Evaluating Plant Water Stress with Canopy Temperature Differences

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Abstract
The temperature stress day (TSD) has been used as a remotely sensed indicator of plant water stress without a thorough knowledge of TSD behavior. The purpose of this study was to demonstrate a dependence of the TSD on net radiation (Rm), air temperature (Ta), and atmospheric water vapor pressure deficit (VPD), and to evaluate a replacement water status index. The dependence on these parameters was proven by the following methods: (i) theoretically, using the Penman‐Monteith resistance equation; (ii) empirically, using the non‐water‐stressed baseline equation relating canopy and air temperatures to VPD; and (iii) in a field experiment with alfalfa [Medicago sativa (L.)] on an Anthropic Torrifluvent soil. The effect of these parameters caused the field‐measured TSD to vary up to 7 °C, rendering its utility as a plant stress indicator questionable. A modification of the crop water stress index (CWSI), which accounts for Rn, Ta, and VPD, was proposed as the TSD replacement. The modification consisted of using a measured instead of estimated well‐watered canopy temperature (Tcl) in the canopy temperature ratio defined by the CWSI. This modification obviates the need for the canopy resistance value required to calculate the theoretically based CWSI. The modified CWSI responded to imposed irrigation regimes as indicated through yield and evapotranspiration comparisons. The modified CWSI appears to be a suitable replacement for the TSD by accounting for environmental dependence while maintaining the measurement simplicity of the TSD.
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