Geomorphology-Based Water-Sediment-Pollutant Model And Its Application

One of the most challenge issues in the current work is the scarce of water resources, and the serious pollution of water bodies enhance such situation, of which the non-point source pollution (NPSP) contributes the most significant portion. The NPSP topic received a lot of research interesting during the past several decades, and mathematical simulation approach of coupled water, sediment and pollutant serves as not only a way to understand the mechanism and processes of pollution, but an effective tool in basin management and environmental protection.The coupled processes of water, sediment and pollutant in watershed are dominated by geomorphological features. In this paper, a distributed hydrological model coupling water, sediment and pollutant at the catchment scale based on geomorphological features is developed, which named as GBHM-CWSP, after current GBHM model by Yang et al. (1997). The GBHM is used to simulate hydrological process, and the other processes including erosion over hillslope, sediment routing in stream, pollutant discharge between soil and runoff inter-face, pollutant transport and transformation in soil profile and in stream, are all coupled in this integrated model. In GBHM-CWSP model, the physically based equations are used to describe most aspects of water, sediment and pollutant. Moreover, geomorphological similar elements are used as discretization and simulation units based on DEM, which make the computation more rapidly and effective than regular division as grid cells.The characteristics of sediment yield in rocky mountain area is greatly different from that in loess area, because the sediment quantity is limited, which may cause a phenomenon of"sediment exhaust"after a heavy storm event, and not accumulated enough for erosion in the next storm. Therefore the"potential of erosion quantity"is used to control the actual erosion in hillslope unit and in stream. By this way the simulation result will be more precise. In the simulation module of pollutant movement in hillslope unit, an exchange function between water soil and runoff is constructed to depict the solute discharge from soil to runoff at the soil-water interface during rainfall-runoff. The convection- dispersion equation (CDE) is adopted to describe the solute transportation on the soil profile. The combining of mechanic equations in large-scale basin model enhance the physical basis.The Chaobai river basin, located in the upstream of Miyun reservoir, is chosen as case study area in this paper. Based on the DEM, landuse, soil data, etc., the GBHM-CWSP model is build up. After the calibration and validation by observed data of runoff, sediment, nitrogen and phosphorus loadings from 2000 to 2005 at two stations: Xiahui and Zhangjiafen, this integrated model is applied to analyze and identify the effects of changes of landuse-cover and climate factors on runoff, sediment, nitrogen and phosphorus loadings. By scenario analysis, the contributions of landuse change and climate factors change are evaluated. The results show that the change of climate factors has predominant contribution to runoff and sediment changes, both more than 90%. While for nitrogen and phosphorus, the effect of climate factors changes is decreased as 51% and 70% respectively. Especially for nitrogen, the farmland is the major source of nutrient loss, thus landuse changes have significant effects on nitrogen loadings. Using GBHM-CWSP model and IPCC data, an attempt of prediction of runoff, sediment and pollutant loadings in the future in Chaobai river basin is performed.