University of Birjand-Research Group of Drought and Climate Change Journal of Drought and Climate change Research 3092-6076 Articles in Press 2026 06 30 Regional Calibration of the Hargreaves-Samani Method and Potential Evapotranspiration projection in Iran under Climate Change Scenarios Regional Calibration of the Hargreaves-Samani Method and Potential Evapotranspiration projection in Iran under Climate Change Scenarios 4071 10.22077/jdcr.2026.10664.1197 FA Mohammad Mohammadlou PH.d student in watershed management engineering 0000-0001-5146-5391 Abdolreza Bahremand Department of Watershed Management and Desertification Control, Gorgan University of Agricultural Sciences and Natural Resources Shervan Gharari Department of Civil Engineering, Schulich School of Engineering, University of Calgary, Calgary, Alberta, Canada Amir Sadoddin Department of Watershed Management and Desertification Control, Gorgan University of Agricultural Sciences and Natural Resources Iman Babaian Climate Research Center, Research Institute for Meteorology and Atmospheric Sciences, Mashhad, Iran Journal Article 2025 12 16 Potential evapotranspiration (ET₀) is a fundamental component of the water cycle and is highly sensitive to climate change. In this study, the Hargreaves-Samani (HS) method was calibrated regionally for Iran for the first time, using observational data from 152 synoptic stations during the baseline period 1995–2014 against the FAO56 Penman-Monteith reference method. Four calibration approaches (daily and monthly scales, including linear and second-order fits) were evaluated on the aggregated data of six climatic-altitudinal zones. The monthly linear method showed the highest performance, with a mean R² of 0.89 and a mean RMSE of 0.77 mm day⁻¹, and was selected as the most suitable option. Consequently, correction coefficients were derived for the six zones and applied to estimate future ET₀ (2030–2090) using outputs from three CMIP6 models (ACCESS-CM2, IPSL-CM6A-LR, and CNRM-CM6-1) under three emission scenarios (SSP1-2.6, SSP2-4.5, SSP5-8.5). Results indicate an increasing trend of ET₀ across all regions of Iran, with average increases during the far future period (2071–2090) ranging from 58% to 129% relative to the baseline. The highest sensitivity is observed in high-latitude low-altitude zones and dry central areas, while mountainous regions show the smallest increases. The largest absolute increases occur in summer months (up to 15.5 mm month⁻¹), whereas the largest relative increases occur in winter (up to 182%). These findings provide a serious warning for future water stress in the agricultural sector and underscore the urgent need to revise cropping patterns—particularly in central arid regions during summer—and to adopt climate-adaptive water management strategies. Potential evapotranspiration (ET₀) is a fundamental component of the water cycle and is highly sensitive to climate change. In this study, the Hargreaves-Samani (HS) method was calibrated regionally for Iran for the first time, using observational data from 152 synoptic stations during the baseline period 1995–2014 against the FAO56 Penman-Monteith reference method. Four calibration approaches (daily and monthly scales, including linear and second-order fits) were evaluated on the aggregated data of six climatic-altitudinal zones. The monthly linear method showed the highest performance, with a mean R² of 0.89 and a mean RMSE of 0.77 mm day⁻¹, and was selected as the most suitable option. Consequently, correction coefficients were derived for the six zones and applied to estimate future ET₀ (2030–2090) using outputs from three CMIP6 models (ACCESS-CM2, IPSL-CM6A-LR, and CNRM-CM6-1) under three emission scenarios (SSP1-2.6, SSP2-4.5, SSP5-8.5). Results indicate an increasing trend of ET₀ across all regions of Iran, with average increases during the far future period (2071–2090) ranging from 58% to 129% relative to the baseline. The highest sensitivity is observed in high-latitude low-altitude zones and dry central areas, while mountainous regions show the smallest increases. The largest absolute increases occur in summer months (up to 15.5 mm month⁻¹), whereas the largest relative increases occur in winter (up to 182%). These findings provide a serious warning for future water stress in the agricultural sector and underscore the urgent need to revise cropping patterns—particularly in central arid regions during summer—and to adopt climate-adaptive water management strategies.