Modeling of reference temperatures for calculating crop water stress indices from infrared thermography

Abstract Leaf temperature (TL) is tightly coupled with the rate of transpirational water loss from the leaf. The temperatures of wet and dry reference leaf surfaces (Twet and Tdry, respectively) are commonly used to normalize temperature measurements for current environmental conditions and then calculate a crop water stress index (CWSI). Since it is often impractical to directly measure Tdry and Twet, the goals of this work were to: i) determine a suitable artificial reference surface that makes application of the CWSI faster and easier in the field, ii) develop a model for Tdry and Twet based on the reference surface temperature that allows for calculation of standard CWSIs, iii) test the technique for a range of weather conditions and tree species, and iv) analyze the sensitivity of these two models to Tdry and Twet, and their impact on the estimation of four different CWSIs. Our results showed that both Tdry and Twet are linearly related to the thermal temperature of green paper across a wide range of environmental conditions. Although there was a significant effect of the light conditions on Tdry and Twet, the same models could be used in both the sun and shade to relate Tdry and Twet to Tref. Moreover, results indicated that a new CWSI dependent only on TL and Twet was least sensitive to errors in Twet, but most sensitive to TL.