Future projections of reference evapotranspiration in semi-arid areas: CMIP6 bias correction, climate trends, and implications for irrigation in the Chichaoua agricultural basin in Morocco

Study Region The study was conducted in the Haouz plain of Morocco, focusing on five rural municipalities in the province of Chichaoua (Saidate, Sidi M’Hamed Dallil, Majjat, Lamzoudia, and Ahdil), a semi-arid agricultural region located in the foothills of the western High Atlas. Study Focus Climate change is increasing pressure on water resources in semi-arid regions where irrigated agriculture remains essential to food security. This study provides a detailed analysis of future changes in reference evapotranspiration and the meteorological parameters governing it using a bias-corrected CMIP6 multi-model ensemble. Fourteen CMIP6 models were first evaluated over the historical period 1981–2014, and five best-performing models were selected to generate future projections under the SSP245 and SSP585 scenarios. Climate variables were interpolated at the barycenters of 37 agricultural plots distributed across the five municipalities and bias-corrected against ERA5-Land using an empirical non-parametric quantile mapping method. Monthly ET₀ was computed following the FAO-56 Penman-Monteith formulation. Future anomalies and temporal trends were analyzed using Mann-Kendall tests and Sen slope estimators, while the relative contribution of climatic drivers was quantified using standardized multiple regression. New hydrological insights for the region The projections indicate a robust increase in ET₀ across all municipalities, with stronger anomalies during spring and summer and under the SSP585 scenario. Average ET₀ is projected to increase by about 3.5 mm month⁻¹ (≈2.7%) for 2031–2060 under SSP245 and up to about 8.3 mm month⁻¹ (≈6.4%) by 2071–2100 under SSP585. Trend analyses reveal significant positive annual and seasonal trends, intensifying toward the end of the century. Attribution analysis shows that air temperature is the dominant driver, explaining approximately 40–60% of the projected increase in ET₀, while decreasing relative humidity further amplifies atmospheric evaporative demand. These changes imply increasing irrigation water requirements and growing pressure on water resources in the Haouz plain. The results provide scientific evidence to support water planning, crop calendar adjustments, irrigation optimization, and agroclimatic adaptation strategies in semi-arid agricultural systems.