Water use dynamics of almond and pistachio crops in the Mediterranean region amid climate change

Almond and pistachio are socio-economically significant tree crops grown in California’s San Joaquin Valley (SJV). Continuing commercial production with efficient irrigation planning under changing climate requires an extensive understanding of their biophysical interactions such as altered phenology, biomass accumulation, transpiration, and other processes. This study examines the phenology and water use (ETc) responses of almonds and pistachios under two scenarios: climate change (warming-only) and climate change with elevated atmospheric CO 2 (eCO 2 ), using an agrohydrological model, VIC-CropSyst. The model was calibrated and validated using observations on crop evapotranspiration. The simulated crop responses to warming-only and eCO 2 revealed compounding effects on chill accumulation, phenology, biomass accumulation, and eventually on crop water use. The active growth period from bud break to dormancy was prolonged by 10–25 days for both crops under simulated Representative Concentration Pathways (RCP4.5 and RCP8.5) scenarios compared to the baseline. The annual ETc under warming-only scenarios showed a slight increase for almonds (1 % under RCP4.5 and 3 % under RCP8.5) and a higher increase for pistachios (8 % under both RCP4.5 and RCP8.5) by the mid-21 st century. These increases in ETc translate to 260 (almonds) and 180 (pistachios) million m 3 /year of additional water for irrigation in the absence of eCO 2 effects. The eCO 2 is expected to moderately reduce irrigation demand by 35 million m 3 /year for almonds and increase by 140 million m 3 /year for pistachios over the baseline. The outcomes can guide decision-makers to anticipate potential threats and concoct adaptive strategies by managing irrigation and choosing suitable cultivars or alternate crops. • Climate change driven increase in seasonal water use in almond and pistachio crops. • Enhanced atmospheric CO 2 can entirely offset warming effect on almonds’ water use. • Crops’ growth cycle and chill requirements determined their response. • Higher vulnerability for pistachios was revealed under climate change.