Water stress causes stomatal closure but does not reduce canopy evapotranspiration in almond

We investigated the degree of physiological control of transpiration at the leaf and canopy-level in commercial almond orchards in California during periods of water stress ranging from −0.8 to −2.0 MPa of stem water potential. Field measurement of evapotranspiration using the residual of the energy balance method failed to detect a decrease of actual evapotranspiration (ETa) relative to standardized reference evapotranspiration for short canopies (ETo) during weekly periods of water stress. Although a decline in the ratio Ka = ETa/ETo is expected in presence of water stress as a result of stomatal regulation, a flat response to water stress was observed for both daily and average midday Ka. A substantial decrease in Ka was observed after harvest, perhaps caused by the decrease in leaf area resulting from harvest operations. Weekly cycles of irrigation caused a clear reduction in midday stem water potential, a sensitive indicator of water status in plants. At the leaf level, there was a continuous decline in stomatal conductance with declining stem water potential for both sunlit and shaded leaves. Shaded leaves had lower conductance values and a flatter slope than sunlit leaves. The results suggest that a decrease in conductance of roughly 50% is expected at the leaf level for the observed range of stem water potential. A scaling-up exercise together with a model of the energy balance indicated the expected magnitude the reduction in Ka for the observed range of stem water potential but did not account for the apparent disconnect between the leaf scale and the canopy scale. Our results underscored the complications associated with estimating plant water stress by measuring field evapotranspiration, especially when grass ETo is used as the reference. From a water management point of view, this study suggests that inducing mild to moderate water stress in almond may not produce substantial water savings.