Effect Simulation Of Ecological Restoration In Bailong River Basin Based On SWAT Model

Bailong River Basin with severe water loss and soil erosion belongs to the high-incidence debris flow disasters region in China. Ecological restoration is an important method to improve the fragile ecological environment, to prevent and control the soil erosion and debris flow disasters in Bailong River Basin. It is a vital scientific basis for the ecological restoration to carry out simulation researches according to the remediation efficiencies of different ecological restoration scenarios. On the basis of the distributed hydrological model SWAT which based on the physical mechanism, and the scenarios simulation method, this paper used surface runoff depth as a key indicator to simulate and analyze the runoff changes under different ecological restoration scenarios from the perspective of ecological and hydrological processes. Furtherly, this paper took Goulinping, a small typical gully of debris flow in Bailong River Basin, as an example to analyze the prevention and mitigation effect of debris flow disaster under different scenarios of ecological restoration based on the debris flow starting threshold of surface runoff depth.Main conclusions are as follows:(1)The SWAT model was suitable for Bailong River Basin. By calibrating and verifying the flow volume of Wudu hydrological station in middle reaches of Bailong River and the one of Bikou hydrological station in lower reaches of Bailong River, the efficiency coefficient and correlation coefficient were found to have reached the essential requirements of simulation in SWAT, which demonstrated that it could simulate the Bailong River Basin runoff and surface runoff depth objectively and accurately.(2)Through the sensitivity analysis of parameters in SWAT model, several parameters which had greatly influence on the runoff were gotten, including CN2, SURLAG, CH_K2, etc.(3)This paper simulated the effects of ecological restoration of the debris flow disaster in Bailong River Basin: based on the national policy of returning farmland to forestland or grassland, 35 scenarios of ecological restoration were designed, including 5 extreme scenarios and 30 reasonable scenarios.(4) The changes of average annual surface runoff depth under five extreme scenarios showed that the most effective way to reduce the surface runoff depth was to return all the farmland to evergreen forest, and its decline rate reached 12.79% which was the best value among the five extreme scenarios. And then, compared with the surface runoff depth under the current situation scenario, the monthly changes of average surface runoff depth under extreme scenarios were analyzed. The surface runoff depth under each extreme scenario declined all year round and even did not appear individual rise, which explained that the simulation results were of high accuracy.(5)The changes of annual surface runoff depth under 30 reasonable scenarios in Bailong River basin were analyzed and the results showed that if it was the same type of vegetation that farmland was returned to, the reduction of the surface runoff depth in the scenarios that returned the farmland of which slope was greater than 15 degree was less than the reduction in the extreme scenarios that all farmland was returned, but more than the reduction in the scenarios of which the slope was greater than 25 degrees. Analysis concerning farmland area showed that the highest efficiency of ecological restoration in reducing surface runoff depth was in the scenarios that slope of returning farmland was greater than 15 degrees. Take the scenario of returning farmland to evergreen forest as an example, surface runoff depth would decrease by 0.0095 mm if 1 km2 farmland was returned. Finally, the scenarios of returning farmland to different types of vegetation in different slope levels were analyzed, and it could be revealed that the surface runoff depth declined the most when the farmland was returned to the combination of evergreen forest and shrub.(6) The effects simulation of the ecological restoration on debris flow disaster in Goulinping watershed was carried out: based on the national policy of returning farmland to forestland or grassland, nine ecological restoration scenarios were designed under the consideration of the distribution of natural vegetation. The analysis about the changes of annual average surface runoff depth in all scenarios showed that it could be the most obvious restoration effect when returned farmland to forest land, in which annual average surface runoff depth decreased by 21.15%, followed by the restoration scenarios of returning to the combination of the forest and shrub.(7) According to the study of debris flow starting threshold of precipitation and the 2013 debris flow disasters records in Goulinping watershed, this paper arrived the result that Goulinping surface runoff depth of debris flow starting threshold is 9.49 mm.(8) Based on the starting threshold of surface runoff depth and the debris flow occurred in May 24,2013, this paper simulated the change characteristics of surface runoff depth under different ecological restoration scenarios in Goulinping debris flow gully. The results showed that all the nine ecological restoration scenarios designed in this paper could reduce the surface runoff depth and make it below the threshold. Meanwhile, the scenario returning farmland to forest land and shrub land reduced the most of surface runoff depth, which meant that human could contain the starting of debris flow, and reduce the risk of disasters through ecological restoration project, and thus to prevent and control debris flow disasters effectively.