2018 ASHS Annual Conference

Crop coefficient (Kc) values commonly used in California for irrigating pistachio were developed in an earlier study (1985) conducted in a commercial, sprinkler-irrigated orchard of Kerman cultivar grafted onto P. Atlantica rootstock. Nowadays, the majority of pistachio in the San Joaquin Valley of California is grown with drip-irrigation, which could reduce the evaporative water losses and increase irrigation efficiency and water productivity. Our team of researchers from the University of California Cooperative Extension conducted a three-year field study (2015-2017) to estimate the actual evapotranspiration (ETa) of a well-watered mature (30-year old, 75% canopy cover) commercial pistachio orchard (Kerman onto PGI) grown on a sandy clay loam soil with drip irrigation in the San Joaquin Valley. We used the residual of energy balance method to estimate the actual water use, with a combination of eddy covariance and surface renewal equipment to measure the sensible heat flux density. The collected field data confirm the ability of pistachio trees to use large water volumes when irrigated for full production, with average ETa of 7.5 mm day^-1 during the hottest months (June and July) and daily peak water use reaching up to 10 mm in late June. The highest Kc values reached 0.90 during the period from mid-May to mid-July. Kc was 0.80 from mid-April to mid-May and during August. In September, a steep decrease of Kc was observed, reaching values around 0.50, most likely as a result of pre-harvest reduced water applications by the grower. The Kc values documented with the earlier study varied from 0.43 at the end of April to 1.19 in mid-July and decreased to 0.67 in October. The newly developed Kc trends should enable pistachio growers to achieve higher water productivity and improve the resource efficiency and pistachio production in the San Joaquin Valley.