Assessment of canopy temperature-based water stress indices for irrigated and rainfed soybeans under subhumid conditions

Crop stress indicators can serve as a basis for demand-oriented irrigation management. Canopy temperaturebased algorithms in combination with nondestructive infrared thermometer measurements are widely used, mainly in arid regions. Due to the impact of climate change on agricultural production, canopy temperaturebased indices are increasingly applied in humid areas of Central Europe. This requires an evaluation of the algorithms under local environmental conditions. The objective of this study was to assess the Crop Water Stress Index (CWSI) and the Degrees Above Canopy Threshold (DACT) as irrigation scheduling methods in a subhumid agricultural area in Austria. The study was performed in 2018 and 2019 on a soybean field that was divided into four plots under different water management conditions. One of the plots was irrigated by means of sprinklers (SI plot), another one by drip lines (DI plot) and a third one by a hose reel boom (BI plot). The fourth plot (NI) was rainfed. To be able to relate crop water status to soil water status, soil matric potential (psi m) was monitored at 20, 40 and 60 cm depths. Both CWSI and DACT reflected the different plant water conditions accordingly. The highest stress levels were found in the NI plot, followed by the SI, DI and BI plots. The interpretation of the indices regarding cloudy skies was improved by using the maximum CWSI and DACT observed between 1000 h and 1600 h. Overall, the plots with lower seasonal mean indices produced larger yields, and vice versa. The psi m correlation results suggested that under severe stress, a better representation of soil water availability down to 60 cm can be obtained when using the DACT compared to the CWSI. The DACT was considered a more practical alternative than the CWSI since it requires less effort and proved to be equally effective.