Evidence suggests that climate change include rising CO2 resulting in temperature changes and rainfall patterns, would change considerably, within different years and areas. These changes will create uncertain regional crop yield sustainability and water resource planning over the Lake Urmia Basin in northwestern Iran. The Soil and Water Assessment Tool (SWAT) used to evaluate the sugar beet growing in the future (2024–2050), under two CO2 emission scenarios (RCP 4.5 and RCP 8.5) applying daily weather data involve rainfall, maximum and minimum temperatures from a General Circulation Model (GCM). We built three separate scenarios in the SWAT model for both present and future using 23 hydrometric and 53 weather data stations. The GCM data were downscaled and bias-corrected at a 0.5o grid using a statistical method. The Nash-Sutcliffe Efficiency (NSE) and PBIAS used respectively, monthly river discharge and crop productivity, as model performance criteria. The results showed a reasonable agreement between observed and simulated outflows in most selected crop areas (NSE=0.37 and PBIAS=27%). According to the results, the warmer temperature (range 1.8 to 2.3) with dispersion in precipitation (range +7% to +48%) predicted for the future. The runoff would decrease (range of -6% to -14%), and the annual evaporation (ET) would increase (range 6% to 8%). All scenarios predicted a decrease in sugar beet yield (range -23% to -55%) besides the increase in annual variability. Given the significant increase in the frequency of days with temperatures over 35°C by 2050, the current water crisis in the study area appears to be increased, and sugar beet farming in the basin severely will be limited by rising temperature, which accounts for 7% of the agricultural crop income in the region. Overall, adaptation strategies for this highly water-consuming crop will likely be necessary soon enough, include: using of irrigation, changing the sowing date, and possibly of different, slower maturing cultivars to maintain the sugar beet yield.

Keywords: Lake Urmia Basin, SWAT, CO₂ concentration, GFDL, Crop yield