Using Sap Flow to Calculate Integral Water Capacity in Pistachio Trees

Integral Water Capacity (IWC), as one of the recently defined criteria for soil water availability, uses weighting functions to takes into account various soil physical limitations for calculating plant available water. Also, the weighting functions can be calculated from plant responses. In this study, in a pistachio orchard, surface (DI) and subsurface (SDI) drip irrigation systems were established, afterwards soil physical properties and water and salinity distribution in root zone were measured. IWC were calculated using soil physical properties and sap flow (SF) as one of the plant responses features. The result of plant measurements indicated that SF decreased progressively during the time between successive irrigation periods, but the decrease in DI was greater than SDI. By considering equal depth of irrigation water applied to DI and SDI, the more favorable moisture and salinity distribution in the root zone of SDI compared to DI treatment was the reason for more water availability in SDI. The calculated IWC by Groenevelt weighting function (  in DI and SDI showed the lowest amount (0.0557 and 0.0545  respectively) but  values were calculated as 0.176 and 0.191, respectively, and were close to IWC calculated without any salinity effect (0188 ). Van Genuchten equation led to more realistic estimates of salinity limitations on water availability (  were obtained as 0.146 and 0.151 ) and the integral energy based on  was very close to integral energy calculated by SF. These findings indicate that the salinity weighting functions used in  had the best estimates of soil physical conditions to consider water uptake energy and plant water availability. It is found that based on all soil water availability criteria, SDI provided more available water and less integral energy than DI.