Assessment of Climate Change Impact on Spring-Sown Sugar Beet (Beta vulgaris L.) Net Irrigation Water Requirement and Potential Yield in Karaj, Iran (Climate Classification: BSk)

Climate change is an undeniable phenomenon, which affects virtually every aspect of life on Earth. The agricultural sector is heavily connected to the environment and thus, gets affect by climate change the most. Climate change affects agriculture by affecting temperature and rainfall. The elevated air temperature increases the potential evapotranspiration. Variation in precipitation is generally unfavorable. Increase in precipitation causes waterlogging and subsequently erosion in soil, while decreased precipitation causes water stress in crops. It is necessary to assess the climate change impact on agriculture in order to change policies accordingly. This paper sought to assess the impact of climate change on sugar beet net irrigation water requirement and potential yield. In this research, the climatic information of the meteorological station located in Mohammadshahr, Karaj for the period 1970-2014 and the crop parameter of the sugar beet research farm located in Mehrshahr, Karaj were used. Future climatic data from six global climate models, namely ACCESS-ESM1-5, CanESM5, EC-EARTH3, IPSL-CM6A-LR, MRI-ESM2-0 and NorESM2-LM, under three optimistic, intermediate and pessimistic scenarios (SSP126, SSP245 and SSP585) for the period 2015-2100 were downscaled for Karaj using empirical quantile mapping method. Taylor diagram was used to evaluate the downscaling results. Temperature and precipitation data as well as reference evapotranspiration, which was calculated by Hargreaves-Samani method, were given to AquaCrop model. The effective rainfall calculation approach was set to USDA-SCS method in AquaCrop model. The trend and fluctuations of the climatic and crop variables were examined using the Mann-Kendall and Pettitt tests, respectively.The results indicate that the sugar beet sowing and harvest dates advance by 39 and 71 days, respectively, which means a decrease of 29 days in the growing season length (from 171 to 142 days). The reference evapotranspiration under SSP585 will increase by 14.8% (6.5% under SSP126) at the end of the 21st century in Karaj. The water requirement can increase up to 7.8% under SSP585 (3.7% under SSP126). The amount of irrigation water requirement in the future period will increase up to 10.5% under SSP585 (5.8% under SSP126). The biomass and yield variations at the 21st century ending period will be 11.8% and 19.2% increase under SSP585 (4.2% and 5.9% increase under SSP126), respectively. All the climatic and crop variables showed strongly significant trend in the study period (1995-2100) under the pessimistic scenario, while the fluctuations of the variables were not as significant as their corresponding trends under the same scenario.According to the results of this paper, the increasing temperature caused by the elevating atmospheric CO2 concentration, under the pessimistic scenario, increases the sugar beet net irrigation water requirement by increasing the crop evapotranspiration. The decreasing precipitation also contributes to the obtained result. The potential yield showed a contradicting result, i.e., the elevated CO2 under the pessimistic scenario favors the yield production due to the fact that more CO2 contributes to more efficient photosynthesis. The results can be used in climate change adaptation policies regarding water allocation and optimal planting date determination for sugar beet cultivation in Karaj.