The Development And Application Of Watershed Scale DEM-Based Soil Erosion Model For A Single Rainfall Event

Accelerated soil erosion has been globally recognized as a serious problem since people took up agriculture. It is becoming one of the most serious environmental problems in the world. Soil erosion affects soil productivity by changing soil properties, and particularly by destroying topsoil structure, reducing soil volume and water holding capacity, reducing infiltration, increasing run-off and washing away plant nutrients such as nitrogen, phosphorous, and organic matter. The resulting sediments themselves act as the carrier of pollutants including heavy metal, nutrient, pesticide and others. In a word, soil erosion degrades the soil resources that human sustain on and threatens the environment that people living in. Soil erosion model, which predicts total amount of eroded soil, helps to understand the erosion process and distribution, thus to guide the installations of soil conservation measures, received great attention both from the scientific field and from the governmental sectors. The soil erosion model can be generally divided into two categories, the empirical model and the physical-based model. The former is derived through the extensive analysis of numerous experimental data while the latter is developed upon the concrete physical processes, which confines the applications of empirical models to the areas that they derived from and give ways to physical-based ones for modeling in a wide range of areas. The physical-based models are usually distributed to account for the spatial variations of rainfalls and ground features. The development of distributed models was greatly pushed in the later 1990s as the advance of spatial information technologies. Remote sensing (RS) was believed to be a good data source to acquire ground cover information and digital elevation model (DEM) was proved to be useful data to derive topographical parameters of the watershed. Geographic Information Systems (GIS) have changed our way to handle regionally distributed information.China is one of the countries that were seriously suffered from soil erosion. In China, about 1,790,000 km2 land is suffered from water erosion, which accounts for 18.3% of China's total area. As a result, 5 billion tons of soil was eroded and over 667 km2 arable land was lost each year. However, the soil erosion modeling techniques in China is not so advanced for the lack of good models and the difficulty in modeling over the complex topography in the country. The empirical models are frequently employed in soil erosion modeling since there were only a few distributed physical-based models developed. And they were mainly focused on the loess plateau areas. There are no distributed physical-based models available in Southern China, where soil erosion by water is also very seriously. So it's urgent to develop physical-based soil erosion model that's suitable for the Southern China area, esp. the Poyang Lake watershed.After a throughout review of physical-based models, this paper proposed a physical-based distributed soil erosion model to calculate total soil erosion amount and distribution after a single rain event. The calibration of the model was conducted using the data collected from the gauge stations. And the calibrated model was used to model the soil erosion after a storm and gave satisfactory results. This paper mainly include the following sections:1. The extraction of watershed parameters based on DEM. The extraction is consisting of the following steps. 1) The treatment of DEM to remove the sinks or flat area, 2) the assignment of flow direction of the DEM grids based on D8 algorithm, 3) the calculation of flow accumulation grids based on flow direction grids. The slope cells and channel cells were separated by setting a threshold to the flow accumulation grids. And the watershed boundary was identified by iterative search from the watershed outlet based on flow direction grids. This paper reviewed the DEM reprocessing methods and presented an effective algorithm based on three-direction search. Comparisons were made between the ArcHydro method and the proposed method. The results showed that the proposed algorithm can eliminate the parallel rivers as frequently occurred with ArcHydro method, and the algorithm made fewer elevation modifications to the original DEM.2. The study of the distributed hydrological module. Since soil erosion by water is closely related to rainfall and runoff, erosion modeling can't be separated from the procedures used to model the generation of runoff and its routing down a hillside and through the river channel network. In this paper, saturation excess runoff was modeled after satisfaction of rainfall interception by vegetation and soil infiltration. The runoff was routed from the watershed to the outlet using the ranked-grid based routing method. Kinematic wave and diffusion wave were employed in the flow routing according to the slopes of the cells.3. The modeling of soil erosion processes. The basis for describing soil erosion processes in Southern China is rather limited. In this paper, we borrowed the equations derived overseas and in the loess plateau areas. Three main erosion processes such as soil detachment by raindrop impact, soil detachment by rill and inter rill flows were considered and modeled accordingly. Flow transport capacity was also modeled to calculate the total soil erosion yield in each cell. The net sediment yield in each cell was then routed downstream to the outlet using continuity equation.4. The calibration of the model. The model can't be applied to study area before it was properly calibrated because some of the model parameters are empirically based. There are several calibration methods available. However, taking the massive data volume of distributed model into consideration, human-machine interactive method was selected to calibrate the model along with the rainfall and hydrological and sediment data collected from the gauge stations.5. The validation and application of the model. ZaoKouShui watershed, located in the upstream of Xiu River in the Poyang Lake watershed in Southern China was chosen as the study area because it's one of the watershed that is suffered from severe soil erosion and it's possible to collect the detailed rainfall and hydrological and sediment data of the watershed. The model was properly validated using the measured data. The statistics showed that the model was capable of modeling water and sediment yield at watershed scale. The modeling of soil erosion was conducted after one storm and gave the distribution of the erosion and total sediment yield at the outlet. The results were satisfactory compared with the measured data. And it was also revealed that the main sediment comes from the agriculture land cultivated on slopes higher than 15%.