Journal of Watershed Management Research

Extended Abstract
Background: Groundwater resources play an important role in the development of agriculture, industry, and health. Due to the easy access to these resources, overexploitation lowers the water table and depletes groundwater reserves. On the other hand, water has a significant impact on agricultural stability and production. The impact of climate change on the optimal management of water and soil resources is a challenging issue in many arid regions of the world. Therefore, this study aims to evaluate the effects of climate change on the Ravansar-Sanjabi Plain aquifer and to optimally allocate land to crops in the cropping pattern to achieve the highest profit in line with the optimal management of water resources and soil conservation.
Method: The Ravansar-Sanjabi study area in western Iran is an open aquifer with a thickness of about 70 meters, and the water table varies between 1317 and 1352 m from the open water table from the southwestern to the eastern parts. The main surface flow of this plain is the Qarasu River. The optimization problem was modeled, analyzed, and solved using the simplex linear programming model and Lingo software (V.11) for three management scenarios and three climate scenarios. The WEAP software was used to simulate the resources and uses (drinking, agricultural, and environmental) in the statistical period 2006-2019. To study the hydrological effects of climate change on surface water resources, the IHACRES precipitation-runoff model (V2.1) was used to simulate the future runoff of the Qarasu River in the Ravansar-Sanjabi area. To study the effect of climate change on groundwater resources, the results of the LARS-WG exponential downscaling model were used under RCP 2.6, RCP 4.5, and RCP 8.5 scenarios for the future period of 2021-2040. The LARS-WG model was implemented using daily statistics from the years 1986-2005 of the station in question.
Results: The results of the linear programming section showed that, except for onion, rainfed chickpea, orchard, vegetables, and alfalfa, the rest of the crops were removed from the cropping pattern of the region in all management and climate scenarios. The most observed change was the substitution of rainfed chickpea for rainfed wheat and onion for irrigated wheat. Moreover, changing the cropping pattern in the Ravansar-Sanjabi Plain, while observing all the constraints in the region, increases the production profit (x ̅=3.54) in all the scenarios compared to the current situation. The results of the simulation of the study area by WEAP software showed that the drinking water demand was fully met during the statistical period 2006-2019, and the agricultural demand of the area faced a shortage of supply in most of the statistical period years, which was due to a decrease in the surface water resources of the Qara Su River. The results of the effect of climate change on the groundwater level showed that it decreased by about 20 to 60 cm in all months. The results of the monthly average minimum temperature in the base period (1986-2005) and the future period (2021-2040) showed an increase in the minimum temperature in the future period compared to the base period in the HadGEM2-ES, CanESM2, and CSIRO-MK3-6-0 climate prediction models under all three emission scenarios. The monthly average maximum temperature in all months also showed an increasing trend, which is expected to have a greater increase than the minimum temperature in the future period (2021-2040) than in the base period. The results of the precipitation assessment showed that the RCP8.5 emission scenario had the highest precipitation reduction and the RCP2.6 emission scenario had the lowest precipitation reduction in all three selected climate models.
Conclusion: According to the results obtained from the climate scenarios defined in the study, it can be concluded that policymakers and managers should use the present study as a reference for the development, exploitation, and optimal management of water and soil resources in line with comprehensive watershed management. Finally, to improve the groundwater situation and maximize agricultural profits, the results showed that several scenarios were developed to reduce and stabilize aquifer extraction. The results of the groundwater simulation show that if the trend continues, it will cause serious crises. The results of the integrated model also show that if the current situation continues, 15 to 25% of agricultural needs will not be met in some months, which, taking into account the effect of climate change, will increase this shortage by about 1 to 3% in the long term. The results of the optimization model section indicate that, except for onion, rainfed chickpea, garden, vegetables, and alfalfa, the rest of the crops should be removed from the cropping pattern of the region in all management and climate scenarios. However, if the optimization model is implemented and aquifer extraction is reduced from 20% to 50%, farmers can make more profit than in the current situation. Therefore, to maintain the status of groundwater resources and the environment of the region, and to pay attention to the farmers' profit, regional managers should always take serious measures to limit the use of groundwater resources and modify the cropping pattern and cropping area based on the results of this research.