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 CO2 (eCO2), 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 eCO2 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-21st century. These increases in ETc translate to 260 (almonds) and 180 (pistachios) million m3/year of additional water for irrigation in the absence of eCO2 effects. The eCO2 is expected to moderately reduce irrigation demand by 35 million m3/year for almonds and increase by 140 million m3/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.