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

Leaf temperature (T-L) is tightly coupled with the rate of transpirational water loss from the leaf. The temperatures of we and dry reference leaf surfaces (T-wet and T-d(ry), 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 T-dry and T-wet, 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 T-dry and T-wet 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 T-dry, and T-wet, and their impact on the estimation of four different CWSIs. Our results showed that both T-dry, and T-wet 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 T-dry and T-wet, the same models could be used in both the sun and shade to relate T-dry and T-wet to T-ref. Moreover, results indicated that a new CWSI dependent only on T-L and T-w(e)t was least sensitive to errors in T-wet, but most sensitive to T-L.