Models for Estimating Evapotranspiration of Irrigated Eucalypt Plantations

Evapotranspiration, a major component of water balance and net primary productivity in plant-based terrestrial production systems at local and regional scale, is difficult to measure. In order to better understand tree growth and water-use relationships, and to design plantations and optimize their irrigation schedules, it is important to estimate the climatically induced evapotranspiration demand of tree crops. This demand, considered as the maximum evapotranspiration (ETm), is regulated by the resistances imposed by canopy surfaces during the process of evapotranspiration. This chapter describes several simple methods that have been proposed previously to estimate ETm and compares various process-based estimates of ETm with water-use rates determined from a water balance study. The observations from the study conducted at Forest Hill near Wagga Wagga, NSW, Australia, show that ETm can be estimated from standard meteorological parameters as a one-step approach using the Penman-Monteith equation. In the absence of required climatic data, ETm can be estimated from the radiation using Priestley-Taylor technique. For irrigation scheduling, however, ETm may be estimated from pan evaporation data using an estimated pan factor. This factor is site specific and varies with the season and the age of the plantations. For purposes of design and scheduling of irrigation, monthly pan factors can also be determined from climatic data using the Penman-Monteith equation.