ارزیابی پیامد‌های درونی‌سازی اثرات جانبی آلودگی آب بر مدیریت کمی و کیفی حوضه آبریز زاینده‌رود

 
Introduction: The increase in water demand and the expansion of water pollution due to the development of agricultural, urban and industrial activities have led to a serious risk of water quality in many places. Therefore, its rational and logical management has become very difficult and complicated. In recent decades, concerns about water pollution from agricultural activities and its consequences have been growing. The existing regulations are not sufficient to limit the water pollution of the agricultural sector and to achieve the desired environmental consequences. Thus, economic tools have increasingly been proposed as an affordable way to limit pollution. Therefore, the side effects of water consumption in agriculture are vital issues for controlling and managing water pollution. The recent challenges in water resources of the Zayandehroud basin have led to the fact that this area has not been safe from water pollution and also the supply of high quality water is a major challenge in this basin. Therefore, providing a purposeful cropping pattern by reducing the side effects of water pollution caused by agricultural activities for the Zayandehroud basin can play an effective role in the quantitative and qualitative management of watershed resources.
Materials and Methods: In this study, the water resources management system of Zayandehroud basin has been modeled based on a multi-objective programming model. This model includes hydrological functions, land allocation, resource transfer and exploitation capacities, and the objective function is to maximize the net present value of the total benefits at the basin level. Also, the amount of water available in different sub-basins, the crop yield and net water requirements was simulated using the WEAP model for the 2040 horizon. This data was used as input in the economic model. In the next step, the side effects of water pollution are estimated and internalized in the economic model using permissible limit of water pollution in constraint and the cost of water pollution in the objective function. The mechanism designed to internalize the side effects of water pollution is simulated using the SWAT model and added to the integrated water management model of the basin as environmental constraint and cost of nitrate losses in objective function. Therefore, by comparing the results of these two models, it is possible to evaluate the internalization of the side effects of water pollution on farmers' livelihoods and the cropping pattern in the basin.
Results and Discussion: Cropping pattern under basic conditions was applied in WEAP software for different regions. Yield and net water requirements of products were simulated using MABIA tools. The results were extracted by each region and then estimated at basin level. The results of the optimal cropping pattern after simulating the hydrological parameters of the basin showed that the gross margin compared to the current conditions for Najafabad, North Mahyar, Lenjanat, Kuhpayeh-Segzi, Isfahan-Borkhar and Ben-Saman regions was 14, 5, 15,18, 15 and 20 million Rials per hectare, respectively. The increase in the share of irrigation technologies in the economic model compared to the current model for Najafabad, Lenjanat, Kuhpayeh-Segzi, Isfahan-Borkhar and Ben-Saman regions was 40, 57, 35, 45 and 91 percent, respectively.Therefore, it can be expected that by changing the cropping pattern and also increasing the use of new irrigation systems, it is possible to improve the livelihood of farmers in the basin according to the current and future hydrological conditions. But these changes have increased the side effects of pollution on the basin. Therefore, it is necessary to provide a model that, in addition to improving the benefit, also reduces the cost of nitrate losses. The results of the optimal bioeconomic model indicate that with the application of this model, the rate of gross margin and the cost of nitrate losses have been obtained at 58 and 28 million Rials per hectare, respectively. Comparison of this model and the optimal economic model shows that farmers' gross margin and nitrate losses have decreased about 3 and 2 million Rials per hectare, respectively. Comparison of the current and optimal bioeconomic model also shows that while improving the gross margin by 12 million Rials, the rate of nitrate losses below the allowable level in the basin can be realized.
Conclusion: Zayandehroud basin is one of the most important watersheds in the country, which is facing the challenge of quantitative and qualitative water shortages. The main purpose of this study is to quantitatively and qualitatively manage water resources and evaluate the consequences of internalizing the side effects of water pollution on this type of management. The results of the study showed that using the optimal economic cropping pattern will increase the farmers' gross margin and improve their livelihoods.  Also, comparing the results of the economic model with the multi-objective bioeconomic model showed that considering the side effects of water resources pollution in some areas is effective and in others is ineffective. Therefore, it is recommended that in future studies, considering the effective role of different policies in the field of water resources quality, the effects of different scenarios of climate change, drought, population growth, etc. be examined and analyzed.