Method for calculating nitrogen and phosphorus release flux of vegetation in reservoir drawdown zone

Abstract

The application provides a method for calculating nitrogen and phosphorus release fluxes of reservoir drawdown zone vegetation, which is based on continuous positioning monitoring in previous years and corresponding remote sensing image interpretation data, constructs a drawdown zone remote sensing image-biomass inversion empirical model, realizes rapid estimation of large-area vegetation biomass, and saves a large amount of time and manpower; through in-situ water flooding test, an empirical model of nitrogen and phosphorus release rates of different types of vegetation under real water conditions is established, the model parameters are determined, and the model can be directly used to evaluate the influence of nitrogen and phosphorus release of vegetation on local water quality; when the nitrogen and phosphorus release fluxes of different types of vegetation in the drawdown zone are calculated, the reservoir operation rules and the spatial and temporal specificity of vegetation distribution are fully combined, and the applicability is strong.

Description

Technical Field

[0001] This invention belongs to the field of water environment monitoring and software algorithms, specifically relating to a method for calculating the nitrogen and phosphorus release flux of vegetation in the drawdown zone of a reservoir. Background Technology

[0003] The drawdown zone of a reservoir, also known as the drawdown area, rise and fall zone, etc., is a land area between the highest and lowest water levels, formed by seasonal and temporal fluctuations in the water levels of rivers and lakes due to uneven occurrences of rainfall over time, whether controlled by human intervention or natural rainfall. It is characterized by alternating submersion and exposure.

[0004] As primary producers, vegetation in the drawdown zone plays a crucial role in the biogeochemical cycling of nutrients within the drawdown zone ecosystem. During the outcrop period, vegetation fixes carbon and releases oxygen through photosynthesis, absorbing nitrogen and phosphorus from the soil and converting them into biological organic matter. During periods of high water levels, the plants decompose under submerged conditions, releasing nutrients such as nitrogen and phosphorus into the overlying water bodies. This may increase the risk of water pollution in the reservoir area, especially in the bays, and consequently, the likelihood of environmental pollution events such as algal blooms.

[0005] Current patents primarily focus on methods for purifying river and lake water using plants, providing technical support for the ecological restoration and treatment of eutrophic and polluted water bodies. For example, one method (application number 202010229781.7) utilizes aquatic plants to purify nitrogen and phosphorus nutrients in river water. This method detects and compares total nitrogen, total phosphorus, and ammonia nitrogen, identifying the optimal combination of emergent, floating, and submerged plants for water purification. Another method (application number 202410944434.0) uses optimal plant combinations to reduce nitrogen and phosphorus pollution in riverbank buffer zones in plain areas. This method uses scenario simulation to select the most effective plant combinations for reducing nitrogen and phosphorus pollution, regulating the nitrogen and phosphorus pollution reduction function of riverbank buffer zones in plain areas. A quantitative control method for submerged plants in urban rivers and lakes based on plant growth and death-nitrogen and phosphorus absorption and release simulation (application number 202410047652.4) sets different harvesting scenarios for submerged plants to determine the optimal harvesting scheme, thereby purifying nitrogen and phosphorus water quality in urban rivers and lakes. Although there are also experimental methods to simulate nitrogen and phosphorus release in plants, such as an experimental device for nitrogen and phosphorus nutrient release from the immersion of herbaceous plants in the drawdown zone (application number 201721652122.4), which can reproduce the nitrogen and phosphorus nutrient release process of plants in the drawdown zone after flooding to the greatest extent.

[0006] However, these patents only present simulations of the nitrogen and phosphorus release process from plants, without quantitatively assessing the amount of nitrogen and phosphorus released during immersion. Furthermore, indoor simulated flooding experiments cannot accurately replicate in-situ water pressure and the physicochemical properties of the water, as reservoir water is open, flowing, and deeply submerged. For example, during high-water-level operation of the Danjiangkou Reservoir, vegetation in the drawdown zone is submerged to a depth of 20 meters for over 300 days. Therefore, quantitatively assessing nitrogen and phosphorus release in the drawdown zone can provide forward-looking, scientific, and precise support for water conservancy management. Summary of the Invention

[0007] To address the above problems, this invention provides a method for controlling nitrogen and phosphorus release fluxes from vegetation in the drawdown zone of a reservoir, characterized by comprising the following steps: S1: Collect hydrological data of the reservoir; S2: Divide the reservoir drawdown zone into several flood duration zones according to the different durations of flooding; S3: Investigate the nitrogen and phosphorus content of existing vegetation in each flooded area of ​​the drawdown zone; S4: Interpret the remote sensing images of the drawdown zone to determine the existing vegetation biomass and area of ​​land cover types in each flood duration area of ​​the drawdown zone. S5: Determine the nitrogen and phosphorus stock of existing vegetation in the land cover types within each flood duration area; S6: By measuring the nitrogen and phosphorus release rates of different crop types under different flooding durations; S7: Based on the vegetation nitrogen and phosphorus stock in each vegetation type area, the duration of flooding, and the nitrogen and phosphorus release rates under different flooding durations, calculate the nitrogen flux and phosphorus flux released in each vegetation type area, and then sum them up to obtain the total nitrogen flux and total phosphorus flux of the drawdown zone.

[0008] In one specific implementation, in S2, the reservoir drawdown zone is divided into a high flood duration zone, a medium flood duration zone, and a low flood duration zone.

[0009] In one specific implementation scheme, in S3, the land feature type includes one or more combinations of grassland, cultivated land, forest land, beach land, and water use land; The spatial distribution characteristics of the vegetation include vegetation height, cover, community species distribution and the dominance of each species, and the dominant species are identified. The formula for calculating dominance is as follows: Y i = P i × f i ; P i =N i / N ; N i For species i Importance value; N It is the sum of the importance values ​​of all species within the quadrat. f i For species i Frequency of occurrence at each sampling point; Where, importance value = (relative coverage + relative height) / 2 Relative cover = (Cover of a specific plant species / Sum of cover of all plant species within the quadrat) × 100% Relative height = (height of a certain plant species / sum of heights of all plant species in the quadrat) × 100%.

[0010] In one specific implementation, S4 includes the following steps: S41: Samples were taken from the drawdown zone of the reservoir, and the nitrogen and phosphorus contents of the existing vegetation at the sampling points were measured. S42: Correlate the historical vegetation biomass data of the sampling points with the corresponding remote sensing imagery Normalized Difference Vegetation Index (NDVI), Difference Vegetation Index (DVI), Ratio Vegetation Index (RVI), Enhanced Vegetation Index (EVI), and Greenness Vegetation Index (GNDVI) to obtain simulation models of each vegetation index and biomass. Based on R... 2 The magnitude of RMSE and the scatter plot of predicted and measured values ​​were compared to determine the characteristic index of inverted biomass and its optimal model.

[0011] S43: Remote sensing interpretation of the existing vegetation biomass and area of ​​the land cover types in the reservoir drawdown zone.

[0012] In one specific implementation plan, in S43, using high-resolution remote sensing imagery as the base and Tianditu as an aid in ArcGIS, the vegetation biomass and area of ​​grassland, cultivated land, forest land, beach land, and water use land in each flood duration area of ​​the reservoir drawdown zone are interpreted according to the inversion biomass empirical model, and verified in conjunction with the field sampling results in S41.

[0013] In one specific implementation plan, the nitrogen and phosphorus stock in the land cover type areas within each flood duration area are calculated according to the following two formulas:

[0014]

[0015] In the formula: F 氮存量 B represents the total nitrogen storage (t) of vegetation in the drawdown zone. i fori Land cover type, regional vegetation biomass per unit area (t / km²) 2 ), V i氮 for i Average nitrogen content per unit area of ​​vegetation in land cover type region (g / m²) 2 ), A i氮 for i Area of ​​land cover type (km²) 2 ). F 磷存量 The total phosphorus stock in the vegetation of the drawdown zone (t). V i磷 for i Average phosphorus content per unit area of ​​vegetation in land cover type region (g / m²) 2 ), A i磷 Plant type i Area (km) 2 ).

[0016] In one specific implementation plan, the plant nitrogen and phosphorus release flux and release rate are calculated according to the following two formulas:

[0017]

[0018] In the formula: R i植氮 for i Land cover type, regional vegetation nitrogen release rate (t / km) 2 d), F i植氮t0 The nitrogen stock of plants in area i, before the experiment (t). F i植氮t After being flooded for t(d) i Regional plant nitrogen stock (t) by land cover type. R i植磷 for i Phosphorus release rate of vegetation in land cover type area (t / km) 2 d), F i植磷t0 Before the experiment i Phosphorus stock of plants in the region by land cover type (t). F i植磷t After being flooded for t(d) i Phosphorus stock of plants in the region of land cover type (t).

[0019] In one specific implementation plan, the nitrogen and phosphorus release fluxes from vegetation in the drawdown zone at time t of annual flooding are calculated using the following two formulas:

[0020]

[0021] In the formula: F 氮通量t The nitrogen release flux from vegetation (t) is given when the reservoir is operating at a high water level (t). F 磷通量t The value represents the phosphorus release flux from vegetation (t). R i氮 Ground feature types i nitrogen release rate per unit area of ​​vegetation, phosphorus R i磷 Ground feature type i Phosphorus release rate per unit area of ​​vegetation in the region.

[0022] This invention provides a method for calculating nitrogen and phosphorus release fluxes from vegetation. Based on continuous location monitoring and corresponding remote sensing image interpretation data over the years, it constructs an empirical model for remote sensing image-biomass inversion in the drawdown zone, thereby enabling rapid estimation of biomass over large areas of vegetation and saving significant time and manpower. Through in-situ flooding experiments, an empirical model for nitrogen and phosphorus release rates from vegetation in the drawdown zone is established to simulate real hydrological conditions. Key parameters of the model are determined, and it can be directly used to assess the impact of nitrogen and phosphorus release from vegetation on the water quality of local water bodies. When calculating nitrogen and phosphorus release fluxes from different types of vegetation in the drawdown zone, the method fully considers the reservoir scheduling and operation patterns and the spatiotemporal specificity of vegetation distribution, giving it strong applicability and dynamic response capabilities. Attached Figure Description

[0023] Figure 1 This describes the pattern of water level fluctuations in the reservoir.

[0024] Figure 2 The duration of flooding in different elevation areas of the drawdown zone.

[0025] Figure 3 The fitted curve of fresh weight of vegetation in the drawdown zone versus NDVI.

[0026] Figure 4 This is a scatter plot of measured and predicted vegetation values ​​in the drawdown zone.

[0027] Figure 5 This represents the area of ​​land use types in different elevation regions.

[0028] Figure 6 The amount of nitrogen remaining in vegetation in regions with different land use types.

[0029] Figure 7 The existing phosphorus content in vegetation in regions with different land use types.

[0030] Figure 8 The platform was flooded in situ to prevent nitrogen and phosphorus release from vegetation in the drawdown zone.

[0031] Figure 9 The variation of nitrogen and phosphorus release rates of vegetation in different land use types of the drawdown zone.

[0032] Figure 10 The variation of nitrogen and phosphorus release rates of vegetation in different land use types of the drawdown zone.

[0033] Figure 11 Changes in the percentage of nitrogen and phosphorus release from different vegetation life forms in the drawdown zone.

[0034] Figure 12 Interannual variation of nitrogen release flux from vegetation in the drawdown zone.

[0035] Figure 13 Interannual variation of phosphorus release flux from vegetation in the drawdown zone. Detailed Implementation

[0036] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0037] Taking the Danjiangkou Reservoir as an example, the method principle and spirit of this invention will be illustrated by example.

[0038] 1. Collect data Collect data on the functional positioning of Danjiangkou Reservoir, its hydrological rhythm, daily average water level, as well as characteristic water levels such as flood control limit water level, normal storage water level, and dead water level.

[0039] Characteristic water levels: normal storage level 170 m, autumn flood season flood limit level 163.5 m, dead water level 150 m.

[0040] like Figure 1 As shown, the actual exposure pattern of the drawdown zone of Danjiangkou Reservoir is not very obvious due to various factors such as reservoir operation regulations, inflow and diversion conditions. Water levels are low from March to mid-September. In 2016 and 2019, the water level dropped to 150 m, exposing areas above 150 m elevation in the drawdown zone. In late September, the water level gradually rises, gradually submerging the drawdown zone. The water level is highest in early November, then gradually drops, reaching its lowest point in March-April of the following year, with the largest exposed area of ​​the drawdown zone. During the summer flood season (June 21 to August 20), areas above 160 m elevation in the drawdown zone are exposed; during the autumn flood season (September 1 to October 10), areas above 163.5 m elevation in the drawdown zone are exposed. From October 1, depending on the flood situation of the Han River and hydrological and meteorological forecasts, the reservoir gradually fills, gradually submerging the drawdown zone. After October, the water level gradually stabilizes and drops, reaching 160 m by June 20 of the following year.

[0041] 2. Divide the areas into different elevation zones. Duration of continuous flooding at different water levels (days) = Number of days above a certain water level (water level 1, water level 2, dead water level, etc.) within one flooding-outflow cycle (usually one year).

[0042] Statistics on the duration of flooding in areas at different elevations, such as Figure 2 As shown, the average duration of flooding in areas with elevations of 150-160, 160-165, and 165-170m was 336 days, 141 days, and 35 days, respectively.

[0043] The area was divided into three elevation zones based on the reservoir's operation mode, flood control limit level, and actual water level fluctuations: 150-160 m, 160-165 m, and 165-170 m.

[0044] 3. Investigate the spatial distribution characteristics of vegetation in the drawdown zone. A survey was conducted once before the drawdown zone was submerged. Three flood duration zones were established, with four paper cup quadrats in each zone: 1m x 1m quadrats for herbaceous plants and 20m x 20m quadrats for shrubs and trees. The height and cover of each plant species in each quadrat were recorded, along with the land cover type (grassland, cultivated land, woodland, water use land, etc.). Dominant species were analyzed, with species having a dominance score > 0.02 considered dominant. Dominant species included perennial herbaceous plants such as Bermuda grass, Paspalum notatum, and water hyacinth; annual herbaceous plants such as Artemisia annua, Xanthium sibiricum, and Polygonum hydropiper; trees such as mulberry, willow, and pond cypress; and crops such as corn.

[0045] Importance value = (relative coverage + relative height) / 2 Relative cover = (Cover of a specific plant species / Sum of cover of all plant species within the quadrat) × 100% Relative height = (Height of a specific plant species / Sum of heights of all plant species within the quadrat) × 100% Dominance, which represents a species' position and role in a community, Y i = P i × f i ; In the formula Pi = Ni / N ; Ni For species i Importance value; N It is the sum of the importance values ​​of all species within the quadrat. fi For species i Frequency of occurrence at each sampling point.

[0046] 4. Measure the nitrogen and phosphorus content of existing vegetation. The average nitrogen content of vegetation in the existing grassland, cultivated land, forest land, beach land, and water utilization land in the drawdown zone of Danjiangkou Reservoir is 3.93, 6.31, 3.00, 1.94, and 1.39 t / km², respectively. 2 The average phosphorus content was 0.75, 0.63, 0.58, 0.33, and 0.29 t / km, respectively. 2 .

[0047] 5. Construct an empirical model for remote sensing image-biomass inversion. The vegetation biomass data of sampling points over the years (2009-2023) were correlated with five types of vegetation indices, namely, the Normalized Difference Vegetation Index (NDVI), Difference Vegetation Index (DVI), Ratio Vegetation Index (RVI), Enhanced Vegetation Index (EVI), and Greenness Vegetation Index (GNDVI) of the corresponding remote sensing images (Table 1).

[0048] A total of 319 samples were used. 223 samples (approximately 70% of the total samples) were randomly selected as the training set, and the remaining 96 samples were used as the validation set. The R-values ​​of each model were compared. 2 And RMSE, quadratic or cubic models built with RVI, and R of quadratic or cubic models built with NDVI. 2 The index is relatively high and the RMSE is relatively small, but compared with the fitted curve, the trends of the quadratic and cubic curves of RVI (the higher the index, the lower the fresh weight) do not conform to reality. Therefore, the quadratic model established by NDVI is the optimal one. Figure 3 and Figure 4 The model is as follows: Biomass = 65.88 + 718.60 * NDVI + 1774.20 * NDVI*NDVI, where R 2 The value was 0.535, and the RMSE was 410.2 g / m. 2 .

[0049] Table 1 Biomass estimation model

[0050] 6. Remote sensing interpretation of land cover types and areas in the drawdown zone In ArcGIS, high-resolution remote sensing imagery was used for multi-scale segmentation, and the normalized water index (NDVI) was combined to extract approximate water level boundaries at 150, 160, and 170 m. Then, the slope β was calculated using the ALOS digital elevation model, and the buffer distance was calculated according to the trigonometric function of slope β to obtain more accurate water level boundary lines at elevations of 150, 160, 165, and 170 m. Isolated island patches were obtained by subtracting the actual 170 m water surface from the complete surface vector covering the 170 m water surface. Using high-resolution remote sensing imagery as the base and Tianditu (a map-based system) as an auxiliary tool in ArcGIS, the existing vegetation biomass of drawdown zone grasslands, cultivated land, forest land, beaches, and water use areas was interpreted based on the biomass inversion empirical model and the corresponding normalized vegetation index (NDVI).

[0051] Remote sensing interpretation results show that the average existing vegetation biomass in the drawdown zone of Danjiangkou Reservoir (elevation 150-170 m) for grassland, cultivated land, forest land, beach land, and water utilization land are 819.9, 1059.6, 572.9, 484.6, and 332.7 t / km², respectively. 2 The areas of grassland, cultivated land, forest land, tidal flats, and water use land are 136.68, 114.97, 29.46, 88.49, and 1.84 km², respectively. 2 ( Figure 5 ).

[0052] 7. Determine the nitrogen and phosphorus stock of vegetation in areas with different land cover types. Calculate nitrogen and phosphorus stock using the following two formulas:

[0053]

[0054] In the formula: F 氮存量 B represents the total nitrogen storage (t) of vegetation in the drawdown zone. i for i Land cover type, regional vegetation biomass per unit area (t / km²) 2 ), V i氮 for i Average nitrogen content per unit area of ​​vegetation in land cover type region (g / m²) 2 ), A i氮 for i Area of ​​land cover type (km²) 2 ). F 磷存量 The total phosphorus stock in the vegetation of the drawdown zone (t). V i磷 The average phosphorus content per unit area of ​​vegetation in region i (g / m²) 2), A i磷 Plant type i Area (km) 2 ).

[0055] The nitrogen stock of vegetation in grassland, cultivated land, forest land, beach land, and water utilization area in the Danjiangkou drawdown zone were 537.12, 724.92, 267.97, 171.41, and 2.56 t, respectively. Figure 6 The phosphorus stocks were 102.91, 72.07, 69.73, 64.26, and 0.53 t respectively. Figure 7 ).

[0056] 8. In-situ flooding experiments on vegetation of different land cover types to clarify the nitrogen and phosphorus release rates under different flooding durations.

[0057] An in-situ flooding experimental platform was constructed in the field. This platform consisted of steel pipes and floating containers, with plastic pots containing plants secured to the iron frame using ropes. Figure 8 Adjusting the position of the plastic basin allows for the design of the flooding depth and different flooding times.

[0058] Calculate the nitrogen and phosphorus release flux and release rate from plants using the following two formulas:

[0059]

[0060] In the formula: R i植氮 for i Land cover type, regional vegetation nitrogen release rate (t / km) 2 d), F i植氮t0 The nitrogen stock of plants in area i, before the experiment (t). F i植氮t After being flooded for t(d) i Regional plant nitrogen stock (t) by land cover type. R i植磷 for i Phosphorus release rate of vegetation in land cover type area (t / km) 2 d), F i植磷t0 Before the experiment i Phosphorus stock of plants in the region by land cover type (t). F i植磷t After being flooded for t(d) i Phosphorus stock of plants in the region of land cover type (t).

[0061] In the initial stage of flooding (5 days), the release rates of nitrogen and phosphorus from cultivated land plants were relatively rapid, at 0.33 and 0.32 t / (d·km), respectively.2 The next slowest release rates are found in grasslands and woodlands, while water use areas and beaches have slower release rates. Figure 9 and Figure 10 As the flooding time increased, the nitrogen and phosphorus release rate showed a decreasing trend, and after 120 days, the release of nitrogen and phosphorus from plants tended to slow down. At 120 days of flooding, the average percentage of nitrogen and phosphorus release from grassland plants was 36.9%, from cultivated land crops was 50.2%, and from forest plants was 41.3%. Figure 11 ).

[0062] 10. Calculate the nitrogen and phosphorus release fluxes from vegetation in the drawdown zone. Based on the spatial distribution characteristics of nitrogen and phosphorus content in the drawdown zone vegetation and the results of in-situ monitoring experiments on nitrogen and phosphorus release from plants, the nitrogen and phosphorus release flux of the drawdown zone vegetation was calculated.

[0063] Calculate the annual nitrogen and phosphorus release flux from plants using the following two formulas:

[0064]

[0065] In the formula: F 氮通量t The nitrogen release flux from vegetation (t) is given when the reservoir is operating at a high water level (t). F 磷通量t The value represents the phosphorus release flux from vegetation (t). R i氮 Ground feature types i Nitrogen release rate per unit area of ​​vegetation (t / km) 2 d), phosphorus R i磷 Ground feature type i Phosphorus release rate per unit area of ​​vegetation (t / km) 2 d).

[0066] The scale of nitrogen and phosphorus release from plants in the drawdown zone of Danjiangkou Reservoir is significantly correlated with the duration of flooding. The average duration of flooding in the 156–160 m, 160–165 m, and 165–170 m elevation areas is 336 days, 141 days, and 35 days, respectively, resulting in significant differences in nitrogen and phosphorus release fluxes from plants in the drawdown zone at different elevations.

[0067] The nitrogen release flux from vegetation in the drawdown zone of Danjiangkou Reservoir in 2020, 2021, 2022, 2023, 2024, and 2025 were 656.34t, 805.9t, 717.9t, 774.8t, 735.3t, and 786.7t, respectively. Figure 12 The phosphorus release fluxes were 92.6 t, 114.2 t, 101.8 t, 109.8 t, 104.2 t and 111.6 t, respectively. Figure 13 ).

[0068] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for accounting for the nitrogen and phosphorus release flux of the vegetation in the water level fluctuation zone of a reservoir, characterized in that, Includes the following steps: S1: Collect hydrological data of the reservoir; S2: Divide the reservoir drawdown zone into several flood duration zones according to the different durations of flooding; S3: Investigate the nitrogen and phosphorus content of existing vegetation in each flooded area of ​​the drawdown zone; S4: Interpret the remote sensing images of the drawdown zone to determine the existing vegetation biomass and area of ​​land cover types in each flood duration area of ​​the drawdown zone. S5: Determine the nitrogen and phosphorus stock of existing vegetation in the land cover types within each flood duration area; S6: By measuring the nitrogen and phosphorus release rates of different vegetation types under different flooding durations; S7: Based on the vegetation nitrogen and phosphorus stock in each vegetation type area, the duration of flooding, and the nitrogen and phosphorus release rates under different flooding durations, calculate the nitrogen flux and phosphorus flux released in each vegetation type area, and then sum them up to obtain the total nitrogen flux and total phosphorus flux of the drawdown zone.

2. The method of claim 1, wherein, In S2, the reservoir drawdown zone is divided into a high flood duration zone, a medium flood duration zone, and a low flood duration zone.

3. The method of claim 1, wherein, In S3, the land feature types include one or more combinations of grassland, cultivated land, forest land, beach land, and water use land; The spatial distribution characteristics of the vegetation include vegetation height, cover, community species distribution and the dominance of each species, and the dominant species are identified. Wherein, the calculation formula of the dominance is: Y i = P i × f i ; P i = N i / N ; N i Importance value of species i ; N Sum of importance value of all species in quadrat, f i Frequency of species i occurrence in each sampling point; Where, importance value = (relative coverage + relative height) / 2 Relative cover = (Cover of a specific plant species / Sum of cover of all plant species within the quadrat) × 100% Relative height = (height of a certain plant species / sum of heights of all plant species in the quadrat) × 100%.

4. The method of claim 1, wherein, S4 includes the following steps: S41: Take samples from the drawdown zone of the reservoir and measure the existing vegetation biomass; S42: Correlate the historical vegetation biomass data of the sampling points with the corresponding remote sensing imagery Normalized Difference Vegetation Index (NDVI), Difference Vegetation Index (DVI), Ratio Vegetation Index (RVI), Enhanced Vegetation Index (EVI), and Greenness Vegetation Index (GNDVI) to obtain simulation models of each vegetation index and biomass. Based on R... 2 By comparing the magnitude of RMSE and the scatter plot of predicted and measured values, the characteristic index of inverted biomass and its optimal inverted biomass model are determined. S43: Remote sensing interpretation of the existing vegetation biomass and area of ​​the land cover types in the reservoir drawdown zone.

5. The method of claim 4, wherein, In S43, using high-resolution remote sensing imagery as the base in ArcGIS and Tianditu as an auxiliary tool, the vegetation biomass and area of ​​grassland, cultivated land, forest land, beach land, and water use land in each flooding duration area of ​​the reservoir drawdown zone are interpreted based on the aforementioned inversion biomass empirical model, and verified in conjunction with the field sampling results in S41.

6. The method according to claim 1, characterized in that, Calculate the nitrogen and phosphorus stock in the land cover types within each flood duration area using the following two formulas: In the formula: F 氮存量 The total nitrogen content (t) in the vegetation of the drawdown zone. for i Land cover type, regional vegetation biomass per unit area (t / km²) 2 ), V i氮 for i Average nitrogen content per unit area of ​​vegetation in land cover type region (g / m²) 2 ), A i氮 for i Area of ​​land cover type (km²) 2 ), F 磷存量 The total phosphorus stock in the vegetation of the drawdown zone (t). V i磷 for i Average phosphorus content per unit area of ​​vegetation in land cover type region (g / m²) 2 ), A i磷 Plant type i Area (km) 2 ).

7. The method according to claim 6, characterized in that, Calculate the nitrogen and phosphorus release flux and release rate from plants using the following two formulas: In the formula: R i植氮 for i Land cover type, regional vegetation nitrogen release rate (t / km) 2 d), F i植氮t0 The nitrogen stock of plants in area i, before the experiment (t). F i植氮t After being flooded for t(d) i Regional plant nitrogen stock (t) by land cover type. R i植磷 for i Phosphorus release rate of vegetation in land cover type area (t / km) 2 d), F i植磷t0 Before the experiment i Phosphorus stock of plants in the region by land cover type (t). F i植磷t After being flooded for t(d) i Phosphorus stock of plants in the region of land cover type (t).

8. The method according to claim 6, characterized in that, Calculate the nitrogen and phosphorus release fluxes from vegetation in the drawdown zone at time t of annual plant flooding using the following two formulas: In the formula: F 氮通量t The nitrogen release flux from vegetation (t) is given when the reservoir is operating at a high water level (t). F 磷通量t The value represents the phosphorus release flux from vegetation (t). R i氮 Ground feature types i nitrogen release rate per unit area of ​​vegetation, phosphorus R i磷 Ground feature type i Phosphorus release rate per unit area of ​​vegetation in the region.

Images