Determination of Crop Water Stress Index for Pistachio Trees under Subsurface Drip Irrigation Using Canopy-Air Temperature Difference

This study was conducted to determine the possibility of a crop water stress index (CWSI) to schedule irrigation for Pistachio tree under subsurface drip irrigation. The effects of two irrigation intervals consisting 3 (I3) and 14 (I14) days based on measuring soil water content were investigated on the CWSI. From 22 May (DOY 143) to 22 September (DOY 266), the evapotranspiration in I3 and I14 treatments varied from 795.45 to 490.79 mm, respectively. The CWSI was calculated using the empirical approach from measurements of infrared canopy temperatures, air temperatures and vapor pressure deficit values for the two irrigation levels. The lower baseline (non-water stressed baseline) was calculated as (Tc – Ta)LL = -1.642 VPD + 1.432 with R2 = 0.49. This equation had a correlation coefficient equal to 0.77 (P<0.01). An average threshold CWSI value of about 0.5 and 0.49 was calculated for I3and I14 treatments, respectively. Duncan test showed the effect of irrigation levels on CWSI was significant at P<0.05. Linear relationship between evapotranspiration and monthly CWSI calculated as ET = -804.7 CWSI + 680.1 with determination coefficient equal to 0.6 (P<0.05). The CWSI can be employed as a useful tool for assessing crop water stress for Pistachio.