Plain river network area in-river pollution analysis method based on watershed hydrology and water quality model

Abstract

The invention discloses a plain river network area in-river pollution analysis method based on a watershed hydrology and water quality model, and belongs to the technical field of water environment comprehensive treatment. According to the method, aiming at the problems that complicated hydrological characteristics and pollution sources in a plain river network area are difficult to trace to the source, accurate analysis of pollutants entering a river is realized through riverway topological relation carding, sub catchment partition division, basin hydrological water quality model construction, pollution load calculation and pollution source composition analysis; the method comprises the specific technical steps of river channel screening and topological relation carding, ArcSWAT-based sub catchment partition division, basin hydrology and water quality model building and simulation, pollution load calculation and pollution source identification. The method can reasonably divide catchment zones, simulate the flushing and river entering processes of pollutants, improve the accuracy of pollution source analysis, significantly improve the treatment efficiency, realize accurate traceability and quantitative analysis of pollutants in plain river network areas, and provide powerful data support and theoretical basis for accurate comprehensive treatment of water environments.

Description

Technical Field

[0001] The present invention relates to the technical field of comprehensive water environment management, and in particular to a method for analyzing river pollution in plain river network areas based on a watershed hydrological and water quality model. Background Art

[0002] At present, the concept of water environment governance has gradually changed from the traditional idea of ​​"end-of-pipe governance" to a water pollution control model of "source control, process blocking, and end-of-pipe governance" for the whole process of prevention and control. The comprehensive governance of river basin water environment has become the main direction of water pollution control. With the continuous advancement of the comprehensive governance of water environment, the number of projects for comprehensive governance of river basin water environment is increasing. However, from the perspective of the effect of project implementation, most projects have not fully combined the current status and characteristics of river basin pollution for in-depth technical analysis, resulting in the phenomenon of applying a unified governance plan template in multiple projects, which is not only uneconomical, but also greatly reduces the effect in actual application.

[0003] In order to improve the efficiency of governance, it is necessary to formulate a more targeted and effective comprehensive governance plan based on the regional status. Formulating such a plan requires a comprehensive understanding of the basin situation, including the source of pollution, the proportion of various pollution sources, the pollution load generated in different regions, and the pollution characteristics of the river. However, the types of basin pollution are complex and diverse, with many discharge paths, and the source of water pollution is often difficult to trace directly. These problems are particularly prominent in plain river network areas.

[0004] The plain river network area has a flat terrain, densely interwoven river networks, and complex water systems. Usually, dikes are built to form their own enclosures. Control facilities such as gates and pumps are often installed on the river channel, so that the water flow is largely controlled by humans. Pollution in the same dike area may flow to different water bodies. Due to these complex hydrological characteristics, it is often difficult to accurately calculate the pollution load entering a certain important water body when conducting statistical analysis of pollutants entering the river in the plain river network area.

[0005] Traditional analysis methods are powerless when faced with complex situations in plain river network areas and cannot meet the needs of precise pollution control. Therefore, it is necessary to use numerical simulation technology to simulate the water environment, hydrology and water quality processes of the entire river basin for a long time. Through detailed characterization of the river basin, complex water environment problems can be analyzed and calculated, thus providing a strong guiding basis for the design of comprehensive water environment management solutions.

[0006] However, the current watershed models are more applied to dendritic water system areas with large terrain undulations, top-down water flow direction, and no human control. In these areas, the model software can easily identify the water system and reasonable watershed division according to the elevation data. However, in plain river network areas, due to the flat terrain and the large human influence between water systems, it is difficult to directly divide the water system and reasonable watershed division according to the actual situation using the existing watershed model software.

[0007] In summary, there are many unreasonable aspects in directly using the existing watershed model to construct the hydrological and water quality model of the plain river network area. It is urgent to establish a watershed-level river pollution analysis method that is more suitable for the plain river network area to solve the current technical challenges and limitations of existing technical methods. Summary of the invention

[0008] The technical problem to be solved by the present invention is to provide a method for analyzing river pollution entering a plain river network area based on a watershed hydrological and water quality model, so as to solve the technical problems in water pollution control under the complex hydrological characteristics of the plain river network area. Specifically, due to the flat terrain, dense and interwoven river networks, complex water systems and the great influence of human regulation in the plain river network area, traditional pollution analysis methods are difficult to accurately calculate the pollution load entering important water bodies, which in turn affects the accuracy and effectiveness of water environment management. The present invention aims to overcome the limitations of the existing technology that it is impossible to accurately trace the source and calculate the pollution load in the plain river network area by providing a method for analyzing river pollution entering a plain river network area based on a watershed hydrological and water quality model.

[0009] In order to solve the above technical problems, the technical solution adopted by the present invention is: a method for analyzing river pollution in plain river network areas based on a watershed hydrological and water quality model, comprising the following steps: Step 1: Based on the project goals, needs and requirements, the river channels are screened in combination with water system data, image maps and planning data, and the key rivers to be simulated are selected. The river channel topological relationship is sorted out in combination with the gate pump dispatching rules, polder area planning, water system status and other data; Step 2: Carry out sub-catchment division, adopt the principle of large basin division, sub-area division and hierarchical division, reasonably divide the plain river network area and obtain reasonable catchment division; Step 3: Combine land use and soil type data to divide the hydrological response unit as the minimum calculation unit of the model; Step 4: Combine meteorological data, agricultural planting data, livestock and poultry breeding, aquaculture, industrial pollution, sewage plant tailwater and other data, based on the sub-catchment areas and hydrological response units divided in Step 2~Step 3, extract area, slope, river depth, width, length, slope and other information, and build a watershed hydrological and water quality model; Step 5: Use the established basin hydrology and water quality model to simulate the infiltration, evaporation, overflow of water, and the flushing, degradation, sedimentation and entry of pollutants into the river, and calibrate the model parameters in combination with real-time monitoring data; Step 6: Based on the model results, statistically analyze the total river load, pollution source composition and proportion, and identify key pollution areas.

[0010] In the preferred scheme, the specific method of reasonable division in Step 2 includes: for areas with polder areas, the polder areas are used as water catchment units, and the outlet positions of the water catchment subareas are set according to the locations of the drainage stations; if the polder areas are large and there are multiple rivers in them, they are further cut into smaller water catchment units according to the elevation data; the water catchment areas of the remaining areas are divided automatically and manually according to the elevation data and the river network conditions; during the division process, if aquaculture fish ponds or lakes are encountered, they are divided as separate water catchment subareas.

[0011] In a preferred solution, the division of sub-catchment areas in Step 2 also involves cutting the fish ponds that span two polder areas into sub-basins according to the specific locations of the aquaculture fish ponds and the boundaries of the polder areas.

[0012] In the preferred solution, the watershed hydrology and water quality model in Step 5 adopts the LSPC (Loading Simulation Program C, watershed modeling system) watershed hydrology and water quality model to simulate the loss or increase of water volume and load in various migration processes such as infiltration, evaporation, scouring, degradation, and sedimentation.

[0013] In the preferred scheme, the watershed hydrological and water quality model in Step 5 includes a rainfall runoff calculation model and a pollutant flushing calculation model, wherein the rainfall runoff calculation model uses the Chezy-Manning equation and the empirical expression of the relationship between outflow depth and retention volume to calculate the slope flow; the pollutant flushing calculation model is used to calculate the flushing amount of surface pollutants.

[0014] In a preferred solution, the model parameter calibration process in Step 5 includes adjusting parameters using hydrological and water quality monitoring data to ensure the accuracy and reliability of the model.

[0015] The present invention provides a method for analyzing river pollution in plain river network areas based on a watershed hydrological and water quality model, which has the following beneficial effects: 1. The present invention solves the technical difficulties in water pollution control under the complex hydrological characteristics of plain river network areas, and overcomes the limitations of the prior art that it is impossible to accurately trace the source and calculate the pollution load in plain river network areas; 2. The present invention takes into account the hydrological characteristics of plain river network areas and establishes a method of "large basin division, sub-area division, and hierarchical division" to divide sub-catchment zones. It fully considers the complexity of the flat terrain, polder scheduling, densely crisscrossed river networks, and many lakes in the plain river network area, and reasonably divides the catchment zones to ensure that the surface runoff and pollutant convergence paths are reasonable, coming from the same catchment zone and converging into the same river channel, ensuring the rationality and correctness of the basin model simulation; 3. The present invention is based on ArcSWAT (ArcGIS for Soil and Water Assessment Tool, a watershed hydrological model and environmental simulation software), and can automatically divide the watershed into zones by combining elevation data and the current status of the water system. At the same time, it can be manually adjusted according to the current status of the region. No additional operating tools are required, and the input format required by the watershed hydrological and water quality model can be directly output, which is easy to operate; 4. The present invention is based on the basin hydrological and water quality model, fully considering the scouring and river entry of pollutants on land, and can reasonably simulate the process of pollutant generation and entry into the river, and can achieve accurate source tracing; 5. The present invention can accurately simulate the hydrological and water quality processes in plain river network areas by reasonably dividing the watershed areas and constructing the watershed hydrological and water quality model, and realize the precise tracing of pollutants and accurate calculation of pollution loads; 6. The present invention provides scientific data support and theoretical basis for comprehensive water environment management projects, which helps to formulate targeted management plans, thereby improving management efficiency and effectiveness; 7. The present invention is based on ArcSWAT, combines elevation data and water system status to perform automatic zoning, and supports manual adjustment. It is easy to operate and has high universality. 8. The present invention provides a method for analyzing river pollution in plain river network areas based on a watershed hydrological and water quality model, which significantly improves the accuracy of pollution analysis and the efficiency of governance, and provides strong support for the comprehensive governance of the water environment; 9. The present invention realizes the precise tracing and quantitative analysis of pollutants in plain river network areas, providing powerful data support and theoretical basis for the precise and comprehensive management of water environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The present invention will be further described below in conjunction with the accompanying drawings and implementation examples: Figure 1 It is a working process diagram of the present invention; Figure 2 This is a schematic diagram of the sub-catchment division result of the present invention; Figure 3 This is the pollution space distribution map of the present invention. DETAILED DESCRIPTION

[0017] The technical solution of the present invention is further described below in conjunction with the accompanying drawings and embodiments: Example 1 like Figure 1 As shown, a method for analyzing river pollution in plain river network areas based on a watershed hydrological and water quality model includes the following steps: Step 1: Based on the project goals, needs and requirements, the river channels are screened in combination with water system data, image maps and planning data, and the key rivers to be simulated are selected. The river channel topological relationship is sorted out in combination with the gate pump dispatching rules, polder area planning, water system status and other data; Step 2: Carry out sub-catchment division, adopt the principle of large basin division, sub-area division and hierarchical division, reasonably divide the plain river network area and obtain reasonable catchment division; Step 3: Combine land use and soil type data to divide the hydrological response unit as the minimum calculation unit of the model; Step 4: Combine meteorological data, agricultural planting data, livestock and poultry breeding, aquaculture, industrial pollution, sewage plant tailwater and other data, based on the sub-catchment areas and hydrological response units divided in Step 2~Step 3, extract area, slope, river depth, width, length, slope and other information, and build a watershed hydrological and water quality model; Step 5: Use the established basin hydrology and water quality model to simulate the infiltration, evaporation, overflow of water, and the flushing, degradation, sedimentation and entry of pollutants into the river, and calibrate the model parameters in combination with real-time monitoring data; Step 6: Based on the model results, statistically analyze the total river load, pollution source composition and proportion, and identify key pollution areas.

[0018] In this embodiment, the specific method of reasonable division in Step 2 includes: for areas with polder areas, the polder areas are used as water catchment units, and the outlet positions of water catchment subareas are set according to the locations of drainage stations; if the polder areas are large and there are multiple rivers in them, they are further cut into smaller water catchment units according to elevation data; the water catchment areas of the remaining areas are divided automatically and manually according to elevation data and river network conditions; during the division process, if aquaculture fish ponds or lakes are encountered, they are divided as separate water catchment subareas.

[0019] Furthermore, the division of sub-catchment areas in Step 2 also involves cutting the fish ponds that span two polder areas into sub-basins according to the specific locations of the aquaculture fish ponds and the boundaries of the polder areas.

[0020] Furthermore, the watershed hydrology and water quality model in Step 5 adopts the LSPC (Loading Simulation Program C, watershed modeling system) watershed hydrology and water quality model to simulate the loss or increase of water volume and load in various migration processes such as infiltration, evaporation, scouring, degradation, and sedimentation.

[0021] Furthermore, the watershed hydrological and water quality model in Step 5 includes a rainfall runoff calculation model and a pollutant flushing calculation model, wherein the rainfall runoff calculation model uses the Chezy-Manning equation and the empirical expression of the relationship between outflow depth and retention volume to calculate the slope flow; the pollutant flushing calculation model is used to calculate the flushing amount of surface pollutants.

[0022] Furthermore, the model parameter calibration process in Step 5 includes adjusting parameters using hydrological and water quality monitoring data to ensure the accuracy and reliability of the model.

[0023] Furthermore, the construction of the watershed hydrological and water quality model in Step 5 specifically includes the following steps: Step 5.1: Use the land use type map and soil type map to divide the hydrological response units and calculate the area of ​​each hydrological response unit in each sub-catchment area; Step 5.2: Extract the characteristic values ​​and parameters required by the watershed hydrological and water quality model, including river length, slope, flow direction, width, depth, area and slope of the catchment area, underlying surface type, agricultural planting types and fertilizer application, livestock and poultry breeding types and quantities, aquaculture information, pollution source location information, meteorological station altitude and meteorological data; Step 5.3: Create meteorological files, point source files, and flow boundary input files; Step 5.4: Build a basin hydrological and water quality model and calibrate the model based on hydrological and water quality monitoring data; Step 5.5: Construct a rainfall runoff calculation model, using the Chezy-Manning equation and the empirical expression of the relationship between outflow depth and retention volume, and the slope outflow calculation formula is: (1) (2) (3) (4) (5) (6) (7) Among them, SURO is surface outflow; DELT is the time step; SURSM is the average surface retention in the simulation time interval; SURSE is the equilibrium surface retention under the current supply; SSUPR is the water supply rate of overland flow; PSUR is the potential surface retention; SURS is the initial surface storage; NSUR is the slope Manning coefficient; LSUR is the slope length; SLSUR is the slope gradient, SRC and DEC are intermediate variables; Equation (1) represents the situation where the overland flow increases, and equation (2) represents the situation where the surface water volume is in equilibrium or the overland flow decreases; Construct a pollutant washoff calculation model, and the surface pollutant washoff calculation formula is: (8) (9) Among them, SOQO is the amount of pollutants washed from the surface; SQO is the pollutant storage capacity on the surface that can be directly washed away by overland flow; WSFAC is the sensitivity parameter between pollutant amount and washoff; ADFX is dry deposition or total deposition flux; PREC is rainfall; ADCN is atmospheric wet deposition concentration; SURO is surface outflow.

[0024] Example 2 In another preferred embodiment, based on the above embodiment 1, Figure 1 As shown in the figure, a method for analyzing river pollution in plain river network areas based on watershed hydrological and water quality models is presented. This technology is based on the watershed hydrological and water quality models, combined with ArcSWAT to divide sub-watersheds, and follows the division principles of "large watershed division, sub-area division, and hierarchical division". It can reasonably divide the watershed into sub-areas and build a watershed hydrological and water quality model for the plain river network area. It can reasonably simulate regional pollution sources, calculate river inflow loads, identify the composition and proportion of pollution sources, and identify key pollution areas, providing support for precise governance and control decisions in watersheds.

[0025] The method comprises the following steps: S1. Sorting out river channel topological relationships Based on the project goals, needs and requirements, the river channels were screened in combination with water system data, image maps and planning data, and the key rivers to be simulated were selected. The topological relationship of the river channels was sorted out in combination with the gate and pump dispatching rules, polder area planning, water system status and other data to determine the pollution destination.

[0026] S2. Subcatchment division The basin division is based on ArcSWAT, and the division principle is "large basin division, sub-area division, and hierarchical division". The specific division method is as follows: For areas with polder areas, the polder areas are used as water catchment units. The process of water in the polder area being discharged into the river network is affected by the drainage station. The outlet position of the water catchment division is set according to the location of the drainage station into the river network. If the polder area is relatively large and there are multiple rivers in the polder area, the polder area is further cut into smaller water catchment units according to the elevation data in the polder area and the river network in the polder area. The outlet position of the sub-basin is defined according to the location of the drainage station.

[0027] The remaining areas are divided into catchment areas automatically and manually according to elevation data and river network conditions. In the process of division, if there is an aquaculture fish pond, the fish pond will be divided as a separate catchment area. If a fish pond spans two polder areas, the sub-basin will be cut according to the polder area boundary. Lakes are also divided as separate catchment areas.

[0028] S3. Division of hydrological response units The hydrological response units are divided according to the land use and soil type data, which serve as the minimum calculation unit of the model.

[0029] S4. Construction of watershed model Based on the catchment division and river channel topology, a basin hydrological and water quality model is constructed. This model depicts the process of surface water flushing into the river, expresses the formation and discharge process of various pollution sources, and simulates the spatiotemporal variation of COD (Chemical Oxygen Demand), ammonia nitrogen, total nitrogen, and total phosphorus. The specific steps are as follows: The land use type map and soil type map are used to divide the hydrological response unit as the smallest calculation unit of the model, and the area of ​​each hydrological response unit in each sub-catchment area is calculated.

[0030] Extract the characteristic values ​​and parameters required for the basin hydrological and water quality model, including river length, slope, flow direction, width, depth, catchment area and slope, underlying surface type, agricultural planting types and fertilizer application, livestock and poultry breeding types and quantities, aquaculture information, pollution source location information, meteorological station altitude and meteorological data, etc.

[0031] Create meteorological files, point source files, and flow boundary input files.

[0032] Build a watershed hydrological and water quality model, and calibrate the model based on hydrological and water quality monitoring data.

[0033] S5. Pollution load simulation and analysis The established basin hydrology and water quality model is used to simulate the infiltration, water distribution, evaporation, overflow of water, and the flushing, degradation, sedimentation and inflow of pollutants into the river, including the following calculation models: A rainfall runoff calculation model was constructed, and the slope runoff calculation formula was as follows using the Chezy-Manning equation and the empirical expression of the relationship between outflow depth and retention: (1) (2) (3) (4) (5) (6) (7) Among them, SURO is surface outflow; DELT is the time step; SURSM is the average surface retention in the simulation time interval; SURSE is the equilibrium surface retention under the current supply; SSUPR is the water supply rate of overland flow; PSUR is the potential surface retention; SURS is the initial surface storage; NSUR is the slope Manning coefficient; LSUR is the slope length; SLSUR is the slope gradient, SRC and DEC are intermediate variables; Equation (1) represents the situation where the overland flow increases, and equation (2) represents the situation where the surface water volume is in equilibrium or the overland flow decreases; Construct a pollutant washoff calculation model, and the surface pollutant washoff calculation formula is: (8) (9) Among them, SOQO is the amount of pollutants washed from the surface; SQO is the pollutant storage capacity on the surface that can be directly washed away by overland flow; WSFAC is the sensitivity parameter between pollutant amount and washoff; ADFX is dry deposition or total deposition flux; PREC is rainfall; ADCN is atmospheric wet deposition concentration; SURO is surface outflow.

[0034] S6. Pollution analysis Combined with the gate and pump dispatching rules, the load and proportion of various pollution sources entering the river are statistically analyzed, and the pollution characteristics in the area are analyzed. Taking a certain area as an example, after dividing the watershed into reasonable sub-divisions (such as Figure 2 As shown in Figure 1), based on the divided catchment areas, a regional watershed hydrological and water quality model was built. The model simulated the flushing and inflow of pollutants into the river and described the spatiotemporal variation of terrestrial hydrological and water quality. The spatial distribution of inflow load (see Table 1) and pollution load in the study area (see Figure 1) was obtained. Figure 3 ), identify key pollution sources and key pollution areas.

[0035] Through the above-mentioned specific implementation methods, this application realizes the precise tracing and quantitative analysis of pollutants in plain river network areas, providing strong data support and theoretical basis for the comprehensive management of water environment.

[0036] Table 1 Pollution load statistics

[0037] In the preferred scheme, the specific method of reasonable division in Step 2 includes: for areas with polder areas, the polder areas are used as water catchment units, and the outlet positions of water catchment subareas are set according to the locations of drainage stations; if the polder area is large and there are multiple rivers in it, it is further cut into smaller water catchment units according to elevation data; the water catchment areas of the remaining areas are divided automatically and manually according to elevation data and river network conditions; during the division process, if aquaculture fish ponds or lakes are encountered, they are divided as separate water catchment subareas; the above settings ensure the independence of each water catchment subarea and the convenience of management; at the same time, GIS (Geographic Information System) technology is used to assist analysis, accurately simulate water flow paths, optimize drainage system layout, and improve overall drainage efficiency and flood control capabilities.

[0038] In the preferred scheme, the process of dividing the sub-catchment zones in Step 2 also involves sub-basin cutting of fish ponds spanning two polder areas according to the specific locations of the aquaculture fish ponds and the boundaries of the polder areas; the above settings ensure the accuracy of the sub-catchment zones and effectively avoid confusion and omissions in water resources management; at the same time, customized division strategies are adopted for special terrain and fish pond layouts to improve water resource utilization efficiency.

[0039] In the preferred solution, the watershed hydrology and water quality model in Step 5 adopts the LSPC (Loading Simulation Program C, watershed modeling system) watershed hydrology and water quality model to simulate the loss or increase of water volume and load in various migration processes such as infiltration, evaporation, scouring, degradation, and sedimentation; the above settings can accurately capture the dynamic changes of hydrology and water quality in the watershed, and provide a scientific basis for water resources management and environmental protection; at the same time, the model is also highly flexible and can adjust parameters according to actual needs to ensure the accuracy and practicality of the simulation results.

[0040] In the preferred scheme, the watershed hydrological and water quality model in Step 5 includes a rainfall runoff calculation model and a pollutant flushing calculation model, wherein the rainfall runoff calculation model adopts the Chezy-Manning equation and the empirical expression of the relationship between outflow depth and retention volume to calculate the slope flow; the pollutant flushing calculation model is used to calculate the flushing amount of surface pollutants; the above settings can comprehensively simulate the hydrological cycle and water quality change process in the watershed, enhance the decision-making support capabilities for water resources management and environmental protection, and ensure data accuracy and the effectiveness of model predictions.

[0041] In the preferred scheme, the model parameter calibration process in Step 5 includes adjusting parameters using hydrological and water quality monitoring data to ensure the accuracy and reliability of the model; the above settings also include verifying the calibrated model, testing its predictive ability through independent data sets, further adjusting the optimization parameters until the established standards are met, and finally locking in the best parameter combination to provide a solid foundation for subsequent applications.

[0042] In summary, the present invention provides a method for analyzing river pollution in plain river network areas based on a watershed hydrological and water quality model, which solves the technical difficulties in water pollution control under the complex hydrological characteristics of plain river network areas, and overcomes the limitations of the prior art that it is impossible to accurately trace the source and calculate the pollution load in plain river network areas; the present invention takes into account the hydrological characteristics of plain river network areas, and establishes a method of "large basin division, sub-area division, and hierarchical division" to divide sub-catchment areas, fully considering the complexity of the flat terrain, polder scheduling, densely crisscrossed river networks, and many lakes in the plain river network area, and reasonably divides the catchment areas to ensure that the surface runoff and pollutant convergence paths are reasonable, come from the same catchment area, and flow into the same river channel, thereby ensuring the rationality and correctness of the basin model simulation; the present invention is based on ArcSWAT, and can automatically divide the catchment areas in combination with elevation data and the current status of the water system. At the same time, it can be manually adjusted according to the current regional status, without the need for additional operating tools, and can directly output flow The input format required by the basin hydrological and water quality model is easy to operate; based on the basin hydrological and water quality model, the present invention fully considers the scouring and river entry process of pollutants on land, can reasonably simulate the process of pollutant generation-entry into the river, and can achieve accurate tracing; by reasonably dividing the watershed areas and constructing the basin hydrological and water quality model, the present invention can accurately simulate the hydrological and water quality processes in the plain river network area, realize the accurate tracing of pollutants and accurate calculation of pollution load; the present invention provides scientific data support and theoretical basis for the comprehensive management project of water environment, and is helpful to formulate targeted management plans, thereby improving management efficiency and effectiveness; based on ArcSWAT, the present invention combines elevation data and the current status of the water system for automatic zoning, and supports manual adjustment, which is easy to operate and has high universality; by providing a method for analyzing river entry pollution in plain river network areas based on the basin hydrological and water quality model, the present invention significantly improves the accuracy of pollution analysis and management efficiency, and provides strong support for comprehensive management of water environment.

Claims

1. A method for analyzing river pollution in plain river network areas based on a watershed hydrological and water quality model, characterized in that: The method comprises the following steps: Step 1: Based on the project goals, needs and requirements, the river channels are screened in combination with water system data, image maps and planning data, and the key rivers to be simulated are selected. The river channel topological relationship is sorted out in combination with the gate pump dispatching rules, polder area planning, water system status and other data; Step 2: Carry out sub-catchment division, adopt the principle of large basin division, sub-area division and hierarchical division, reasonably divide the plain river network area and obtain reasonable catchment division; Step 3: Combine land use and soil type data to divide the hydrological response unit as the minimum calculation unit of the model; Step 4: Combine meteorological data, agricultural planting data, livestock and poultry breeding, aquaculture, industrial pollution, sewage plant tailwater and other data, based on the sub-catchment areas and hydrological response units divided in Step 2~Step 3, extract area, slope, river depth, width, length, slope and other information, and build a watershed hydrological and water quality model; Step 5: Use the established basin hydrology and water quality model to simulate the infiltration, evaporation, overflow of water, and the flushing, degradation, sedimentation and entry of pollutants into the river, and calibrate the model parameters in combination with real-time monitoring data; Step 6: Based on the model results, statistically analyze the total river load, pollution source composition and proportion, and identify key pollution areas.

2. The method for analyzing river pollution in plain river network areas based on a watershed hydrological and water quality model according to claim 1 is characterized in that: The specific method of reasonable division in the Step 2 includes: for areas with polder areas, the polder areas are used as watershed units, and the outlet positions of watershed subareas are set according to the locations of drainage stations; if the polder areas are large and there are multiple rivers within them, they are further cut into smaller watershed units according to elevation data; the watershed areas of the remaining areas are divided automatically and manually according to elevation data and river network conditions; during the division process, if aquaculture fish ponds or lakes are encountered, they are divided as separate watershed subareas.

3. The method for analyzing river pollution in plain river network areas based on a watershed hydrological and water quality model according to claim 1 is characterized by: The process of dividing the sub-catchment areas in Step 2 also involves cutting the fish ponds that span two polder areas into sub-basins according to the specific locations of the aquaculture fish ponds and the boundaries of the polder areas.

4. The method for analyzing river pollution in plain river network areas based on a watershed hydrological and water quality model according to claim 1, characterized in that: The watershed hydrology and water quality model in Step 5 adopts the LSPC watershed hydrology and water quality model to simulate the loss or increase of water volume and load in various migration processes such as infiltration, evaporation, scouring, degradation, and sedimentation.

5. The method for analyzing river pollution in plain river network areas based on a watershed hydrological and water quality model according to claim 1 is characterized by: The watershed hydrology and water quality model in Step 5 includes a rainfall runoff calculation model and a pollutant flushing calculation model, wherein the rainfall runoff calculation model uses the Chezy-Manning equation and an empirical expression of the relationship between outflow depth and retention volume to calculate the slope flow; The pollutant washoff calculation model is used to calculate the washoff amount of surface pollutants.

6. The method for analyzing river pollution in plain river network areas based on a watershed hydrological and water quality model according to claim 1 is characterized by: The model parameter calibration process in Step 5 includes adjusting parameters using hydrological and water quality monitoring data to ensure the accuracy and reliability of the model.

7. The method for analyzing river pollution in plain river network areas based on a watershed hydrological and water quality model according to claim 1, characterized in that: The construction of the watershed hydrological and water quality model in Step 5 specifically includes the following steps: Step 5.1: Use the land use type map and soil type map to divide the hydrological response units and calculate the area of ​​each hydrological response unit in each sub-catchment area; Step 5.2: Extract the characteristic values ​​and parameters required by the watershed hydrological and water quality model, including river length, slope, flow direction, width, depth, area and slope of the catchment area, underlying surface type, agricultural planting types and fertilizer application, livestock and poultry breeding types and quantities, aquaculture information, pollution source location information, meteorological station altitude and meteorological data; Step 5.3: Create meteorological files, point source files, and flow boundary input files; Step 5.4: Build a basin hydrological and water quality model and calibrate the model based on hydrological and water quality monitoring data; Step 5.5: Construct a rainfall runoff calculation model, using the Chezy-Manning equation and the empirical expression of the relationship between outflow depth and retention volume, and the slope outflow calculation formula is: （1） （2） （3） （4） （5） （6） （7） Among them, SURO is surface outflow; DELT is the time step; SURSM is the average surface retention in the simulation time interval; SURSE is the equilibrium surface retention under the current supply; SSUPR is the water supply rate of overland flow; PSUR is the potential surface retention; SURS is the initial surface storage; NSUR is the slope Manning coefficient; LSUR is the slope length; SLSUR is the slope gradient, SRC and DEC are intermediate variables; Equation (1) represents the situation where the overland flow increases, and equation (2) represents the situation where the surface water volume is in equilibrium or the overland flow decreases; Construct a pollutant washoff calculation model, and the surface pollutant washoff calculation formula is: （8） （9） Among them, SOQO is the amount of pollutants washed from the surface; SQO is the pollutant storage capacity on the surface that can be directly washed away by overland flow; WSFAC is the sensitivity parameter between pollutant amount and washoff; ADFX is dry deposition or total deposition flux; PREC is rainfall; ADCN is atmospheric wet deposition concentration; SURO is surface outflow.

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