ارزیابی و پهنه‌بندی فرسایش خاک در مناطق کارستی با استفاده از مدل Smlrk (مطالعه موردی: حوضه الوند، استان کرمانشاه)

فرسایش خاک پدیده مخربی است که نتایج زیانباری چون بیابان‌زائی، کاهش حاصلخیزی خاک، افزایش سیلاب‌ها و بار رسوب آنها و تاثیرگذاری بر سازه‌های هیدرولیکی را به دنبال خواهد داشت. رخداد این پدیده در مناطق حساس و شکننده کارستی منجر به بیابان‌زائی سنگی، تاثیر بر سیکل هیدرولوژی کارست و کاهش کیفیت و کمیت منابع آب کارست می‌شود. حوضه‌ رودخانه‌ الوند در غرب استان کرمانشاه از تیپیک‌ترین مناطق کارستی ایران بوده که ژئومورفولوژی کارست بخش وسیعی از سطح این حوضه را تشکیل می‌دهد و فرسایش تهدیدی جدی برای منابع آب و خاک این حوضه است. در این پژوهش با استفاده از مدل smlrk و با در نظرگرفتن شرایط ژئومورفولوژی کارست حوضه به ارزیابی و پهنه بندی فرسایش خاک در سطح حوضه پرداخته شده است. از مدل smlrk برای تهیه نقشه فرسایش خاک در دو حالت دخیل کردن پارامتر نفوذ (کارست) و عدم دخالت آن استفاده شده است. نتایج نشان داد در حالت عدم دخالت و حالت دخالت پارامتر نفوذ در مدل، به ترتیب حدود 55% و 61% مساحت حوضه الوند در طبقات فرسایشی متوسط، زیاد و خیلی زیاد قرار می گیرند. نتایج مدل smlrk در حالت دخالت پارامتر نفوذ و شرایط کارستی حوضه، از تطبیق بیشتری با وضعیت طبیعی حوضه برخوردار بوده و این مناطق دارای میزان فرسایش خاک بیشتری می‌باشند. شیب و بارش زیاد و خاک کم ضخامت، مهمترین دلایل فرسایش زیاد خاک در نواحی کارستی حوضه بوده است.

Evaluation and zoning of soil erosion in karst areas by SMLRK Model (case study of Alvand basin of Kermanshah province)

Introduction Soil erosion is a process of moving soil by water or wind when the soil particles are detached and is one of the main forms of soil destruction around the world which restricts sustainable development of the earth resources by creating negative environmental effects (Marques et al, 2018). Every year more than 75 billion tons of soil is eroded from the earth’s surface which is amounted to 134 tons per square kilometer (Sobhani, 2002,18). In Iran too nearly 2 billion tons of valuable soils are lost annually and damages resulted from soil erosion is amounted to 56 billion dollar (Gholami, 2002,20). Karst areas have their special conditions due to their special hydrological and geomorphological conditions. Alvand basin is a distinguished karset basin in the west of Kermanshah province and the problem of soil erosion has existed in this basin for long time and exerted destructive effects on the environment. It is feared that by continuing current trend and its intensification due to human activities and destruction of oak forests, this basin faces with stone desertification in addition to disturbance in karst water resources and hydraulic constructions of the basin face with problems resulted from sediment. Meanwhile no study has been conducted in the Alvand basin surface to assess soil erosion by taking in to account karst condition, then conducting this research is essential and necessary. The goal of this study is to evaluate and zone soil erosion by taking karst geomorphological condition in Alvan basin in to account. Methods and materials Library, descriptiveanalytical, field methods as well as SMLRK model were used. Above model includes influential internal parameters (infiltration, erodibility and morphology) and external parameters (land use and rainfall erosivity). In this study above model is conducted in two modes with and without involvement of infiltration parameters. For verification of the maps error matrix of two maps were calculated. Research data includes Landsat satellite image, 10 meters DEM, geological and pedological map and rainfall data from climate stations of the region. Results and Discussion In the zoning map obtained from equation (5) infiltration parameter which covers karst aspects has been neglected (Fig.7). Classes with low and very low erosion constitute 44.68% of the basin area, class with moderate erosion 21.9% of the basin area and classes with high and very high erosion 33.4% of the Alvan basin area. Totally, nearly 55% of the Alvand basin area is exposed to the risk of moderate to high soil erosion. Low erosion class conforms to plains’ surface and hilly regions of the basin. These regions generally have agricultural and rangeland and semidense forests use. Texture of the soil in these regions is heavy with lot of mineral materials and very deep. This karst class morphologically has low sleep and the directions are back to the sun and are less in the realm of the karst regions. Zone with moderate erosion conforms to erosive plains in the west of the basin, hilly regions and low hillsides of the zone’s mountains. The usage of these regions is usually rangeland forest and includes parts of the karst regions of the basin. From morphological point of view, it has medium slope and soil texture is moderate with mineral materials and moderate depth. Zone with grate erosion exists both in karst and nonkarst areas. These regions are usually steep and have rangeland and thin forest use with light soil texture and shallow. Karset regions are located in Sirvan, Ghalaje, Sarkesh, Deh Nooshak, Navakooh and Dalahou heights. Highly erosive zones conforms to rocky and steep regions with thin rangeland use, soil texture is light and very shallow (Fig.7B).  Low and very low erosive zones occupied 38.75% , moderate erosive zone 29.86% and high and very highly erosive zones 31.37% of the Alvan basin surface (table. 8). Land use, morphology, geomorphological, rainfall and soil condition in each of these zones is the same as erosive zones of the previous map. Conclusion In SMLRK model unlike most common models, karst geomorphological conditions have been involved in its subparameters. In this research final map of the erosion results from this model has been provided in two ways with and without involvement of the infiltration parameters. In the final maps of the soil erosion obtained from two methods, there are five erosive zones. Providing soil final erosive map for the Alvand basin using infiltration parameters causes changes in the surface of the erosive zones and their special distribution. With involvement of the infiltration parameters in modeling soil erosion of the Alvand basin, surface of low and high erosive zones in the map with five factors compared to the soil erosion maps with four factor has decreased and  surface of the very low, moderate and very high erosive zone has increased 2.04%, 7.96% and 5.98% respectively. In order to evaluate exactitude of the obtained maps their error matrix and similarity degree were calculated (Table 9). Accordingly, two maps and their classes were compared based on pixel and total sum of table diagonal is divided to total sum of the tables which expresses similarity degree of two maps. Similarity degree between two maps is 54.85% which reflects the main effect of infiltration parameter in amending result. Indeed, karst developed areas in east and north heights of the basin with involvement of infiltration parameter are located in highly erosive zone. Therefore, karst zoning results obtained from involvement of the infiltration parameter and consequently karst geomorphology have better performance and the results of this mode of model reflects more soil erosion in the area having karst geomorphology.