Yield, quality and crop water stress index relationships for deficit-irrigated soybean [Glycine max (L.) Merr.] in sub-humid climatic conditions

Field experiments were conducted for 2 years to evaluate the use of the crop water stress index (CWSI) for irrigation scheduling of soybeans under a sub-humid climate of Bursa, Turkey. Additionally, statistical relationships between CWSI and seed yield, quality parameters, crop evapotranspiration (ETc) and water use efficiency (WUE) were investigated. Irrigations were scheduled based on the replenishment of 100 (T100), 75 (T75), 50 (T50), 25 (T25), and 0% (T0) of soil water depletion from a soil depth of 90 cm using a 7-day irrigation interval. To compute CWSI, lower (nonstressed) and upper (stressed) baselines were developed based on the canopy temperature (Tc) measurements of fully irrigated and rain-fed treatments, respectively. According to results, CWSI could be used to determine the irrigation time of soybean for sub-humid climate and 0.22 could be offered as a threshold value. Statistically significant relationships were determined between CWSI and seed yield, protein yield, oil yield, ETc and WUE. The polynomial relationship between WUE and CWSI demonstrated that highest WUE could be obtained under CWSI close to 0.6. Consequently, an irrigation schedule that considers water stress could be employed when the cost of water is high and/or water is scarce. However, because of the yield reduction, all economic aspects of water limited irrigation scheduling should be considered before making this decision. We conclude that the CWSI could be used to evaluate crop water stress and improve irrigation scheduling for soybeans under sub-humid climatic conditions.

► Base lines of CWSI for soybeans under sub-humid climate conditions were determined.
► CWSI values of five irrigation treatments were calculated during two seasons.
► CWSI can be used to evaluate water stress of soybeans under sub-humid conditions.
► CWSI is a useful parameter to prepare irrigation schedule for soybeans.
► CWSI could be used as an indicator for seed yield, protein yield and oil yield.