Use of Crop Water Stress Index for Irrigation Scheduling of Soybean in Mediterranean Conditions

Canopy temperature measured via infrared thermometers is an important parameter to determine crop water stress. The crop water stress index (CWSI) is the most often used index based on difference of canopy-air temperature and vapor pressure deficit (VPD) to detect crop water stress and to schedule irrigation for field crops. The aim of this study is to determine the relationship between the canopy-air temperature difference and the vapor pressure deficit in order to calculate the CWSI value in soybean plants. The study is carried out in randomized complete block design with six different irrigation treatments and three replications at the Batı Akdeniz Agricultural Research Institute (BAARI), Antalya, Turkey. Plots were irrigated when the cumulative evaporation in Class A pan is 25±5 mm using drip irrigation system based on the plant-pan coefficient (kpc) of 0, 0.25, 0.50, 0.75, 1.00, and 1.25. Before and after each irrigation, canopy temperature was measured using a portable infrared thermometer in all treatments between 11.00 to14.00. Throughout the season, before irrigation, soil moisture content was measured. The CWSI values were determined using empirical approach. When using this technique in Antalya conditions, it is suggested to keep the seasonal mean CWSI value approximately 0.26 and index value of 0.40 can be used to start irrigations. Additionally, it is suggested that the amount of irrigation can be equal to the amount of evaporation measured until the index value reach 0.40. According to the results obtained, it is concluded that infrared thermometer can be used to schedule irrigation of the soybean plant under Mediterranean (Antalya) conditions of Turkey.