Abstract:
The estimation of crop water use is critical for accurate irrigation scheduling and
water licenses. However, the direct measurement of crop water use is too expensive
and time consuming to be performed under all possible conditions, which
necessitates the use of water use models. The FAO-56 procedure is a simple,
convenient and reproducible method, but as canopy cover and height vary greatly
among different orchards, crop coefficients may not be readily transferrable from one
orchard to another. Allen and Pereira (2009) therefore incorporated a procedure into
the FAO-56 approach which estimates crop coefficients based on a physical
description of the vegetation and an adjustment for relative crop stomatal control
over transpiration. Transpiration crop coefficients derived using this procedure and
fixed values for citrus, did not provide good estimates of water use in three citrus
orchards. However, when mean monthly leaf resistance was taken into account, good agreement was found with measured values. A relationship between monthly
reference evapotranspiration and mean leaf resistance provided a means of
estimating mean leaf resistance which estimated transpiration crop coefficients with
a reasonable degree of accuracy. The use of a dynamic estimate of mean leaf
resistance therefore provided reasonable estimates of transpiration in citrus.
