Estimation method of total nitrogen and phosphorus mass in shallow lakes

Abstract

The present invention relates to a method for estimating the total mass of nitrogen and phosphorus in shallow lakes. By collecting vertical samples from the lake, the nitrogen and phosphorus concentrations at different depths are tested, and the total mass of nitrogen and phosphorus in the unit water column is calculated in two layers, an upper and lower layer, and an algorithm for the total mass of nitrogen and phosphorus in the unit water column is established. The spatial distribution of nutrients in the lake is obtained by interpolation, and the total mass of nitrogen and phosphorus in the lake is calculated in combination with the lake bottom topography and water level data. The method of the present invention can be used to calculate the total mass of nitrogen and phosphorus in the lake through historical water quality monitoring data, and simulate the changing trend of the total mass of nitrogen and phosphorus. This method enriches the connotation of lake nutrient status assessment, expands the theoretical framework, and provides a reference for lake water environment management and basin nutrient salt emission control.

Description

Technical Field

[0001] The present invention relates to the field of lake water environment, and in particular to a method for estimating the total mass of nitrogen and phosphorus in shallow lakes. Background Art

[0002] To assess the extent of eutrophication, nutrient concentration surveys have been conducted worldwide for decades (Chen et al., 2019; Huang et al., 2021). Due to the high cost of traditional survey methods, the use of remote sensing technology to determine the spatiotemporal distribution of nutrient concentrations in water bodies has gradually expanded since the 21st century (Chang et al., 2013; Xiong et al., 2022). High-frequency remote sensing observations have shown that the spatial distribution of water quality in shallow lakes can significantly change within a short period of time due to the influence of algal blooms, hydrodynamics, and meteorological factors (Du et al., 2016; Qi et al., 2018; Shi et al., 2017). This uneven and unstable distribution of nutrients leads to a series of problems, including inaccurate lake surveys and ineffective eutrophication control measures. The total mass of lake nutrients reflects the overall nutrient level of a lake and is influenced by the regulatory effects of surface sediments (Xie et al., 2003a; Xu et al., 2017). Furthermore, the longer water exchange cycles in eutrophic lakes increase the retention time of nutrients in the lake (Zhu et al., 2019). Therefore, estimating the total mass of nitrogen and phosphorus in lakes can provide new insights into lake eutrophication management.

[0003] Estimating the total mass of nitrogen and phosphorus in water bodies first requires clarifying their vertical distribution characteristics. Because shallow lakes are susceptible to wind and wave disturbances, which can lead to mixing between upper and lower layers, studies of vertical water quality distribution are generally conducted in the ocean (Wirtz and Smith, 2020) and deep lakes (Liu et al., 2019). Currently, vertical water quality research in shallow lakes primarily focuses on water temperature and phytoplankton (Hu et al., 2021; Yang et al., 2018), with little research on the vertical distribution of nitrogen and phosphorus concentrations. Studies in deep lakes have shown that thermal stratification of lake water temperature significantly influences vertical variations in water quality, such as dissolved oxygen (Zhang et al., 2015), while dissolved oxygen concentration is a key factor influencing phosphorus adsorption and desorption by sediments (Ni et al., 2020). Although shallow lakes are susceptible to wind and wave disturbances, diurnal water temperature stratification can occur under specific weather conditions in spring and summer, lasting no more than 24 hours (Yang et al., 2018). Water temperatures warm most significantly at a depth of 1.5 m (Chen Zheng et al., 2021; Zhang Yuchao et al., 2008). Therefore, it can be assumed that shallow lakes are generally in a mixed state. When stratification occurs, the 1.5 m boundary is used to estimate the total nitrogen and phosphorus mass of the upper and lower layers. Furthermore, the vertical distribution of phytoplankton also significantly influences the vertical distribution of water quality. In particular, the pumping effect during phytoplankton blooms can lead to elevated nutrient concentrations (Xie et al., 2003b). Research on the vertical structure of phytoplankton in lakes is relatively mature (Xue et al., 2015; Xue et al., 2017). This method generally uses surface chlorophyll concentrations to estimate the total chlorophyll content within a unit water column, and then calculates the algal biomass within that unit water column. This method has been applied in shallow lakes such as Chaohu Lake (Li et al., 2017) and Dianchi Lake (Bi et al., 2019), and has been used to determine the temporal and spatial variations in algal biomass. Therefore, the same approach can be used to estimate the total nitrogen and phosphorus mass.

[0004] In summary, research on the total mass of nitrogen and phosphorus in lakes is scarce. Estimating the total mass of nitrogen and phosphorus in shallow lakes is a groundbreaking research topic, and currently there are not many reports to refer to. Estimating the total mass of nutrients per unit area of ​​the water column based on nitrogen and phosphorus concentrations in field-collected water samples, and then estimating the changes in the total mass of nitrogen and phosphorus in lakes based on this, and exploring the trends and mechanisms of change in the total mass of nitrogen and phosphorus in lakes, will not only enrich the scope of lake nutrient status assessment and expand the theoretical framework, but also provide a reference for lake water environment management and watershed nutrient emission control. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for estimating the total mass of nitrogen and phosphorus in shallow lakes, which can use water quality monitoring data to simulate the total mass of nitrogen and phosphorus in lakes and their changing trends.

[0006] In order to achieve the above object, the technical solutions adopted by the present invention are as follows:

[0007] Methods for estimating the total mass of nitrogen and phosphorus in shallow lakes include:

[0008] Collect vertically stratified water samples and mixed layer samples from the lake to measure total nitrogen and total phosphorus concentrations;

[0009] The vertically stratified water quality samples are divided into upper layer samples and lower layer samples using d as the depth dividing line:

[0010] a. For upper layer samples, a model for estimating the total mass of nitrogen and phosphorus in the upper layer was established, using the measured nitrogen and phosphorus concentrations in the mixed layer samples as the independent variable and the measured total mass of nitrogen and phosphorus in the upper layer as the dependent variable;

[0011] b. For the lower layer samples, calculate the rate of change of the concentration at point d and the concentration at the lowest layer. Calculate the measured total mass of nitrogen and phosphorus in the lower layer based on the water level and lake bottom topography on that day. Establish a simulation calculation model for the concentration at point d using the measured total mass of nitrogen and phosphorus in the lower layer as the dependent variable and the measured nitrogen and phosphorus concentrations of the mixed layer samples as the independent variables, thereby obtaining an estimation model for the total mass of nitrogen and phosphorus in the lower layer.

[0012] Obtain the total nitrogen and phosphorus mass estimation model of the sampled water column based on the established upper nitrogen and phosphorus total mass estimation model and the lower nitrogen and phosphorus total mass estimation model;

[0013] According to the measured total nitrogen and total phosphorus concentrations in the mixed layer, the spatial distribution of total nitrogen and total phosphorus concentrations in the whole lake was obtained by interpolation, and the total nitrogen and phosphorus mass estimation model of the unit water column was used to obtain the total nitrogen and phosphorus mass of the lake.

[0014] As a preferred embodiment, when the vertical stratified water quality samples are collected, 8-10 depth levels are divided according to the results of the lake water depth measurement, and water samples are extracted at each depth level. In order to prevent the disturbance of the water surface during the ship's travel, it is necessary to wait until the water surface is stable after the ship stops, and then measure the water depth. According to the results of the water depth measurement, 8-10 depth levels are divided, with 0.1m below the water surface as the first layer and the area close to the bottom of the lake as the lowest layer. The water pump is slowly lowered into the water, and water samples are extracted from top to bottom at each level. The collected water samples are refrigerated and kept away from light and brought back to the laboratory, and the total nitrogen and total phosphorus are determined by the alkaline potassium persulfate high-temperature digestion method.

[0015] As a preferred embodiment, when using a water pump to pump water, tie a rope to the water pump and mark the rope with scales to facilitate recording the pumping depth when sampling. If the water pump is not heavy enough, hang a heavy object (such as a lead fish) under the water pump to ensure that the water pump sinks vertically to the lake surface as much as possible.

[0016] As a preferred embodiment, the mixed layer sample is a sample obtained by mixing water samples from the surface layer, bottom layer, and half the water depth, wherein the surface layer generally refers to 0.2 m below the water surface and the bottom layer refers to 0.2 m above the bottom mud.

[0017] In a preferred embodiment, the depth boundary is determined based on the thermal stratification of the lake. Above the thermal boundary is the upper layer, which is susceptible to disturbances from wind and waves, and the total mass of nitrogen and phosphorus is calculated by integration. The lower layer is more stable, and the rate of change of concentration at the thermal boundary and the concentration in the lowest layer is calculated, taking into account the water level and lake bottom topography to calculate the total mass of nitrogen and phosphorus.

[0018] As a preferred embodiment, the measured total mass of upper nitrogen and phosphorus is obtained by integrating the total nitrogen and total phosphorus concentrations of vertically stratified water quality samples of the lake.

[0019] As a preferred embodiment, in said a, a part of the measured data is selected as a training set, and the rest is a validation set; the nitrogen and phosphorus concentrations of the mixed layer samples in the training set are used as independent variables, and the upper layer total mass estimation algorithm is established respectively using linear, exponential, and power functions, through multiple groups of randomly selected training sets and validation sets, based on R 2 The algorithm performance was evaluated by R and RMSE, and the algorithm with the highest accuracy was selected as the final estimation model for the total nitrogen and phosphorus mass in the upper layer of the unit water column. Preferably, 75% of the samples were randomly selected for training and 25% for validation. Through 5 sets of randomly selected training sets and validation sets, the R 2 The algorithm performance was evaluated by RMSE and RMSE, and the algorithm with the highest accuracy was selected as the final algorithm for estimating the total mass of nitrogen and phosphorus in the upper layer of the unit water column. The total mass of nitrogen and phosphorus in the lower layer of the unit water column was directly derived from the water level and lake bottom topography. Similarly, 75% of the samples were randomly selected for training and 25% for validation. Through 5 sets of randomly selected training and validation sets, the total mass of nitrogen and phosphorus in the lower layer of the unit water column was directly derived from the water level and lake bottom topography. 2 The performance of the algorithm was evaluated using RMSE.

[0020] As a preferred embodiment, in said b, the total mass of nitrogen and phosphorus in the lower layer is calculated based on the following formula:

[0021]

[0022]

[0023] Where, TP mass2 TN mass2 are the total phosphorus and total nitrogen masses in the lower layer respectively; A is the area of ​​the water column; TP d TN d are the total phosphorus and total nitrogen concentrations at water depth d, respectively, simulated using mixed layer samples; H is the water depth; λ TP ,λ TNThe values ​​are the total phosphorus and total nitrogen concentrations at point d and the rate of change of the total phosphorus and total nitrogen concentrations in the lowest layer, respectively, calculated using the sampling data. The water column can be calculated as a unit water column, that is, the water column per unit area (1 square meter).

[0024] As a preferred embodiment, kriging interpolation is used to obtain the spatial distribution of total nitrogen and total phosphorus concentrations in the entire lake. Preferably, the kriging interpolation method is ordinary kriging, the model type is stable, and to reduce the amount of calculation, the output is raster data with a resolution of 100m. This operation is completed in ArcGIS 10.4.

[0025] As a preferred implementation method, it also includes using historical water quality monitoring data to simulate the total mass of nitrogen and phosphorus in lakes over a long period of time.

[0026] As a preferred embodiment, the method for estimating the total mass of nitrogen and phosphorus in shallow lakes is applied to areas with a water depth of ≥1.5 m. Areas with a water depth of less than 1.5 m are mainly distributed along the lakeshore, have a large amount of aquatic vegetation, and are affected by human activities on land, so they are not considered.

[0027] This method collects vertical lake samples, measures nitrogen and phosphorus concentrations at different depths, and calculates the total mass of nitrogen and phosphorus within a unit water column in two layers. It then establishes an algorithm for calculating the total mass of nitrogen and phosphorus within a unit water column. This algorithm uses interpolation to determine the spatial distribution of lake nutrients, and combines lake bottom topography and water level data to calculate the total mass of nitrogen and phosphorus in the lake. This method enriches the connotations of lake nutrient status assessment, expands the theoretical framework, and provides a reference for lake water environment management and watershed nutrient emission control.

[0028] As can be seen from the technical solutions of the present invention, the present invention provides a method for estimating the total mass of nitrogen and phosphorus in a lake using the nitrogen and phosphorus concentrations of mixed layer samples. This method can be based on historical water quality monitoring data or remote sensing inversion nitrogen and phosphorus data, combined with water level and lake bottom topography, to simulate the spatiotemporal distribution of the total mass of nitrogen and phosphorus in a lake. Long-term monitoring of the total mass of nitrogen and phosphorus in a lake helps to scientifically assess the nutrient status of the lake, accurately describe changes in the ecological environment of the lake water body, and provide technical support for lake eutrophication monitoring and water environment protection and management.

[0029] It should be understood that all combinations of the foregoing concepts and the additional concepts described in more detail below, as long as such concepts are not mutually inconsistent, can be considered part of the inventive subject matter of this disclosure. In addition, all combinations of the claimed subject matter are considered part of the inventive subject matter of this disclosure.

[0030] The foregoing and other aspects, embodiments, and features of the present invention will be more fully understood from the following description in conjunction with the accompanying drawings. Other additional aspects of the present invention, such as features and / or beneficial effects of the exemplary embodiments, will become apparent from the following description or through practice of specific embodiments according to the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The accompanying drawings are not intended to be drawn to scale. In the drawings, each identical or nearly identical component shown in various figures may be represented by the same reference numeral. For the sake of clarity, not every component is labeled in every figure. Embodiments of various aspects of the present invention will now be described by way of example and with reference to the accompanying drawings, in which:

[0032] Figure 1 This is a flow chart for estimating the total mass of nitrogen and phosphorus.

[0033] Figure 2 This is a map of Taihu Lake sampling points and water quality monitoring points.

[0034] Figure 3 It is the water depth below the average water level of Taihu Lake.

[0035] Figure 4 It is a vertical stratified sampling device.

[0036] Figure 5 Vertical changes in nitrogen and phosphorus concentrations in Taihu Lake: (a) total nitrogen; (b) total phosphorus.

[0037] Figure 6 Estimation accuracy of the total mass of nitrogen and phosphorus in a unit water column: (a) Estimation accuracy of the total mass of nitrogen in the upper layer; (b) Estimation accuracy of the total mass of phosphorus in the upper layer; (c) Estimation accuracy of the total mass of nitrogen in the lower layer; (d) Estimation accuracy of the total mass of phosphorus in the lower layer.

[0038] Figure 7 Figure 3. Spatial distribution of total nitrogen and phosphorus mass in Lake Taihu from 2006 to 2015: (a) total nitrogen mass; (b) total phosphorus mass.

[0039] Figure 8 Statistical results of the total mass of nitrogen and phosphorus in Taihu Lake from 2006 to 2015: (a) total mass of nitrogen; (b) total mass of phosphorus.

[0040] In the aforementioned Figures 1-8, the coordinates, symbols or other expressions expressed in English are all well known in the art and will not be described in detail in this example. DETAILED DESCRIPTION

[0041] In order to better understand the technical content of the present invention, specific embodiments are given and described below with reference to the accompanying drawings.

[0042] Various aspects of the present invention are described in this disclosure with reference to the accompanying drawings, in which a number of illustrative embodiments are shown. The embodiments of the present disclosure are not necessarily intended to include all aspects of the present invention. It should be understood that the various concepts and embodiments introduced above, as well as those described in more detail below, can be implemented in any of many ways, and that the concepts and embodiments disclosed herein are not limited to any implementation. In addition, some aspects of the present disclosure may be used alone or in any appropriate combination with other aspects of the present disclosure.

[0043] Example 1

[0044] This embodiment takes Taihu Lake, a typical shallow lake, as an example to further describe the technical solution of the present invention.

[0045] The method measures lake depth, divides the lake into vertical layers, collects water samples, and tests nitrogen and phosphorus concentrations at different depths to calculate the total nitrogen and phosphorus mass of a unit water column. The unit water column is divided into an upper and lower layer at a depth of 1.5 meters. The nitrogen and phosphorus concentrations in the mixed layer are used to estimate the total nitrogen and phosphorus mass of the upper and lower layers, respectively. This method establishes an algorithm for the total nitrogen and phosphorus mass of a unit water column. Kriging interpolation is used to determine the spatial distribution of nitrogen and phosphorus concentrations in the lake, and combined with lake bottom topography and water level data, the total nitrogen and phosphorus mass of the lake is calculated. Finally, the algorithm is applied to historical water quality monitoring data from Taihu Lake to simulate the spatiotemporal distribution of total nitrogen and phosphorus mass in Taihu Lake from 2006 to 2015.

[0046] As an exemplary description, the implementation of the above method is described in detail below with reference to the accompanying drawings.

[0047] Step 1: Collect vertically stratified water samples from the lake and measure the concentrations of total nitrogen (TN) and total phosphorus (TP);

[0048] The sampling points of Taihu Lake are as follows: Figure 2 As shown in the figure, vertical sample collection experiments were conducted in November 2020 and November 2021, and 232 and 208 vertical samples, 29 and 26 mixed samples were collected respectively. When collecting vertical samples, in order to prevent the disturbance of the water surface during the boat's movement, it is necessary to wait until the boat stops and the water surface is stable. First, use a rope to hang a lead fish and place it on the bottom of the lake. Mark the position of the rope on the water surface, and measure the distance between the lead fish and the mark, which is the water depth; the average water level of Taihu Lake is 3.2m, and the average water depth is about 2m ( Figure 3 ), so it can be divided into 8 levels of 0.1, 0.2, 0.3, 0.5, 0.7, 1.0, 1.5 and 2m, and the pump is slowly lowered into the water to extract water samples from each level from top to bottom. Figure 4 As shown, a rope is tied to the water pump, and the rope is marked with scales to facilitate recording the pumping depth during sampling. The water pump used in this embodiment weighs 3 kg. When sampling, the water surface is calm and there are no waves, and the water pump can sink vertically to the lake surface.

[0049] When collecting vertical stratified samples, mixed layer samples are collected at the same time. Using the same method, water samples are collected at 0.2m below the water surface (surface layer), 0.2m above the bottom mud (bottom layer), and half the water depth (middle layer). After mixing in equal proportions, they are mixed layer samples.

[0050] The collected water samples were refrigerated and protected from light and brought back to the laboratory for determination of total nitrogen and total phosphorus using the alkaline potassium persulfate high-temperature digestion method.

[0051] Step 2: Analyze the vertical distribution characteristics of nitrogen and phosphorus concentrations, calculate the total mass of nitrogen and phosphorus in the unit water column, and establish an algorithm for estimating the total mass of nitrogen and phosphorus in the unit water column using the total nitrogen and total phosphorus concentrations of the mixed layer sample;

[0052] The vertical distribution characteristics of nitrogen and phosphorus concentrations are as follows: Figure 5 As shown, TN and TP concentrations decrease slowly with depth, with the TN concentration decreasing more significantly. The ratio of surface TN concentration to bottom TN concentration is 1.158, and the ratio of surface TP concentration to bottom TP concentration is 1.058. It is important to emphasize that the ratios vary across layers, with the ratio increasing toward the surface. For example, the ratio of TN concentration at 10 cm to 50 cm is 1.068, and the ratio of TN concentration at 150 cm to 200 cm is 1.026. The ratio of TP concentration at 10 cm to 50 cm is 1.034, and the ratio of TP concentration at 150 cm to 200 cm is 0.98.

[0053] The total mass of nitrogen and phosphorus is calculated with 1.5m as the dividing line and divided into two parts, upper and lower parts, to establish the total mass of nitrogen and phosphorus estimation model:

[0054] 1) The upper layer is susceptible to wind and wave disturbances, and the total mass of nitrogen and phosphorus in the upper layer is calculated by integration. Then, the nitrogen and phosphorus concentrations of the mixed layer samples are used as independent variables, and the total mass of nitrogen and phosphorus in the upper layer is used as the dependent variable. Linear, exponential, and power functions are used to establish upper layer total mass estimation models. 75% of the samples are randomly selected for training and 25% of the samples are used for validation. Through 5 sets of randomly selected training and validation sets, the R 2 The performance of the algorithm was evaluated by RMSE. The results showed that the linear algorithm was the optimal algorithm. The verification accuracy of the total mass of nitrogen and phosphorus in the upper layer reached 0.96 and 0.97 respectively. Figure 6 );

[0055] 2) The lower layer is relatively stable. The concentration change rate was calculated based on the concentrations at 1.5 m and 2.0 m. The total nitrogen and phosphorus mass in the measured lower layer was calculated using the concentration at 1.5 m, the concentration change rate, the water level on that day, and the lake bottom topography of Taihu Lake. The water level data were obtained from the monthly water regime report published by the Taihu Basin Administration of the Ministry of Water Resources (http: / / www.tba.gov.cn / slbthlyglj / sj / sj.html).

[0056] The total mass of nitrogen and phosphorus in the lower layer was used as the dependent variable and the concentration of mixed samples was used as the independent variable to fit the nitrogen and phosphorus concentration at a depth of 1.5 m. The total mass of nitrogen and phosphorus in the lower layer was estimated by combining the above-mentioned change rate, water level and topography data. 75% of the samples were randomly selected for training and 25% for validation. Five sets of randomly selected training and validation sets were used to estimate the total mass of nitrogen and phosphorus in the lower layer. 2 The algorithm performance was evaluated by RMSE and the results showed that the verification accuracy of total nitrogen mass and total phosphorus mass reached 0.91 and 0.93 respectively ( Figure 6 ).

[0057] In summary, the nitrogen and phosphorus concentrations of mixed layer samples can accurately obtain the total mass of nitrogen and phosphorus in the water body. The algorithm formula is:

[0058] TN mass1 =A×(1.468×TN M +0.0395)

[0059] TP mass1 =A×(1.3382×TP s +0.0111)

[0060]

[0061] TN 150 =1.0111×TN M -0.0792

[0062]

[0063] TP 150 =1.0793×TP M -0.0119

[0064] TN mass =TN mass1 +TN mass2

[0065] TP mass =TP mass1 +TP mass2

[0066] Where, TN mass1 and TP mass1Represents the total mass of TN and TP in the upper water column (0-150cm depth water column) per unit area (mg / m 2 ); A represents the area of ​​the water column. The unit area of ​​the water column in this study is 1m 2 ; M represents the mixed concentration (mg / L); TN mass2 and TP mass2 Represents the total mass of nitrogen and phosphorus per unit area of ​​the lower water column (water column below 150 cm in depth) (mg / m 2 );TN 150 and TP 150 They represent the total nitrogen and total phosphorus concentrations at 150 cm, which can be calculated from the mixed concentration; H represents the depth (cm), λ TN and λ TP TN represents the concentration change rate of total nitrogen and total phosphorus below 150 cm (mg / cm), which is -0.0007 and 0.00004, respectively, calculated based on the sampling data; M TP M Respectively represent the concentrations of TN and TP in the mixed layer; TN mass TP mass Respectively represent the total mass of TN and TP in the unit water column.

[0067] Step 3: Use Kriging interpolation to interpolate the mixed layer samples to obtain the spatial distribution of total nitrogen and total phosphorus concentrations in the entire lake. Combined with the total nitrogen and phosphorus mass estimation algorithm, the total nitrogen and phosphorus mass of the lake is obtained. Historical water quality monitoring data are used to simulate the total nitrogen and phosphorus mass of the lake over a long period of time.

[0068] The kriging interpolation method is ordinary kriging, and the model type is stable. To reduce the amount of calculation, the output result is raster data with a resolution of 100m. This operation is completed in ArcGIS 10.4.

[0069] The calculation area of ​​total nitrogen and phosphorus mass in the lake does not include areas with a water depth of less than 1.5m. The areas in Taihu Lake where the water level is always below 1.5m are mainly distributed along the shore of East Taihu Lake ( Figure 5 ), has abundant aquatic vegetation, and is affected by human activities on land.

[0070] According to the Taihu Lake water quality monitoring data from 2006 to 2015 provided by the Taihu Lake Ecosystem Research Station, the monitoring points are as follows: Figure 2 As shown, the monitoring period is February, May, August and November each year. Using the algorithm of this embodiment, based on the interpolation results, combined with the water level and lake bottom topography during the monitoring period, the spatial distribution of the total nitrogen and phosphorus mass of Taihu Lake is obtained ( Figure 7 ) and statistical results ( Figure 8). The results show that the multi-year average total nitrogen mass of Taihu Lake is 11,727 tons, showing a slow downward trend before 2010 and gradually stabilizing after 2010. The maximum total nitrogen mass in the year mostly occurs in May, and the minimum value mostly occurs in November. The multi-year average total phosphorus mass is 512 tons, showing a slow downward trend before 2010 and a slow upward trend after 2010. The maximum total phosphorus mass in the year mostly occurs in August, and the minimum value mostly occurs in February and May. Generally speaking, the changing trends of the upper and lower layer total masses are basically the same, and the upper layer total mass is higher than the lower layer total mass. However, in August 2009, the changing trends of the upper and lower layer total masses were inconsistent, and the lower layer total mass exceeded the upper layer total mass. This was because an extreme precipitation event occurred in the Yangtze River Delta region at that time, and the water level of Taihu Lake reached 4.2 meters, exceeding the multi-year average water level of 3.26 meters by 29%, far higher than the water levels in other sampling periods. It is worth noting that although the water volume of Taihu Lake has increased significantly, the total mass of nitrogen and phosphorus is not significantly higher than the total mass of nitrogen and phosphorus in that year, indicating that the rising water level will not significantly change the total mass of nitrogen and phosphorus, and the total mass can be maintained at a certain level in a short period of time.

[0071] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Those skilled in the art will appreciate that various modifications and variations can be made without departing from the spirit and scope of the present invention.

Claims

1. A method for estimating the total mass of nitrogen and phosphorus in shallow lakes, characterized in that: include: Collect vertically stratified water samples and mixed layer samples from the lake to measure total nitrogen and total phosphorus concentrations; The thermal dividing line is determined based on the thermal stratification of the lake. The water depth d where the thermal dividing line is located is used as the depth dividing line to divide the vertically stratified water quality samples into upper layer samples and lower layer samples: a. For the upper layer samples, the measured nitrogen and phosphorus concentrations of the mixed layer samples were used as the independent variable and the measured total nitrogen and phosphorus mass of the upper layer was used as the dependent variable. The following method was used to establish the total nitrogen and phosphorus mass estimation model for the upper layer: A part of the measured data was selected as the training set, and the rest was the validation set. The nitrogen and phosphorus concentrations of the mixed layer samples in the training set were used as independent variables, and the linear, exponential, and power functions were used to establish the upper layer nitrogen and phosphorus total mass estimation algorithms. Through multiple groups of randomly selected training sets and validation sets, the R 2 The algorithm performance was evaluated by RMSE and RMSE, and the algorithm with the highest accuracy was selected as the final estimation model for the total nitrogen and phosphorus mass in the upper layer of the unit water column; b. For the lower layer samples, calculate the rate of change of the concentration at point d and the concentration at the lowest layer. Combined with the water level and lake bottom topography on that day, calculate the measured total mass of nitrogen and phosphorus in the lower layer. Establish a simulation calculation model for the concentration at point d using the concentration at point d as the dependent variable and the measured nitrogen and phosphorus concentrations of the mixed layer samples as the independent variables. This yields an estimation model for the total mass of nitrogen and phosphorus in the lower layer as follows: ； ； Where, 、 are the total phosphorus and total nitrogen masses in the lower layer, respectively; is the area of ​​the water column; 、 are the total phosphorus and total nitrogen concentrations at water depth d, respectively, which are simulated using mixed layer samples; H is the water depth; 、 are the change rates of total phosphorus and total nitrogen concentrations at point d and the total phosphorus and total nitrogen concentrations at the bottom layer, respectively, which are calculated using the sampling data; Obtain the total nitrogen and phosphorus mass estimation model of the sampled water column based on the established upper nitrogen and phosphorus total mass estimation model and the lower nitrogen and phosphorus total mass estimation model; Based on the measured total nitrogen and total phosphorus concentrations in the mixed layer, the spatial distribution of total nitrogen and total phosphorus concentrations in the entire lake was obtained by interpolation, and the total nitrogen and phosphorus mass estimation model of the sampled water column was used to obtain the total nitrogen and phosphorus mass of the lake.

2. The method according to claim 1, characterized in that When collecting the vertically stratified water quality samples, the lake is divided into 8-10 depth layers according to the results of the lake water depth measurement, and water samples are extracted at each depth layer.

3. The method according to claim 1, characterized in that The mixed layer sample is a sample obtained by mixing water samples from the surface layer, bottom layer and half the water depth.

4. The method according to claim 1, wherein The measured total mass of upper nitrogen and phosphorus is obtained by integrating the total nitrogen and total phosphorus concentrations of vertically stratified water quality samples of the lake.

5. The method according to claim 1, characterized in that The spatial distribution of total nitrogen and total phosphorus concentrations in the entire lake was obtained using Kriging interpolation.

6. The method according to claim 1, characterized in that It also includes the use of historical water quality monitoring data to simulate the total nitrogen and phosphorus mass of lakes over long periods of time.

7. The method according to claim 1, characterized in that The method for estimating the total mass of nitrogen and phosphorus in shallow lakes is applicable to areas with a water depth of ≥1.5m.

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