اثر احداث پخش سیلاب در آبدهی قنوات پایین‌دست (مطالعۀ موردی: سامانۀ پخش سیلاب مهریز یزد)

با توجه به رشد جمعیت، کمبود بارندگی در مناطق بیابانی و نظر به اهمیت منابع آب‌های زیرزمینی، طرح‌های مختلفی برای تقویت این ذخایر اجرا شده است. یکی از طرح‌های مذکور، ایجاد و احداث پروژه‌های پخش سیلاب است. ارزشیابی اثرات پروژه‌های اینچنینی در قبل از احداث معمولاً میسر نیست و بعد از اجرا نیز کمتر مورد توجه قرار می‌گیرد. لذا در این پژوهش سعی بر آن است تا با بررسی پروژۀ پخش سیلاب ابراهیم‌آباد مهریز که در سال 1997 اجرا شده است، اثرات آن در افزایش آبدهی قنوات پایین‌دست شامل قنات‌های خیرآباد، بغدادآباد، مزویرآباد و محمدآباد بررسی شود تا بتوان از نتایج آن برای مدیریت منابع آب استفاده کرد. بدین منظور در دو دورۀ ده‌ساله قبل و بعد از اجرای پروژه، آمار آبدهی قنوات در اطراف پروژه پخش سیلاب مورد تجزیه و تحلیل قرار گرفت. نتایج پژوهش با استفاده از آزمون مقایسۀ نمونه‌های جفتی و تحلیل هیدرولوژیک میزان آبدهی منابع پایین‌دست نشان می‌دهد تاثیر اجرای طرح بر قنات‌های بغدادآباد، خیرآباد و مزویرآباد مثبت بوده و در قنات محمدآباد تاثیری نداشته است. نتایج آزمون کای اسکوئر نیز نشان می‌دهد که اجرای طرح بر کاهش تعداد سیلاب‌ها و افزایش آبدهی قنوات پایین‌دست تاثیر مثبت داشته است.

Investigating the Effect of Flood Displacement on Downstream Qanat Drainage (Case study: Mehriz flood spreading)

Expanded abstractsIntroduction: In recent decades, floodspreading and artificial flood recharge have been used at the proper time and place for optimal utilization of floods in arid and semiarid regions. The importance of these projects in Iran is mainly due to the excessive use of groundwater resources and recent droughts, which have rapidly decreased the level of underground aquifers. Therefore, in a country like Iran, which is regarded as an arid and semiarid region, performing such projects is essential for water resources management and flood control. Evaluation of the performance of watershed management projects in different countries worldwide has a history of more than 80 years. Thus, this study sought to determine the longterm effects of this project on flood control and underground water recharges, especially in dry years, through a technical investigation of the Mehriz floodspreading project.Material and Method: This study used technical evaluation methods to evaluate the Mehriz floodspreading project, which was carried out in two ways: statistical analysis and fieldwork. In the first method, all necessary data were collected and analyzed. In the second method, field visits to the project were made, the pros and cons of the note plan were identified, and the technical evaluation of the plan was performed. This way, the discharge of qanats was investigated before and after applying the waterspreading project. Result: The comparison of average annual discharge in Fakhrabad Riverchr('39')s hydrograph before and after the project shows that from July to November, the flow rate in each period was equal to zero, and from December to April, the riverchr('39')s flow rate was less in the preproject period than what it was in the postproject period. Moreover, the average discharge rate in the month before the project began was lower than the rate reported after the project finished. Also, the average rate of postproject discharge in May and June was less than the rate reported for the preproject period. The studychr('39')s results indicate that while most preproject floods occurred in late spring and early summer, the postproject floods occurred in late winter and early spring. Discussion and conclusion: As mentioned earlier, this study investigated the effect of the Mehriz floodspreading project on qanat discharge. According to the studychr('39')s results, Baghdadabadchr('39')s qanat in preproject years was less than that of control 1 (Ibrahim Abad) and control 2 (Henza). During the years since the project was implemented, these qanatschr('39') discharge rate was higher than that of both control qanats. A comparison of the threeyear mean discharge of the Baghdad qanats in May, August, and February in two tenyear periods before and after the implementation of the project showed that these qanatschr('39') discharge rate was increased in preproject years. Therefore, implementing the floodspreading project has helped increase the volume of water in those qanats. Also, the result of the statistical ttest showed that this aqueductchr('39')s flow rate has increased in preproject years.The changing trend of the Khairabad qanatchr('39')s discharge in Yazd indicated the positive effect of implementing the floodspreading project on the qanatchr('39')s discharge in postproject years. On the other hand, the result of the ttest for Mohammadabad qanat before and after the implementation of the project suggested that the qanatchr('39')s discharge rate in preproject years was less than the rate reported for the postproject years. Also, a hydrological comparison of the average quarterly discharge rates of the qanats with those of the control qanats pre and postproject years showed that the discharge rates of Khairabad and Mohammadabad qanats had gone through a downward trend from 1987 to 2007, indicating no effect of the floodspreading project on increasing the discharge rate, which is considered as one of the drawbacks of the flood distribution plan. This could be attributed to the fact that the project area is located in a place where flood water naturally spreads and nourishes most channels. Moreover, diverting the water of the Fakhrabad channel and spreading the flood water to a particular place prevents some downstream qanats (such as Mohammadabad) from being fed by the floodwater, which, in turn, may upset the floodschr('39') balance and negatively affect the behavior of the basinchr('39')s residents.