Study On Cumulative Effects Of Cascade Reservoirs On Downstream Flow And Sediment In Watershed Outlet

The conflict between basin cascade development and river ecological environment has become the biggest problem that should be urgently resolved in watershed harnessing at present. In order to achieve river's harmonization with humans, and to keep river health, the cumulative effects of cascade reservoirs on ecosystem and environment should be profoundly studied, which will be very important on theory and application.The water resources development focused on assessing individual construction project in the second half period of 20th century in China. It is more to research on effects of individual reservoir on ecological environment, which has space time limitation, whereas it is rare to analyze the cumulative effects of cascade reservoirs from perspective of watershed ecosystem integrality. Furthermore, there is something lacking in theoretical research and methods system.Hydrology and sediment are two important habitat factors in river ecosystems. It should study from the basic factors: hydrology and sediment, and then extend to ecosystem. Based on this research guideline, the paper preliminary discusses the cumulative effects of cascade reservoirs on hydrology and sediment in theory, and then qualitatively analyzes adaptable response of river ecosystem due to the changes in flow and sediment regimes.The key issues discussed in this paper are shown as follows:Firstly, new approach and framework modeling watershed flow and sediment are given in order to profoundly study the cumulative effects process and mechanism of cascade reservoirs, and to quantitative seek the change law of flow and sediment. Comprehensive watershed model, including rainfall-runoff model, reservoir operation model and fluvial sediment transportation model has been developed through integrating multi-methods and multi-models from whole catchment's aspect.Secondly, on the basis of analyzing cumulative effects on ecosystem and environment, the cumulative effects on downstream flow, bed forming discharge and sediment are deeply analyzed. As to the change of hydrological behavior, averaging coefficient, which is an important physical measure of cumulative effect degree, is defined and the method for calculation is provided. Sediment trapping coefficient is also defined and the method for calculation is provided to compute tapping efficiency of cascade reservoirs.Thirdly, the developed comprehensive watershed model was applied for simulating the runoff distribution and sediment production of the region from gauging station Huatan to Pingshan. The errors between simulated and observed results were acceptable. The developed comprehensive watershed model was also applied to analyze cumulative effects of cascade reservoirs on downstream flow and sediment in lower Jinshajiang.Finally, the concept and connotation of river ecosystem and its subsystems are depicted, and its features are also analyzed. The regulation of water quality is responded to flow variability and sediment trapping. It has become increasingly clear that flow disturbance has been accompanied by major changes in structure of biotic communities, such as alien species invading, biodiversity shrinking, and area of organisms spawning destroying. The special countermeasures are bought forward to maintain the sustainable development of rive health.