بررسی اثر خشکسالی هیدرولوژیک و بهره‌برداری سنتی (توزیع، انتقال) از منابع آب سطحی بر روند تخریب زمین و بیابان‌زایی در مناطق خشک (مطالعۀ موردی: دشت سیستان)

هدف از اجرای این پژوهش، بررسی اثر خشکسالی هیدرولوژیک و بهره‌برداری سنتی از سیلاب بر روند تخریب زمین و بیابان‌زایی در منطقۀ سیستان است. بدین منظور در ابتدا به بررسی وضعیت خشکسالی هیدرولوژیک در رودخانۀ هیرمند پرداخته شد. با استفاده از شاخص جریان رودخانه (sdi) خشکسالی هیدرولوژیک برای یک دورۀ 22 ساله محاسبه شد. به‌منظور بررسی تاثیر نحوۀ بهره‌برداری از منابع آب سطحی بر روند بیابان‌زایی و تخریب اراضی در منطقه، نقشۀ تغییرات کاربری‌ اراضی و الویت دسترسی مناطق به منابع آب سطحی با یکدیگر هم‌پوشانی شد. نتایج بررسی شاخص جریان رودخانه (sdi) بیانگر وقوع 18 سال خشکسالی در طول دورۀ مورد مطالعه می‌باشد که طی سال‌‌های اخیر شدت آن نیز افزایش یافته است. با مقایسۀ مساحت کلاس‌های کاربری در طی سال‌های عدم وقوع (1382) و وقوع (1397) خشکسالی، نیز نتایج بیانگر افزایش 41 درصدی سطح اراضی بایر و همچنین کاهش به‌ترتیب 84، 67، 57 و 31 درصدی پهنه‌های آبی، مرتع، سطح زیر کشت محصولات کشاورزی و مناطق مسکونی بوده است. در بررسی نحوۀ‌ بهره‌برداری از جریان‌های سیلابی، یافته‌های این پژوهش نشان داد که توزیع و انتقال سیلاب‌های ورودی به منطقۀ سیستان با توجه به فصلی بودن و تداوم نامشخص آن‌ها، محدودیت دسترسی به این منابع آب سطحی را در مناطقی از جمله شهرستان‌های هیرمند (بخش قرقری)، نیمروز (بخش پشت‌آب) و زابل (بخش‌ مرکزی) به همراه داشته است. همچنین نتایج بیانگر آن است که بیشترین تغییرات‌ کاربری‌ها مربوط به تبدیل پهنه‌های آبی (معادل با 12581 هکتار)، اراضی کشاورزی سیلابی (معادل با 22 /3073 هکتار) و مراتع ضعیف (معادل با 8/4495 هکتار) به اراضی بایر بوده که از لحاظ فضایی منطبق با مناطقی است که در دسترسی به جریان‌های سیلابی با محدودیت همراه‌اند. بر این اساس با توجه به نتایج به‌دست‌آمده، خشکسالی هیدرولوژیکی و بهره‌برداری سنتی از جریان‌های سیلابی با توجه به نظام آبیاری و شبکۀ آبرسانی، تغییرات محسوس کاربری اراضی را با تخریب اکوسیستم‌های وابسته به آب به‌ همراه داشته که باعث گسترش مناطق بیابانی و نقاط بحرانی در سیستان شده است.

Investigating the Effect of Hydrological Drought and Traditional Utilization (Distribution and Transmission) of Water Resources (Flood Streams) on Land Degradation and Desertification in Drylands: A Case Study of Sistan Plain

Introduction: Water is crucial for survival and development. Longterm access to water resources provides the ground for scientific and technological developments. Geographically, Iran is located in an arid part of the world. Water scarcity and mismanagement could create a water crisis in the world, and that droughts may exacerbate different environmental and socioeconomic aspects of life. As revealed in a report issued by the United Nations rsquo; Environment Programme, 80 percent of the world rsquo;s population currently live in places with no water security, and that 1.5 billion people throughout the world are facing anhydrous risk. As Iran has, in recent years, been affected by drought, access to water for agricultural, industrial, and drinking purposes is now of great concern for inhabitants of the country rsquo;s drylands. According to the reports issued by the World Bank, mismanagement and inefficiency of irrigation systems are the main reasons for water insecurity in the world. In recent years, mismanagement of available water resources in the arid region has followed by the destruction of natural ecosystems that are dependent on water resources. As a result, changes in land use over a long period and the expansion of desert areas have led to environmental degradation. Sistan is located in an arid region in Iran where difficulty to access surface water resources due to drought in recent years has brought about environmental degradation in the region. Low precipitation (50mm), high temperature (48o), high transpiration (5000mm), and 120day winds are among the specific climatic conditions that characterize the region. Sistan is also hit by floods for a limited period when the Hirmand river flows as a result of torrential rainfall in Afghanistan, providing the required water for its 120,000 ha agricultural land. Materials and methods: This study sought to investigate the environmental impacts of hydrological drought and traditional utilization of floods streams on Land Degradation and Desertification in Sistan. Located at the southeastern border of Iran and Afghanistan, Sistan forms part of the DashteLut desert land, possessing an extreme arid climatic regime. The region is a roughly flat and featureless arid plain with an average height of 475 -500m ASL in its eastern parts. To realize the purpose of the research, the hydrological drought status of the Hirmand river was investigated. Using the Streamflow Drought Index (SDI), the hydrological drought was then calculated for 22 years. The distribution and utilization of flood streams in the Sistan area were examined via Infield operation. On the other hand, the Irrigation system of the area was examined through library studies and the factors affecting the circulation of surface water resources were identified. Weighting these variables via Analytic Hierarchy Process (AHP) and integration of data layers, access priority map of sectors to the water resource was estimated. The landuse map of the years when the hydrological drought occurred and didn rsquo;t occur were extracted through the Landsat satellite images of the years 2002 and 2018. The identification of changes in the region was then carried out by assessing the accuracy of the images and their classification, using the comparison approach. Moreover, the landuse change maps were overlapped whit the access priority map of sectors to the water resource to investigate the effect of the utilization method on desertification and land degradation. Result: The findings of this study showed that the severity of drought in Sistan has increased over the past 18 years. On the other hand, the findings for the areas with years of hydrological drought indicated that barren land had increased by 41 percent, and that agricultural, residential, and water zone areas had decreased by 61, 31, and 84 percent respectively. It was also found that the distribution and transmission of incoming floods to the Sistan area lead to limited access to these surface waters in areas such as Hirmand (Gregory sector), Nimroz (post ab sector), and Zabul (Central sector) because of their seasonality and uncertain continuity. The most landuse changes were found to be the conversion of water zone, (equivalent to 12581 hectares) agricultural lands (equivalent to 3073/22 ha), and poor rangelands (equivalent to 4495/8 hectares) into barren lands where access to floodwaters is limited.Discussion and Conclusion: This study proved the clear environmental effects of hydrological drought and traditional utilization of water resources on the Sistan region. Reduction of streamflow in the Hirmand river due to the recent occurrence of hydrological droughts and limited access to water resources because of the irrigation system and the traditional methods of utilizing water resources led to the landuse change as a result of the destruction of waterdependent ecosystems, increasing the desert areas and critical points in the Sistan area accordingly. Having said that, breaking the drought, improving environmental conditions, and using efficient production methods require new approaches for proper utilization and management of water resources in the Sistan area.