Soil erosion estimation in a part of Gheshlagh dam watershed using Thornes model

Determining the amount of soil erosion and its spatial distribution can effectively provide appropriate management practices to reduce erosion problems. Direct measurement of soil erosion takes a lot of time and money, and results are often regional and limited. This issue has led to the development of various models for estimating soil erosion and sediment production. Since most of the watersheds in Iran lack sufficient and accurate information, models that require relatively few inputs make more sense than models with high inputs. Along with empirical and process-based models, a number of erosion estimation models are known as hybrid models. One of the hybrid models is the Thornes model, which requires relatively little input data, and there has been no research related to this model in Iran. Therefore, this study was conducted to determine spatial variation the soil erosion rate and sediment yield in a part of the Sanandaj Gheshlagh Dam watershed using the Thornes model utilizing GIS and remote sensing (RS).

Two hundred soil samples were collected using a stratified random sampling method and the parameters of soil texture, particle size distribution, organic carbon, and soil CaCO3 equivalent in the laboratory were measured by conventional methods. In order to determine the input parameters of the studied model, laboratory results, Landsat 8 satellite images during a 6-year period, and some monthly meteorological data including precipitation, precipitation days number and potential evapotranspiration were analyzed. Finally, using ArcMap, Saga GIS, and ENVI software, the required parameters, and maps of the models were prepared, and the studied models were run. Sediment delivery ratios (SDRs) are assumed to be a function of the travel time of surface runoff from catchment cells to the nearest downstream channel.

The results showed that the estimated average erosion of the Thornes model was 0.76 mm per year or 10.24 tons per hectare per year (assuming a bulk density of 1.4 gr/cm3), and according to the calculated SDR, the estimated sediment was calculated by the model as 4.34 tons per hectare per year. The results of the Thornes model showed that the sensitivity of this model to some parameters, including potential evapotranspiration and potential water storage capacity, was very high. Small changes in these parameters caused a significant difference in the results, which would reduce the efficiency of the model.

According to the field observations and the investigation of erosion and sedimentation situations in the studied watershed, as well as long-term information about sediment discharged from the hydrometric station (3.10 tons per hectare per year), it seems that the Thornes model has been a relatively reasonable estimate of erosion in many parts of the studied watershed. Although, the type of model used to estimate the sediment delivery ratio and then the sediment yield can have a significant effect on the model's efficiency. Considering that this model requires relatively few data, it may be possible to use it to predict erosion in watersheds with no information or with poor information.