برآورد نرخ سالانه فرسایش خاک در حوضه آبریز الشتر استان لرستان (مقاله علمی وزارت علوم)
درجه علمی: نشریه علمی (وزارت علوم)
آرشیو
چکیده
فرسایش خاک یکی از مشکلات زیستمحیطی بسیاری از کشورها به ویژه در مناطق خشک و نیمه خشک مانند ایران است که سالانه خسارات قابل توجهی در کشور به بار می آورد ، لذا شناسایی مناطق با فرسایش زیاد در جهت حفاظت خاک و کنترل فرسایش از مسائل مهم برنامه ریزی و مدیریتی کشور است. در این پژوهش هدف برآورد نرخ سالانه فرسایش خاک در حوضه آبریز الشتر در استان لرستان است که برای این منظور از مدل LAMPT استفاده شد. این مدل اساساً مبتنی بر معادله جهانی فرسایش خاک است. پارامترهای محیطی مدل شامل داده های اقلیمی، پوشش زمین و ژئومورفولوژی از ادارات هواشناسی، منابع طبیعی و تصاویر ماهواره ای سنجنده Sentinel-2 سال 2023 به دست آمدند. برای تکمیل داده ها ، بررسی های میدانی انجام شد و برای تهیه نقشه کاربری زمین نمونه داده های آموزشی نیز انتخاب شدند. نتایج ارزیابی فرسایش خاک نشان داد که میانگین فرسایش ویژه سالانه در سطح حوضه 42/9 تن در هکتار در سال است که در مقایسه با میانگین رسوب دهی حوضه در ایستگاه سراب سید علی (خروجی حوضه) ، مدل دقت مناسبی دارد. همچنین طبقه بندی نقشه فرسایش خاک نشان داد که 35 درصد حوضه فرسایش سالانه ای بیش از 10تن در هکتار دارد. ارزیابی میزان فرسایش خاک در کاربرهای اراضی نشان داد که نرخ فرسایش خاک در مراتع با پوشش تاج ضعیف، متوسط و متراکم به ترتیب 7/17، 3/11 و 1/9 تن در هکتار در سال است. با توجه به میزان وسعت مراتع در حوضه ، نرخ فرسایش بالای خاک در آن ها به خصوص در مراتع با پوشش تاج ضعیف، برنامه ریزی کاربری اراضی و استفاده بهینه از مراتع به منظور کاهش نرخ فرسایش خاک درحوضه ضروری است. انجام عملیات حفاظت خاک و آبخیزداری در زیرحوضه های شمالی و شمال شرقی و نیز در مراتع با پوشش تاج ضعیف از جمله اقدامات اساسی اولویت دار است.Assessment of Annual Soil Erosion Losses in the Alashtar Watershed in Lorestan Province
Introduction Soil erosion is a spatiotemporal phenomenon influenced by variable processes, necessitating multiple observations over time and space. These measurements inherently contain a level of uncertainty and are costly and time-consuming. With the advancement of spatial technologies, Geographic Information Systems (GIS), interpolation methods, and the increasing range of environmental data and remote sensing, soil erosion models play a crucial role in designing and implementing soil conservation management and strategies. The most significant empirical models for estimating annual soil erosion losses include the USLE and its modified version, RUSLE, widely used globally for soil erosion estimation. These models consider factors such as raindrop erosivity, soil erodibility, slope length and steepness, cover management, and conservation practices, with most parameters now readily obtainable through high-quality remote sensing data. Methodology The study area, the Alashtar watershed plain, spans 80305 hectares in the northern part of Lorestan province and Selseleh county. Geomorphologically, it lies within the elevated or folded Zagros unit, characterized by thrust faulting and multiple fractures. The watershed predominantly features a mountainous landscape, with hills and mountains covering 65.39% of the area. The minimum elevation is 1500 meters, the maximum is 3600 meters, and the average elevation is 2100 meters. According to the De Martonne method, the watershed has a Mediterranean climate with an average annual rainfall of 506 millimeters. The study area's aquifer is free-flowing, with all wells situated in the alluvial aquifer. In the mountainous part of the Alashtar watershed, carbonate formations, fracture systems, and weathering in the form of snow have developed karstic aquifers, which are the source of the Kehman River. The Kehman River, after originating from the southern heights of Green and hydrating the Alashtar plain, joins the Simreh and then the Karkheh rivers. The Alashtar plain, at the center of the watershed, is a graben surrounded by the Green, Varkhash, Mahab, Sarakheh, Darikanan, and Nashate heights, with geological formations dating back to the Mesozoic and Cenozoic eras. The predominant Jurassic-Cretaceous limestone rocks cover a significant portion of the Alashtar watershed, serving as the primary recharge units (karstic water sources). The soils of Alashtar belong to the brown soil group with a clay concentration horizon, characterized by very deep, dark brown to reddish-brown soils with a heavy texture, containing 3-15% coarse gravel in the surface layers and relatively high amounts of hard limestone grains in the sublayers. The LAMPT model is basically based on the global soil erosion equation, which calculates the net rate of soil erosion. The environmental parameters of the model, including climatic data, land cover and geomorphology, were obtained from meteorological departments, natural resources and Sentinel-2 sensor satellite images during the target year (2023). To complete the data, field surveys were conducted and training data samples were also selected to prepare the land use map. The basis of LAMPT model is RUSLE model, in this model Sediment delivery ratio index (SDR) is used to show the spatial patterns of distribution and performance of pure soil sediments at the basin level. This model integrates the general characteristics of the basin landscape (land use classes, landform parameters, soil types, land cover and management) and precipitation values to simulate the gross soil erosion rate, SDR and net sediment yield at the basin level. Results and Discussion The findings indicate that the average annual soil loss across the watershed is 9.42 tons per hectare, which is consistent with the sediment yield measured at the Sarab-e-Seyyed Ali station (watershed outlet) with an average annual rate of 10.1 tons. The model demonstrates adequate precision in estimating soil erosion and resulting sedimentation. An assessment of soil erosion losses across different land use classes, derived from Sentinel-2 satellite images, shows that 60% of the Alashtar watershed area comprises pastures. The soil erosion rates in pastures with weak, moderate, and dense canopy cover are 17.7, 11.3, and 9.1 tons per hectare per year, respectively. Given the extent of pastures in the Alashtar watershed and the high erosion rates, particularly in those with weak canopy cover, land use planning and optimal utilization of pastures are essential to reduce soil erosion rates in this area. Conclussion In this research, the LAMPT model based on climate, land cover and geomorphological data was used to estimate the annual net rate of soil erosion in the Alashtar watershed. Estimating the net rate of soil erosion during 1402 using this model showed that the average annual soil loss at the basin level is 9.42 tons per hectare per year. The classification of the soil erosion map shows that 35% of the basin has an annual soil loss of more than 10 tons. In agricultural uses, rainfed fields have the highest rate of soil erosion during the year with the amount of 7.5 tons per hectare during the year. Considering the area of pastures in the Elashtar catchment area, the high rate of soil erosion in them, especially pastures with weak canopy, planning land use and optimal use of pastures is necessary to reduce the rate of soil erosion in this basin. The evaluation of the accuracy of the LAMPT model in comparison with the annual average sedimentation rate at the sediment measuring station of Sarab Said Ali (outlet of the basin) showed that the LAMPT model has a high accuracy in estimating the soil erosion and the sediment caused by it, and its difference with the ground data is small, so the use of This model is recommended to calculate soil erosion and its annual losses. According to the findings of the research, it is suggested that the northern and northeastern sub-basins of the basin should be prioritized for protection and watershed management measures. In addition to this, pastures with weak crown cover should be given special attention in terms of land use for watershed management and pasture management.
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