A method for dynamically calculating release risk of internal source phosphorus in a shallow lake

Abstract

This invention discloses a method for dynamically calculating the risk of endogenous phosphorus release in shallow lakes. The steps are as follows: First, acquire lake data; second, based on the acquired lake hydrodynamic data, construct a wave-current coupling model to calculate the intensity of hydrodynamic disturbance and calculate the bottom shear stress; third, based on the acquired sediment characteristic data, determine the critical conditions for sediment resuspension and assess the phosphorus release potential of different sediment forms; fourth, couple the calculated hydrodynamic disturbance intensity and the assessed sediment phosphorus release potential to construct a dynamic calculation model for endogenous phosphorus release flux; fifth, based on the calculated endogenous phosphorus release flux and combined with the critical phosphorus load threshold for eutrophication, assess the risk level of endogenous phosphorus release. The beneficial effects of this invention are: it can fully utilize multi-source measured data, dynamically couple hydrodynamic processes with the biogeochemical processes of various forms of phosphorus in sediments, establish a mechanism-driven high-precision model, and achieve dynamic, accurate, and quantitative calculation of endogenous phosphorus release flux in shallow lakes.

Description

Technical Field

[0001] This invention relates to the field of lake environmental protection and ecological governance technology, and more specifically, to a method for dynamically calculating the risk of endogenous phosphorus release from shallow lakes. Background Technology

[0002] Shallow lakes, due to their shallow depth, are easily affected by hydrodynamic disturbances such as wind and waves at the sediment-water interface, leading to sediment resuspension and the release of endogenous phosphorus, thus exacerbating eutrophication. Phosphorus exists in various forms in sediments, including exchangeable phosphorus (NH4Cl-P), reducible phosphorus (BD-P), metal oxide-bound phosphorus (NaOH-SRP), organic phosphorus (NaOH-NRP), calcium-bound phosphorus (HCl-P), and residual phosphorus (Res-P), etc. Different forms of phosphorus have different release potentials and environmental behaviors.

[0003] Currently, most endogenous phosphorus release assessment methods do not consider real-time changes in hydrodynamic conditions such as wind, waves, and flow velocity, making it impossible to provide early warnings for single disturbances or short-term risks. They often analyze hydrodynamic conditions and sedimentary physicochemical properties in isolation, lacking quantitative models that dynamically couple the two. The shear forces calculated by hydrodynamic models cannot be compared in real-time with key parameters reflecting sediment erosion resistance (such as critical shear stress). When assessing endogenous phosphorus release, total phosphorus (TP) in sediments is typically used as an indicator, ignoring the significant differences in bioavailability and release kinetics of different chemical forms of phosphorus (such as reducible phosphorus and organic phosphorus), leading to inaccurate risk assessment results. Existing methods largely remain at the academic research level, lacking systematic computational tools and failing to achieve dynamic, accurate, and quantitative calculations of endogenous phosphorus release risks.

[0004] Therefore, there is an urgent need for a dynamic calculation method and system that can dynamically, comprehensively, and accurately assess the risk of endogenous phosphorus release in shallow lakes by coupling hydrodynamics and phosphorus speciation in sediments, so as to provide decision support for the precise management of lakes. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing technologies by providing a method and system for dynamically calculating the risk of endogenous phosphorus release in shallow lakes. This method can fully utilize multi-source measured data, dynamically couple hydrodynamic processes with the biogeochemical processes of various forms of phosphorus in sediments, establish a mechanism-driven high-precision model, and achieve dynamic, accurate, and quantitative calculation of endogenous phosphorus release flux in shallow lakes.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: a method for dynamically calculating the risk of endogenous phosphorus release from shallow lakes, comprising the following steps: Step S1: Acquire lake hydrodynamic data, sediment characteristic data, sediment-water interface environmental parameter data, and lake morphology data; Step S2: Based on the lake hydrodynamic data obtained in step S1, construct a wave-current coupling model of hydrodynamic disturbance intensity and calculate the bottom shear stress; Step S3: Based on the sediment characteristic data obtained in Step S1, determine the critical conditions for sediment resuspension and evaluate the phosphorus release potential of different sediment forms. Step S4: Couple the wave-current coupling model of hydrodynamic disturbance intensity constructed in step S2 with the phosphorus release potential of different sediment morphologies assessed in step S3 to construct a dynamic calculation model for endogenous phosphorus release flux and calculate the endogenous phosphorus release flux. Step S5: Based on the endogenous phosphorus release flux calculated in Step S4 and combined with the critical phosphorus load threshold for eutrophication of the water body, assess the endogenous phosphorus release risk level; based on the assessed endogenous phosphorus release risk level, formulate corresponding endogenous pollution control strategies for the lake.

[0007] Further, step S1: Obtain lake hydrodynamic data, sediment characteristic data, sediment-water interface environmental parameter data, and lake morphology data; the specific steps are as follows: Step S11, Acquisition of lake hydrodynamic data: Wind speed and direction data are acquired by setting up buoy-type meteorological and hydrological observation stations on the lake surface; bottom current velocity U is acquired by an acoustic Doppler current profiler installed on the lake bottom. z Wave height H and wave period T are obtained using a pressure wave meter. Step S12, Sediment Characteristic Data Acquisition: A gridded sampling method was used to obtain lakebed sediment samples using a sediment sampler; the lakebed sediment samples were analyzed in the laboratory to obtain the median grain size of the sediments. Phosphorus content in different forms and sediment porosity φ; Step S13, Acquisition of sediment-water interface environmental parameter data: The concentration of dissolved active phosphorus C0 in the pore water at the sediment-water interface and the concentration of dissolved active phosphorus C in the overlying water were determined by molybdenum-antimony spectrophotometry. ws The thickness δ of the diffusion boundary layer was measured using a sediment motion measurement instrument. Step S14, lake morphology data acquisition: measure the lake water depth h using a depth sounder.

[0008] Furthermore, in step S2, a wave-current coupling model of the hydrodynamic disturbance intensity is constructed, and the bottom shear stress is calculated; the specific steps are as follows: Step S21: Calculate the wavelength L using the wave dispersion relation; use the Eckert approximation formula for efficient calculation, as shown in formula (1): (1); Where L is the wavelength, Where is the acceleration due to gravity, T is the wave period, 2π represents the radians corresponding to a complete waveform, tanh() is the hyperbolic tangent function, π represents the radians corresponding to half a waveform, and h is the lake depth; Step S22, Wave Bottom Trajectory Velocity Calculation: Calculate the peak trajectory velocity U of near-bottom water particles caused by the wave. w As shown in formula (2): (2); Among them, U w Let H be the peak orbital velocity of near-bottom water particles caused by waves, H be the wave height, sinh() be the hyperbolic sine function, and k be the wave number, k=2π / L; Step S23, Calculation of bottom shear stress caused by waves: Calculate the bottom shear stress τ caused by waves. w As shown in formula (3): (3); Where, τ w Let f be the bottom shear stress caused by waves, ρ be the density of water, and f be the density of water. w Let be the wave friction coefficient, as shown in formula (4): (4); in, Let A represent an exponential function with base e. w This represents the half-amplitude of the wave orbital motion. k N k represents the roughness length of the substrate. N = 2.5*d 50 d 50 The median grain size of the sediment; Step S24, calculate the friction speed: as shown in formula (5): (5); Among them, u * Let U be the friction velocity, K be the von Kármán constant, taken as 0.4, and U be the friction velocity. z The bottom velocity is the velocity measured at height z, where z is the height of the velocity measurement point of the acoustic Doppler velocity profiler from the bottom bed. Step S25, calculate the bottom shear stress caused by the water flow: as shown in formula (6): (6); Where, τ c This refers to the bottom shear stress caused by the water flow. Step S26, calculate the combined bottom shear stress under the combined action of waves and water flow: as shown in formula (7): (7); Where, τ cw The combined bottom shear stress is the combined effect of waves and water flow, and θ is the angle between the direction of water flow and the direction of wave propagation.

[0009] Furthermore, in step S3, the critical conditions for sediment resuspension are determined, and the phosphorus release potential of different sediment forms is evaluated; specifically: Step S31, calculate the critical erosion shear stress of the sediment: as shown in formula (8): (8); Where, τ cr The critical erosion shear stress of the sediment. ρ is the critical Shields number parameter. s This refers to the density of sediment particles; Critical Shields number parameter The result was obtained by using a fitting formula, as shown in formula (9): (9); Among them, D * The particle size is dimensionless and is calculated using formula (10): (10); Wherein, ε is the kinematic viscosity of water, obtained by interpolation based on measured water temperature; Step S32, based on the combined bottom shear stress τ under the combined action of waves and water flow cw and the critical erosion shear stress τ of sediments cr The sediment resuspension flux E is calculated as shown in formula (11): (11); Where E is the resuspension flux, representing the dry weight of eroded sediment per unit area per unit time, M is the erosion rate parameter, and α is the erosion index parameter; the erosion rate parameter M and the erosion index parameter α were calibrated through indoor direct shear experiments. Step S33: Based on the resuspension flux E calculated in S32, an indoor simulation resuspension experiment is conducted to simulate actual hydrodynamic conditions, namely the critical erosion shear stress τ of the sediment. cw The proportion of different forms of phosphorus released from suspended sediments into the aqueous phase was measured, i.e., the effective release factor EF. i .

[0010] Specifically, in indoor experiments, the shear stress conditions (τ) equivalent to those in the field are reproduced. cw This process causes sediment resuspension. By monitoring changes in the concentration of dissolved active phosphorus in the overlying water, and combining this with the known amount of resuspended sediment and the content of various phosphorus forms, the effective release factor EF is calculated.i .

[0011] Furthermore, in step S33, the effective release factor is determined by calibrating the lake sediments through indoor simulated resuspension experiments, specifically as follows: Step S331: Collect sediment samples from the target lake, transport them back to the laboratory at low temperature and in the dark; after homogenization pretreatment, determine the sediment porosity and the content of six forms of phosphorus, including exchangeable phosphorus NH4Cl-P, reducible phosphorus BD-P, metal oxide bound phosphorus NaOH-SRP, organic phosphorus NaOH-NRP, calcium bound phosphorus HCl-P, and residual phosphorus Res-P. In step S332, the pretreated sediment sample and the overlying water filtered through a 0.45 μm filter membrane are used to reconstruct the sediment-water interface system in a reaction bottle to form an experimental group. The liquid-solid ratio is controlled at 5:1. A blank control group and a sediment background control group are set up. All reaction bottles are pre-cultured in the dark under constant temperature conditions for 48 hours to allow the sediment-water interface system to reach physicochemical equilibrium. Step S333: After the pre-cultivation is completed, an initial water sample is collected. Then, a calibrated oscillation test device is used to apply a constant bottom shear stress to the experimental group that is higher than the critical erosion shear stress of the sediment to simulate a continuous hydrodynamic disturbance process for 6 hours. Step S334: After the disturbance is completed, let it stand for 5 minutes to allow the coarse particles to settle. Then, collect the upper water sample of the experimental group and filter it through a 0.45 μm filter membrane. The concentration of dissolved reactive phosphorus in the water sample is determined by the molybdenum antimony spectrophotometric method. Step S335: Calculate the effective release factor using the mass balance formula, as shown in formula (12): (12); Among them, EF i C is the effective release factor for the i-th form of phosphorus. sample C sample_t0 The concentrations of soluble reactive phosphorus in the supernatant water before and after shaking in the experimental group are C, respectively. blank C blank_t0 The values ​​represent the dissolved reactive phosphorus concentrations in the overlying water at the corresponding time points in the blank control group, respectively. V is the volume of the overlying water in the reaction flask, and M is the volume of the overlying water. dry C is the dry weight of the sediment in the reaction flask. i The content of phosphorus in the i-th form in the sediment; Step S336: The experiment is conducted in parallel at least three times, and the effective release factor EF of the i-th form of phosphorus is finally taken. i The average value was used as the calibration result for different forms of phosphorus.

[0012] Furthermore, in step S4, a dynamic calculation model for endogenous phosphorus release flux is constructed to calculate the endogenous phosphorus release flux; the specific steps are as follows: Step S41, based on the resuspension flux E and the effective release factor EF of the i-th phosphorus form. i The phosphorus release flux under resuspension is calculated as shown in formula (13): (13); Among them, F res denoted as phosphorus release flux under resuspension, and n is the number of phosphorus species in the sediment. Step S42, calculate the phosphorus release flux under diffusion, as shown in formula (14): (14); Among them, F diff φ represents the phosphorus release flux due to diffusion, φ represents the sediment porosity, and D represents the phosphorus release flux due to diffusion. s C0 is the effective diffusion coefficient of phosphorus in sediment pore water, and C0 is the concentration of dissolved active phosphorus in the pore water at the sediment-water interface. ws δ represents the concentration of dissolved active phosphorus in the overlying water, and δ represents the thickness of the diffusion boundary layer. The effective diffusion coefficient D of phosphorus in sediment pore water s The formula is: (15); Where D0 is the molecular diffusion coefficient of phosphate ions in infinitely diluted water; Step S43, the total endogenous phosphorus release flux F is calculated as shown in formula (16): (16); Where F represents the total endogenous phosphorus release flux.

[0013] Furthermore, in step S5, based on the endogenous phosphorus release flux calculated in step S4 and combined with the critical phosphorus load threshold for eutrophication of the water body, the risk level of endogenous phosphorus release is assessed; the specific steps are as follows: Step S51, critical phosphorus load threshold L for eutrophication of water body critical Based on the phosphorus concentration limits corresponding to the respective water quality categories, the allowable phosphorus load threshold is calculated using parameters such as the average lake depth and hydraulic retention time. Step S52, Risk Assessment and Classification: Combine the comprehensive endogenous phosphorus release flux F calculated in step S4 with the critical phosphorus load threshold L for eutrophication of the water body. critical Comparisons were made to classify the risk levels of endogenous phosphorus release. Low risk: F < 0.5 L critical ; Medium risk: 0.5 L critical≤ F < L critical ; High risk: F ≥ L critical .

[0014] Low-risk ecological significance: The endogenous release flux is far below the critical level, the sediments play a dominant role as a phosphorus sink, the water body has sufficient self-purification capacity, and the risk of eutrophication is extremely low. Medium-risk ecological significance: The endogenous release flux is close to the critical level, and the phosphorus exchange at the sediment-water interface is in equilibrium. It may transform into a high-risk state when the external input increases or the hydrodynamic conditions change. High-risk ecological significance: When the endogenous release flux reaches or exceeds the critical level, the function of sediment as a phosphorus "source" becomes dominant. Even if the external pollution is controlled, the eutrophication state of the water body may still be maintained.

[0015] The beneficial effects of this invention are as follows: The method for dynamically calculating the risk of endogenous phosphorus release in shallow lakes by coupling hydrodynamics and phosphorus speciation in sediments provides decision support for the precise management of lakes; it can make full use of multi-source measured data, dynamically couple hydrodynamic processes with biogeochemical processes of various phosphorus speciation in sediments, establish a mechanism-driven high-precision model, and realize dynamic, accurate, and quantitative calculation of endogenous phosphorus release flux in shallow lakes. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the structure of the present invention. Detailed Implementation

[0017] like Figure 1 As shown, the technical solution adopted in this invention is: a method and system for dynamically calculating the risk of endogenous phosphorus release in shallow lakes by coupling hydrodynamics and sediment phosphorus speciation, comprising the following steps: Step S1: Acquire lake hydrodynamic data, sediment characteristic data, sediment-water interface environmental parameter data, and lake morphology data; Step S2: Based on the lake hydrodynamic data obtained in step S1, construct a wave-current coupling model to calculate the intensity of hydrodynamic disturbance and calculate the bottom shear stress; Step S3: Based on the sediment characteristic data obtained in Step S1, determine the critical conditions for sediment resuspension and evaluate the phosphorus release potential of different sediment forms. Step S4: Couple the hydrodynamic disturbance intensity calculated in step S2 and the sediment phosphorus release potential assessed in step S3 to construct a dynamic calculation model for endogenous phosphorus release flux; Step S5: Based on the endogenous phosphorus release flux calculated in Step S4, and combined with the eutrophication threshold of the water body, assess the risk level of endogenous phosphorus release. Step S6: Based on the endogenous phosphorus release risk level assessed in Step S5, formulate corresponding endogenous pollution control strategies for the lake.

[0018] Furthermore, in step S1, lake hydrodynamic data, sediment-water interface environmental parameter data, and lake morphology data are obtained, specifically as follows: Acquire lake hydrodynamic data, sediment characteristic data, sediment-water interface environmental parameter data, and lake water depth data; Step S11, Acquisition of lake hydrodynamic data: Wind speed and direction data are acquired by setting up buoy-type meteorological and hydrological observation stations on the lake surface; bottom current velocity U is acquired by using an acoustic Doppler current profiler (ADCP) installed on the lake bottom. z Wave height H and wave period T are obtained using a pressure wave meter. Step S12, Sediment Characteristic Data Acquisition: A gridded sampling method was used to obtain lakebed sediment samples using a Petersen sediment sampler; the lakebed sediment samples were analyzed in the laboratory to obtain the median grain size. The content of different forms of phosphorus (exchangeable phosphorus (NH4Cl-P), reducible phosphorus (BD-P), metal oxide bound phosphorus (NaOH-SRP), organic phosphorus (NaOH-NRP), calcium bound phosphorus (HCl-P), and residual phosphorus (Res-P)) was determined; the porosity φ of the sediments was obtained through physical experiments. Step S13, Acquisition of sediment-water interface environmental parameter data: The concentration of dissolved active phosphorus C0 in the pore water at the sediment-water interface and the concentration of dissolved active phosphorus in the overlying water were determined by molybdenum-antimony spectrophotometry. The thickness δ of the diffusion boundary layer was measured using a SediMeterSM4 sediment kinematics instrument. Step S14, lake morphology data acquisition: measure the lake water depth h using a depth sounder.

[0019] Furthermore, in step S2, a wave-current coupling model for calculating the intensity of hydrodynamic disturbance is constructed, and the bottom shear stress is calculated, specifically as follows: Step S21, Wavelength Calculation: Based on the wave period T obtained directly from step S11 and the known lake depth h, the wavelength L is calculated using the wave dispersion relation. The following Eckart approximation formula is used for efficient calculation, as shown in formula (1): (1); Where L is the wavelength (m), g is the gravitational acceleration (taken as 9.8 m / s²), 2π represents the radians corresponding to a complete waveform, T is the wave period (s), h is the lake depth (m), and tanh() is the hyperbolic tangent function. This approximate formula avoids iterative calculations and has sufficient engineering accuracy, making it suitable for the shallow lake environment described in this invention.

[0020] Step S22, wave bottom trajectory velocity calculation: Based on the wave height H and wave period T directly monitored in step S11 and the wavelength L calculated in step S21, calculate the maximum trajectory velocity U of the bottom water particles caused by the wave. w As shown in formula (2): (2); Among them, U w Let H be the peak orbital velocity of near-bottom water particles caused by waves (m / s), H be the wave height (m), T be the wave period (s), k be the wave number (rad / m) (k=2π / L, L is the wavelength, 2π represents the radians corresponding to a complete waveform), h be the lake depth (m), and sinh() be the hyperbolic sine function. Step S23, Calculation of bottom shear stress caused by waves: Based on the U calculated in step S22 w Calculate the bottom shear stress τ caused by waves. w As shown in formula (3): (3); Where, τ w U is the bottom shear stress caused by waves (N / m²), ρ is the density of water (taken as 1000 kg / m³), and U is the density of water. w f represents the peak orbital velocity (m / s) of near-bottom water particles caused by waves. w Let be the wave friction coefficient (dimensionless), as shown in formula (4): (4); Among them, A w The half-amplitude (m) of the wave orbital motion. U w denoted as peak orbital velocity (m / s) of near-bottom water particles caused by waves, T as wave period (s), and 2π as the radians corresponding to a complete waveform. N For the bed roughness length (m) , (Median grain size of sediment (m)); Step S24, calculate the friction velocity: based on the flow velocity U directly monitored in step S11. z And the bed roughness length k calculated in step S23 NCalculate the friction speed 𝑢 ∗ As shown in formula (5): (5); Among them, u * U is the friction velocity (m / s), k is the von Kármán constant, taken as 0.4 (dimensionless), and U z The velocity (m / s) measured at height z is the ADCP velocity measurement point above the substrate (m), and k is the velocity measured at height z. N For the bed roughness length (m) , (Median grain size of sediment (m)); Step S25, calculate the bottom shear stress caused by the water flow: based on the calculated friction velocity obtained in step S24. Calculate the bottom shear stress τ caused by water flow. c As shown in formula (6): (6); Where, τ c Let ρ be the bottom shear stress caused by water flow (N / m²), ρ be the density of water (taken as 1000 kg / m³), and u be the density of water. * Friction speed (m / s); Step S26, calculate the combined bottom shear stress under the combined action of waves and current: based on the bottom shear stress τ caused by waves calculated in step S23. w The bottom shear stress τ caused by the water flow calculated in step S25 c Calculate the combined bottom shear stress τ under the combined action of waves and currents. cw As shown in formula (7): (7); Where, τ cw τ is the combined bottom shear stress (N / m²) caused by the combined action of waves and currents. c τ is the bottom shear stress caused by water flow (N / m²). w The bottom shear stress caused by the wave (N / m²) is θ, and the angle (°) between the direction of water flow and the direction of wave propagation is θ (obtained by vector calculation after measuring the bottom water flow direction by ADCP and the wave direction by pressure wave meter). Furthermore, in step S3, the critical conditions for sediment resuspension are determined, and the release potential of different forms of phosphorus is evaluated, specifically as follows: Step S31, Calculate the critical erosion shear stress: Based on the sediment characteristic data obtained in step S1, calculate the critical erosion shear stress τ of the sediment. cr As shown in formula (8): (8); Where, τ cr The critical erosion shear stress (N / m²) is the critical erosion shear stress. ρ is the critical Shields parameter (dimensionless). s ρ is the density of sediment particles (taken as 2650 kg / m³), ρ is the density of water (taken as 1000 kg / m³), g is the acceleration due to gravity (taken as 9.8 m / s²), and d 50 The median grain size (m) of the sediment; Step S311, the critical Shields parameter The results were obtained using the fitting formula provided by Soulsby (1997), as shown in formula (9): (9); Step S312, the dimensionless particle size D * The result is obtained from formula (10): (10); Among them, D * ρ is a dimensionless particle size, ε is the kinematic viscosity of water (m² / s), which can be obtained by interpolation based on measured water temperature, and ρ s ρ is the density of sediment particles (taken as 2650 kg / m³), ρ is the density of water (taken as 1000 kg / m³), g is the acceleration due to gravity (taken as 9.8 m / s²), and d 50 The median grain size (m) of the sediment; Step S32, based on the combined bottom shear stress τ under the combined action of waves and water flow calculated in step S26. cw The critical erosion shear stress τ of the sediment calculated in step S31 cr The resuspension flux E is evaluated as shown in Equation (11): (11); Where E is the resuspension flux (kg / m³) 2 / s), M is the erosion rate parameter (calibrated through indoor erosion experiments, kg / m 2 / s), α is the erosion index parameter (calibrated through indoor erosion experiments, dimensionless), τ cw Combined bottom shear stress (N / m) under the combined action of waves and water flow 2 ), τ cr Critical erosion shear stress of sediment (N / m) 2 ); Step S321, the erosion rate parameter M and the erosion index parameter α are obtained through indoor direct shear experiments.

[0021] Step S33, determine the effective release factor (EF) i Calibration of sediments from specific lakes requires indoor simulated resuspension experiments. The core objective is to quantify the proportion of different phosphorus forms released into the aqueous phase during resuspension events. Specifically: Step S331: Collect sediment columnar samples from the target lake area, transport them back to the laboratory at low temperature and in the dark. After homogenization pretreatment, determine the sediment porosity and the content of various phosphorus forms (C). i It includes six forms: NH4Cl-P, BD-P, NaOH-SRP, NaOH-NRP, HCl-P, and Res-P. In step S332, the pretreated sediment sample and the overlying water (taken from the lake area) filtered through a 0.45 μm filter membrane were used to reconstruct the sediment-water interface system in a reaction bottle. The liquid-solid ratio was controlled at 5:1. A blank control group and a sediment background control group were set up. All reaction bottles were pre-cultured in the dark under constant temperature conditions for 48 hours to allow the sediment-water interface to reach physicochemical equilibrium. Step S333: After the pre-culture is completed, an initial water sample is collected. Then, a calibrated shaking experimental device (such as a magnetic stirrer) is used to apply a constant bottom shear stress to the experimental group that is higher than the critical shear stress of the sediment to simulate a continuous hydrodynamic disturbance process for 6 hours.

[0022] Step S334: After shaking, let stand for 5 minutes to allow coarse particles to settle. Then, collect the upper water sample and filter it through a 0.45μm filter membrane. The concentration of dissolved reactive phosphorus (SRP) in the water sample is determined by the molybdenum antimony spectrophotometric method.

[0023] Step S335: Calculate the effective release factor (EF) using the mass balance formula. i As shown in formula (12): (12); Among them, EF i C is the effective release factor (dimensionless) for the i-th form of phosphorus. sample C sample_t0 The concentrations (μg / L) of soluble reactive phosphorus in the supernatant water before and after shaking in the experimental group are shown in Figure 1. blank C blank_t0 The values ​​represent the SRP concentration (μg / L) at the corresponding time points in the blank control group, V is the volume of water covering the reaction flask (L), and M is the volume of water covering the reaction flask. dry C is the dry weight (g) of the sediment in the reaction flask. i The content of phosphorus of the i-th form in the sediment (μg / g).

[0024] Step S336: The experiment should be performed in parallel at least three times, and the final EF value should be taken. iThe average value was used as the calibration result for this form of phosphorus.

[0025] Step S4: Dynamic Calculation of Endogenous Phosphorus Release Flux: Coupled with the hydrodynamic disturbance intensity calculated in Step S2 and the phosphorus release potential of various sediment forms assessed in Step S3, a dynamic calculation model for endogenous phosphorus release flux is constructed, as follows: Step S41, based on the resuspension flux E calculated in step S32 and the effective release factor EF calculated in step S33. i Calculate the phosphorus release flux F under resuspension. res As shown in formula (13): (13); Among them, F res The phosphorus release flux under buoyancy (mg / m³) 2 / s), E is the resuspension flux (kg / m³) 2 / s), EF i C is the effective release factor (dimensionless) for the i-th form of phosphorus. i denoted as the content (mg / kg) of the i-th form of phosphorus in the sediment, and n is the number of phosphorus forms in the sediment; Step S42, calculate the phosphorus release flux F under diffusion. diff As shown in formula (14): (14); Among them, F diff Phosphorus release flux due to diffusion (mg / m³) 2 / s), φ is the sediment porosity (dimensionless), D s The effective diffusion coefficient of phosphorus in sediment pore water (m 2 / s), C0 is the concentration of dissolved active phosphorus in the pore water at the sediment-water interface (mg / L). δ represents the concentration of dissolved active phosphorus in the overlying water (mg / L), and δ represents the thickness of the diffusion boundary layer (m). Step S421, the effective diffusion coefficient Ds of phosphorus in the pore water of the sediment is calculated according to formula (15), the specific formula is as follows: (15); Among them, D s The effective diffusion coefficient of phosphorus in sediment pore water (m 2 / s), D0 is the molecular diffusion coefficient of phosphate ions in infinitely diluted water (taken as ). φ is the porosity of the sediment (dimensionless); Step S43: Based on the calculation formulas in steps S41 and S42, the comprehensive endogenous phosphorus release flux F is obtained, as shown in formula (16): (16); Where F represents the total endogenous phosphorus release flux, F res The phosphorus release flux under buoyancy (mg / m³) 2 / s), E is the resuspension flux (kg / m³) 2 / s), EF i C is the effective release factor (dimensionless) for the i-th form of phosphorus. i F represents the content (mg / kg) of the i-th form of phosphorus in the sediment. diff Phosphorus release flux due to diffusion (mg / m³) 2 / s), φ is the sediment porosity (dimensionless), D s The effective diffusion coefficient of phosphorus in sediment pore water (m 2 / s), C0 is the concentration of dissolved active phosphorus (mg / L) in the pore water at the sediment-water interface, C ws δ represents the concentration of dissolved active phosphorus in the overlying water (mg / L), and δ represents the thickness of the diffusion boundary layer (m). Step S5: Based on the daily average phosphorus release flux F calculated in Step S4, and combined with the critical phosphorus load threshold L_critical for eutrophication of the water body, a dynamic quantitative assessment and classification of the risk of phosphorus release from the lake's internal sources is performed, as detailed below: Step S51, Determination of critical load threshold: The critical phosphorus load threshold L for eutrophication of the water body critical The phosphorus concentration limit corresponding to the corresponding water quality category in the national "Eutrophication Evaluation Method and Classification Technical Regulations for Lakes (Reservoirs)" was adopted and converted into the allowable phosphorus load threshold by parameters such as the average water depth and hydraulic retention time of the lake. Step S52, Risk Assessment and Classification: The daily average phosphorus release flux F calculated in step S4 is compared with the critical phosphorus load threshold L. critical Based on comparisons, the risk levels of endogenous phosphorus release are classified according to the following criteria: Low risk: F < 0.5 L critical Ecological significance: The endogenous release flux is far below the critical level, the sediments play a dominant role as a phosphorus sink, the water body has sufficient self-purification capacity, and the risk of eutrophication is extremely low.

[0026] Medium risk: 0.5 L critical ≤ F < L critical Ecological significance: The endogenous release flux is close to the critical level, and the phosphorus exchange at the sediment-water interface is in equilibrium. It may turn into a high-risk state when the external input increases or the hydrodynamic conditions change.

[0027] High risk: F ≥ L criticalEcological significance: When the endogenous release flux reaches or exceeds the critical level, the function of sediment as a phosphorus "source" becomes dominant. Even if the external pollution is controlled, the eutrophication state of the water body may still be maintained.

Claims

1. A method for dynamically calculating the risk of endogenous phosphorus release from shallow lakes, characterized in that: Includes the following steps: Step S1: Acquire lake hydrodynamic data, sediment characteristic data, sediment-water interface environmental parameter data, and lake morphology data; Step S2: Based on the lake hydrodynamic data obtained in step S1, construct a wave-current coupling model of hydrodynamic disturbance intensity and calculate the bottom shear stress; Step S3: Based on the sediment characteristic data obtained in Step S1, determine the critical conditions for sediment resuspension and evaluate the phosphorus release potential of different sediment forms. Step S4: Couple the wave-current coupling model of hydrodynamic disturbance intensity constructed in step S2 with the phosphorus release potential of different sediment morphologies assessed in step S3 to construct a dynamic calculation model for endogenous phosphorus release flux and calculate the endogenous phosphorus release flux. Step S5: Based on the endogenous phosphorus release flux calculated in Step S4, and combined with the critical phosphorus load threshold for eutrophication of the water body, assess the endogenous phosphorus release risk level; based on the assessed endogenous phosphorus release risk level, formulate corresponding endogenous pollution control strategies for the lake. Specifically, step S3 includes: Step S31, calculate the critical erosion shear stress of the sediment: as shown in formula (8): （8）； in, The critical erosion shear stress of the sediment. ρ is the critical Shields number parameter. s ρ is the density of sediment particles; ρ is the density of water. Let d be the acceleration due to gravity. 50 The median grain size of the sediment; Critical Shields number parameter The result was obtained by using a fitting formula, as shown in formula (9): （9）； where D * is the dimensionless particle diameter, calculated from equation (10): （10）； Wherein, ε is the kinematic viscosity of water, obtained by interpolation based on measured water temperature; Step S32, based on the combined bottom shear stress caused by the combined action of waves and water flow and critical erosion shear stress of sediments The sediment resuspension flux E is calculated as shown in formula (11): （11）； Where E is the resuspension flux, representing the dry weight of eroded sediment per unit area per unit time, M is the erosion rate parameter, and α is the erosion index parameter; the erosion rate parameter M and the erosion index parameter α were calibrated through indoor direct shear experiments. Step S33: Based on the resuspension flux E calculated in S32, an indoor simulation resuspension experiment is conducted to simulate actual hydrodynamic conditions, namely the critical erosion shear stress of the sediment. The proportion of different forms of phosphorus released from suspended sediments into the aqueous phase was measured, i.e., the effective release factor EF. i ; Specifically, in indoor experiments, shear stress conditions equivalent to those in the field are reproduced. This process causes sediment resuspension. By monitoring changes in the concentration of dissolved active phosphorus in the overlying water, and combining this with the known amount of resuspended sediment and the content of various phosphorus forms, the effective release factor EF was calculated. i .

2. The method for dynamically calculating the risk of endogenous phosphorus release from shallow lakes according to claim 1, characterized in that: Step S1: Obtain lake hydrodynamic data, sediment characteristic data, sediment-water interface environmental parameter data, and lake morphology data; the specific steps are as follows: Step S11, lake hydrodynamic data acquisition: wind speed and direction data are obtained by setting a buoy type meteorological and hydrological observation station on the lake surface; bottom flow velocity U is obtained by an acoustic Doppler current profiler installed at the bottom of the lake z ; wave height H and wave period T are obtained by a pressure wave meter; Step S12, Sediment Characteristic Data Acquisition: A gridded sampling method was used to obtain lakebed sediment samples using a sediment sampler; the lakebed sediment samples were analyzed in the laboratory to obtain the median grain size of the sediments. Phosphorus content in different forms and sediment porosity φ; Step S13, Acquisition of sediment-water interface environmental parameter data: The concentration of dissolved active phosphorus C0 in the pore water at the sediment-water interface and the concentration of dissolved active phosphorus C in the overlying water were determined by molybdenum-antimony spectrophotometry. ws The thickness δ of the diffusion boundary layer was measured using a sediment motion measuring instrument. Step S14, lake morphology data acquisition: measure the lake water depth h using a depth sounder.

3. The method for dynamically calculating the risk of endogenous phosphorus release from shallow lakes according to claim 2, characterized in that: In step S2, a wave-current coupling model of the hydrodynamic disturbance intensity is constructed, and the bottom shear stress is calculated; the specific steps are as follows: Step S21: Calculate the wavelength L using the wave dispersion relation; use the Eckert approximation formula, as shown in formula (1): （1）； Where L is the wavelength, Where is the acceleration due to gravity, T is the wave period, 2π represents the radians corresponding to a complete waveform, tanh() is the hyperbolic tangent function, π represents the radians corresponding to half a waveform, and h is the lake depth. Step S22, Wave Bottom Trajectory Velocity Calculation: Calculate the peak trajectory velocity U of near-bottom water particles caused by the wave. w As shown in formula (2): （2）； Among them, U w Let H be the peak orbital velocity of near-bottom water particles caused by waves, H be the wave height, sinh() be the hyperbolic sine function, and k be the wave number, k=2π / L; Step S23, Calculation of bottom shear stress caused by waves: Calculate the bottom shear stress caused by waves. As shown in formula (3): （3）； in, Let f be the bottom shear stress caused by waves, ρ be the density of water, and f be the density of water. w Let be the wave friction coefficient, as shown in formula (4): （4）； in, Let A represent an exponential function with base e. w This represents the half-amplitude of the wave orbital motion. k N k represents the roughness length of the substrate. N = 2.5*d 50 d 50 The median grain size of the sediment; Step S24, calculate the friction speed: as shown in formula (5): （5）； Among them, u * Let U be the friction velocity, K be the von Kármán constant, taken as 0.4, and U be the friction velocity. z The bottom velocity is the velocity measured at height z, where z is the height of the velocity measurement point of the acoustic Doppler velocity profiler from the bottom bed. Step S25, calculate the bottom shear stress caused by the water flow: as shown in formula (6): （6）； in, This refers to the bottom shear stress caused by the water flow. Step S26, calculate the combined bottom shear stress under the combined action of waves and water flow: as shown in formula (7): （7）； in, The combined bottom shear stress is the combined effect of waves and water flow, and θ is the angle between the direction of water flow and the direction of wave propagation.

4. The method for dynamically calculating the risk of endogenous phosphorus release from shallow lakes according to claim 3, characterized in that: In step S33, the effective release factor is determined by calibrating lake sediments through indoor simulated resuspension experiments. Specifically: Step S331: Collect sediment samples from the target lake and transport them back to the laboratory at low temperature and away from light; After homogenization pretreatment, the porosity of the sediment and the content of six forms of phosphorus were measured. The six forms of phosphorus included exchangeable phosphorus NH4Cl-P, reducible phosphorus BD-P, metal oxide bound phosphorus NaOH-SRP, organic phosphorus NaOH-NRP, calcium bound phosphorus HCl-P, and residual phosphorus Res-P. In step S332, the pretreated sediment sample and the overlying water filtered through a 0.45 μm filter membrane are used to reconstruct the sediment-water interface system in a reaction bottle to form an experimental group. The liquid-solid ratio is controlled at 5:

1. A blank control group and a sediment background control group are set up. All reaction bottles are pre-cultured in the dark under constant temperature conditions for 48 hours to allow the sediment-water interface system to reach physicochemical equilibrium. Step S333: After the pre-cultivation is completed, an initial water sample is collected. Then, a calibrated oscillation test device is used to apply a constant bottom shear stress to the experimental group that is higher than the critical erosion shear stress of the sediment to simulate a continuous hydrodynamic disturbance process for 6 hours. Step S334: After the disturbance is completed, let it stand for 5 minutes to allow the coarse particles to settle. Then, collect the upper water sample of the experimental group and filter it through a 0.45 μm filter membrane. The concentration of dissolved reactive phosphorus in the water sample is determined by the molybdenum antimony spectrophotometric method. Step S335: Calculate the effective release factor using the mass balance formula, as shown in formula (12): （12）； Among them, EF i C is the effective release factor for the i-th form of phosphorus. sample C sample_t0 The concentrations of soluble reactive phosphorus in the supernatant water before and after shaking in the experimental group are C, respectively. blank C blank_t0 The values ​​represent the dissolved reactive phosphorus concentrations in the overlying water at the corresponding time points in the blank control group, respectively. V is the volume of the overlying water in the reaction flask, and M is the volume of the overlying water. dry C is the dry weight of the sediment in the reaction flask. i The content of phosphorus in the i-th form in the sediment; Step S336: The experiment is conducted in parallel three or more times, and the effective release factor EF of the i-th form of phosphorus is finally taken. i The average value was used as the calibration result for different forms of phosphorus.

5. The method for dynamically calculating the risk of endogenous phosphorus release from shallow lakes according to claim 4, characterized in that: In step S4, a dynamic calculation model for endogenous phosphorus release flux is constructed to calculate the endogenous phosphorus release flux. The specific steps are as follows: Step S41, based on the resuspension flux E and the effective release factor EF of the i-th phosphorus form. i The phosphorus release flux under resuspension is calculated as shown in formula (13): （13）； Among them, F res denoted as phosphorus release flux under resuspension, and n is the number of phosphorus species in the sediment. Step S42, calculate the phosphorus release flux under diffusion, as shown in formula (14): （14）； Among them, F diff The phosphorus release flux due to diffusion. D represents the porosity of the sediment. s C0 is the effective diffusion coefficient of phosphorus in sediment pore water, and C0 is the concentration of dissolved active phosphorus in the pore water at the sediment-water interface. ws δ represents the concentration of dissolved active phosphorus in the overlying water, and δ represents the thickness of the diffusion boundary layer. The effective diffusion coefficient D of phosphorus in sediment pore water s The formula is: （15）； Where D0 is the molecular diffusion coefficient of phosphate ions in infinitely diluted water; Step S43, the total endogenous phosphorus release flux F is calculated as shown in formula (16): （16）； Where F represents the total endogenous phosphorus release flux.

6. The method for dynamically calculating the risk of endogenous phosphorus release from shallow lakes according to claim 5, characterized in that: In step S5, based on the endogenous phosphorus release flux calculated in step S4 and combined with the critical phosphorus load threshold for eutrophication of the water body, the risk level of endogenous phosphorus release is assessed; the specific steps are as follows: Step S51, critical phosphorus load threshold L for eutrophication of water body critical Based on the phosphorus concentration limits corresponding to the respective water quality categories, the allowable phosphorus load threshold is calculated using parameters such as the average lake depth and hydraulic retention time. Step S52, Risk Assessment and Classification: Combine the comprehensive endogenous phosphorus release flux F calculated in step S4 with the critical phosphorus load threshold L for eutrophication of the water body. critical Comparisons were made to classify the risk levels of endogenous phosphorus release. Low risk: F < 0.5 L critical ; Medium risk: 0.5 L critical ≤ F < L critical ; High risk: F ≥ L critical .

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