Canopy temperature versus soil water pressure head for the prediction of crop water stress

• We present an unpublished comparison between two plant drought stress indexes.
• The crop water stress index is based on air and leaf temperature observations.
• The soil-based method of Feddes et al. (1978) is widely used by soil physicists.
• Unique measurements of soil water pressure head in the very dry soil were used.
• We confirmed a clear link between soil and atmosphere-based methods.

Plant water stress is linked to both above- and below-surface parameters; with above-surface parameters being generally easier to measure. Models utilizing above-surface parameters, such as canopy temperature to identify plant water stress, frequently employ the crop water stress index (CWSI). Alternatively, the regular usage of the transpiration reduction function (FRF), as proposed by Feddes in the 1970s, requires more difficult to measure below-surface parameters such as soil water pressure head. In order to assess the agreement between these models, we experimentally compared plant water stress predicted by the CWSI and FRF using Common Bean grown in Brazil under full and deficit irrigation; the sensitivity of the models to the key parameters, water stressed baseline and limiting soil water pressure head, was also evaluated. The simple equation 1 − CWSI = Tr provided a good fit when relating CWSI to the relative transpiration Tr as predicted by the Feddes model. We show that above ground measurements that are combined within the CWSI are just as effective at predicting plant water stress as soil-based factors like soil water pressure head. The models show high sensitivity to variations in key parameters, implying significant differences will result in the identification of the onset of plant water stress; with sensitivity highest for the CWSI under dry conditions.