Comparison of Some Crop Water Stress Measurement Methods¹

The measurement of crop water stress under field conditions is fraught with technical and logistical problems. Although leaf water potential has become a standard measurement it has spatial and temporal sampling limitations. In the current study, a rice ( Oryza sativa L.) crop was used to compare eight measurements indicating crop water status, namely leaf water potential ψ leaf ), stomatal resistance (r a , transpiration rate (T), net photosynthesis rate (P N ), canopy temperature (T c ), canopy minus air temperature (T c — T a ), crop water stress index (CWSI), and visual leaf rolling score (LRS). The eight measurements were compared across seven water stress levels created by a line source sprinkler irrigation system. The methods were compared for accuracy, sensitivity, sampling time, and the destructive‐disruptive nature of the sampling procedure. Accuracy was estimated by comparison with ψ leaf and by the interaction between water stress level and time of day. All methods except P N were significantly correlated with ψ leaf at the 1% level. However, T c , T c — T a and CWSI showed less interaction between water stress level and time of day when total variance was partitioned into its relative components; water stress level, time, and the interaction between water stress level and time of day. All methods, with the exception of P N , were equally sensitive to the water stress gradient when “goodness of fit” response functions across the water stress gradient were compared. The visual LRS was the most rapid while the canopy temperature‐based measurements, T c , T c — T a . and CWSI, were nearly three times faster than the gas exchange techniques and about two and a half times faster than ψ leaf . Leaf water potential sampling was both destructive and disruptive to the crop plant community. The gas exchange methods were nondestructive but repetitive sampling was disruptive. Only the remotely sensed T c T c — T a , CWSI and LRS were neither destructive nor disruptive to the crop. The interpretive value of various measurements is discussed. The CWSI was found to be highly correlated with mean daily P N and represents a significant advancement in crop level detection and measurement of water stress.