Climate change is a perilous threat to the world's water resources; it directly alters hydrological cycles, thermal regimes, and precipitation patterns. These disturbances subsequently affect reference evapotranspiration (ET0) and crop water requirements, particularly in arid and semi-arid regions such as agricultural regions, where sustainability is already vulnerable. The effects of climate change during this period were examined regarding evapotranspiration and water demands of main crops such as barley, wheat, Alfalfa, and cotton on Garmsar plain in the western part of Iran during 2025-2100. Local climatic variables were downscaled using the Statistical Downscaling Model (SDSM) on the outputs from three global circulation models (GCMs): CanESM5, MPI-ESM1-2-HR, and NorESM2-MM for four shared socioeconomic pathways (SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5) to estimate local-scale climate variables. Reference evapotranspiration (ETo) was modeled using the Hargreaves-Samani method. Results showed an increasing trend in both maximum and minimum temperatures, causing the elevation of ET0 as well as crop evapotranspiration in all scenarios. High to moderate increases in ET0 were evident for all seasons, with autumn and summer showing the most significant seasonal increments, while winter had the lowest. Both had very long growing seasons; Alfalfa and cotton were water demands that showed the most significant increases in water use among the crops studied. In addition, the findings reiterate the variability implied in model and scenario-based climate projections, emphasizing the need to incorporate uncertainty analysis into future climate impact assessments. This study highlights the importance of adaptive agricultural planning, better cropping patterns, and the utilization of water resources in less water-available zones such as Garmsar due to the effects of climate change.