اثر تغییر کاربری اراضی بر رواناب بخشی از حوضه ی آبخیز قره سو اردبیل

تغییرات کاربری اراضی، یکی از عواملی است که بر خصوصیات رواناب سطحی و سیلاب در سطوح مختلف حوضه‌ها، تاثیر می‌گذارد. هدف از پژوهش حاضر بررسی اثر تغییرات بوجود آمده در کاربری و پوشش اراضی بخشی از حوضه ی آبخیز قره سو اردبیل در بازه ی زمانی 2012-1992 بر روی ارتفاع رواناب حوضه است. بدین منظور ابتدا با استفاده از نقشه ی گروه هیدرولوژیکی خاک و نقشه ی کاربری اراضی مربوط به سال های 1992، 2002 و 2012، شماره ی منحنی منطقه به دست آمد و سپس با استفاده از این نقشه ها، میزان نگهداشت ویژه ی خاک (s) محاسبه و میزان رواناب از طریق روش scs برآورد شد. نتایج نشان داد که در طول دوره های مورد مطالعه، مساحت کاربری های اراضی جنگل، زراعت آبی و زمین بایر به ترتیب2.54، 16.69 و 1.19 درصد کاهش پیدا کرده و مساحت کاربری های مرتع، زراعت دیم و مناطق مسکونی به ترتیب 5.74، 12.39 و 2.29 درصد افزایش یافته است. این تغییرات باعث افزایش شماره ی منحنی از 78.57 در سال 1992 تا مقدار79.77 در سال 2012 شده و به دنبال آن نگهداشت ویژه از 69.28 در سال 1992 به مقدار 64.42 در سال 2012 کاهش پیدا کرده است. تغییرات این عوامل باعث افزایش رواناب از مقدار 263.4 میلی‌متر در سال 1992 تا 297.07 میلی‌متر در سال 2012 گردیده است که بیانگر افزایش 33.67 میلی متری (11.33 درصد) رواناب در طول دوره ی مورد مطالعه می‌باشد. محاسبه ی ضریب همبستگی بین کاربری های مختلف و شماره ی منحنی و ارتفاع رواناب نشان داد که این متغیرها با کاربری های مرتع، زراعت دیم و مناطق مسکونی رابطه ی مستقیم دارند، در حالی که با کاربری های جنگل، زراعت آبی و زمین بایر، دارای رابطه ی معکوس هستند.

The Effects of Land Use Change on Surface Runoff in a Part of Gharehsu Watershed in Ardabil

1IntroductionGiven the growing population and the increasing need for food, water and soil conservation are of great value. In the context of conservation of soil and water resources, information on the amount of runoff production and erosion to achieve sustainable development is the basis for planning and decision making. Therefore, careful investigation of surface runoff and floods is an important and key step in planning and managing optimal water resources. One of the factors affecting the characteristics of surface runoff is landuse changes at different basin levels (Melesse and Shih, 2002). Land use is influenced by two components of human needs and environmental processes. Inaccurate landuse changes will disrupt the water cycle from natural equilibrium, resulting in devastating floods, including economic damage, loss of life, loss of water, and consequently reduced water resources (Jakeman et al., 2005). During the last two decades, the Qharehsu watershed, particularly its downstream, has been experiencing rapid growth in the construction and expansion of residential structures. Human activities and changes in the basin have affected the natural arrangement of stream processes that transmit water and sediment from upstream to downstream. Human interventions are one of the major hazardous issues in this basin that causes changes in the pattern of surface currents and natural conditions of the catchments and encroachment on rivers and streams. In this regard, the present study aimed to investigate the role of landuse change on runoff in a part of Qharehsu watershed in Ardebil province over a period of almost 20 years due to the availability of information and access to satellite images of different time periods.2MethodologyThe study area consists of a part of Qharehsu watershed located in Ardabil province with an area of 2162.6283 km2. The minimum and maximum elevation of the mentioned watershed are 1280 and 3829 m respectively, and its average slope is 11.57%. Land use in this area often includes dry and irrigated agriculture, pasture, forest, and residential areas. The aim of the current research is to study the effect of different land uses and its changes during the years 19922012 on the surface runoff in a part of Qharehsu, Ardabil watershed. At first, the maps of land use and curve number in the mentioned years were gathered and the area of each of the units was extracted. In the following, the process of landuse changes in the cases of the study period and its effect on changing the specific retention (S) and curve number were calculated and the height of runoff was estimated using the SCS method.3ResultsThe results showed that during the case of the study period, area of forest, water farming, and wasteland land uses were decreased by 2.54%, 16.69%, and 1.19% respectively and the area of the rangeland, dry farming, and urban land uses were increased by 5.74%, 12.39%, 2.29% respectively. These changes have caused the increase of curve number from 78.57 to 79.77 in the years 1992 and 2012, respectively and following the decrease of the specific retention (S) from 69.28 mm in the year 1992 to 64.42 mm in the year 2012. Also, runoff height has increased from 263.4 mm in the year 1992 to 297.07 mm in the year 2012 (11.33%). Calculation of correlation coefficient between different land uses and curve number and runoff height showed that these variables have a direct relationship with rangeland, dry farming, and  urban land uses while they have an inverse relationship with the forest, water farming, and wasteland.4Discussion and conclusionIn the present study, the results of the study showed that landuse change due to its effect on the curve number of the studied basin causes a change in the surface runoff. During this 20year period, land use has changed and this landuse change has tended to decrease from 1992 to 2012 land use including residential, pasture and dryland areas increased by 2.29%, 12.39% and 5.74% respectively, as well as forest, water and wastewater land use decreased by 2.54%, 16.69% and 1.19%, respectively. As a result, its curve number has increased, followed by a runoff height of 11.33%. This shows that in a natural ecosystem, land use and environmental changes, especially vegetation and land use affect the hydrological responses such as flooding and erosion and sedimentation rate in the area. Ultimately, it will cause severe economic and social damages. Changes in the total volume of runoff and changes in hydrological balance are the most important effects of landuse change on watershed hydrology.