Reference evapotranspiration (ET0) and drought variability serve as critical indicators for comprehending climatic conditions in arid and semi-arid regions. This study assesses the efficacy of global climate model (GCM) outputs, compiled in the WorldClim open-access climate dataset, for estimating ET0 and supporting standardised precipitation evapotranspiration index SPEI drought analysis in the Al-Madinah region. Monthly WorldClim temperature and precipitation data were initially compared with recorded historical station data (1970-2018) to evaluate accuracy. The results showed strong to very strong agreement, with R values ranging from 0.937 to 0.999 and R2 values ranging from 0.878 to 0.999, although slight underestimation was observed for both temperature and precipitation. Then WorldClim-derived Hargreaves ET0 was validated against station-based FAO-56 Penman-Monteith and station-based Hargreaves ET0, also showing strong agreement. Future ET0 was projected under SSP2-4.5 and SSP5-8.5 scenarios for four 20-year periods: 2021-2040, 2041-2060, 2061-2080, and 2081-2100. The projections indicated a gradual increase in ET0 under both SSP scenarios, with stronger increases under SSP5-8.5, especially during 2081-2100. In this period, SSP5-8.5 showed an average monthly ET0 of about 182.47 mm/month, equivalent to 2189.7 mm/year, representing an increase of 359.70 mm/year relative to the historical baseline. Baseline ET0 was higher in the eastern and southeastern areas and lower in the northwest. Future projections showed the same spatial pattern, with increasing ET0 values across the region, especially under SSP5-8.5 in the late century. SPEI projections indicated a shift from near-normal conditions in the early century to increasing moisture deficit after mid-century, particularly under SSP5-8.5. These results highlight the value of WorldClim-based climate projections for assessing climate variability, future climate-change impacts, drought susceptibility, and evaporative conditions, particularly in arid and semi-arid regions.
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