Empirical validation of the relationship between the crop water stress index and relative transpiration in almond trees

Abstract

ABSTRACT There is growing interest in the use of canopy temperature to evaluate the water status of crops for irrigation water management. One of the main indicators currently used is the Crop Water Stress Index (CWSI). In this index, the canopy temperature is normalized by the environmental conditions to account for the evaporative demand of the atmosphere. The theoretical framework, based on the Penman-Monteith equation, defined the CWSI as one minus the ratio between actual and potential water use (Jackson et al., 1981). For the first time in tree crops, we have related the actual transpiration of almond trees measured with heat-pulse sap flow probes with the CWSI, calculated using an empirical Non-Water Stress Baseline. The relationship obtained between the CWSI and the relative transpiration fitted the theoretical relationship, although it showed a large scatter (R2=0.69; RMSE=0.13). The variability in micrometeorological conditions among different parts of the canopy, the scatter of the NWSB, or inherent measurement errors are identified as probable causes of this scatter. Finally, the effect of this scatter on the accurate assessment of actual transpiration from canopy temperature is analyzed for practical irrigation management purposes. We found an error of about 10% in the relative transpiration, which seems acceptable for irrigation management applications.