2018 ASHS Annual Conference

Pistachio acreage is rapidly expanding in California on salt-affected areas thanks to its economic profitability and salt tolerance. However, no information is currently available to growers on the actual water use of mature pistachio orchards grown on soils with high salinity. Our team of researchers from the University of California Cooperative Extension conducted a field study during the 2016 and 2017 crop seasons to determine the actual evapotranspiration (ETa) and crop coefficient (Ka) trends of one non-salt affected and two salt affected commercial pistachio orchards grown with drip irrigation in the San Joaquin Valley. We used the residual of energy balance method with a combination of eddy covariance and surface renewal to measure the sensible heat flux density. In addition, we collected the percentage of photosynthetically active radiation intercepted by the tree canopies (fPAR) and midday stem water potential (Ψ_stem) data to quantify tree canopy size and tree water status. The fPAR decreased from 75% for the non-salt affected orchard to 25% for the worst salt affected orchard, which had 10 to 30% lower seasonal ETa. The radiation interception was lower but the relative contribution of sensible heat flux to ETa increased to a maximum of 18% in the most salt impacted sites. The Ψ_stem values in the non-salt affected orchard were constantly above -1.5 MPa, whereas values Ψ_stem of around -3.0 MPa were reached in the salt affected orchards. The Kc of the salt affected orchards was 0.40 in April, between 0.60 and 0.80 from May to mid-July, and decreased to around 0.25 in October. Soil analysis highlighted that the electro conductivity (EC) did not explain the differences in ETa and fPAR among sites and suggested that the secondary effect of sodicity on soil physical properties (de-flocculation, crusting, compaction, reduced infiltration and aeration) may have a large effect on pistachio performance. Irrigation water volumes currently applied to salt affected orchards were as much as 33% higher than the measured seasonal ET values. This over application of “non beneficial” water not only decreases irrigation efficiency but also likely harms the trees as a result of reduced infiltration and water logging due to high sodicity and poor soil structure. Our results highlight the necessity to better understand the long term response of pistachio water use in salt affected soils to improve water management and orchard performance.