بررسي تاثير تغييرات پوشش و كاربري زمين در قابليت فرسايش خاك – مطالعه موردي حوضه قره سو گرگانرود

Investigation about the influence of land-cover and land use changes on soil erodibility potential, case study: Gharesou, Gorganrood

فرسايش خاك يكي از ريسك هاي اصلي تهديدكننده منابع آب و خاك در ايران است كه رابطه اي قوي با نوع پوشش و كاربري زمين دارد. در اين پژوهش بوسيله مدل RUSLE با بهره گيري از تصاوير سنجنده هايTM ، ETM و OLI ماهواره لندست در يك بازه 30 ساله براي سه سال 1985 ، 2000و 2015 تاثير تغييرات پوشش زمين بر پتانسيل فرسايش خاك در حوضه آبخيز قره سو مورد بررسي قرار گرفت. نتايج تغييرات پوشش زمين نشان دهنده كاهش پوشش هاي جنگل متراكم، جنگل با تراكم كم و باغ و مرتع در برابر افزايش سطوح كشاورزي، زمين هاي بدون پوشش و سكونتگاه هاي انساني در طي بازه سي ساله است. همچنين نتايج مدل RUSLE سير افزايشي پتانسيل فرسايش خاك درحوضه آبخيز قره سو را نشان مي دهد، هرچند كه در نواحي جلگه اي با كاربري كشاورزي روندي معكوس با روند كلي در نتيجه اصلاح و تغيير الگوي كشت و رشد كشاورزي آبي ديده مي شود. ميانگين پتانسيل فرسايش خاك برآورد شده درحوضه قره سو براي 1985 ، 2000و 2015 به ترتيب 102.02، 103.11و 103.76 تن در هكتار در سال است. همچنين در كلاس هاي بيش از 100 (تن در سال در هكتار) يا طبقات با پتانسيل خيلي زياد و بحراني اين مقدار از 43.8 درصد به 45.5 درصد از مساحت حوضه در سال 2015 افزايش يافته است. اين روند افزايشي در سطح زير حوضه ها نيز مورد آزمون قرار گرفت و در اكثر آنها پتانسيل فرسايش خاك بر اساس روند تغييرات كاربري زمين رو به افزايش است.

Investigation about the influence of land-cover and land use changes on soil erodibility potential، case study: Gharesou، Gorganrood Land use and land cover (LUC) change associated with climatic and geomorphologic conditions of the area have an accelerating impact on the land degradation. Natural as well as human-induced land use land cover change (LUCC) has significant impacts on regional soil degradation، including soil erosion، soil acidification، nutrient leaching، and organic matter depletion. Since the last century، soil erosion accelerated by human activities has become a serious environmental problem. It has a manifold environmental impact by negatively affecting water supply، reservoir storage capacity، agricultural productivity، and freshwater ecology of the region. In recent years، many researchers have highlighted the environmental consequences of soil erosion. Soil erosion estimation at a regional scale is influenced by the complexity of the soil erosion process and the availability of data describing the soil erosion factors. In the last decade، regional and national level assessments of soil erosion were carried out using different approaches، ranging from indicator or factor-based approaches to process-based models. However، the revised universal soil loss (RUSLE) and its modifications are still widely used because of its simplicity and a greater availability of input parameters. Gharesou basin is one of the sub-basins of Gharesou، it suffered from severe erosion in some areas over the past years. This erosion has occurred for different reasons and one of them is land use change and weak management of water and soil resources. The purpose of this research is to investigate the effects of land-cover changes on the potential of soil erosion in Gharesou Basin، a sub-basin of Gorganrood، in Golestan province. For this، we have employed RUSLE Model and used landsat satellite images from the sensors of TM، ETM، and OLI for 1985، 2000، and 2015. The potential soil erosion in this study was estimated using RUSLE model، which can be described using following equation: A = R × K × LS × C × P where A is amount of soil erosion calculated in tons per hectare per year، R is rainfall factor ، K is soil erodibility factor ، L is slope length factor، S is slope steepness factor، C is cover and management factor، and P is erosion control practice factor. To run the RUSLE model in GIS، first، rainfall raster layer، soil، slope، Digital Elevation Model، and also layers of soil protection range were created. Each of the involved factors was calculated in separate units in the basin level. In this research، Gharesou basin was analyzed based on raster network data with 30 meters cell size، because، from one hand it's small enough to show heterogeneity of the basin and on the other hand، it matches pixel dimensions of landsat satellite images. The results of land-cover changes have revealed a decrease in dense forest areas، low forest areas and the mixture of orchard، forest and pastures in a thirty years period. According to the results of RUSLE، changes of the classes indicate a general trend to the soil loss in the basin. Therefore، Gharesou basin is a basin with increasing soil erosion potential. In the plain and coastal plain areas of the basin، that is the mainly cultivated area، the amount of erosion is different from the other areas، and soil loss process is decreasing. It's due to the changes of cultivation method from traditional to modern، increase of irrigated farming area، choosing more environmentally friendly plants، and also، increase in the area of cities and villages from 7.14 percent to 29.04 percent during 30 years. In the study classes، for output of RUSLE model، in every 3 years of study، the maximum area relates to the classes of 100 to 200 Ton per year that is more seen in the mountainous regions. In these regions، all factors except vegetation are toward soil loss. Also، during 30 years، the amount of dense vegetation decreased from 34.56 to 31.55. In fact the only factor in protecting soil in (prone to erosion) areas has given its place to less effective vegetation، so، the area of this region has increased and Gharesou basin is in danger of soil loss in mountainous and forest parts. Also، areas with more than 200 Ton in hectare، with the lowest amount، have had a tangible increase during 30 year of study and its amount has increased from 11.74 to 12.50. These areas are usually located in mountainous parts with no vegetation. Also، the average of soil erosion potential estimated in Gharesou basin for 1985، 2000 and 2015 is 102.02، 103.11، and 103.76 (ton per hectare per year). This amount was found in the sub-basins too and except the sub-basin 4 located in coastal plain areas of the basin، with farming use، the amount of other sub-basins is increasing. According to the results of study، mountainous parts of Gharesou basin، has the most damage due to the accumulation of involved factors in the potential increase of soil loss. So، the necessity of watershed management is observed. Also modification of cultivation pattern and soil conservation training in farming lands of foothills and hillsides are required.