Spatiotemporal variability and climatic determinants of reference evapotranspiration in Saudi Arabia

ABSTRACT Accurate estimation of reference evapotranspiration (ET0) is essential for water resource management in arid and semi-arid regions such as Saudi Arabia, where high evaporative demand shapes agricultural and environmental strategies. Given the country's reliance on irrigation-dependent farming, analysing ET0 trends and their climatic drivers can enhance irrigation scheduling, reduce over-irrigation, and support sustainable water use in agriculture. This study investigates the temporal variability, long-term trends, and climatic sensitivity of ET0 across 25 meteorological stations distributed throughout Saudi Arabia between 2000 and 2024. Daily ET0 values were estimated using the FAO-56 Penman–Monteith method, with trend analysis performed through the Mann–Kendall test and Sen's slope estimator. Results reveal that the majority of stations experienced statistically significant increasing trends in ET0, particularly during the summer months, highlighting intensified atmospheric water demand coinciding with peak solar radiation and low humidity periods. Stations such as Wadi Al-Dawasser, Sharorah, and Najran exhibited the strongest upward trends, with Sen's slope values exceeding 4 mm per month during summer. Sensitivity analysis using standardised multiple linear regression indicated that mean relative humidity was the most influential climatic driver of ET0, contributing 27.2% to its variability, followed by wind speed and maximum air temperature.