Study On The Impact Of Environmental Changes On Runoff And Reservoir Flood Control Risk In The Chengbi River Karst Basin,Guangxi

Climate and land-use change are two major driving factors affecting the hydrological processes and the operational safety of water conservancy projects.The complex substrate characteristics of karst basins make the runoff and flood processes sensitive to the changing environment,which affects the development and use of water resources and the flood risk of reservoir projects.Therefore,it is typical and urgent to study the influence of changing environment on runoff and flood risk of reservoir projects in karst areas.Based on this,this thesis takes the Chengbi River Basin,a karst basin in southwest China,and the Chengbi River Reservoir,the controlling project in the basin,as the research object and the main work and results are as follows.（1）The influence of historical environmental changes on runoff and reservoir flood risk was explored using the climate elasticity coefficient method,Copula function theory,and the flood risk rate model.The results show that climate change plays a major role in influencing the change of runoff values,land-use change is the main factor for the increase of runoff concentration and concentration period,while climate change plays a dominant role in the increase of runoff seasonal index;as the degree of impact of environmental changes deepens,the reservoir flood risk ratio decreases from 0.11211%to 0.05419%and 0.01420%to0.00720%under design and calibration conditions respectively,and the corresponding flood protection standards increase from 892 years to 1845 years and 7042 years to 13889 years respectively.（2）The analysis of variance method and TOPSIS integrated evaluation method were used to study the sources of uncertainty and preferences of future climate modeling results,and the FLUS model was applied to predict future land use scenarios,and finally,a coupled future climate and land-use scenario for the basin was constructed.The results show that DSM is the main source of uncertainty for each representative climate indicator,followed by GCM*SSP*DSM and GCM*DSM;from SSP1-2.6 to SSP5-8.5,the optimal simulations of rainfall are P_2-1-4,P_3-2-4,P_4-3-4,and P_3-4-2,and the optimal simulations of daily maximum temperature are P_3-1-4,P_7-2-5,P_7-2-5,and P_7-4-2.The optimal simulations of daily maximum temperature are P_3-1-4,P_7-2-5,P_7-3-1,P_5-4-4,and daily minimum temperature are P_3-1-4,P_7-2-4,P_1-3-4,P_4-4-4;The FLUS model was able to simulate the spatial distribution of land use in the watershed with a Kappa coefficient of 0.826802.（3）The SWAT model was improved by adding groundwater matrix storage reservoirs and conduit storage reservoirs in karst areas.The results show that the simulation accuracy of the improved SWAT hydrological model has been effectively improved,and its Nash efficiency coefficient has increased from 0.7593-0.7799 to 0.8204-0.8466;the factors affecting the accuracy of the model simulation include the model rate and validation method as well as the variation of the model parameters during the peak and dry seasons;the optimal model rate and validation solution are to use the observed flow data from Pingtang station for single-site staged simulation;the values of the recharge distribution coefficient and receding constant for matrix storage reservoirs are 0.0219 and 0.01 larger during the dry period than during the rich period,respectively,while the difference between the receding constant for conduit storage reservoirs during the dry and rich periods is small（0.0003）,and changes in hydrological abundance and depletion have little effect on the receding capacity of conduit storage reservoirs.（4）The runoff and reservoir flood control risk under the future climate and land use coupling scenario are studied by the Linear trend analysis method,risk rate model of overtopping dam,and the risk assignment model of overtopping dam.The results show that:the average annual runoff depth of the basin in the future years is characterized by an increasing trend（19.453mm/10a）in the SSP5-8.5 scenario and a decreasing trend in the other scenarios;under the checking and design conditions,the reservoir has the lowest flood risk rate under the SSP1-2.6 scenario（0.00581%and 0.04438%）,which corresponds to the highest flood protection standard（17212 years and 2253 years）,while the reservoir has the highest flood risk rate under the SSP5-8.5 scenario（0.01589%and 0.11396%）and the lowest flood protection standards（6293 and 878 years）;the risk grade of overtopping dam for each scenario is very low when the wave proof wall of the reservoir is in normal operation,but when the wave proof wall of reservoir fails,the overtopping risk rates for the SSP3-7.0 and SSP5-8.5 scenarios are 2.80184 x 10^-5and 4.26143 x 10^-5respectively,with corresponding danger degree of 0.4867 and 0.5269,both of which are moderate.（5）It is recommended to improve and upgrade the hydro-meteorological dynamic monitoring system,optimize the land use pattern of the basin,prepare water resources allocation schemes,construct and improve karst hydrological simulation schemes,review and revise the relevant design results and dispatching schemes of reservoirs,and strengthen the implementation of reservoir de-risking and reinforcement work,to provide a solid guarantee for the sustainable development of water resources,flood and drought disaster prevention and the safety of water supply in the center of Baise City under changing environments.