ارزیابی کیفی و پهنه‌بندی فرسایش خاک با رویکرد شاخص خطر در حوضه کارستی الوند، استان کرمانشاه

فرسایش خاک از تهدیدکننده های اصلی منابع آب وخاک در جهان و ازجمله کشور خشک و نیمه خشک ایران است که ارتباطی قوی به ویژه با نوع پوشش و کاربری زمین دارد. این مسئله به‌خصوص در مناطق کارستی منجر به رخداد پدیده بیابان زایی سنگی و گرفتگی مجاری کارستی می‌شود. در نوشتار پیش رو با استفاده از مدل شاخص ریسک فرسایش و استفاده از تصاویر ماهواره ای لندست تی. ام. و اُ.ال.آی. در بازه زمانی 28 ساله، سطح حوضه کارستی رودخانه الوند در غرب استان کرمانشاه برای سه سال 1990، 2005 و 2018 به منظور ارزیابی کیفی و پهنه‌بندی فرسایش خاک با تاکید بر نوع پوشش و کاربری زمین بررسی شده است. نتایج نشان می دهد در بازه زمانی 28 ساله، میزان تراکم پوشش گیاهی در سطح حوضه الوند از روند نزولی برخوردار بوده و از سال 1990 تا 2018 مساحت آن بیش از 30% کاهش یافته است. تعداد طبقات کاربری اراضی و میزان مساحت آن ها در این بازه زمانی تغییرات قابل توجّهی داشته است. پهنه فرسایش بسیارکم در هیچ کدام از نقشه های نهایی فرسایش خاک در سال‌های انتخابی در حوضه وجود نداشته است. پهنه فرسایش متوسّط در هر سه نقشه نهایی فرسایش خاک، بیشترین مساحت و پهنه های فرسایش زیاد و کم، به ترتیب دومین و سومین پهنه فرسایشی وسیع در هر سه نقشه نهایی فرسایش خاک بوده اند. پهنه فرسایش بسیارزیاد منطبق بر مناطق کارستی حوضه است. شیب و بارش زیاد و خاک کم ضخامت، مهم ترین دلایل فرسایش زیاد خاک در مناطق کارستی حوضه بوده است.

Qualitative Assessment and Risk Zoning of Soil Erosion with a Risk Index Approach in Alvand Karstic Basin, Kermanshah Province

Soil erosion is one the main threats to water and soil resources in the world, as well as arid and semiarid country of Iran. Soil erosion have a strong relationship with vegetation type and land use. Soil erosion especially in the karst areas led to phenomenon of Rock desertification and to block the channels of karst. The present paper aims at studying the karstic basin of Alvand River in the west of Kermanshah province for three years 1990، 2005 and 2018 by applying Erosion risk index model and satellite images Thematic Mapper (TM) and Operational Land Imager. The findings reveal that the vegetation of Arvand basin has had a decreasing rate in which 30% of the area has decreased from 1990 to 2018. The number of land use classes and their area during this time period have had significant changes. In the selected years, there was no very low erosion zone in any final maps of erosion in the basin. The medium erosion zone has the largest area in all three final maps of the soil erosion. The high and the low erosion zones have been the second and the third wide erosion zones in all the three final maps of the soil erosion, respectively. The very high erosion zone has been consistent with the karst area of the basin. Slope, heavy rainfall, and lowthickness soil have been the most important reasons for high soil erosion in the karst areas of the basin.Extended Abstract1Introduction The process of soil erosion leads to some changes in soil, the loss of nutrients and the decrease in its dewatering capacity. Soil erosion and its loss of productivity limit the structure and distribution of land use and exacerbate tension between human and land resources. Land use and land cover are two important factors affecting the formation and severity of runoff and soil erosion. The destructive phenomenon of soil erosion in sensitive karst areas has the consequence of rock desertification. The desertification in the karstic areas refers to the process of land degradation, in which the soil completely eroded. A large part of the Alvand river basin in the west of Kermanshah province has developed karst geomorphology, which has had an effect on the erosion of the basin. Due to the lack of appropriate and comprehensive quantitative data at the catchment area, a qualitative assessment of soil erosion at the surface of this karstic zone is necessary for the management of the environment. The purpose of this study is to assess and qualify soil erosion in the Alvand basin using Erosion risk index model in a 28year time series2Materials and MethodsThis study uses Erosion risk index model to assess and qualitatively determine soil erosion in Alvand karstic basin. In this model, the topographic slope parameters, annual precipitation, soil texture, cover and land cover are used to prepare a quality erosion zoning map and each parameter is divided into five different levels with assigned values. In the model, the erosion zoning map for the years 1990, 2005, and 2018 is provided. Two parameters of land use and vegetation cover as variable parameters in the selected years are combined with three other constant parameters. Table (1) shows the five classes of each of the five selected parameters and the privileges assigned to them. Finally, based on the quality of soil erosion risk index (Eq. 4), an alvetian erosion risk map is prepared. After calculating the erosion rate according to table (4), the final soil erosion map is classified into five classes. Data were collected from Landsat satellite imagery, ten meters DEM, weather data stations and regional soil maps.Equation 4:Erosion risk index = 0.33* rainfall ‏+ 0.27 * vegetation coverage+ ‏ 0.2 * slope ‏+ 0.13 *landcover +‏ 0.07 * soil type3Results and DiscussionAll three soil erosion mapping maps have four low risk, medium, high and high risk levels, and there is very little erosion class in any of the maps. The average erosion zone in each of the three final maps has the highest soil erosion (Table 10). The high and low erosion zones are respectively the second and third largest erosion zones in all three final maps of soil erosion. The erosion zone in the selected years has an area less than one square kilometer from the Alvand Basin. The low erosion zone in all three maps of the final erosion is consistent with the agricultural land and gardens of the plains of GilaneGharb, Deira, Qal’ShahinSarePuleZahab, Khafar and Pathagh The average erosion zone corresponds to most of the basin areas, including plains, hillsides and mountainous regions. The high erosion zone corresponds to the slopes of the mountains in the north and east of the basin. The low erosion zone is affected by slope parameters, soil texture, and low rainfall, vegetation cover of more than 45% and agricultural use and gardens. Moderate erosion zone is also more affected by two parameters of land use and vegetation. Because of the texture of the soil, rainfall and steepness of the topography of this range are very unstable. The high erosion zone is also affected by topographic slope parameters, soil texture and precipitation. High erosion areas mainly correspond to the alveolar karstic areas, which indicates that in the karst areas of the basin, due to sloping, shallow soil and high rainfall, soil erosion is more severe than other areas of the basin.4ConclusionThe results showed that vegetation cover variations in selected years had a high correlation with the NDVI index and in the period of 28 years, vegetation density in the Alvand basin had a downward trend. The study of the number of land use classes and their area in the Alvand Basin during the period from 1990 to 2018 has been significantly altered. Residential and lake dwellings have increased over the years and their area in 2018 has increased by about 8 km2 compared to 1990. Agricultural, pasture and forest use decreased by 7, 14 and 17 square kilometers, respectively. The final quality maps of Alvand basin erosion indicate that the basin environmental conditions are suitable for the occurrence of soil erosion process. During the selected years, the erosion of soil in 2005 was more severe than in the years 1990 and 2018, so that the extent of the erosion area is about 8% more than the other two years. In the dry year 2005, vegetation was lower than other years, and the erosion rate was more severe than the other two years. The land use parameter has been limited over the past 28 years, and the parts that have lost the ownership of the residential and Lake Dam have not played a role in exacerbating erosion. In terms of spatial distribution, the erosion zones in the three years of selection show the same overall trend. The erosion zone corresponds to the karstic areas of the basin in the northern and eastern highlands of the basin. Slope, high rainfall and lowdensity soil are the most important reasons for soil erosion in the basin’s karstic areas.Precision estimation of final soil erosion maps with error matrices and their degree of similarity reflects the effect of changes in vegetation and land use in soil erosion.