Olive area expansion under climate change: a CMIP6-SWAT+ basin-scale water balance and demand assessment in southern Portugal

In recent decades, olive groves have continuously expanded across the Sado River Basin (SRB, southern Portugal), with the support of inter-basin water transfers. However, projected climatic aridification could increase olive water demand and pressure local water reserves. This study quantifies the response of SRB water balance and olive water demand to olive grove expansion under the SSP5–8.5 scenario. The SWAT+ agro-hydrological model was calibrated and validated against observed river flow at two hydrometric stations over contrasting decades (1962–1971, 1981–1990), and forced with 10-km CMIP6-derived atmospheric data and unlimited irrigation. Simulations were run for 1981–2010 and 2041–2070, combining unchanged (F1) and expanded (F2, +7% of basin area) olive groves. Results showed that ~100 mm annual precipitation decreases and ~2°C warming could increase potential evapotranspiration by ~9% and decrease surface runoff, ground flow, and aquifer percolation by 29%, 33–47%, and 31–37%, respectively. Consequently, ~90% of annual precipitation could be routed to evapotranspiration. Olive annual irrigation variability range and total-volume irrigation may increase by 24%/34% and 17%/141% under F1/F2 relative to reference values of 1400 m 3 /ha and 90 hm 3 , respectively. Resolving the climate-water-crop nexus at basin-scale revealed that climate aridification could increase SRB olive water demand and further decrease local soil moisture, channel flow, and groundwater infiltration. However, a definitive answer regarding the impacts of SRB olive grove expansion on local water reserves depends on additional mid-century seasonal water availability research. This approach can be extrapolated to other Mediterranean regions experiencing perennial crop expansion under climate aridification.