Transpiration from canopy temperature: Implications for the assessment of crop yield in almond orchards

This paper evaluates the usefulness of the Crop Water Stress Index (CWSI) for monitoring transpiration and water status in almond trees, and proposes a methodology for assessing crop yield derived from the relation between canopy temperature and transpiration. For this purpose, a Non-Water Stress Baseline (NWSB) was developed from canopy temperature measurements taken with Infrared Thermometers (IRT) installed permanently over well-watered trees for three years. Tree transpiration was measured continuously with sap flow probes installed in the same trees than the IRT sensors. The calculated CWSI was closely related to water potential and stomatal conductance measured during kernel filling, as well as with transpiration and the ratio kT/GC (the transpiration coefficient over the ground cover). Taking into consideration this relation and the water production function recently published, the seasonal CWSI was compared to final yield and the regression yielded good results (R-2 = 0.80). An empirical relationship between the CWSI acquired remotely from two flights performed during the kernel filling stage and crop yield was determined for this orchard. The estimated yield from the proposed methodology was compared to ground-truth measurements of crop yield measured in 80 trees during 2014 and 2015. The result obtained a RMSE that yielded 1.54 kg/tree. This study thus demonstrates that CWSI is closely related to the transpiration and the ratio kT/GC. This relation settles the basis for the development of methodologies for estimating water-limited crop yield from thermal derived information.