ارزیابی هزینه اثربخشی سیاست اعطای کمک بلاعوض سامانه‌های آبیاری تحت فشار در حفاظت از منابع آب زیرزمینی

برداشت بی‌رویه از منابع آب زیرزمینی در سال‌های اخیر موجب بدتر شدن وضعیت این منابع ارزشمند شده است. یکی از دشت‌های ممنوعه کشور که شاهد افت شدید سطح تراز آب زیرزمینی بوده است، دشت قزوین می‌باشد. امروزه سیاست‌های مختلفی به منظور حفاظت از منابع آب زیرزمینی در جهان رایج است. از جمله سیاست‌هایی که در کشور ما نیز برای صرفه‌جویی آب آبیاری و حفاظت منابع آب تاکید زیادی بر آن شده و در قانون برنامه پنجم توسعه نیز بدان اشاره شده است، اعطای کمک بلاعوض سامانه‌های آبیاری تحت فشار می‌باشد. در مطالعه حاضر تلاش شده است با تشکیل یک مدل ریاضی بهینه‌سازی پویا و معادله تعادل آب ‌زیرزمینی در آن تاثیر اجرای این سیاست بر صرفه‌جویی آب توسط کشاورزان و حفاظت آب زیرزمینی در دشت قزوین ارزیابی گردد. نتایج این تحقیق نشان داد که با افزایش سهم پرداخت کمک بلاعوض سامانه آبیاری و در نتیجه بهبود کارایی آبیاری، مصرف کل آب سطحی و زیرزمینی در کل منطقه کاهش یافته و موجب صرفه‌جویی آب می‌شود. این نتیجه‌گیری برای پمپاژ آب زیرزمینی نیز صادق بود. لذا، اجرای این سیاست برای کاهش پمپاژ آب اثربخش تشخیص داده شد. از سوی دیگر، به علت کاهش نفوذ آب به عمق با افزایش سهم پرداخت دولت، کسری ذخیره آبخوان کاهش می‌یابد. به عبارت دیگر، اجرای این سیاست منجر به حفاظت از منابع آب زیرزمینی نمی‌شود و بررسی معیار هزینه اثربخشی نشان داد که این سیاست برای حفاظت آب زیرزمینی اثربخش نمی‌باشد.

The Costeffectiveness Evaluation of the Grant Policy for under Pressure Irrigation Systems in Conservation of Groundwater Resources

Introduction: Today, governments have to adopt different policies to offset water scarcity and balance groundwater resources. Among the policies that have been emphasized in this regard, are subsidies and incentive payments to use modern irrigation technology. In Iran, the policy of grants for under pressure irrigation systems is mentioned in the budget law of the country. It is generally believed that the implementation of this policy can save water and would lead to the conservation of groundwater resources. But, in practice this issue has not yet been proven. It is generally believed that the implementation of this policy can save water and lead to the conservation of groundwater resources. But, in practice this issue has not yet been proven. So, the Qazvin plain as one of the biggest and the depleting aquifers in Iran is selected as case study.;Materials and Methods: Since, the irrigation water is supplied through the canal in the modern irrigation network area and the agricultural wells in the traditional cultivations, the objective function of the dynamic programming model is considered to maximize the net present value of cultivating in the irrigation network area and the land of Qazvin plain area. The crop yield in the objective function of the model is the functional of the quantity of water available for plant, which is expressed in the quadratic form. The groundwater pumping costs were considered as a function of pump lift. By pumping groundwater, the saturated thickness would decrease in two area. Therefore, the equation of the groundwater balance was formed to allow changes in aquifer stock from one period to another. The components of this equation were considered as the difference in the inflow to the aquifer from the outflow of the aquifer. The maximum groundwater extraction in the model is limited to the natural capacity of pumping from wells and access to the canal water is confined by the longterm average canal water. It is named the first scenario. The upper limit of access to canal water that supplied from combined wells and artificial recharge was simulated according to rainfall variations and then its average longterm value was calculated. For this purpose, the probability of occurrence of dry, normal and wet years was calculated by the standard precipitation index (SPI) using monthly data of 60 years.;Results and Discussion: This paper presents an analytic model of the effectiveness of groundwater conservation policies on irrigated agriculture in Qazvin plain such as grant for under pressure irrigation systems. The results indicated that by increase in the share of grant, the groundwater used per hectare in the modern irrigation system is lower than the flood irrigation system and gradually the modern system is chosen instead of the flood system. With increasing share of grant the yield and net profit will be increased for each crop until the yield reaches saturation. The results showed that in the whole area, by increasing the share of grant, total water consumption and total groundwater are decreasing and so water will be saved. On the other hand, by increasing the share of grant, the deep percolation into the depth decreases. Investigating the groundwater balance showed that in the normal conditions, the increase in the share of grant cause to increase aquifer stock deficit and reduce the groundwater head and saturated thickness. The empirical findings in Qazvin plain showed that the long term average of the aquifer 's reservoir volume is decreasing with increasing the share of grants. In the final, the cost effectiveness analysis (CEA) was calculated for total saved water and the aquifer stock to the total amount of government payments.;Conclusion: A review of the effect of grant policy for investing in the modern irrigation systems on the water use was considered efficient on saving water. It means that with increasing the share of grant the overall water saving in the area will increase. But, the implementation of this policy does not conserve groundwater resources. So, for each a million tomans payment, 665 and 598 cubic meters of water are saved, respectively. However, this policy is not effective for the conserve of groundwater resources because in the share of 100% for each a million tomans payment, the aquifer stock is reduced to 2912 cubic meters. As a final remark, it is suggested that grant policy be accompanied by a limitation on groundwater pumping and crop pattern.