Application of a new method to evaluate crop water stress index

Optimum water management and irrigation require timely detection of crop water condition. Usually crop water condition can be indicated by crop water stress index (CWSI), which can be estimated based on the measurements of either soil water or plant status. Estimation of CWSI by canopy temperature is one of them and has the potential to be widely applied because of its quick response and remotely measurable features. To calculate CWSI, the conventional canopy-temperature-based model (Jackson's model) requires the measurement or estimation of the canopy temperature, the maximum canopy temperature (Tcu), and the minimum canopy temperature (Tcl). Because extensive measurements are necessary to estimate Tcu and Tcl, its application is limited. In this study, by introducing the temperature of an imitation leaf (a leaf without transpiration, Tp) and based on the principles of energy balance, we studied the possibility to replace Tcu by Tp and reduce the included parameters for CWSI calculation. Field experiments were carried out in a winter wheat (Triticum aestivum L.) field in Luancheng area, Hebei Province, the main production area of winter wheat in China. Six irrigation treatments were established and soil water content, leaf water potential, soil evaporation rate, plant transpiration rate, biomass, yield, and regular meteorological variables of each treatment were measured. Results indicate that the values of Tcu agree with the values of Tp with a regression coefficient r=0.988. While the values of CWSI estimated by the use of Tp are in agreement with CWSI by Jackson's method, with a regression coefficient r=0.999. Furthermore, CWSI estimated by the use of Tp has good relations with soil water content and leaf water potential, showing that the estimated CWSI by Tp is a good indicator of soil water and plant status. Therefore, it is concluded that Tcu can be replaced by Tp and the included parameters for CWSI calculation can be significantly reduced by this replacement..