Plain river network polder area non-point source pollution water inlet load accounting method

Abstract

The invention relates to a plain river network polder area non-point source pollution water inlet load accounting method. The method comprises the steps that the runoff yield of a pollution production unit is calculated in a water volume module; calculating the non-point source pollution discharge amount of each pollution production unit in the water quality module; calculating a non-point source pollution in-water body coefficient of each pollution production unit in a water body migration module; and accounting to obtain the total load of the non-point source pollutants entering the water body of the pollution production unit. According to the method, polder area non-point source pollution characteristics, the distance between a pollution production unit and a receiving water body and the interception effect of forests and grassland on pollutants are fully considered, and meanwhile the runoff yields of different underlying surfaces under the rainfall condition are combined; therefore, the influence of different hydrological driving conditions and the land migration interception process on the non-point source pollutant entering the water body can be more accurately represented, the key time period and key area risk of the non-point source pollutant entering the water body can be identified, and the method has good applicability in actual implementation.

Description

Technical Field

[0001] The present invention relates to a method for calculating the load of non-point source pollution entering water bodies in the plain river network polder area, and belongs to the technical field of ecological environmental protection. Background Technique

[0002] Plain polder areas are widely distributed in the areas along rivers and lakes in the south of China. As a unique hydrological unit in the plain river network area, the polder areas are flat, with dense river network systems, shallow groundwater burial, diverse land use types, and the water flow exchange being affected by human factors. These characteristics make the runoff generation and concentration process show its own particularity and complexity, directly resulting in differences in the generation and migration processes of pollutants compared with non-polder areas and mountainous areas in the plain. Considering the characteristics of non-point source pollution in the polder area, constructing a method for calculating the load of non-point source pollution entering water bodies based on the non-point source pollution migration process and carrying out quantitative research on non-point source pollution in the polder area can provide a theoretical basis for non-point source pollution control and scientific management of the water environment in the plain river network area, and at the same time provide relevant decision-making basis for the formulation of water environmental protection policies, which has important theoretical and practical significance for the improvement of the water environment in the plain river network area.

[0003] The research on non-point source pollution at home and abroad mainly focuses on basins with relatively large topographic slopes such as mountains and hills, while the research on plain river network polder areas is relatively less. However, the polder areas are densely populated and economically developed, and their water ecological and environmental problems have increasingly attracted people's attention (Huang Jiacong and Gao Junfeng, 2015; Lai et al., 2018; Su and Luo, 2019), and many scholars have also begun to explore the characteristics and mechanisms of the pollutant migration and transformation process in the polder area. At present, the research on non-point source pollution load in the polder area mainly includes methods such as field in-situ observation, empirical model, and mechanism model estimation.

[0004] In terms of in-situ field observations, it is mainly based on the sampling and monitoring of runoff or water body pollutants in the study area to analyze the interaction mechanism between non-point source pollution and environmental factors in the plain river network area. Current research mainly focuses on the impacts of hydrometeorological conditions, land use types, soil characteristics, agricultural cultivation methods, etc. on the non-point source pollution output in polder areas (Yuan et al., 2013). Affected by driving conditions such as rainfall, Xu Ailan, Wang Peng and others took Dize Lianwei in Xishan District of Wuxi City as a representative study area, studied the nitrogen and phosphorus migration characteristics under the drive of natural rainfall-runoff, and established a quantitative relationship between the nitrogen and phosphorus transport in paddy fields, runoff flux and fertilization (Wang Peng et al., 2008; Xu Ailan et al., 2008). There are differences in the non-point source pollution load characteristics under different underlying surface conditions and different time scales. Zhang Mingkui et al. (2011) adopted the method of field runoff plot location research, selected 27 farmlands in Shaoxing County, Zhejiang Province, and studied the runoff loss characteristics, loads and influencing factors of nitrogen and phosphorus in 7 planting types of farmlands in the water network plain area under natural rainfall conditions; Chu Yin et al. (2013) based on 3-year field monitoring in the polder area along the Chaohu Lake, explored the non-point source pollution transport characteristics in the oil wheat season and rice season in the polder area; Ma Fafan et al. (2019) also took the Chaohu polder area as the research object, and explored the output characteristics and spatio-temporal distribution rules of nitrogen and phosphorus in the drainage of farmland ditches in the polder area. Since the actual measurement method has high labor and material costs and is only applicable to small watersheds with simple pollution source composition and clear water collection paths, it is difficult to be popularized on a large scale. Although this method is based on experiments, it cannot reflect the migration path and process of non-point source pollutants in the polder area, and is limited by the number of sampling times and observation costs, and cannot comprehensively reflect the spatio-temporal variation law of non-point source pollution in the polder area.

[0005] In terms of mechanism models, mechanism models can better describe the migration and transformation of non-point source pollutants as well as complex spatio-temporal transmission processes, and are relatively accurate in quantitatively estimating loads. At present, a large number of non-point source models have been developed at home and abroad. Widely used models include SWAT, AnnAGNPS, HSPF and other models. In the plain river network area, Shepherd et al. (1999) analyzed 14 characteristics available for phosphorus loss models, and combined with the model application objectives, the spatial scale of the study area and the data basis, and determined that the SWAT model is most suitable for the long-term scale simulation of phosphorus loss in the northern plain area of the UK. Lam et al. (2012) calibrated the parameters of the SWAT model based on measured data and applied it to non-point source pollution simulation at different spatial scales in northern Germany. In China, Xu Ailan et al. (2010) superimposed the digitized actual river network water system on the DEM, simulated the drainage pattern of the polder area with a "virtual reservoir", and used the SWAT model to simulate the non-point source pollution in a typical polder area of the Taihu Lake Basin, and analyzed the spatio-temporal variation law of its pollutant output. Zhang et al. (2010) and Su and Luo (2019) adopted a model preprocessing method similar to that of Xu Ailan, and used the SWAT model to simulate the agricultural non-point source pollution in the plain river network area. Sun Jinhua et al. (2013) studied the non-point source pollution in the plain river network polder area of Wujin District, Changzhou City, Taihu Lake Basin based on the hydrological monitoring data of rainfall events, and used the AnnGNPS model to estimate its annual pollution emission load. Many scholars have improved the reliability and practicability of the model in non-point source pollution simulation in the plain river network area through the improvement of existing models and the coupling of different modules. Schoumans et al. (2002) developed the SIMPLE and PLEASE models successively by combining elements such as topography, land use, soil type and hydrology, and simulated the agricultural non-point source pollution in plain areas such as the Netherlands and Denmark. Wang Peng (2006) based on the Dutch pollution load model WLM (Waste Load Model), changed the traditional method of allocating pollution loads according to the land width in the pollution confluence process in the polder area, and combined "virtual connection" with the scheduling principles in different situations to develop a distributed pollution load model (DWLM) for the plain river network area. Wang Tingting et al. (2011) used the Xin'anjiang model and the MUSLE soil erosion model to calculate the runoff and sediment yield in the plain polder area, and then combined with the drainage patterns of paddy fields and polder areas and the pollution load equivalents of livestock and poultry breeding, rural residential areas, etc. to study the non-point source pollution load in the Dongting Lake Plain polder area. Huang and Yan et al. (Huang et al., 2018; Yan et al., 2019) took Jianwei in Liyang City, Lake West District as the research object, identified the key processes of hydrology and pollutant transport in the polder area, and constructed a hydrological and pollutant transport process model for the plain polder area including key processes such as runoff generation and confluence, artificial controlled outflow, and nutrient retention in farmland-ditch-pond.The results of model simulation methods are relatively reliable, but they require high-quality data, have many parameters, and are difficult to calibrate. Moreover, common mechanistic models such as SWAT are insufficient in characterizing the hillslope hydrological transfer process from the plot scale to the sub-basin scale, and it is difficult to express the impact of the differences in runoff paths of different plots on the pollutant input coefficient into the river.

[0006] With the development of technology and the in-depth research, the quantitative research on non-point source pollution has developed from simple empirical models to complex mechanistic models. However, mechanistic models have high requirements for data accuracy, and most of them are constructed by referring to foreign models, lacking the expression of the regional characteristics of the study area, and it is difficult to be widely promoted and applied on a large scale. Empirical models require less input data and are simple and easy to operate. However, traditional non-point source pollution empirical models mainly consider the generation amount of non-point source pollutants, and consider less about the processes of migration, transformation, and reduction of non-point source pollutants, resulting in low simulation accuracy. Generally using the production and discharge amounts of non-point source pollutants to characterize their impact on water bodies will lead to deviations in the assessment of water body pollution loads, and at the same time will cause the distortion of the results of pollution source investigations, covering up the impact of other important pollution sources on the water environment, and affecting the effectiveness and pertinence of pollution control policy guidance. Therefore, it is crucial to comprehensively and objectively reflect the non-point source pollution input load into water bodies and accurately grasp the contribution degree of non-point source pollutants to environmental quality in the plain river network polder area. Summary of the Invention

[0007] The main object of the present invention is to overcome the problems existing in the prior art and provide a method for calculating the non-point source pollution input load into water bodies in the plain river network polder area. It fully considers the non-point source pollution characteristics in the polder area, the distance from the pollution source unit to the receiving water body, and the interception effect of forest and grassland on pollutants. At the same time, it combines the runoff generation amounts of different underlying surfaces under rainfall conditions, so as to be able to more accurately characterize the impact of different hydrological driving conditions and land migration and interception processes on non-point source pollutants entering water bodies, identify the key periods and key regional risks of non-point source pollutants entering water bodies, and has good applicability in actual implementation.

[0008] The technical solution adopted by the present invention to solve its technical problems is as follows:

[0009] A method for calculating the non-point source pollution input load into water bodies in the plain river network polder area, characterized by comprising the following steps:

[0010] The first step is to construct a water volume module; in the water volume module, based on the precipitation and underlying surface characteristics of the target plain river network polder area and different underlying surface runoff generation models, calculate the runoff generation amount of the pollution source unit.

[0011] The second step is to construct a water quality module; based on the results obtained in the first step, in the water quality module, calculate the non-point source pollution loads within a preset period for different non-point source pollutions in the target plain river network polder area, that is, the non-point source pollution emissions of each pollution source unit.

[0012] Step 3: Construct the module for migration into water bodies. In the module for migration into water bodies, the land attenuation process of non-point source pollutants is characterized according to the first-order reaction kinetics equation. Meanwhile, considering the interception effect of pollutants during the migration process on different underlying surfaces, the non-point source pollution inflow coefficients of each pollution source unit are calculated based on the confluence path of the pollution source units.

[0013] Step 4: Multiply the non-point source pollution emissions of each pollution source unit obtained in Step 2 by the non-point source pollution inflow coefficients of each pollution source unit obtained in Step 3 to obtain the non-point source pollutant load into the water body of each pollution source unit. Then, summarize and calculate to obtain the total non-point source pollutant load into the water body of the pollution source units.

[0014] This method fully considers the characteristics of non-point source pollution in the polder area, the distance between the pollution source unit and the receiving water body, and the interception effect of forest and grassland on pollutants. Meanwhile, it combines the runoff yields of different underlying surfaces under rainfall conditions, so as to more accurately characterize the impact of different hydrological driving conditions and land migration and interception processes on the non-point source pollutants entering the water body, identify the key periods and key area risks of non-point source pollutants entering the water body, and has good applicability in actual implementation.

[0015] Preferably, the specific process of Step 1 includes: calculating the runoff yields of the pollution source units of each underlying surface within a preset time period for different underlying surfaces in the target plain river network polder area, and then calculating the total runoff yield of the pollution source units of the target plain river network polder area in combination with the areas of each underlying surface.

[0016] More preferably, in Step 1, the underlying surface includes at least one of water area, paddy field, aquaculture pond, dry land, and construction land; the total runoff yield of the pollution source units of the target plain river network polder area is:

[0017] R S =A W R W +A R R R +A A R A +A L R L +A IP R IP ;

[0018] In the formula: R S is the total runoff yield of the pollution source units of the target plain river network polder area, with the unit of mm; A W is the weight of the water area underlying surface area in the total area of the target plain river network polder area; R W is the runoff yield within the preset time period of the water area underlying surface, with the unit of mm; A R is the weight of the paddy field underlying surface area in the total area of the target plain river network polder area; R RThe runoff volume during the preset period for the paddy field underlying surface, unit: mm; A A The weight of the aquaculture pond underlying surface area in the total area of the target plain river network polder area; R A The runoff volume during the preset period for the aquaculture pond underlying surface, unit: mm; A L The weight of the dry land underlying surface area in the total area of the target plain river network polder area; R L The runoff volume during the preset period for the dry land underlying surface, unit: mm; A IP The weight of the construction land underlying surface area in the total area of the target plain river network polder area; R IP The runoff volume during the preset period for the construction land underlying surface, unit: mm.

[0019] If the target plain river network polder area does not contain a certain underlying surface, the weight of that underlying surface is counted as 0. For example, if the target plain river network polder area does not contain aquaculture ponds, then the corresponding weight A A is counted as 0, and so on.

[0020] More preferably, in the first step, the runoff volume during the preset period for the water area underlying surface is calculated using the water body water balance principle, and the specific process is as follows:

[0021] W E = W S +(P - α*E);

[0022] When W E ≤ W M , no runoff occurs, R W = 0;

[0023] When W E > W M , R W = W E - W M ;

[0024] In the formula, W E is the water storage volume of the water surface at the end of the period, unit: mm; W S is the water storage volume of the water surface at the beginning of the period, unit: mm; W M is the water storage capacity of the water surface in the polder area, unit: mm; P is the rainfall, unit: mm; E is the evaporation, unit: mm; α is the evaporation conversion coefficient (specifically, the evaporation pan conversion coefficient of the basin where the target plain river network polder area is located can be used, the same below).

[0025] The runoff volume during the preset period for the paddy field underlying surface during the irrigation period is calculated using the field water balance principle, and the specific process is as follows:

[0026] The field water balance equation with a daily period is:

[0027] H2 = H1 + P + M i -R R -K C *α*E - β;

[0028] When H2 > H P then, R R = H2 - H P , M i = 0;

[0029] When H D < H2 < H P then, R R = 0, M i = 0;

[0030] When H2 < H D then, R R = 0, M i = H U ;

[0031] Wherein, H1 and H2 are the water depths of the paddy field at the beginning and end of the period, in mm; K C is the water requirement coefficient of rice in each growth period; H P is the waterlogging tolerance depth of rice in each growth period, in mm; H U is the upper limit of the suitable water depth of rice in each growth period, in mm; H D is the lower limit of the suitable water depth of rice in each growth period, in mm; M i is the irrigation water volume required for the paddy field during the period, in mm; P is the rainfall, in mm; E is the evaporation, in mm; α is the evaporation conversion coefficient; β is the daily seepage volume of the paddy field, in mm.

[0032] The runoff generation calculation of the paddy field underlying surface during the preset period in the non-irrigation period is treated as that of dry land.

[0033] The runoff generation in the preset period of the underlying surface of the aquaculture pond is calculated by the following formula:

[0034]

[0035] H A2 = P - E A + H A1 - I A ;

[0036] Wherein: H A2 is the water depth of the pond at the end of the calculation period, in mm; H Aflood is the waterlogging tolerance depth, in mm; P is the rainfall, in mm; E A is the evaporation of the pond, in mm; H A1 is the water depth of the pond at the beginning of the calculation period; IA The infiltration water volume in paddy fields, unit: mm.

[0037] More preferably, in the first step, the runoff generation volume within the preset time period of the dry underlying surface is calculated using the full-infiltration runoff generation principle. The specific process is as follows:

[0038] When P - E ≤ 0, the runoff generation volume is: R = 0;

[0039] When P - E + a < WMM, the runoff generation volume is:

[0040]

[0041] When P - E + a > WMM, the runoff generation volume is: R = P - E - (WM - W);

[0042] In the formula, R is the runoff generation volume, unit: mm; WM is the regional average water storage capacity, unit: mm; P is the rainfall, unit: mm; E is the evaporation, unit: mm; WMM is the regional maximum point water storage capacity, unit: mm; W is the known initial soil water content in the region, unit: mm; B is the regional water storage capacity curve index and is an empirical constant; a is the ordinate value corresponding to W on the regional water storage capacity curve, unit: mm.

[0043] The calculation formula of a is as follows:

[0044]

[0045] The calculation formula of WMM is as follows:

[0046]

[0047] In the formula: IMP is the ratio of the impervious area in the region to the total area of the region, that is, the impervious area ratio coefficient;

[0048] The calculation formula of E is as follows:

[0049] E = K × EM;

[0050] In the formula: K is the evaporation and transpiration conversion coefficient; EM is the measured water surface evaporation, unit: mm.

[0051] More preferably, in the first step, the runoff generation volume within the preset time period of the construction land underlying surface is calculated using the runoff coefficient method. The specific process is as follows:

[0052] R IP = p1 × R IP1 + p2 × R IP2 ;

[0053] In the formula: p1 is the area ratio of the permeable area of the construction land underlying surface; p2 is the area ratio of the impermeable area of the construction land underlying surface; RIP1 The runoff volume of the permeable area of the underlying surface for construction, with the unit of mm, is calculated according to the treatment of dry land; R IP2 is the runoff volume of the impermeable area of the underlying surface for construction, with the unit of mm.

[0054] R IP1 The calculation process is carried out according to the treatment of dry land.

[0055] R IP2 Is calculated according to the following formula: R IP2 = C × P - d;

[0056] In the formula: C is the runoff coefficient of the impermeable surface; P is the rainfall, with the unit of mm; d is the depression storage volume, with the unit of mm.

[0057] After adopting the above preferred solution, the specific technical features of the first step can be further optimized.

[0058] Preferably, in the second step, the non-point source pollution includes urban non-point source pollution and / or agricultural non-point source pollution.

[0059] More preferably, in the second step, the urban non-point source pollution includes urban domestic non-point source pollution and / or surface runoff pollution of construction land.

[0060] The load of urban domestic non-point source pollution is: L t = C t × A t ;

[0061] In the formula: L t is the load of urban domestic non-point source pollution, with the unit of kg; C t is the coefficient of urban domestic non-point source pollution, with the unit of g / (person·d); A t is the urban population quantity in the region, with the unit of person.

[0062] The load of surface runoff pollution of construction land is: L c = R c × C c × A c ;

[0063] In the formula: L c is the load of surface runoff pollution of construction land, with the unit of kg; R c is the surface runoff volume of construction land, with the unit of mm; C c is the pollutant concentration of runoff generation on construction land, with the unit of mg / L; A c is the area of construction land, with the unit of km 2 .

[0064] More preferably, in the second step, the agricultural non-point source pollution includes at least one of planting pollution, livestock and poultry breeding pollution, and rural domestic pollution.

[0065] For planting pollution, the emissions of planting pollutants are calculated according to the pollution loss coefficients of different land use types and the areas of each land use type.

[0066] For livestock and poultry breeding pollution, using the statistical yearbook of the research area and field research data, the number of each livestock and poultry species and the number (or proportion) of large-scale and scattered breeding are determined, and combined with the pollutant production and discharge coefficients of livestock and poultry breeding, the emissions of livestock and poultry breeding pollutants are determined.

[0067] For rural life, combining the statistical yearbook and research data, the rural population is determined, and using the pollution production coefficient, the emissions of rural domestic pollutants are calculated.

[0068] The agricultural non-point source pollution load is:

[0069]

[0070] In the formula: L is the load intensity of agricultural non-point source entering the water body, unit kg / a; n is the type of land use or livestock, population; E i is the output coefficient of pollutants in the i-th land use type, unit kg·hm -2 ·a -1 , or the output coefficient of livestock, population, unit kg·ind -1 ·a -1 ; A i is the area of the i-th land use type, unit hm 2 , or the number of livestock, population; I i is the input amount of the i-th pollution source, unit kg; P is the amount of nutrients input by rainfall, unit kg.

[0071] Among them, the non-point source pollution load related to rainfall runoff is:

[0072] L i =R i ×C i ×A i

[0073] In the formula, L i is the non-point source pollution load of different land use types, unit kg; R i is the runoff volume of different land use types, unit mm; C i is the pollutant concentration of runoff of different land use types, unit mg / L; A i is the area of different land use types, unit km 2 .

[0074] The rural domestic pollution load is:

[0075] L r = Q r × C r

[0076] In the formula, L r is the rural domestic pollution load, with the unit of kg; Q r is the discharge of rural domestic sewage, with the unit of m 3 ; C r is the concentration of pollutants discharged from rural domestic sewage treatment facilities, with the unit of mg / L.

[0077] After adopting the above preferred solution, the specific technical features of the second step can be further optimized.

[0078] Preferably, in the third step, the non-point source pollution inflow system coefficient is:

[0079]

[0080] In the formula, k is the attenuation coefficient of pollutants, with the unit of s -1 ; A is the length of the water flow path from the pollution source to the river, with the unit of m; v is the surface runoff velocity, with the unit of m / s; α is the interception coefficient of pollutants, that is, the interception effect of different underlying surfaces on pollutants on the runoff confluence path.

[0081] The pollution source is divided into several grid units, and the velocity v and migration time t of surface runoff flowing through the grid units are calculated by coupling the dynamic wave equation and the Manning equation:

[0082]

[0083] In the formula, L is the grid length, with the unit of m; n is the Manning coefficient of the grid plot; tanβ is the slope of the grid plot; Q is the depth of surface runoff in the grid plot, with the unit of mm; t e is the surface runoff generation time, with the unit of s.

[0084] After adopting the above preferred solution, the specific technical features of the third step can be further optimized.

[0085] Compared with the prior art, the method for calculating the non-point source pollution load into water bodies in the plain river network polder area of the present invention considers the non-point source pollutant output characteristics of different underlying surfaces under different hydrological driving conditions. At the same time, based on the distances between different pollution-producing units and the receiving water bodies, a water volume module, a water quality module, and a migration model into water bodies are respectively constructed, and a method for calculating the non-point source pollution load into water bodies is coupled and constructed. This calculation method fully considers the non-point source pollution characteristics of the polder area, the distance between the pollution-producing unit and the receiving water body, and the interception effect of forest and grasslands on pollutants. At the same time, it combines the runoff generation of different underlying surfaces under rainfall conditions, so as to more accurately characterize the impact of different hydrological driving conditions and land migration and interception processes on the non-point source pollutants entering water bodies, identify the key periods and key area risks of non-point source pollutants entering water bodies. Through actual simulation verification, this calculation method has good applicability in actual implementation. BRIEF DESCRIPTION OF THE DRAWINGS

[0086] Figure 1 It is a schematic diagram of the regional water storage capacity curve in Embodiment 2 of the present invention.

[0087] Figure 2 It is a water level verification diagram of 3 typical rainfall events in Yue Moulianwei in Embodiment 2 of the present invention.

[0088] Figure 3 It is a verification diagram of the non-point source pollution TN load into water bodies in Yue Moulianwei in Embodiment 2 of the present invention.

[0089] Figure 4 It is a comparison diagram of non-point source TN and TP loads calculated by different methods in Yue Moulianwei in Embodiment 2 of the present invention.

[0090] Figure 5 It is a change diagram of the non-point source pollutant loads into water bodies in Yue Moulianwei from 2018 to 2022 in Embodiment 2 of the present invention.

[0091] Figure 6 It is a spatial distribution diagram of the non-point source pollutant loads into water bodies in Yue Moulianwei in 2020 in Embodiment 2 of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0092] In specific implementation, the method for calculating the non-point source pollution load into water bodies in the plain river network polder area of the present invention contains all the technical features (including the features of the preferred solutions) recorded in the foregoing invention content, and the specific content will not be repeated here.

[0093] The present invention will be further described in detail below with reference to the drawings and in combination with embodiments. However, the present invention is not limited to the given examples.

[0094] Embodiment 1

[0095] This embodiment is the analysis of non-point source pollution characteristics in the plain river network polder area.

[0096] The specific content of this embodiment is as follows:

[0097] The spatial structure and hydrological characteristics of the polder area lead to special laws in the output of non-point source pollution load. Non-point source pollutants from various pollution sources do not directly enter the water body. Generally, they are first retained in the soil and will enter the adjacent water body along with rainfall runoff when effective rainfall runoff occurs. The amount of pollution generated by the pollution source is the pollution generation amount, and the amount of pollution discharged into the external river is the pollution discharge amount. The entire non-point source pollution process in the polder area is generally divided into three processes: generation, retention, and discharge. Generation refers to the generation of non-point source pollution by various pollution sources in the polder area under the condition of rainfall or irrigation runoff generation. Retention refers to the non-point source pollution generated by farmland, aquaculture water bodies, etc. in the polder area stagnating (accumulating) in farmland or water bodies at all levels such as ditches, canals, ponds, and puddles for a period of time and then gradually entering the inland river. Discharge refers to the situation where during the flood season, when the rainfall is large and prone to waterlogging or the water surface in the field is too high, the non-point source pollution accumulated in the ditches and inland river channels is discharged into the river outside the polder along with the drainage runoff by manually opening the sluice gates and pumping stations.

[0098] (1) Sources of non-point source pollution in the plain river network polder area

[0099] Polder areas are mostly distributed in the areas along the rivers and lakes in the middle and lower reaches of the rivers in southern China. The climate is humid, the soil water content is large, and the groundwater depth is relatively shallow, making it easier to form surface runoff after rainfall. Combining the investigation and research of agricultural production and rural life and the main types of underlying surfaces in the polder area, the main sources of non-point source pollution in the polder area are divided into different sources such as farmland runoff pollution, livestock and poultry breeding pollution, aquaculture pollution, rural domestic pollution, and surface runoff pollution from construction land.

[0100] (2) Influencing factors of non-point source pollution entering water bodies in the plain river network polder area

[0101] During the process of non-point source pollutants in the plain river network polder area from the pollution generation unit to entering the water body, they are affected by multiple processes and factors. The key processes of non-point source pollution mainly include four, namely, precipitation process, surface runoff process, underground leaching process, and soil erosion process. In addition, non-point source pollutants are also affected by the driving of topographic conditions and the interception effect of vegetation systems such as forests and grasses during the migration process. These processes are interrelated, interact with each other, and influence each other. Due to the flat terrain in the plain area, the topographic driving force is small, and the soil erosion intensity is relatively low.

[0102] (3) Transmission characteristics of non-point source pollution in the plain river network polder area

[0103] The non-point source pollution characteristics in polder areas are similar to those in general river basins. However, due to the particularity of the water flow exchange between polder areas and the outside world, the laws of nutrient generation, migration, and transformation in polder areas are unique and complex. Previous studies on polder areas have mostly been limited to aspects such as polder design, polder construction, polder drainage, and polder runoff generation mechanisms, etc., and less attention has been paid to the specific laws of non-point sources in polder areas.

[0104] Polder areas are different from non-polder areas with natural outflows. Due to the obstruction of polder dikes, only sluice gates or pumping stations can be used to exchange the water volume inside and outside the polder area. Therefore, whether the pollution load inside the polder can be discharged into the river network is mainly controlled by humans. Usually, the polder gate remains closed, and the river inside the polder is not connected to the river outside the polder. The runoff generated inside the polder flows into the drainage ditch nearby and then converges into the river inside the polder adjacent to it. The non-point source pollution generated by various pollution sources inside the polder accumulates in water bodies at all levels. Only when the runoff volume generated by heavy rain exceeds the water storage capacity of the polder area or the water level in the ditch reaches a certain height, the water in the ditches and rivers inside the polder is discharged outside the polder through the sluice gate for drainage, and the pollutants contained in the runoff are also discharged into the outer river along with the discharged runoff and then flow into large rivers and lakes.

[0105] (4) Summary

[0106] This embodiment summarizes the main sources of non-point source pollution in the plain river network polder area. Combining the relevant investigation and research on production and life in the polder area and the main types of underlying surfaces in the polder area, the main sources of non-point source pollution in the polder area are divided into different sources such as farmland runoff pollution, livestock and poultry breeding pollution, aquaculture pollution, rural domestic pollution, and surface runoff pollution from construction land. The process of non-point source pollutants entering water bodies in the plain river network polder area is affected by multiple processes and factors. The key processes mainly include four, namely, precipitation process, surface runoff process, underground leaching process, and soil erosion process; non-point source pollutants are also affected by the driving of topographic conditions and the interception effect of vegetation systems such as forests and grasslands during the migration process. The non-point source pollution characteristics in polder areas are similar to those in general river basins. Due to the particularity of the water flow exchange between polder areas and the outside world, the laws of nutrient generation, migration, and transformation in polder areas are unique and complex.

[0107] Example 2

[0108] This embodiment is for the accounting of the load of non-point source pollution entering water bodies in the plain river network polder area.

[0109] The specific content of this embodiment is as follows:

[0110] This embodiment mainly considers the impact of land-based non-point source pollutants on the water environment quality of the plain river network polder area, and calculates the load of non-point source pollution entering water bodies in the polder area by constructing a method for accounting the load of non-point source pollutants entering water bodies in the plain river network polder area.

[0111] The main idea of this embodiment is as follows:

[0112] 1) Determine the types and sources of pollutants entering water bodies from land areas:

[0113] Based on the pollutant sources and pollutant characteristics within the region, determine the types and sources of pollutants entering water bodies from land areas, such as agricultural non-point source pollution, urban runoff pollution, etc.

[0114] 2) Estimate the pollutant generation amounts entering water bodies from land areas:

[0115] Adopt methods such as statistical analysis, questionnaires, on-site monitoring, etc. to estimate the pollutant generation amounts entering water bodies from land areas, such as farmland area, crop planting situation, fertilization amount, pesticide usage amount, population quantity, etc.

[0116] 3) Determine the pollutant migration pathways and routes:

[0117] Based on factors such as topography, rainfall, soil type, etc., determine the pathways and routes of pollutant migration from land areas to water bodies, such as surface runoff, groundwater flow, soil erosion, etc.

[0118] 4) Establish a mathematical model:

[0119] According to the actual situation, establish a mathematical model to simulate the pollutant migration process. The model can adopt hydrological models, water quality models, etc. to simulate the migration and transformation processes of pollutants on different pathways and routes and calculate their loads.

[0120] 5) Calibrate the model:

[0121] By collecting measured data, calibrate the accuracy and reliability of the model and further improve the model parameters and assumption conditions.

[0122] 6) Calculate the pollutant loads:

[0123] Use the model to calculate the loads of pollutants entering water bodies, including annual average loads, pollutant input fluxes, pollutant concentrations in water bodies, etc.

[0124] (I) Construction of the accounting method for non-point source pollution loads entering water bodies

[0125] For the accounting method of non-point source pollution loads entering water bodies in the plain river network polder area, consider the non-point source pollutant output characteristics of different underlying surfaces under different hydrological driving conditions. At the same time, based on the distances between different pollutant generation units and the receiving water bodies, construct a water volume module, a water quality module, and a migration model into water bodies respectively, and couple them to construct the accounting method for non-point source pollution loads entering water bodies.

[0126] 1.1 Water volume module

[0127] The runoff calculation of the plain polder area water cycle process module is carried out separately for different underlying surfaces: for water areas, the water volume balance principle of water bodies is used to calculate runoff; for paddy fields, the field water volume balance principle is used to calculate runoff during the irrigation period, and the full storage runoff principle is used to calculate runoff during the non-irrigation period (i.e., treated as dry land); for dry land, the full storage runoff principle is used to calculate runoff, and the runoff coefficient method is used to estimate the runoff of construction land. Since the ditches are dense in the plain river network area and the overland flow time on the land surface is short, the model in this embodiment does not consider the overland runoff concentration time in the polder area. After obtaining the runoff calculation results of various underlying surfaces, the total runoff volume of the polder area can be calculated by area-weighted averaging of various underlying surfaces.

[0128] (1) Runoff generation in water areas

[0129] In the runoff generation calculation method for water areas in this embodiment, for each water conservancy sub-region in the plain area, the regulation effect of water bodies during surface runoff generation is considered, and the calculation process is as follows:

[0130] W E =W S +(P - α*E)

[0131] When W E ≤W M , no runoff is generated, that is

[0132] R W =0

[0133] When W E >W M , the runoff volume is:

[0134] R W =W E -W M

[0135] In the formula: W E is the water storage volume at the end of the time period of the water surface water body (mm); W S is the water storage volume at the beginning of the time period of the water surface water body (mm); W M is the water storage capacity of the water surface water body in the polder area (mm); P is the rainfall (mm); E is the evaporation (mm); α is the evaporation conversion coefficient. Taking the Taihu Lake Basin as an example, the E-601 evaporation pan conversion coefficient is shown in Table 1 below.

[0136] Table 1. E-601 evaporation pan conversion coefficient in the Taihu Lake Basin

[0137]

[0138] (2) Runoff generation in paddy fields

[0139] The runoff generated in paddy fields during the irrigation period is calculated according to the principle of field water balance. To ensure the normal growth of rice, a certain water layer depth needs to be maintained on the paddy field surface during different growth periods of rice. Among them, the controlling water layer depths of paddy fields include the upper limit of suitable water depth, the lower limit of suitable water depth, and the waterlogging tolerance depth, etc. The lower limit of suitable water depth mainly controls that rice will not wither due to insufficient water depth in the paddy field and affect the yield due to water loss. When the actual water depth in the paddy field is lower than the lower limit of suitable water depth, irrigation needs to be carried out in a timely manner. The upper limit of suitable water depth mainly controls the maximum water depth allowed for the optimal growth of rice, and this depth is used as a limiting condition during each irrigation. The waterlogging tolerance depth mainly controls that the water layer depth in the paddy field cannot exceed this value. When the rainfall is too large and the water layer depth exceeds the waterlogging tolerance depth, the excess water in the paddy field needs to be drained in a timely manner, and the drainage volume of the paddy field is the net rainfall depth generated by the paddy field.

[0140] The field water balance equation with a daily time period is as follows:

[0141] H2 = H1 + P + M i -R R -K C *α*E-β

[0142] When H2 > H P At this time

[0143] R R = H2 - H P ,M i = 0

[0144] When H D < H2 < H P At this time

[0145] R R = 0,M i = 0

[0146] When H2 < H D At this time

[0147] R R = 0,M i = H U

[0148] In the formula: H1 and H2 are the water depths (mm) of the paddy field at the beginning and end of the time period respectively; K C is the water requirement coefficient for each growth period of rice; H P is the waterlogging tolerance depth (mm) of rice for each growth period; H U is the upper limit of suitable water depth (mm) of rice for each growth period; H D is the lower limit of suitable water depth (mm) of rice for each growth period; R R is the drainage volume (i.e., the runoff generated by the paddy field) (mm) of the paddy field during the time period; M iis the irrigation water volume required for paddy fields during a period (mm); P is the rainfall (mm); E is the evaporation (mm); α is the evaporation conversion coefficient; β is the daily seepage volume of paddy fields (mm). The water depth requirements and water demand coefficients at different growth stages of rice are shown in Table 2 below.

[0149] Table 2. Water depth requirements and water demand coefficients at different growth stages of rice

[0150]

[0151] Regarding the seepage volume during the irrigation period of paddy fields, the model in this embodiment considers that a part of the water volume will be used as return water to supplement the soil moisture content of dry land. When the soil moisture content of dry land reaches saturation, this part of the return water volume will directly be a part of the runoff volume of dry land.

[0152] Paddy fields are treated as dry land during the non-rice planting season (i.e., the non-irrigation period), and the runoff calculation is carried out according to the runoff generation method of the dry land underlying surface.

[0153] (3) Runoff generation of aquaculture ponds

[0154] Considering that the growth of freshwater fish and other aquatic products requires certain conditions such as light and water temperature, and when the water level floods over the top of the pond embankment during floods, the fish will escape over the bank, causing losses to farmers. Therefore, aquaculture should have a certain suitable water depth range. When the water depth is too shallow or too deep, farmers need to replenish or drain water by themselves to reduce the losses caused by the escape of aquaculture products. Based on the above considerations, a similar method to paddy field irrigation can be used to simulate the runoff generation process of aquaculture, and a suitable water depth range is set, characterized by the upper limit of the suitable water depth and the lower limit of the suitable water depth: when the water depth is lower than the suitable water depth, water is taken from the river for replenishment until the suitable water depth is reached; when the water depth is higher than the suitable water depth, the pumping station needs to be started for drainage, and the water is also drained to the suitable water depth. Therefore, the runoff generation of aquaculture should be closer to the runoff generation method of paddy fields.

[0155] Referring to the runoff generation mechanism of paddy fields, according to the water demand process of aquaculture, the upper and lower limits of the suitable water depth, the water depth resistant to flooding and other factors, the water balance calculation is carried out day by day to derive the runoff depth of aquaculture. The suitable water depth H s of aquaculture ponds in the study area is 500mm, and the water depth resistant to flooding H Aflood is 1200mm.

[0156] H A2 = P - E A + H A1 - I A

[0157] In the formula, H A2 is the water depth of the pond at the end of the calculation period; H A1 is the water depth of the pond at the beginning of the calculation period; E A is the evaporation of the pond; IA Let \(Q\) be the infiltration volume in paddy fields, and the infiltration volume is taken as \(0.5\) mm / d; \(P\) is the rainfall (mm).

[0158] Calculation of runoff generation in ponds:

[0159]

[0160] In the formula, \(R\) A is the runoff generation in ponds, in mm.

[0161] (4) Runoff generation in dry land

[0162] The Taihu Lake Basin is located south of the Yangtze River, with abundant rainfall and belongs to a typical humid area. The condition for rainfall to form runoff is mainly that the rainfall exceeds the soil water deficit and generates runoff. The characteristics of this rainfall runoff are suitable for using the full - storage runoff generation model to calculate the runoff generated by rainfall.

[0163] Let the initial soil water content of the region be \(W\), and the regional water storage capacity curve be as shown in Figure 1 . Now assume that the initial soil water content \(W\) of the region follows the Figure 1 distribution shown, so there is:

[0164]

[0165] In the above formula, \(W\) is the known initial soil water content of the region, and \(W'\) m is the water storage capacity at the point in the unsaturated zone of the regional soil. Therefore, \(a\) can be solved from the above formula. In this case, according to the regional water storage capacity curve, the formulas for calculating the total runoff of full - storage runoff generation and the increment of soil water content in the regional time period are as follows:

[0166]

[0167] In the formula: \(R\) is the runoff generation (mm); \(P\) is the rainfall (mm); \(E\) is the evaporation (mm); \(\Delta W\) is the increment of soil water content in the time period (mm).

[0168] Practice shows that the regional water storage capacity curve is generally preferably a parabolic curve, that is represented by the following parabola:

[0169]

[0170] In the formula: \(B\) is the index of the regional water storage capacity curve, which is an empirical constant; \(W_{MM}\) is the maximum point water storage capacity of the region (mm); \(f / F\) represents the proportion of the area where the point water storage capacity in the region is less than or equal to the field capacity of the soil vadose zone.

[0171] Applying the above regional water storage capacity curve and related formulas, the following dry - land runoff generation calculation formula can be derived.

[0172] When P - E ≤ 0, R = 0.

[0173] When P - E + a < WMM,

[0174] When P - E + a > WMM, R = P - E - (WM - W).

[0175] Where: R is the runoff yield (mm); WM is the regional average water storage capacity (mm); P is the precipitation (mm); E is the evaporation (mm); WMM is the regional maximum point water storage capacity (mm); W is the known initial soil moisture content of the region (mm); B is the regional water storage capacity curve index, which is an empirical constant.

[0176] a is the ordinate value corresponding to W on the regional water storage capacity curve (mm), and the calculation formula is as follows:

[0177]

[0178] For the regional maximum point water storage capacity, considering the correction of the regional impervious area, the calculation formula is as follows:

[0179]

[0180] Where: IMP is the ratio of the regional impervious area to the total regional area, that is, the impervious area ratio coefficient.

[0181] Under the condition of full - storage runoff yield, runoff is only a function of precipitation, evapotranspiration and the initial regional water storage. Therefore, as long as the values of these three variables are known, the runoff can be calculated. Among these three variables, precipitation and the initial regional water storage are known, and the evapotranspiration is the actual evapotranspiration of the region, and its calculation method is often determined by establishing the relationship between the regional evapotranspiration capacity and the actual evaporation of the region.

[0182] E is the regional evapotranspiration (mm), and the calculation formula is as follows:

[0183] E = K × EM

[0184] Where: K is the evapotranspiration conversion coefficient; EM is the measured water surface evaporation (mm).

[0185] When applying the full - storage runoff yield method to calculate the total runoff yield of dry land, the water balance equation for each time step is:

[0186] W t+1 = P t - R L - E t + W t

[0187] Where: Pt is the rainfall in a time period, given by measured data; W t is the soil water content in the region at the beginning of the time period, which is the known initial condition or the soil water content in the region at the end of the previous time period; E t is the evapotranspiration in the region during the time period, which can be calculated according to the soil water content W in the region at the beginning of this time period t and the measured water surface evaporation during this time period through the above regional actual evapotranspiration calculation model; R L is the total runoff formed by rainfall in the time period; W t+1 Soil water content in the region at the end of the time period.

[0188] (5) Runoff generation on construction land

[0189] The specific steps of the runoff generation calculation method for construction land are as follows:

[0190] A) Divide the underlying surface of the construction land in the polder area into two parts: impervious and pervious, with the area ratios being p1 and p2 respectively;

[0191] B) For the pervious area, use the dryland model to calculate the runoff volume R IP1 ;

[0192] C) The daily runoff formula for the impervious area is

[0193] R IP2 = C × P - d;

[0194] In the formula, R IP2 is the daily surface runoff volume (mm) of the impervious area; C is the impervious surface runoff coefficient (refer to the Taihu Lake Basin model, and 0.8 is adopted); P is the rainfall; d is the depression storage volume (the depression loss on the impervious area in cities in the Taihu Lake Basin is 2 mm).

[0195] D) Calculate the daily runoff volume of the residential area and construction land with area as the weight, and its formula is

[0196] R IP = p1 × R IP1 + p2 × R IP2 .

[0197] (6) Total runoff in the polder area

[0198] The total runoff volume in the time period of the polder area is obtained by adding the total runoff volumes of various underlying surfaces in the time period multiplied by the corresponding area weights of various underlying surfaces, that is

[0199] R S = A W R W + A R R R + A A R A + AL R L +A IP R IP ;

[0200] In the formula: R S is the total runoff volume of the polder area during a period (mm); A W , A R , A A , A L , A IP respectively represent the weights of the underlying surface areas such as water area, paddy field, aquaculture pond, dry land, construction land, etc. in the total area of its water conservancy division; R W , R R , R A , R L , R IP respectively represent the runoff volumes (mm) of the water area, paddy field, aquaculture pond, dry land, and construction land.

[0201] 1.2 Water quality module

[0202] Non-point source pollution is divided into urban non-point source pollution and agricultural non-point source pollution. Urban non-point source pollution includes urban domestic non-point source pollution, surface runoff pollution of construction land, etc. Agricultural non-point source pollution includes planting, rural life, aquaculture, and livestock breeding, etc. For the non-point source pollution related to rainfall runoff, the rainfall runoff model is used to calculate the runoff volumes of different underlying surfaces, and combined with the runoff pollutant concentrations of different underlying surfaces, the non-point source pollution load of surface runoff is calculated; for the non-point source pollution such as urban domestic pollution, rural life, and livestock breeding, the export coefficient model is used to calculate its non-point source pollution load.

[0203] (1) Urban non-point source pollution

[0204] Urban non-point source pollution mainly includes urban domestic non-point source pollution, surface runoff pollution of construction land, etc.

[0205] For the non-point source pollution load of urban domestic pollution, the export coefficient model is used. According to the water pollutant generation coefficient of urban domestic sources in the second national pollutant census, combined with the urban resident population in the study area, and deducting the domestic sewage volume entering the sewage treatment plant, the non-point source pollution load of urban domestic pollution is calculated.

[0206] L t =C t ×A t ;

[0207] In the formula, L t is the non-point source pollution load of urban domestic pollution, kg; C t is the non-point source pollution coefficient of urban domestic pollution, g / (person·d); At is the urban population quantity in the region, person. Taking Jiangsu Province as an example, the generation coefficients of water pollutants from urban domestic sources in Jiangsu Province are shown in Table 3 below.

[0208] Table 3. Generation Coefficients of Water Pollutants from Urban Domestic Sources in Jiangsu Province

[0209]

[0210] The pollution load of surface runoff from construction land is calculated by combining the surface runoff generation of construction land with the concentration of surface runoff pollutants. The concentrations of various pollutants in the surface runoff of construction land refer to Table 4.

[0211] L c = R c × C c × A c ;

[0212] In the formula, L c is the pollution load of surface runoff from construction land, kg; R c is the surface runoff generation of construction land, mm; C c is the concentration of pollutants in the surface runoff of construction land, mg / L; A c is the area of construction land, km 2 .

[0213] Table 4. Concentrations of Pollutants in Surface Runoff from Construction Land (mg / L)

[0214]

[0215] (2) Agricultural non-point source pollution

[0216] Currently, the calculation of agricultural source pollutants mainly uses the statistical number of pollution sources and calculates using the production and pollution discharge coefficients based on the output coefficient model. For planting, the pollutant emissions of planting are calculated according to the pollution loss coefficients of different land use types and the areas of each land use type; for livestock and poultry breeding, the number of each livestock and poultry species and the number (or proportion) of large-scale and scattered breeding are determined using the statistical yearbook of the research area and field research data, and the pollutant emissions of livestock and poultry breeding are determined in combination with the production and pollution discharge coefficients of livestock and poultry breeding; for rural life, the rural population quantity is determined by combining the statistical yearbook and research data, and the pollutant emissions of rural life are calculated using the pollution generation coefficients.

[0217]

[0218] In the formula, L is the load intensity of agricultural non-point source entering the water body, kg / a; n is the type of land use or livestock, population; E i is the output coefficient of pollutants in the i-th land use type (kg·hm -2 ·a -1) or the output coefficient of livestock or population (kg·ind -1 ·a -1 );A i is the area of the i-th land use type, hm 2 or the number of livestock or population; I i is the input amount of the i-th pollution source, kg; P is the amount of nutrients input by rainfall, kg.

[0219] In the calculation of agricultural non-point source pollution load in this embodiment, the calculation of livestock and poultry breeding and rural domestic pollution adopts the output coefficient model, and the planting industry (dry land and paddy field) related to rainfall runoff adopts the calculation of runoff volume and pollutant concentration. The pollutant discharge coefficients are mainly selected from the pollutant discharge coefficients of agricultural sources in the second national pollution census, and are further determined in combination with relevant literature data in the study area. Taking Jiangsu Province as an example, the pollutant discharge coefficients of livestock and poultry farmers in Jiangsu Province are shown in Table 5 below.

[0220] Table 5. Pollutant discharge coefficients of livestock and poultry farmers in Jiangsu Province

[0221]

[0222] Calculation of non-point source pollution related to rainfall runoff:

[0223] L i =R i ×C i ×A i ;

[0224] In the formula, L i is the non-point source pollution load of different land use types, kg; R i is the runoff volume of different land use types, mm; C i is the pollutant concentration of runoff from different land use types, mg / L; A i is the area of different land use types, km 2 .

[0225] Calculation of rural domestic pollution load:

[0226] L r =Q r ×C r ;

[0227] In the formula, L r is the rural domestic pollution load, kg; Q r is the discharge volume of rural domestic sewage, m 3 ; C r is the pollutant concentration discharged from rural domestic sewage treatment facilities, mg / L.

[0228] 1.3 Water body migration module

[0229] Non-point source pollution is finally discharged into the river network along with the rainfall runoff process. Different from point source pollution, the non-point source pollution inflow coefficient is often related to the distance between the pollution source and the river, pollutant leaching, forest and grass interception, etc. The focus of this embodiment is on the attenuation process of non-point source pollution under the action of rainfall runoff, emphasizing how to generalize the concentration attenuation process of pollutants according to the distance between the pollution source and the river, and at the same time considering the interception effect of different underlying surfaces in the land area on pollutants during the migration process.

[0230] Since the key point of generalization is the attenuation process of pollutants generated by non-point source pollution before flowing into the river, considering that the study area belongs to a plain river network area, the runoff volume generated by the catchment area during rainfall is relatively small compared to the river water volume, and the flow velocity is slow during the confluence process. The non-point source pollutant land attenuation process is based on the first-order reaction kinetic equation as follows:

[0231]

[0232] In the formula, C is the pollutant concentration, mg / L; k is the pollutant attenuation coefficient, s -1 。

[0233] Assume that the shape of the pollution source is a rectangle D with length A and width B. Take a pollution-producing grid unit D1 in the pollution source. The distance from it to the river is y, the water production per unit time is q0, the distance from the far end of the rectangle to the river is d2, the distance from the near end to the river is d1, and α is the interception coefficient of pollutants, that is, the interception coefficient of different underlying surfaces on pollutants along the runoff confluence path.

[0234] The time for the runoff from grid D1 to flow into the river network is:

[0235]

[0236] The amount of pollutants generated by grid D1 flowing into the river is:

[0237] M1=(1 - α)q0c0e -kd / v

[0238] Integrate the area D to calculate the total amount of pollution discharged from this pollution source into the river as:

[0239] M = ∫∫ D (1 - α)q0c0e -kd / v dxdy

[0240] After arrangement, we get:

[0241]

[0242] Considering that the non-point source pollution source in the study area is connected to the river, that is, d1 is small, the above formula can be rewritten as:

[0243]

[0244] The total amount of pollutants flowing into the river from this area divided by the runoff volume during this period is the pollutant concentration flowing into the river during the runoff process of this non-point source pollution source:

[0245]

[0246] Considering only the pollutant attenuation process, the non-point source pollution water entry coefficient is:

[0247]

[0248] In the above formulas: k is the pollutant attenuation coefficient, s -1 ; A is the length of the water flow path from the calculation unit to the river, m; c0 is the initial concentration of the pollutant, mg / L; v is the surface runoff velocity, m / s; α is the pollutant interception coefficient, that is, the interception effect of different underlying surfaces on the pollutant on the runoff confluence path.

[0249] The velocity v and migration time t of the surface runoff flowing through the grid are calculated by coupling the dynamic wave equation and the Manning equation:

[0250]

[0251] In the formula, L is the grid length, m; n is the Manning coefficient of the grid plot; tanβ is the slope of the grid plot; Q is the surface runoff depth of the grid plot, mm; t e is the surface runoff generation time, s.

[0252] 1.4 Non-point source water entry load accounting

[0253] Under the action of driving factors such as rainfall and terrain, non-point source pollutants leave the pollution generation unit, and through physical and chemical processes such as migration and transformation, they enter the receiving water body with runoff and sediment. Starting from the whole process of non-point source pollutants from generation to entry into the receiving water body, this embodiment calculates the water production of the calculation unit, depicts the pollutant emissions of the calculation unit, considers the non-point source pollutant migration attenuation and interception process, and constructs a method for accounting the non-point source pollutant water entry load in the plain river network polder area. The implementation steps are as follows:

[0254] (1) Water volume module, combining precipitation and underlying surface characteristics, and calculating the runoff volume of the pollution generation unit based on different underlying surface runoff generation models.

[0255] (2) Water quality module: For non-point source pollution related to rainfall runoff, the non-point source pollution load of surface runoff is calculated by using the runoff yields and runoff pollutant concentrations of different underlying surfaces. For non-point source pollution such as domestic pollution and livestock and poultry breeding, the output coefficient model is used to calculate its non-point source pollution load. Combining with the non-point source pollutant spatial optimization method, the non-point source pollution emissions of each pollutant-producing unit in space are calculated.

[0256] (3) Water body migration module: The land attenuation process of non-point source pollutants is characterized according to the first-order reaction kinetic equation. At the same time, combining the interception effect of pollutants during the migration process of different underlying surfaces on land, the non-point source pollution water entry coefficient is calculated based on the confluence path of pollutant-producing units.

[0257] (4) Based on the non-point source pollutant emissions of pollutant-producing units calculated by the water quality module, combined with the non-point source pollution water entry coefficient of pollutant-producing units, the non-point source pollutant water body load of pollutant-producing units is calculated and determined.

[0258] (2) Verification of non-point source pollution water body load in typical polder areas

[0259] 2.1 Verification of water yield (i.e., runoff yield, the same below)

[0260] To verify the rationality of the non-point source pollutant water body load calculation method, it is necessary to first verify the surface runoff calculated by the water volume module. In this embodiment, Yuejia Lianwei in the plain river network area of Hangzhou Lake is taken as the verification object, and three typical rainfall events in 2022 (20220413, 20220624, and 20220914) are selected. Based on the hourly water level changes in the inner-polder river channels, the water yield verification of Yuejia Lianwei is carried out.

[0261] The results are as Figure 2 shown. The simulated water level process lines of the three typical rainfall events are relatively consistent with the measured water level process lines. The deviation between the simulated water level and the measured water level is within 10 cm, and the relative error is within 5%. The constructed water yield model has good applicability in the study area.

[0262] 2.2 Verification of non-point source pollutant water body load

[0263] To verify the non-point source pollutant water body load, considering the pollutant monitoring situation of the rivers in Yuejia Lianwei, the monitoring period of the water quality automatic station in Yuejia Lianwei in 2022 (from August to December 2022) is selected. Combining with the period when rainfall-induced runoff non-point source pollutants enter the water body, three typical rainfall events in 2022 (20220904, 20220913, and 20221128) are selected. Based on the water quality concentration changes in the inner-polder river channels, the verification of the non-point source pollutant water body load entering Yuejia Lianwei is carried out.

[0264] The results are as Figure 3As shown in the figure, taking the change of TN pollutant concentration in Yuemoulianwei as an example, in the rainfall event on September 4, 2022, the maximum error of the simulated TN concentration was 3.1%; in the rainfall event on September 23, 2022, the maximum error of the simulated TN concentration was 23.25%; in the event on September 4, 2022, the maximum error of the simulated TN concentration was 13.2%. In the three typical rainfall events, the change process of the simulated pollutant concentration was relatively consistent with the measured water quality concentration, and the relative error was within a reasonable range. The constructed non-point source pollutant load accounting method in the water body had good applicability in the study area.

[0265] (3) Accounting for the load of non-point source pollutants entering the water body in typical polder areas

[0266] 3.1 Accounting for the emissions of non-point source pollutants

[0267] Based on the pollutant concentration and surface runoff volume of different land use types in Yuemoulianwei, calculate the emissions of non-point source pollutants in different land uses in the polder area.

[0268] The monitored concentrations of surface runoff pollutants in Yuemoulianwei are shown in Table 6 below. Among different land use types, the average concentrations of TN, TP, and ammonia nitrogen in surface runoff of dry land are the highest, reaching 5.01 mg / L, 0.82 mg / L, and 1.92 mg / L respectively; the average concentrations of TP and ammonia nitrogen in surface runoff of construction land are the lowest. Among different pollutant types, the average concentration of permanganate index in surface runoff of forest land is the highest, reaching 10.77 mg / L, and its variation range is relatively large, with a maximum value of 25.66 mg / L.

[0269] Table 6. Concentrations of surface runoff pollutants in different land use types in the study area Unit: mg / L

[0270]

[0271] The depths of surface runoff of different land use types in Yuemoulianwei from 2018 to 2022 are shown in Table 7 below. The depth of surface runoff is directly related to the rainfall. In 2018, the rainfall was the smallest, and the depth of surface runoff of each underlying surface within the year was the smallest. In 2020, the rainfall reached 1417.0 mm, and the depth of surface runoff of the underlying surface was the largest. Among different land use types, the depth of runoff in water areas is the largest, followed by construction land. In addition to the depth of runoff per unit area, the water yield within the polder is also related to the area of different underlying surfaces. The largest water yield among different land use types in Yuemoulianwei is in construction land, and its water yield in 2020 reached 1.8769 million m 3 ; the water yield in water areas is the smallest, and its minimum value was 232,000 m in 2019 3 .

[0272] Table 7. Depths of surface runoff of different land use types in the study area Unit: mm

[0273]

[0274] According to the surface runoff volume and pollutant concentration of Yuemou Lianwei, the non-point source pollutant emissions in the polder area from 2018 to 2022 were calculated, as shown in Table 8 below. Among the different years, the non-point source pollutant emissions in 2020 were the highest, COD Mn , ammonia nitrogen, TP and TN emissions were 44078.65kg, 5205.68kg, 1801.21kg and 13817.29kg respectively.

[0275] Table 8. Emissions of non-point source pollutants in the study area from 2018 to 2022 Unit: kg

[0276]

[0277] 3.2 Calculation of non-point source pollutants entering water bodies

[0278] Based on the non-point source pollutant emissions of Yuemou Lianwei and the number of non-point source pollutants entering the water system in the weir area, the load of non-point source pollutants entering the water body in the weir area from 2018 to 2022 was calculated.

[0279] Considering the distance between the pollutant-producing unit and the receiving water body and the interception effect of forest and grassland on pollutants, the number of non-point source pollutants entering the water system for different land use types in Yue Lianwei was calculated and determined, as shown in Table 9 below. Among the different land use types, the number of forest land entering the water system is the smallest, and its pollutant attenuation and interception effect is the most significant. Its COD Mn , ammonia nitrogen, TP and TN entering the water system were 0.61, 0.72, 0.65 and 0.67 respectively.

[0280] Table 9. Mean number of non-point source pollutants entering the water system for different land use types in the study area Unit: kg

[0281]

[0282] As shown in Table 10 below, among the non-point source pollutants entering the water body of Yue Lianwei from 2018 to 2022, the non-point source pollutants entering the water body in 2018 were the smallest, and the non-point source pollutants entering the water body in 2020 were the highest. Mn , ammonia nitrogen, TP and TN loads into the water body were 35705.05kg, 4519.91kg, 1490.60kg and 11252.12kg respectively.

[0283] Table 10. Non-point source pollutant load into water bodies in the study area from 2018 to 2022 Unit: kg

[0284]

[0285] 3.3 Comparative Analysis of Non-Point Source Pollution Load Accounting Methods

[0286] (1) Export Coefficient Model

[0287] The non-point source pollution in Yue's Lianwei mainly comes from different land use types. According to the pollutant export coefficients of different land use types and the areas of each land use type, the export coefficient model is sampled to calculate the non-point source pollutant load of different land use types. That is:

[0288]

[0289] In the formula, L is the non-point source load intensity into the water body, kg / a; n is the type of land use type; E i is the export coefficient of pollutants in the i-th land use type (kg·hm -2 ·a -1 ); A i is the area of the i-th land use type, hm 2 ; I i is the input amount of the i-th pollution source, kg; P is the amount of nutrients input by rainfall, kg. Among them, the export coefficients of different land use types in Yue's Lianwei are shown in Table 11 below.

[0290] Table 11. Export Coefficients of Different Land Use Types in the Study Area Unit: kg·hm -2 ·a -1

[0291]

[0292] According to the areas and export coefficients of different land use types in Yue's Lianwei, the non-point source pollutant loads of different land use types in the polder area and the total non-point source pollution load in the polder area are calculated. Among them, the annual output loads of non-point source pollutants of different land use types in Yue's Lianwei are shown in Table 12 below.

[0293] Table 12. Annual Output Loads of Non-Point Source Pollutants of Different Land Use Types in the Study Area Unit: kg

[0294]

[0295] Considering the pollutant load input by wet deposition rainfall, the non-point source pollution loads in the polder area from 2018 to 2022 are statistically calculated (as shown in Table 13 below). Among them, the TP and TN loads are about 1230 kg and 10040 kg respectively, and the inter-annual differences are not significant.

[0296] Table 13. Non-Point Source Pollutant Loads Calculated by the Export Coefficient Model in the Study Area from 2018 to 2022 Unit: kg

[0297]

[0298] (2) Improved output coefficient model

[0299] The existing output coefficient model inadequately considers the influence of hydrological factors, mainly reflected in: (a) The output coefficient uses the multi-year average value, that is, the output coefficient of the same land use type adopts the same value in different years, without considering the influence of rainfall on the output coefficient; (b) The influence on non-point source pollution output under different topographic driving conditions is not considered; (c) The rainfall, runoff generation and concentration processes, etc., which are directly related to the generation of non-point source pollution, are inadequately considered, only considering the pollutants carried by rainwater itself. It can be seen that to make the output coefficient model more accurately predict the non-point source pollution load at different spatial and temporal scales, the influence of hydrological, runoff generation and concentration processes, etc. on non-point sources must be reflected in the model.

[0300] Drawing on the improved output coefficient model, combined with the natural element characteristics (precipitation and terrain) of the pollution generation unit, considering the driving factors of pollutant emissions based on spatial natural attributes, the non-point source pollution load is corrected as follows:

[0301]

[0302] In the formula, L is the non-point source emission load intensity, kg / a; α is the rainfall driving correction factor; β is the terrain driving correction factor; the meanings of the other symbols are the same as above.

[0303] 1) Rainfall driving correction factor

[0304] For the rainfall driving correction factor, consider the influence of the inter-annual difference and spatial distribution of rainfall on the loss of non-point source pollutants. According to the relevant data of the study area, obtain the correlation between rainfall and total nitrogen and total phosphorus.

[0305] L = f(r)

[0306] In the formula: r is the rainfall in the measurement unit; L is the loss amount of pollutants in the measurement unit.

[0307] ① Inter-annual change of rainfall driving factor

[0308] Rainfall inter-annual difference influence factor α i It is expressed as:

[0309]

[0310] In the formula: α i is the rainfall inter-annual difference influence factor; is the regional multi-year average rainfall, mm.

[0311] ② Spatial distribution change of rainfall driving factor

[0312] Differences in the spatial distribution of rainfall result in differences in non-point source pollution caused by rainfall in different regions within the same year, which is mainly reflected by the spatial distribution of rainfall amount. The influencing factor α of rainfall spatial distribution s is expressed as:

[0313]

[0314] In the formula: α s is the influencing factor of rainfall spatial distribution; R j is the annual average rainfall of the basic calculation unit j; is the regional annual average rainfall.

[0315] In summary, the expression of the rainfall driving factor α is:

[0316]

[0317] 2) Topographic driving correction factor

[0318] For the topographic driving correction factor, slope is an important factor affecting pollutant generation. Different slopes affect the loss amount of non-point source pollutants by influencing the runoff volume. Therefore, the influence of slope on non-point source pollutants can be transformed into the relationship between slope and runoff volume.

[0319] The relationship between slope and runoff volume is established as follows:

[0320] Q = aθ b

[0321] In the formula: Q—runoff volume; θ—slope; a, b—constants.

[0322] Through experimental monitoring and data statistical analysis, the relationship formula between slope and the loss amount of non-point source pollutants is established:

[0323]

[0324] In the formula: L is the non-point source pollutant load; is the average slope of the study area; c, d are constants.

[0325] The topographic driving factor β mainly characterizes the difference in the non-point source pollutant load caused by different slopes between the basic units of the watershed and the typical small watershed. The calculation method is as follows:

[0326]

[0327] In the formula: β is the topographic driving factor; is the average slope of the i-th basic unit of the watershed; is the average slope of the study area.

[0328] Yue Moulianwei belongs to the plain river network area with a small regional area, relatively flat terrain, and small slope differences in different areas within the embankment. The improved output coefficient model mainly considers the influence of precipitation.

[0329] L TN = 16.317r - 11217R 2 = 0.92

[0330] L TP = 2.3404r - 1732.8R 2 = 0.93

[0331] In the formula: r is the rainfall in the study area; L is the non-point source pollutant load in the study area.

[0332] Combined with the improved output coefficient model, the non-point source pollutant loads of Yue Moulianwei from 2018 to 2022 are calculated as shown in Table 14 below. The non-point source pollution load is the largest in 2020, with the TP and TN loads being 2130.4 kg and 16486.69 kg respectively; the non-point source pollution load is the smallest in 2018, with the TP and TN loads being 873.98 kg and 7481.67 kg respectively.

[0333] Table 14. Non-point source pollutant loads calculated by the improved output coefficient model in the study area from 2018 to 2022 Unit: kg

[0334]

[0335] (3) Comparative analysis of different non-point source pollution accounting methods

[0336] Compare the output coefficient model, the improved output coefficient model, and the non-point source pollutant load accounting method constructed in this embodiment. As shown in Table 15 and Figure 4 shown, the non-point source pollution loads of Yue Moulianwei calculated by different methods from 2018 to 2022 are significantly different, and the load is the largest in 2020. The annual differences in the non-point source pollutant loads calculated by the output coefficient model are not significant, and the TP and TN loads are about 1230 kg and 10040 kg respectively. The improved output coefficient model considers the influence of annual rainfall differences. The TP and TN non-point source pollution loads in 2020 are 2130.4 kg and 16486.69 kg respectively. For the non-point source pollutant load accounting method into the water body, the non-point source pollutant load into the water body of Yue Moulianwei is the smallest in 2018, with the TP and TN non-point source pollution loads being 582.42 kg and 4950.77 kg respectively.

[0337] Table 15. Non-point source pollution loads of different calculation methods in Yue Moulianwei Unit: kg

[0338]

[0339] The output coefficient model and the improved output coefficient model calculate the non-point source pollutant emissions of pollution-generating units, without considering the migration and transformation effects of pollutants from pollution-generating units during the process of entering the receiving water body, which may overestimate the pollution load of non-point source pollutants entering the water body. At the same time, the output coefficient model is an empirical value of the pollutant emission intensity of different underlying surfaces, and the time scale is generally annual, which cannot effectively characterize the differences in non-point source pollutants under different rainfall driving conditions within a year.

[0340] (4) Key periods and key areas of non-point source pollutants entering the water body in typical polder areas

[0341] 4.1 Key periods and key areas of non-point source pollution entering the water body

[0342] The time distribution change of the non-point source pollutant load entering the water body in the plain river network polder area is mainly affected by precipitation. The changes in the non-point source pollutant loads entering the water body in Yuemoulian Polder from 2018 to 2022 are shown in Figure 5 . On the inter-annual scale, in the wet year (2022), the loads of COD Mn , ammonia nitrogen, TP and TN entering the water body in Yuemoulian Polder were 35705.05 kg, 4519.91 kg, 1490.60 kg and 11252.12 kg respectively, which were more than twice the loads of COD Mn , ammonia nitrogen, TP and TN entering the water body in the dry year (2018). The total load of non-point source pollutants entering the water body in the wet year was significantly higher than that in the dry year and normal year. Due to different rainfall intensities between years, the differences in the loads of non-point source pollutants entering the water body in different rainfall events were large. Among them, on August 10, 2019, the loads of COD Mn , ammonia nitrogen, TP and TN entering the water body reached 4729.83 kg, 606.93 kg, 205.52 kg and 1522.44 kg respectively.

[0343] 4.2 Key areas of non-point source pollutants entering the water body

[0344] The high-intensity areas of the non-point source pollutant load entering the water body in the polder area are mainly affected by the combined action of the non-point source pollutant emission intensity and the water entry coefficient. Among the non-point source pollutant loads entering the water body in Yuemoulian Polder, the average values of the loads of TN, TP and ammonia nitrogen entering the water body in dry land are the highest, and the average value of the load of COD Mn entering the water body in construction land is the highest. Taking the non-point source pollutant load entering the water body in Yuemoulian Polder in 2020 as an example (as shown in Figure 6 ), the areas with higher loads of TN and TP entering the water body are mainly distributed in the middle and northern parts of the polder area, near Beizhuangcun Port and Dongzhoudai. At the same time, near Qinglongqiao Port is a residential gathering area, and the loads of TN and TP entering the water body in this area are also relatively high. The COD MnThe load intensities of pollutants entering the water body are relatively high, being 66.68 kg / ha and 62.03 kg / ha respectively, and these two types of underlying surfaces account for 54% of the polder area. The spatial distribution of the areas with relatively high ammonia nitrogen load intensity entering the water body is similar to that of the areas with high TN load intensity entering the water body. Near rivers such as Yaojiadai, Dongzhoudai, Qinglongqiaogang, and Beizhuangcun Port in the central region, the ammonia nitrogen load intensity entering the water body is relatively high.

[0345] Based on the above contents in (3) and (4), it can be seen that the non-point source pollutant load accounting method constructed in this embodiment takes into account the distance between the pollution source unit and the receiving water body and the interception effect of forest and grasslands on pollutants. At the same time, it combines the water yields of different underlying surfaces under rainfall conditions, and can more accurately characterize the impact of different hydrological driving conditions and land migration and interception processes on non-point source pollutants entering the water body, and identify the key periods and key area risks of non-point source pollutants entering the water body.

[0346] (5) Summary

[0347] Starting from the whole process of non-point source pollutants from generation to entering the receiving water body, this embodiment constructs a non-point source pollutant load accounting method for plain river network polders by calculating the water yield of the calculation unit, depicting the pollutant emissions of the calculation unit, and considering the migration, attenuation, and interception process of non-point source pollutants.

[0348] (1) The non-point source pollutant load accounting method for plain river network polders mainly includes: 1) The water volume module, which combines precipitation and underlying surface characteristics, and calculates the water yield of the pollution source unit based on the runoff generation models of different underlying surfaces; 2) The water quality module, for non-point source pollution related to rainfall runoff, uses the runoff volumes and runoff pollutant concentrations of different underlying surfaces to calculate the non-point source pollution load of surface runoff; for non-point source pollution such as domestic pollution and livestock and poultry breeding, uses the export coefficient model to calculate its non-point source pollution load; combines the non-point source pollutant spatial optimization method to calculate the non-point source pollution emissions of each pollution source unit in space; 3) The water body migration module, which characterizes the land attenuation process of non-point source pollutants according to the first-order reaction kinetic equation, and at the same time combines the interception effect of different underlying surfaces on pollutants during the land migration process, and calculates the non-point source pollution water entry coefficient based on the confluence path of the pollution source unit; 4) Based on the non-point source pollutant emissions of the pollution source unit calculated by the water quality module, combined with the non-point source pollution water entry coefficient of the pollution source unit, calculate and determine the non-point source pollutant load of the pollution source unit entering the water body. The above non-point source pollution load accounting method for plain river network polders takes into account the non-point source pollutant output characteristics of different underlying surfaces under different hydrological driving conditions, and at the same time constructs a water volume module, a water quality module, and a water body migration model respectively based on the distances between different pollution source units and the receiving water body, and couples and constructs a non-point source pollution load accounting method.

[0349] (2) Based on the non-point source pollutant emissions of Yuemoulianwei and combined with the non-point source pollutant water entry coefficient in the polder area, the non-point source pollutant water body load in the polder area from 2018 to 2022 was calculated. The non-point source pollutant water body load was the smallest in 2018 and the highest in 2020. Its COD Mn , ammonia nitrogen, TP, and TN water body load amounts were 35705.05 kg, 4519.91 kg, 1490.60 kg, and 11252.12 kg respectively.

[0350] (3) By comparing the export coefficient model, the improved export coefficient model, and the non-point source pollutant water body load accounting method constructed in this embodiment, the non-point source pollution load amounts of Yuemoulianwei from 2018 to 2022 calculated by different methods were significantly different. The export coefficient model and the improved export coefficient model calculate the non-point source pollutant emissions of the pollution-producing units, without considering the migration and transformation effects of the pollutants in the pollution-producing units during the process of entering the receiving water body, which may overestimate the non-point source pollutant water body pollution load. The export coefficient model is an empirical value of the pollutant emission intensity of different underlying surfaces, and the time scale is generally annual, which cannot effectively characterize the differences in non-point source pollutants under different rainfall driving conditions within a year. The non-point source pollutant water body load accounting method constructed in this embodiment considers the distance between the pollution-producing unit and the receiving water body and the interception effect of forest and grasslands on pollutants, and at the same time combines the water production of different underlying surfaces under rainfall conditions, and can more accurately characterize the impact of different hydrological driving conditions and land migration and interception processes on non-point source pollutants entering the water body.

[0351] (4) The key time and key areas affected by non-point source pollution in the polder area were analyzed. In terms of time, the time distribution change of the non-point source pollutant water body load in the plain river network polder area was mainly affected by precipitation. The total non-point source pollutant water body load in wet years was significantly higher than that in dry years and normal years. The average proportion of the contribution of different pollutants to the water body load during the flood season of Yuemoulianwei accounted for about 61% of the annual load; in terms of space, the areas with higher non-point source pollutant water body load intensity were spatially inconsistent with the areas with higher non-point source pollutant emission intensity and water entry coefficient. Affected by the combined action of non-point source pollutant emission intensity and water entry coefficient, the areas with higher water body load in Yuemoulianwei were mainly distributed in the middle and northern parts of the polder area.

[0352] Combining the above embodiments, the non-point source pollution water body load accounting method for the plain river network polder area of the present invention considers the non-point source pollutant output characteristics of different underlying surfaces under different hydrological driving conditions, and at the same time, based on the distance between different pollution-producing units and the receiving water body, a water volume module, a water quality module, and a water body migration model are respectively constructed, and a non-point source pollution water body load accounting method is coupled and constructed. After actual simulation verification, this accounting method has good applicability in actual implementation.

[0353] The method for calculating the load of non-point source pollution entering water bodies in the plain river network polder area of the present invention takes into account the distance between the pollution source unit and the receiving water body and the interception effect of forest and grassland on pollutants, and at the same time combines the runoff generation of different underlying surfaces under rainfall conditions, which can more accurately characterize the impact of different hydrological driving conditions and land migration and interception processes on the entry of non-point source pollutants into water bodies, and identify the key periods and key area risks of non-point source pollutants entering water bodies.

[0354] In addition to the above embodiments, the present invention may have other implementation manners. All technical solutions formed by equivalent replacement or equivalent transformation fall within the protection scope required by the present invention.

Claims

1. A method for calculating the load of non-point source pollution entering water bodies in a plain river network polder area, characterized in that, It includes the following steps: The first step: Construct a water volume module; In the water volume module, combining the precipitation and underlying surface characteristics of the target plain river network polder area, based on different underlying surface runoff generation models, calculate the runoff volume of the pollution generation unit; The second step: Construct a water quality module; Based on the result obtained in the first step, in the water quality module, for different non-point source pollutions in the target plain river network polder area, calculate the non-point source pollution load within a preset time period for each non-point source pollution, that is, the non-point source pollution discharge amount of each pollution generation unit; The third step: Construct a migration module into the water body; In the migration module into the water body, characterize the land attenuation process of non-point source pollutants according to the first-order reaction kinetics equation, and at the same time, combining the interception effect of pollutants during the migration process of different underlying surfaces, calculate the non-point source pollution into the water body coefficient of each pollution generation unit based on the confluence path of the pollution generation unit; The fourth step: Multiply the non-point source pollution discharge amount of each pollution generation unit obtained in the second step by the non-point source pollution into the water body coefficient of each pollution generation unit obtained in the third step, that is, obtain the non-point source pollutant load into the water body of each pollution generation unit, and then summarize and calculate to obtain the total non-point source pollutant load into the water body of the pollution generation unit.

2. A method for calculating the load of non-point source pollution entering water bodies in a plain river network polder area according to claim 1, characterized in that, The specific process of the first step includes: Calculate the runoff volume of the pollution generation unit of each underlying surface within a preset time period for different underlying surfaces in the target plain river network polder area, and then calculate the total runoff volume of the pollution generation unit of the target plain river network polder area by combining the areas of each underlying surface.

3. A method for calculating the non-point source pollution load into water bodies in a plain river network polder area according to claim 2, Its feature is that in the first step, the underlying surface includes at least one of water area, paddy field, aquaculture pond, dry land, and construction land; the total runoff volume of the pollution generation unit of the target plain river network polder area is: R S = A W R W + A R R R + A A R A + A L R L + A IP R IP ; Where: R S is the total runoff of the pollution generation units in the target plain river network polder area, with the unit of mm; A W is the weight of the water area underlying surface area in the total area of the target plain river network polder area; R W is the runoff generated in the preset period of the water area underlying surface, with the unit of mm; A R is the weight of the paddy field underlying surface area in the total area of the target plain river network polder area; R R is the runoff generated in the preset period of the paddy field underlying surface, with the unit of mm; A A is the weight of the aquaculture pond underlying surface area in the total area of the target plain river network polder area; R A is the runoff generated in the preset period of the aquaculture pond underlying surface, with the unit of mm; A L is the weight of the dry land underlying surface area in the total area of the target plain river network polder area; R L is the runoff generated in the preset period of the dry land underlying surface, with the unit of mm; A IP is the weight of the construction land underlying surface area in the total area of the target plain river network polder area; R IP is the runoff generated in the preset period of the construction land underlying surface, with the unit of mm; If the target plain river network polder area does not contain a certain underlying surface, the weight of this underlying surface is counted as 0.

4. A method for calculating the non-point source pollution load into the water body of a plain river network polder area according to claim 3, its feature is that in the first step, the runoff volume of the water area underlying surface within a preset time period is calculated using the water volume balance principle of the water body, and the specific process is as follows: W E = W S + (P - α * E); When W E ≤ W M no runoff occurs and R W = 0; When W E > W M then R W = W E - W M ; Where, W E is the water storage volume of the water body on the water surface at the end of the time period, with the unit of mm; W S is the water storage volume of the water body on the water surface at the beginning of the time period, with the unit of mm; W M is the water storage capacity of the water body on the water surface within the polder area, with the unit of mm; P is the rainfall, with the unit of mm; E is the evaporation, with the unit of mm; α is the evaporation conversion coefficient; The runoff volume of the paddy field underlying surface within a preset time period during the irrigation period is calculated using the field water volume balance principle, and the specific process is as follows: The field water volume balance equation with a daily time period is: H2 = H1 + P + M i -R R -K C *α*E-β; When H2 > H P , R R = H2 - H P , M i = 0; When H D <H2 < H P then, R R = 0, M i = 0; When H2 < H D then, R R = 0, M i = H U ; Where, H1 and H2 are the water depths of the paddy field at the beginning and end of the period respectively, with the unit of mm; K C is the water requirement coefficient for each growth stage of rice; H P is the waterlogging tolerance depth of rice for each growth stage, with the unit of mm; H U is the upper limit of the suitable water depth of rice for each growth stage, with the unit of mm; H D is the lower limit of the suitable water depth of rice for each growth stage, with the unit of mm; M i is the irrigation water volume required for the paddy field during the period, with the unit of mm; P is the rainfall, with the unit of mm; E is the evaporation, with the unit of mm; α is the evaporation conversion coefficient; β is the daily seepage volume of the paddy field, with the unit of mm; The runoff volume of the paddy field underlying surface within a preset time period during the non-irrigation period is calculated according to the dry land treatment; The runoff volume of the aquaculture pond underlying surface within a preset time period is calculated according to the following formula: H A2 = P - E A + H A1 - I A ; Where: H A2 is the water depth of the pond at the end of the calculation period, in mm; H Aflood is the water depth resistant to flooding, in mm; P is the rainfall, in mm; E A is the evaporation of the pond, in mm; H A1 is the water depth of the pond at the beginning of the calculation period; I A is the infiltration water volume of paddy fields, in mm.

5. A method for calculating the non-point source pollution load into the water body of a plain river network polder area according to claim 3, its feature is that in the first step, the runoff volume of the dry land underlying surface within a preset time period is calculated using the full storage runoff generation principle, and the specific process is as follows: When P - E ≤ 0, the runoff volume is: R = 0; When P - E + a < WMM, the runoff volume is: When P - E + a > WMM, the runoff volume is: R = P - E - (WM - W); In the formula, R is the runoff volume, unit mm; WM is the regional average water storage capacity, unit mm; P is the rainfall, unit mm; E is the evaporation, unit mm; WMM is the regional maximum point water storage capacity, unit mm; W is the known regional initial soil water content, unit mm; B is the regional water storage capacity curve index and is an empirical constant; a is the ordinate value corresponding to W on the regional water storage capacity curve, unit mm; The calculation formula of a is as follows: The calculation formula of WMM is as follows: In the formula: IMP is the ratio of the impervious area of the region to the total area of the region, that is, the impervious area ratio coefficient; The calculation formula of E is as follows: E = K × EM; In the formula: K is the evaporation and transpiration conversion coefficient; EM is the measured water surface evaporation, with the unit of mm.

6. A method for calculating the non-point source pollution load entering water bodies in a plain river network polder area according to claim 3, characterized in that in the first step, the runoff generation amount during the preset period of the construction land underlying surface is calculated by the runoff coefficient method, and the specific process is as follows: R IP = p1 × R II1 + p2 × R IP2 ; Where: p1 is the area proportion of the permeable area of the underlying surface of the construction land; p2 is the area proportion of the impermeable area of the underlying surface of the construction land; R IP1 is the runoff yield of the permeable area of the underlying surface of the construction land, with the unit of mm, and its calculation process is treated as dry land; R IP2 is the runoff yield of the impermeable area of the underlying surface of the construction land, with the unit of mm; R IP1 The calculation process of R IP2 Calculated according to the following formula: R IP2 = C × P - d; In the formula: C is the impervious surface runoff coefficient; P is the rainfall, with the unit of mm; d is the depression storage, with the unit of mm.

7. A method for calculating the non-point source pollution input water body load in a plain river network polder area according to any one of claims 1 to 6, characterized in that, In the second step, the non-point source pollution includes urban non-point source pollution and / or agricultural non-point source pollution.

8. A method for calculating the non-point source pollution load entering water bodies in a plain river network polder area according to claim 7, characterized in that in the second step, the urban non-point source pollution includes urban domestic non-point source pollution and / or construction land surface runoff pollution; The non-point source pollution load of urban life is: L t = C t × A t ; Where: L t is the non-point source pollution load of urban domestic sewage, unit: kg; C t is the non-point source pollution coefficient of urban domestic sewage, unit: g / (person·d); A t is the urban population in the region, unit: person. The surface runoff pollution load of the construction land is: L c = R c × C c × A c ; Where: L c is the pollution load of surface runoff in construction land, unit kg; R c is the surface runoff yield in construction land, unit mm; C c is the pollutant concentration of runoff generation in construction land, unit mg / L; A c is the area of construction land, unit km 2 .

9. A method for calculating the non-point source pollution load entering water bodies in a plain river network polder area according to claim 7, Characterized in that in the second step, the agricultural non-point source pollution includes at least one of planting pollution, livestock and poultry breeding pollution, and rural domestic pollution; For planting pollution, calculate the emissions of planting pollutants according to the pollution loss coefficients of different land use types and the areas of each land use type; For livestock and poultry breeding pollution, use the statistical yearbook of the research area and field investigation data to determine the number of each livestock and poultry species and the number (or proportion) of large-scale and scattered breeding, and combine the pollutant discharge coefficients of livestock and poultry breeding to determine the emissions of livestock and poultry breeding pollutants; For rural domestic use, combine the statistical yearbook and investigation data to determine the rural population, and use the pollution generation coefficient to calculate the emissions of rural domestic pollutants; The agricultural non-point source pollution load is: Where: L is the load intensity of agricultural non-point source entering the water body, with the unit of kg / a; n is the type of land use or livestock and population; E i is the output coefficient of pollutants in the i-th land use type, with the unit of kg·hm -2 ·a -1 , or the output coefficient of livestock and population, with the unit of kg·ind -1 ·a -1 ; A i is the area of the i-th land use type, with the unit of hm 2 , or the number of livestock and population; I i is the input amount of the i-th pollution source, with the unit of kg; P is the amount of nutrients input by rainfall, with the unit of kg; Among them, the non-point source pollution load related to rainfall runoff is: L i = R i × C i × A i In the formula, L i is the non-point source pollution load of different land use types, with the unit of kg; R i is the runoff volume of different land use types, with the unit of mm; C i is the pollutant concentration of runoff from different land use types, with the unit of mg / L; A i is the area of different land use types, with the unit of km 2 ; The rural domestic pollution load is: L r = Q r × C r Where L r is the rural domestic pollution load, in kg; Q r is the rural domestic sewage discharge, in m 3 ; C r is the pollutant concentration discharged from rural domestic sewage treatment facilities, in mg / L.

10. A method for calculating the non-point source pollution load into water bodies in a plain river network polder area according to any one of claims 1 to 6, characterized in that, In the third step, the non-point source pollution into water body coefficient is: where k is the decay coefficient of the pollutant, with the unit of s -1 ; A is the length of the water flow path from the pollution source to the river channel, with the unit of m; v is the surface runoff velocity, with the unit of m / s; α is the interception coefficient of the pollutant, that is, the interception effect of different underlying surfaces on the pollutant on the runoff confluence path; Divide the pollution source into several grid cells, and calculate the flow velocity v and migration time t of the surface runoff flowing through the grid cells by coupling the dynamic wave equation and the Manning equation: Wherein, L is the grid length in m; n is the Manning coefficient of the grid plot; tanβ is the slope of the grid plot; Q is the surface runoff depth of the grid plot in mm; t e is the surface runoff generation time in s.

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