Determining soil loss and sediment yield in borçka dam watershed using wepp model and gis techniques

For many years, excessive and inappropriate uses of natural resources have caused the fragile balance between soil, water and plant to be broken; thus, the problems including accelerated soil erosion, increase in flooding events and sedimentation have reached a serious level. Necessary precautions should be taken against these problems, but in order to reach this goal, first, quantities and reasons of these issues must be determined with sufficient data. To find out quantity and degree of soil erosion problem in an area, 5 main factors (climate, soil properties, topographic conditions, vegetation cover and human interferences) affecting soil erosion must be investigated. However, when considering many sub-factors under these main factors, it becomes very clear that determining soil erosion especially in the field is very difficult because it is both a time consuming and a costly process. Therefore, efforts on estimating and predicting soil erosion of an area have shifted towards using computers and GIS (Geographic Information System) technologies, which was first started with USLE (Universal Soil Loss Equation) and then evolved into recently developed several erosion prediction models including RUSLE (Revised Universal Soil Loss Equation), WARSEM (Washington Road Surface Erosion Model) and WEPP (Water Erosion Prediction Project). However, the number of scientific studies where such models and techniques are applied in Turkey is very limited. One of the areas where these models are often applied is the large dam reservoirs because to determine the data of annual average sediment amounts in a timely and reliable way is important for such reservoirs especially in respect to both their operating life for producing electricity and determining potential problem areas in reservoir watersheds for implementing necessary precautions. Due to the planning of several large dams on it, the Coruh River Valley consists of many example areas, including the research area of this study, the Borcka Dam Watershed, where mentioned models can be used. The main aims of this study were; to determine the amounts of soil loss and sediment yield in a timely and reliable way using WEPP model and GIS techniques and thus initiating watershed-based studies on investigating soil loss phenomenon by applying an erosion prediction modelling within the Coruh River Watershed. Within the whole watershed, a total of 15 model sub-watersheds were selected by considering the features of parent material, land use, slope and aspect. A total of 480 undisturbed (cylinder) and 480 disturbed (composite) soil samples by taking 240 samples from each soil depth of 0 - 10 cm and 10 – 30 cm were collected. On these soil samples, some physical, hydro-physical and chemical properties including texture, bulk density, permeability, water holding capacity, pH and organic matter content were analyzed. The results showed that annual total sediment amount for a 86576.83 ha Borcka Reservoir Watershed was 360431.70 tons, while the total sediment yield was found to be 242.41 ton/ha/yr. In addition, 4.16 ton/ha/yr of sediment was produced per unit area. In addition, the analyzed soil properties were compared based on chosen model sub-watersheds, parent material, land use and soil depth and it was determined that most of these comparisons showed statistically significant differences. The results gathered from this study may be an important example to be used for other large dam projects planned in the Coruh River Watershed. Bu boşluk iki aralık olacaktır. Keywords: Borçka Dam Watershed, soil loss, runoff, sediment yield, GIS, WEPP model.