Projections of Meteorological Drought and Potential Evapotranspiration Using the RegCM4 Over Northeastern Iran

We project future meteorological drought and potential evapotranspiration (PET) for northeastern Iran (Great Khorasan) using the Regional Climate Model RegCM4.4.5.10 driven by CanESM2 under the RCP4.5 scenario (CMIP5). Simulations span a baseline period (1990–2009) and two projection windows (2040–2059 and 2080–2099). PET is estimated using the Hargreaves–Samani method and drought is quantified with the Standardised Precipitation–Evapotranspiration Index (SPEI). Model skill is evaluated against station observations using Kling–Gupta efficiency (KGE), normalised RMSE, coefficient of determination ( R 2 ) and Willmott's d. Relative to 1990–2009, temperatures increase across all stations, with late‐century warming in some locations exceeding +3°C. PET increases consistently, rising by an average of +0.35 mm day −1 in 2040–2059 and reaching up to +0.7 mm day −1 in some stations (e.g., Mashhad) by 2080–2099. Consistent with these changes, regional SPEI shifts toward drier conditions: values range from −1.269 to −1.907 in 1990–2009, −1.446 to −1.996 in 2040–2059 and −1.717 to −2.136 in 2080–2099, indicating an intensification from predominantly moderate–severe drought to severe–extreme drought conditions by the late century. Spatial patterns are modulated by elevation and land‐use/land‐cover gradients; NDVI and climatic water‐deficit diagnostics corroborate increasing evaporative demand and growing water stress. These projections imply escalating risks for water resources and agriculture and support proactive, regionally tailored drought‐risk management in northeastern Iran.