تعيين عوامل مؤثر بر گسترش آبكندهاي حوزه‏ي آبخيز مدرس شوشتر

Determination of Effective Factors in Gullies Development in Modares Watershed of Shushtar

منطقه مورد مطالعه حوزه آبخیز شهید مدرس شوشتر می‌باشد كه بخش‌های زیادی از اراضی كشاورزی آن مورد تهدید فرسایش آبكندی واقع شده است. هدف از این تحقیق شناسایی عوامل مؤثر در پیشروی طولی فرسایش آبكندی و مدل‌سازی آن‌ می‌باشد. به همین منظور ابتدا نقشه پراكنش اقلیمی و آبكندی استان تهیه و سپس از هر طبقه اقلیمی یك حوزه آبخیز كه دارای بیش‌ترین خسارات فرسایش آبكندی بود انتخاب شد و در نهایت حوزه آبخیز هدام از طبقه اقلیمی گرم- خشك بیابانی و حوزه آبخیز شریف از طبقه اقلیمی گرم- نیمه خشك انتخاب شد و از هر حوزه آبخیز 15 عدد آبكند انتخاب و پارامترهای مختلف در این آبكندها بررسی و اندازه‌گیری شد. در نهایت رابطه بین پیشروی طول آبكند با ویژگی‌های آبخیز، ویژگی‌های فیزیكی و شیمیائی خاك، درصد پوشش‌گیاهی، سنگ‌ریزه، خاك لخت، لاشبرگ، شیب، مساحت بالای پیشانی آبكندها، بارندگی و مساحت كاربری‌های مختلف در طی یك دوره 20 ساله (1391-1372) مورد بررسی قرار گرفت. مدل‌سازی پیشروی آبكند با استفاده از رگرسیون چندمتغیره در نرم‌افزار SPSS با استفاده از روش گام به گام انجام گردید. نتایج حاكی از این است كه در حوزه آبخیز هدام بیش‌ترین رشد طولی آبكند در دوره بررسی، مربوط به آبكند شماره A17 به میزان 8/78 متر است و آبكند B2 كم‌ترین میزان رشد طولی را به میزان 8/3 متر داشته است. در حوزه آبخیز شریف بیش‌ترین رشد طولی آبكند مربوط به آبكند شماره B3به میزان 1/108 متر و آبكند B4 كم‌ترین میزان رشد طولی به میزان 1/10 متر را در طی این 20 سال داشته است. نتایج حاصل از خوشه‌بندی آبكندها نشان داد كه آبكندهای حوزه آبخیز هدام در دو خوشه و آبكندهای حوزه آبخیز شریف در سه خوشه قرار گرفتند. معادله نهایی برای گسترش طولی آبكندهای هدام در خوشه یك نشان می‌دهد كه میزان گسترش طولی تابع سه متغیر مساحت بالای نقطه اولیه (X4)، درصد خاك لخت (X6) و شیب نقطه اولیه (X2) است. در خوشه دو، میزان گسترش طولی تابع دو متغیر درصد سیلت (X18) و مقدار اسیدیته (X23) است. معادله نهایی برای پیشرفت آبكندهای شریف در خوشه یك نشان می‌دهد كه میزان پیشرفت تابع مساحت بالای پیشانی (X3) است. در خوشه دو، میزان پیشرفت تابع دو متغیر مساحت بالای نقطه اولیه (X4) و سدیم (X22) است.

Introduction: Khuzestan province with 4.6 million hectares area has suitable agricultural lands that gully erosion destroyed some parts of them. One of the major basins in which much of its agricultural land threatened by gully erosion is located in Modarres watershed of Shushtar. Gully erosion progression in this basin causing the loss of farmland and thus the farmers are leaving farming and consequently would lead to poverty and migration to urban areas and social consequences. Therefore, presenting a model to determine the factors that impact on gully erosion and erosion hazard zonation for conserving and stability of the land need to research and investigate on gullies of Khuzestan province. Materials and Methods: Study area is located in Shahid Modares basin that large parts of these farm lands destroyed by gully erosion. The aim of this project is to identify the main factors affecting on linear gully development and modeling them. For this purpose, distribution maps of climate and the gully was determined. Then a Watershed from each climate class with the highest losses was selected. At last Haddam watershed with warm-arid climate and Sharif watershed with warm-semi arid climate were selected. And 15 gullies were selected from each watershed then some parameters studied and measured in these gullies. Finally relationship between the length and volume development of gully with watershed characteristics, Physio-chemical soil properties, the percentage of vegetation cover, pebbles, bare soil, litter, slope and upstream area of head cuts, rainfall and different land use area a 20-year period (1993-2012) were analyzed. Modeling of the gully development was done using multivariate regression. Results and Discussion: The results showed that in Haddam watershed the gully number A17 had most elongation (78.8 meters) while the gully number B2 had lowest elongation (3.8 meters). In Sharif watershed the gully number B3 had the most elongation (108.1 meters) while the gully number B4 had the lowest elongation (1993-2012). Gully clustering results showed that gullies of Haddam’s watershed were in two clusters and gullies of Sharif’s watershed in the three clusters. The final equation for the development of Haddams gullies in the cluster one shows the amount of development in these gullies depends on three variables including upstream area of initial point (X4), the percentage of bare soil (X6) and slope of initial point (X2). At cluster two the amount of progress in these gullies depends on silt percent (X18) and PH(X23). The final equation for the development of Sharif’s gullies in the cluster one shows the amount of progress in these gullies depends on upstream area of head cut(X3). At cluster two the amount of development depends on two variables upstream area of initial point (X4) and Sodium content (X22). In Haddam’s watershed more than 80 percent of land use is covered by rain fed and irrigated land. Unlike Sharif’s watershed that more than 80 percent of pasture land form. In addition, measurement of upstream watershed area reflects very poor range condition. In this region, due to the slight slope in gully head cut the surface runoff has not velocity but also due to lack of canopy cover in gullies’ watershed and low soil permeability because of silt frequency and high salinity in the soil profile increases the risk of gully development. In this research, slope of head cut upstream and slope of initiates point is less than 5 percent at total gullies that are correlated with development of gully elongation. However, due to poverty and lack of vegetation cover conditions provided for gully linear growth. Conclusions: Gully erosion is a form of progressive erosion that allocated the largest portion of soil degradation in different climates to itself. Gullies behavior against its spreading and growth is affected by different environmental factors. In this research, available data analysis in Haddam’s watershed showed that the most important factors on the gully development were watershed characteristics on gully headcut (area and slope), the fine geological formation (silt), acidity and high soil salinity. These factors along with poor vegetation cover and agricultural land had created an excellent platform for gully elongation so that over a 20 year period have at least 3.8meter per year and a maximum elongation 78.8 m. In Sharif ‘s watershed showed that the most important factors on gully development were upstream area of head cut, upstream area of initiate pointed amount of sodium which caused to 10.1 and 108.1 meters minimum and maximum linear development respectively.