Spatiotemporal Evolution Characteristics And Simulation For Urban Watershed

Rivers are the cradle of urban civilization,and urban rivers are one of the most critical resources and environmental carriers for urban development.However,in the process of urbanization,the environment and ecology of most urban rivers have been polluted and damaged.In recent years,China has strengthened the governance of river and lake water environments and ecosystems.Urban watersheds are undergoing comprehensive management and restoration,making it important to study and evaluate the spatiotemporal evolution laws of urban river environments and ecology.This paper focuses on the key scientific issue of the feedback relationship between the improvement of urban watershed water environment quality and the improvement of the ecological system.Taking Beiyun river（Beijing section）as an example,this study conducts monitoring and research on the spatiotemporal evolution laws of urban water environment and ecology.A comprehensive water environment simulation model is constructed,which takes into account the social governance of the watershed water system,environmental quality,and the health of the ecological system.Based on the comprehensive model,future water environment and ecological comprehensive management plan evaluation and optimization studies are carried out.The main research content and conclusions of this paper are as follows:（1）Research on the temporal and spatial evolution patterns of urban water environment and ecology monitoring.A water environment and ecological monitoring system was established in the Beijing section of Beiyun river.Monitoring data from 2016,2018,and2020 were selected to analyze the spatial and temporal evolution of water resources,water environment,and water ecology in Beiyun river basin at the river scale.In terms of water environment,the COD,NH₃-N,and TP at each monitoring point showed a decreasing trend year by year,but the average COD concentration during flood periods was about 5.6%higher than that during non-flood periods.From the spatial scale analysis,the average concentration of COD,NH₃-N,and TP showed significant fluctuations.The upstream Shaheza section,the middle section of Houlincou,and the downstream sections of Majuqiao and Dingzhuangqiao had higher concentrations.In terms of water ecology,the average bio-density of phytoplankton decreased from 44.78 million indiv/L to 20.85million indiv/L,while the density of zooplankton showed an increasing trend,and the number of fish species increased.This confirms that the water quality in Beiyun river basin has improved,and the water ecology system is gradually recovering.The overall water ecology quality of the basin is in a healthy and damaged state,but some sites such as Yangfangzha are in a mildly damaged state.The comprehensive index of the multi-year water ecology quality in healthy areas is 3.96,while the comprehensive index of mildly damaged areas is 2.92.（2）A comprehensive water environment simulation model considering SEE scenarios.A coupled water dynamic-water quality model for the Beijing section of Beiyun river was established,incorporating the hydrodynamic module（HD）,rainfall-runoff module（RR）,convection-diffusion module（AD）,and water quality module（Ecolab）in the MIKE11software.By integrating the one-dimensional model MIKE11,MIKE URBAN,and two-dimensional model MIKE21,a dynamic coupled model system was established to simulate the rainfall-runoff,pipe network flow,river water dynamics,and water quality processes in typical regions.This enabled the establishment of a comprehensive water environment simulation model considering Social governance,Environment quality,and Ecosystem health（SEE）scenarios for the basin water system.This model can simulate scenarios such as runoff processes and source control measures for sponge city under complex urban underlying surface conditions,overflow pollution control by combined sewer system,and reducing the pollution load into the river basin.By analyzing the general factors related to social and economic development,governance capability,and environmental self-purification ability in the basin,the main factors affecting the water ecological system health and water environment quality stability in the basin can be identified.（3）Reseach on Comprehensive Management Technology for Urban Water Environment and Ecology.Conduct research on urban water environmental governance technology from two aspects:non-point source pollution and point source pollution.The governance of non-point source pollution includes source control,agricultural non-point source control,and urban non-point source pollution control;Point source pollution control includes source point sources and other dispersed point sources.The key consideration in water ecology restoration is underwater ecological restoration.In terms of pollution control in the source area,it is necessary to focus on non-point source pollution prevention and control measures such as rainwater restoration ponds and infiltration-type drainage ditches.For agricultural non-point source pollution control,organic fertilizers should be used to improve soil nutrient supply,increase crop yields,and reduce nitrogen fertilizer application by 20%.In terms of combined sewer overflow pollution control,interception measures are needed to reduce pollutant concentration.The average reduction rates for COD,SS,NH₃-N,TP,and TN are between 60%and 69%.In terms of typical sewage treatment process control,fine process control is necessary to achieve good denitrification and phosphorus removal results.The annual reduction rates of pollutants entering the river are approximately 87.1%for COD,91.5%for NH₃-N,and 89.9%for TP.Regarding water ecology restoration,the combination of species including bitter grass,golden algae,wheel leaf algae,and grass carp can form a relatively complete aquatic ecosystem.（4）Evaluation and Optimization of Comprehensive Ecological Management Scheme for Future Water Environment Based on SEE Model.The evaluation technology of water environment and ecological comprehensive governance based on the SEE model evaluated the effectiveness of water environment and ecological comprehensive governance in different combination scenarios of the North Canal（Beijing section）,and proposed a comprehensive water pollution prevention and control system for the entire basin.Specifically,measures include carrying out ecological clean small watershed construction in the source area,replacing fertilizers with organic fertilizers to reduce agricultural non-point source pollution,systematically controlling overflow pollution to reduce urban non-point source pollution,adopting Orbal oxidation ditch fine technology to regulate and control scattered point source pollution,and carrying out underwater ecosystem restoration.Under this governance system,the predicted results indicate that the pollution load of COD and NH₃-N in the watershed will be 498000 T and 51000 T in 2025,2030,and 2035,497000 T and 51000 T,535000 T and 55000 T respectively.The comprehensive average concentration of COD in the Shahe Sluice section in 2035 will be 19.6 mg/L,meeting the Class III standard.