Penman–Monteith reference evapotranspiration adapted to estimate irrigated tree transpiration

The Penman–Monteith equation as parameterized by the FAO-56 bulletin to compute grass reference evapotranspiration (E0, L ground m−2 d−1) was used to predict transpiration of irrigated orchard apple trees, olives, grapevines, kiwifruit and an isolated walnut tree. Sap flow (S, L plant−1 d−1) measured by the compensation heat-pulse technique was taken as the tree transpiration on a daily time scale, and it proved to be a very good way to connect E0 with the individual plant water use. With the exception of the kiwifruit with large nighttime transpiration, results from 102 days from five independent experiments with the other species substantiate the working hypothesis that S = E0AL/2.88, where AL (m2 of leaf plant−1) is the plant leaf area, and 2.88 is the hypothetical grass leaf area index. This is an indication that under well-irrigated conditions transpiration per unit leaf area is very similar regardless of the plant canopy size or shape. The present scheme eliminates the need of a crop coefficient. Two alternatives for estimating the kiwifruit transpiration are also presented.