ارزیابی سیاست قیمت گذاری آب بر پایداری و حفظ آبخوان: مطالعه آبخوان مهیار شمالی در حوضه زاینده رود

برداشت های بی رویه از منابع آب زیرزمینی در دشت مهیار شمالی واقع در حوضه آبخیز زاینده رود، سطح آب در آبخوان این دشت را به شکل قابل توجهی کاهش داده و منابع آب زیرزمینی به عنوان مهمترین منبع تامین کننده آب مورد نیاز بخش کشاورزی این منطقه را با بیلان منفی مواجه کرده است. بر این مبنا، مدیریت آبخوان از منظر حفظ و پایداری منابع آبی آن از اهمیت ویژه ای برخوردار است، به ویژه برای این منطقه که اقتصاد آن مبتنی بر تولید محصولات کشاورزی باشد. هدف این مطالعه تحلیل و ارزیابی تاثیر سیاست قیمت گذاری آب به عنوان یکی از سیاست های مربوط به حفظ منابع آب زیرزمینی بر متغیرهای اقتصادی و هیدرولوژیکی آبخوان دشت مهیار شمالی می باشد. برای این منظور، یک مدل برنامه ریزی ریاضی مثبت (pmp) به شکل پویا گسترش یافت اثرات مربوط به اعمال سناریوهای مختلف قیمتی بر پارامترهای مورد نظر در طول یک دوره 20 ساله توسط این مدل شبیه سازی شد. نتایج نشان داد در محدوده آبخوان مورد مطالعه و در مقایسه با شرایط فعلی حاکم در منطقه، لحاظ سیاست قیمت گذاری منابع آب زیرزمینی با تغییر الگوی کشت در جهت محدود تر شدن درصد کشت محصولات با آبیاری غرقابی و گسترده تر شدن درصد کشت محصولات با آبیاری تحت فشار و در نتیجه تعدیل روند بهره برداری بی رویه از منابع کمیاب آب، متغیرهای هیدرولوژیکی اعم از بیلان آب زیرزمینی، ضخامت لایه اشباع آبخوان و سطح ایستایی آبخوان می تواند در سطح قابل ملاحظه ای بهبود یابند. با این حال، به دلیل تاثیر منفی افزایش قیمت آب بر منافع اقتصادی در بخش کشاورزی منطقه، ضروری است سیاست قیمت گذاری آب به گونه ای اعمال گردد که علاوه بر بهبود پایداری کشاورزی و حفظ منابع حیاتی آب، منافع اقتصادی و اجتماعی بخش نیز دچار نوسان زیادی نگردد.

Evaluation of Water Pricing Policy on Aquifer Stability and preservation: Study of Northern Mahyar Plain Aquifer in ZayandehRud Basin

Introduction: Irregular consumption of groundwater resources in the northern Mahyar plain that is located in Zayande Rood basin river has considerably decreased the water’s level of aquifer of this plain and groundwater resources as important resources of water supply for agriculture of this region have been faced with a negative balance. On this basis, Aquifer management in terms of preserving and sustainability of water resources is very important. The regional economic dependence on irrigated agriculture and the decline of the Mahyar Aquifer due to agricultural pumping have been much of the basis for the relatively recent governmental interest in developing policy alternatives for conserving water in the aquifer. Especially, for the area 's economy is based on agricultural production. Although governments supportive payments to agricultural activities are fulfilled for the purpose of increasing of social benefits, it will cause some economic and environmental externalities. The objectives of this study were to analyze and evaluate the outcomes of water price increment as specified water conservation policy alternatives on groundwater consumption and cropping patterns on the aquifer of northern Mahyar plain in Isfahan province, using nonlinear optimization models. Materials and Methods: The objective of this study is to analyze the effects of irrigation water pricing scenarios on groundwater balance and the modification in farmers ' cropping patterns in the northern Mahyar plain. The objective function in this study is to maximize net present value to land, management, groundwater, and irrigation systems over a twenty years planning horizon. For these purposes, A Positive Mathematical Programming (PMP) model is considered in a dynamic framework and was calibrated to land use data for a base period of 20042005. The effects of the eight pricing scenarios of water in the range of zero to 1800 (IRR) on parameters over a period of twenty years were simulated by the model. PMP approach produces a constrained nonlinear optimization model that mimics the land and water allocation decision facing producers each year The choice variables in the model are the acreages planted to each of the major crops and the water use by crop. This method is a useful approach in the determination of planning strategies and policies for groundwater systems. This method was intended to collect data and surveybased study of 250 questionnaires, documents, and use of views and experiences of experts. Results and Discussion: The results derived from the model employed in this work showed that water pricing alone can potentially reduce considerably the consumption of aquifer groundwater, and adjust the trend of overexploitation of groundwater by irrigated agriculture. This effect is generated mainly as a result of water pricing providing incentives to the modification of farmers ' cropping patterns through the substitution of crops with higher water requirements by others which use less water. Therefore, another modification in farmers ' cropping patterns is substitution of crops with traditional irrigation by modern irrigation system. In fact, it has been analyzed that under certain structural and economic conditions, the effect of pricing policies can generate significant levels of water savings. The chief generalization is that the high water price scenarios significantly decreased the rate of decline in saturated thickness. Regardless of the price scheme, the saturated thickness will increase so long as the rate of recharge exceeds the irrigation rate. Therefore, it stands to reason that we should observe an increase in the saturated thickness than a base price scenario. A limitation of this proposed model regards the assumption of constant technology for both irrigation systems and pumping engines, which is significant in affecting the results. Conclusions: Water pricing policies can affect water saving, at least in some situations, depending on water tariff rates, external conditions, and type of consumer behavior. Given the initial volume of groundwater in the northern Mahyar plain, it is imaginable that critical conditions will be reached in the near future by continuing the base scenario. A nonlinear dynamic programming model in the framework of PMP techniques has been built to analyze the effects of water pricing scenarios on groundwater conservation of northern Mahyar plain. The Positive Mathematical Programming (PMP) method proved to be very effective. As seen in the data section there was not a plethora of information (data) on this subject. Therefore, it was important that the method chosen for this study could use a minimal amount of data while still maintaining accuracy. The PMP method was able to take the minimal data, calibrate to it, and then effectively reproduce the observed results. Additionally, the production function was able to be incorporated into this process seamlessly while maintaining accuracy. The result shows that under the high price scenarios, water application per acre decreased significantly during the early years of the simulated period. This suggests that high water prices have a significant effect on the rate of increment of the northern Mahyar aquifer stability. This is one of the first times that the PMP method has been incorporated into an applied groundwater management problem such as this, while also incorporating hydrological and agronomic information over time.