of Arizona the ETos value would seem of limited value since we rarely grow turf that is equivalent to the reference surface. However, we get around this problem by adjusting the ETos value to account for differences in turf type, quality and stage of development. This document describes the procedures used to adjust ETos for use on managed turf surfaces in Arizona.