Method for reducing nitrogen and phosphorus loss of slope cropland

Abstract

The invention discloses a method for reducing nitrogen and phosphorus loss of a slope cropland. The method comprises the following steps: step 1, determining a water collecting area of the slope cropland and estimating a nitrogen and phosphorus loss load of an SWAT model; 2, designing a drainage ditch, a rain and sewage diversion device and a diversion pipe according to a model estimation result; 3, designing a vegetation detention pond according to the runoff volume of the slope cropland; the vegetation detention pond effectively reduces the loss of nitrogen and phosphorus in the slope cropland through the absorption of plants, the filtration of sediments and the retention of water flow. According to the method disclosed by the invention, by realizing split-flow treatment of high-concentration nitrogen and phosphorus runoff in the early stage of light rain and heavy rain, the nitrogen reduction rate in the drainage of the slope cropland reaches 70-80%, and the phosphorus reduction rate reaches 80-90%.

Description

Technical Field

[0001] The invention belongs to the field of environmental protection and agricultural environment, and specifically relates to a method for reducing nitrogen and phosphorus loss in sloping cultivated land. Background Art

[0002] Sloping farmland is an important land type in agricultural production. Its topographic conditions make nutrients such as nitrogen and phosphorus in the soil vulnerable to surface runoff caused by rainfall, resulting in significant loss. The loss of nitrogen and phosphorus from sloping farmland is significantly correlated with rainfall. Under the conditions of light rain or early heavy rain, the soil has a long contact time with water, resulting in a high concentration of nitrogen and phosphorus in the surface runoff, which requires purification measures to treat; while under the conditions of mid-to-late heavy rain, the concentration of nitrogen and phosphorus in the surface runoff is low and no reduction treatment is required. At present, bioretention facilities are the key measures to reduce the discharge of nitrogen and phosphorus wastewater from farmland, with a nitrogen removal rate of 30%-70% and a phosphorus removal rate of 60%-90%. However, the existing bioretention facility technology has certain shortcomings in the application of sloping farmland. In terms of construction, since sloping farmland is susceptible to soil erosion and heavy precipitation, the construction of bioretention facilities on sloping farmland needs to consider complex terrain, hydrological conditions and soil characteristics, and requires regular maintenance. In terms of rainwater and sewage separation, traditional facilities such as interception ditches and sedimentation tanks usually receive all farmland drainage. When the drainage concentration is low, it still enters the bioretention facility, resulting in the system's inefficient operation and limiting its ability to treat low-concentration nitrogen and phosphorus drainage. Overall, there are two problems with traditional bioretention facility technology in practical applications: 1) the engineering workload is large, which is not conducive to promotion and application; 2) it is difficult to achieve automatic separation and treatment of drainage with low nitrogen and phosphorus concentrations. Summary of the invention

[0003] The technical problem to be solved by the present invention is to provide a method for reducing nitrogen and phosphorus loss in sloping farmland in view of the deficiencies in the prior art.

[0004] The technical solution of the present invention is as follows:

[0005] A method for reducing nitrogen and phosphorus loss in sloping farmland comprises the following steps:

[0006] Step 1: Determination of the catchment area of ​​sloping farmland and estimation of nitrogen and phosphorus loss loads using the SWAT model;

[0007] Step 2: Based on the model estimation results, design drainage ditches, rainwater and sewage diversion devices and diversion pipes; the drainage ditches are used to lay out rainwater and sewage diversion devices and diversion pipes, and receive drainage with low nitrogen and phosphorus loss concentrations from sloping farmland, such as in the middle and late stages of heavy rain; the rainwater and sewage diversion devices are used to separate drainage with high nitrogen and phosphorus loss concentrations from sloping farmland into diversion pipes, such as in light rain or the early stages of heavy rain; the diversion pipes are used to lead the diverted water to the vegetation retention pond;

[0008] Step 3: Design vegetation retention ponds based on the runoff volume of sloping farmland. Vegetation retention ponds can effectively reduce the loss of nitrogen and phosphorus from sloping farmland through plant absorption, sediment filtration and water retention.

[0009] The method, the step 1 comprises the following steps:

[0010] Step 1.1 Prepare the database required for the SWAT model of sloping farmland;

[0011] Step 1.2: Division of water catchment area of ​​sloping farmland;

[0012] Step 1.3 SWAT model estimates nitrogen and phosphorus loss loads from sloping farmland;

[0013] Step 1.4 Determine the pollution threshold of nitrogen and phosphorus loss from sloping farmland due to rainfall;

[0014] The method described, step 1.1 comprises the following steps:

[0015] 1) DEM data is used to generate topographic characteristics of sloping farmland, including drainage network, slope and catchment area;

[0016] 2) Soil data are used to characterize the physical and chemical properties of soil to simulate the migration and transformation of water and nitrogen and phosphorus pollutants in the soil, including soil layer thickness, soil organic matter content, soil texture (sand, silt, clay content), permeability, and soil water holding capacity;

[0017] 3) Land use data are used to determine surface runoff, evapotranspiration, and nitrogen and phosphorus pollutant loads;

[0018] 4) Meteorological data are used to drive the model’s hydrological and nitrogen and phosphorus pollutant process simulations, including daily precipitation, maximum and minimum temperatures, wind speed, relative humidity, and solar radiation;

[0019] 5) Agricultural management data are used to simulate the impact of different management measures on hydrological processes, soil loss and nitrogen and phosphorus pollutant migration on sloping farmland, including crop planting patterns and rotation cycles, irrigation management, fertilizer and pesticide application, and sloping farmland farming methods.

[0020] The method described, step 1.2 comprises the following steps:

[0021] Step 1) DEM data input and processing: DEM data is pre-processed to fill low-lying areas in the terrain to ensure the accuracy of slope and watershed;

[0022] Step 2) Drainage outlet point selection: When the drainage outlet point is automatically selected, the SWAT model automatically identifies the drainage outlet within the basin based on the slope and flow direction algorithm of the basin; however, in some special terrain or regional conditions, the drainage outlet point needs to be manually selected to ensure accurate drainage path and basin division;

[0023] Step 3) Adjustment of drainage outlet point: In the actual application of sloping farmland, after the SWAT model selects the outlet point, the selection of drainage outlet point will be affected by farmland layout, irrigation channels and land use patterns, and needs to be appropriately adjusted in the model;

[0024] Step 4) Calculation of water flow accumulation: The SWAT model uses the water flow accumulation algorithm of DEM data to calculate the water flow path and water volume distribution of each grid; by calculating the flow direction and confluence path of each cell in the area, the water flow direction of the sloping farmland is simulated;

[0025] Step 5) Delineation of the catchment area: After determining the drainage path, the SWAT model will delineate the catchment area based on the flow direction and slope data;

[0026] Step 6) Setting the catchment threshold; the catchment threshold refers to the minimum area of ​​water accumulation in each grid unit. Only when this area is reached will it be considered as a valid catchment area. By adjusting the catchment threshold, the division of the catchment area is optimized and the impact of too small catchment areas on model accuracy is reduced;

[0027] Step 7) Adjust the density of the watershed area. The density of the watershed area directly affects the number of hydrological response units in the area. When the density is high, the model can simulate the water flow process more finely, otherwise it loses sensitivity to local hydrological characteristics. In the adjustment process, it is necessary to make reasonable settings based on the actual terrain and farmland water management characteristics.

[0028] Step 8) Finally, determine the location of the catchment area based on the zoned water outlet.

[0029] The method described, step 1.3 comprises the following steps:

[0030] Step 1) Division of hydrological response units; Hydrological response unit (HRU) is the basic unit for pollution load estimation. The hydrological characteristics of each HRU are defined by combining factors such as crop planting type, soil type and slope.

[0031] Step 2) Data meteorological data input;

[0032] Step 3) Input of agricultural management data;

[0033] Step 4) SWAT model operation, calibration verification, and data reading;

[0034] Step 5) Read the nitrogen and phosphorus loss load data of sloping farmland, FLOW_OUT is the drainage data, TOT_N is the total nitrogen data, and TOT_P is the total phosphorus data.

[0035] The method described, step 1.4 comprises the following steps:

[0036] Step 1) Simulate the drainage volume and nitrogen and phosphorus loss concentration of sloping farmland under rainfall events; use the SWAT model to simulate the changes in drainage volume and nitrogen and phosphorus loss concentration of sloping farmland under different rainfall scenarios, and analyze the relationship between rainfall intensity and drainage volume and nitrogen and phosphorus loss concentration;

[0037] Step 2) determining the correlation between rainfall and nitrogen and phosphorus loss concentration on sloping farmland; constructing an empirical formula for the relationship between rainfall and nitrogen and phosphorus loss concentration based on regression analysis, and determining the rainfall and drainage volume corresponding to 80% nitrogen and phosphorus load pollutant concentration;

[0038] Step 3) Determine the emission standards for nitrogen and phosphorus loss pollution: According to the relevant farmland drainage pollution control standards, determine the limit values ​​of nitrogen and phosphorus loss concentrations, such as total nitrogen shall not exceed 15 mg / L and total phosphorus shall not exceed 0.3 mg / L;

[0039] Step 4) Determine the threshold value of nitrogen and phosphorus loss pollution caused by rainfall; according to the standard concentration of nitrogen and phosphorus loss in sloping farmland, regression analysis is performed on the corresponding rainfall to determine the drainage time corresponding to the rainfall and the threshold value of nitrogen and phosphorus loss pollution caused by rainfall in sloping farmland.

[0040] The method described, the step 2 comprises the following steps:

[0041] Step 2.1 Drainage ditch layout:

[0042] Step 1) Laying out interception ditches on the cross slopes to receive drainage from sloping farmland, with a slope ratio greater than 1.2;

[0043] Step 2) Border drainage ditches are laid to intercept external water from sloping farmland, with a slope ratio greater than 1.2;

[0044] Step 3) The main drainage ditch is laid out to receive water from the cross slope interception ditch and the boundary drainage ditch, with a slope ratio greater than 1.2; the width of the main drainage ditch is determined according to the drainage corresponding to the maximum rainfall on the sloping farmland simulated by the SWAT model in step 1.4, and the maximum drainage is multiplied by a safety factor, which is generally 1.1 to 1.5;

[0045] Step 2.2 Design of rainwater and sewage diversion device:

[0046] Step 1) Rainwater and sewage diversion devices are arranged at the outlets of the cross slope interception ditch and the boundary drainage ditch, including an upper diversion port and a lower diversion port; the rear part of the lower diversion port is connected to a diversion pipe; an intermediate partition is arranged between the upper diversion port and the lower diversion port, and the intermediate partition and the baffle are connected by two double-link mechanisms; the opening angle of the baffle stays at any position, and the stay angle is automatically adjusted according to the drainage volume; when the nitrogen and phosphorus concentrations in the drainage are low, the water flow is large, the baffle is placed in a fully closed state, and the water flow does not enter the vegetation retention tank; when maintenance or equipment is idle, the baffle is placed in a fully open state; wherein the acceptable water flow impact force of the spring set in the double-link mechanism is determined based on the SWAT model in step 1.4 to calculate the 80% nitrogen and phosphorus load pollutant concentration; when the water flow impact force is above 80% nitrogen and phosphorus load pollutant concentration, the drainage enters the diversion pipe from the lower diversion port; when it is below 80%, the drainage enters the main drainage ditch from the upper diversion port;

[0047] Step 2) The height and opening of the diversion pipe of the main drainage ditch are determined based on the height of the guardrail; when light rain causes the drainage ditch to discharge small amounts but high nitrogen and phosphorus concentrations, the water enters the diversion pipe and is discharged into the vegetation retention pond; when heavy rain causes the drainage ditch to discharge large amounts but low nitrogen and phosphorus concentrations, the water enters the main drainage ditch and is discharged into the next-level drainage ditch or river.

[0048] The method described, the step 3 comprises the following steps:

[0049] Step 3.1 Selection of location for vegetation retention pond: The location is determined based on the drainage collection area of ​​the sloping farmland partition in step 1;

[0050] Step 3.2 Layout size of vegetation retention pond: Design the size of the vegetation retention pond based on the drainage time and nitrogen and phosphorus loss pollution threshold of the sloping farmland catchment area in step 1.4. The depth should be greater than 1.7 meters and the area should be 1-2% of the sloping farmland partition.

[0051] The method, the vegetation retention pond comprises:

[0052] Step 1) Fiber material layer, thickness 2-3 cm, made of plant fiber cloth, natural fiber mat; function: intercepting larger particles, reducing the impact of water flow, preliminary filtration; reduction rate: nitrogen 0%, phosphorus 0%;

[0053] Step 2) Organic covering layer, thickness 4-6 cm, made of fresh straw, rice straw, and shredded bark; Function: create and protect the growth environment of microorganisms, intercept some heavy metals and organic pollutants; Reduction rate: 6-8% for nitrogen, 6-8% for phosphorus;

[0054] Step 3) biological filter material layer, thickness 4-6 cm, materials are activated carbon, natural minerals, bentonite; function: absorb nitrogen and phosphorus pollutants in drainage, provide attachment surface for microorganisms, and promote microbial degradation of pollutants; reduction rate: nitrogen 8-10%, phosphorus 8-10%;

[0055] Step 4) Soil layer, thickness 80-120 cm, made of a mixture of sand, compost and loam, with gramineous plants, legumes, shrubs and wetland plants as the planting crops; function: further absorb nitrogen and phosphorus pollutants, provide physical support and prevent soil erosion; reduction rate: 38-40% for nitrogen and 44-46% for phosphorus;

[0056] Step 5) coarse sand layer, thickness 40-60cm, made of sand with a particle size less than 5mm; function: prevent planting soil from entering the gravel layer, and filter and purify the upper layer of water; reduction rate: nitrogen 10-12%, phosphorus 12-14%;

[0057] Step 6) Gravel layer, thickness is 40-60cm, made of 5-10mm gravel; function: supporting layer, collecting water and temporarily storing part of rainwater; reduction rate: nitrogen 8-10%, phosphorus 10-12%;

[0058] Step 3.4 Drainage system layout:

[0059] Step 1) Drainage pipes are laid out at the lowest point of the vegetation retention pond to ensure that the water flowing out of the upper filter material flows into the river or drainage ditch;

[0060] Step 2) Slope design, usually a slope of 2-3% is enough to ensure that water flows toward the drainage pipe and avoid water accumulation;

[0061] Step 3) Anti-blocking design: Drainage pipes and water collection wells should be designed with anti-blocking systems to avoid poor drainage due to the accumulation of sediments and pollutants;

[0062] Step 4) Inspection port design: Set up inspection ports in drainage pipes or sump pits.

[0063] In summary, compared with the prior art, the present invention has the following beneficial effects:

[0064] 1. Effectively reduce nitrogen and phosphorus loss. The present invention estimates the nitrogen and phosphorus loss load of sloping farmland based on the SWAT model, and proposes the design of a rainwater and sewage diversion device in combination with the relationship between rainfall intensity and nitrogen and phosphorus concentration. By realizing the diversion treatment of high-concentration nitrogen and phosphorus runoff in light rain and the early stage of heavy rain, the nitrogen reduction rate in sloping farmland drainage reaches 70-80%, and the phosphorus reduction rate reaches 80-90%. Among them, fiber material layer, organic cover layer, biological filter material layer, soil layer, coarse sand layer, and gravel layer.

[0065] 2. Unpowered rainwater and sewage diversion. The design of the rainwater and sewage diversion device fully considers the dynamic changes in the nitrogen and phosphorus concentrations of runoff under rainfall conditions. It can automatically separate high-concentration and low-concentration runoff according to rainfall intensity without human intervention, solving the problem that traditional facilities are difficult to achieve automatic separation and treatment, and reducing infrastructure requirements and long-term maintenance costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0066] Figure 1 It is the overall layout diagram of the present invention;

[0067] Among them: 10 boundary drainage ditch, 11 rainwater and sewage diversion device, 12 cross slope interception ditch, 13 rainwater and sewage diversion pipe, 14 main drainage ditch, 15 vegetation retention pond;

[0068] Figure 2 This is a schematic diagram of the design of the rainwater and sewage diversion device and the upper and lower water diversion structure of the waterway of the present invention;

[0069] Figure 3 It is a schematic diagram of the double-link mechanism of the water-controlled self-closing function of the baffle of the present invention (the position of the baffle in the working state);

[0070] Among them: a middle partition plate 1 of the upper and lower waterways, a double-link mechanism 2, a double-link mechanism booster spring 3, and a baffle 4;

[0071] Figure 4 Schematic diagram of the double-link mechanism of the baffle water control self-closing function of the present invention (the position of the baffle in the fully closed state);

[0072] Figure 5 Schematic diagram of the double-link mechanism of the baffle water control self-closing function of the present invention (the position of the baffle in the fully open state);

[0073] Figure 6 The vegetation retention pond structure of the present invention;

[0074] Among them: 151 fibrous material layer, 152 organic cover layer, 153 biological filter material layer, 154 soil layer, 155 coarse sand layer, 156 gravel layer; DETAILED DESCRIPTION

[0075] The present invention is described in detail below in conjunction with specific embodiments.

[0076] A method for reducing nitrogen and phosphorus loss in sloping farmland comprises the following steps:

[0077] Step 1: Determination of the catchment area of ​​sloping farmland and estimation of nitrogen and phosphorus loss loads using the Soil & Water Assessment Tool (SWAT) model

[0078] 1.1 Prepare the database required for the SWAT model of sloping farmland:

[0079] 1) DEM data is used to generate topographic characteristics of sloping farmland, including drainage network, slope and catchment area;

[0080] 2) Soil data are used to characterize the physical and chemical properties of soil to simulate the migration and transformation of water and nitrogen and phosphorus pollutants in the soil, including soil layer thickness, soil organic matter content, soil texture (sand, silt, clay content), permeability, and soil water holding capacity;

[0081] 3) Land use data are used to determine surface runoff, evapotranspiration, and nitrogen and phosphorus pollutant loads;

[0082] 4) Meteorological data are used to drive the model’s hydrological and nitrogen and phosphorus pollutant process simulations, including precipitation (daily), temperature (maximum and minimum), wind speed, relative humidity, and solar radiation;

[0083] 5) Agricultural management data are used to simulate the effects of different management measures (such as irrigation, fertilization, tillage, etc.) on hydrological processes, soil loss and migration of nitrogen and phosphorus pollutants in sloping farmland, including crop planting patterns and rotation cycles, irrigation management (time, frequency and water volume), fertilizer and pesticide application (type, amount and time), and sloping farmland tillage methods.

[0084] 1.2 Division of watershed for sloping farmland:

[0085] 1) DEM data input and processing. DEM data is pre-processed to fill low-lying areas in the terrain to ensure the accuracy of slopes and watersheds;

[0086] 2) Drainage outlet point selection. When the drainage outlet point is automatically selected, the SWAT model automatically identifies the drainage outlet within the basin based on the slope and flow direction algorithm of the basin. However, under some special terrain or regional conditions, the drainage outlet point needs to be manually selected to ensure accurate drainage path and basin division;

[0087] 3) Adjustment of drainage outlet points. In the actual application of sloping farmland, after the SWAT model selects the outlet point, the selection of drainage outlet points will be affected by farmland layout, irrigation channels and land use methods, and appropriate adjustments need to be made in the model;

[0088] 4) Calculation of water flow accumulation. The SWAT model uses the water flow accumulation algorithm of DEM data to calculate the water flow path and water volume distribution of each grid. By calculating the flow direction and confluence path of each cell in the area, the water flow direction of sloping farmland is simulated;

[0089] 5) Delineation of the catchment area. After determining the drainage path, the SWAT model will delineate the catchment area based on the flow direction and slope data;

[0090] 6) Setting the catchment threshold. The catchment threshold refers to the minimum area of ​​water accumulation in each grid unit. Only when this area is reached will it be considered as a valid catchment area. By adjusting the catchment threshold, the division of the catchment area can be optimized and the impact of too small catchment areas on model accuracy can be reduced;

[0091] 7) Adjustment of watershed density. The density of the watershed directly affects the number of hydrological response units in the area. When the density is high, the model can simulate the water flow process more finely, otherwise it loses sensitivity to local hydrological characteristics. In the process of adjustment, it is necessary to make reasonable settings in combination with the actual terrain and the characteristics of farmland water management;

[0092] 8) Finally, determine the location of the catchment area based on the zoned outlet.

[0093] 1.3 SWAT model estimates nitrogen and phosphorus loss loads from sloping farmland:

[0094] 1) Division of hydrological response units. The hydrological response unit (HRU) is the basic unit for pollution load estimation. The hydrological characteristics of each HRU are defined by combining factors such as crop planting type, soil type and slope.

[0095] 2) Data meteorological data input;

[0096] 3) Agricultural management data input;

[0097] 4) SWAT model operation, calibration verification, and data reading. The calibration verification parameters and initial ranges are shown in Table 1;

[0098] 5) Read the nitrogen and phosphorus loss load data of sloping farmland, FLOW_OUT is the drainage data, TOT_N is the total nitrogen data, and TOT_P is the total phosphorus data.

[0099] 1.4 Determination of the pollution threshold of nitrogen and phosphorus loss from sloping farmland due to rainfall:

[0100] 1) Simulate the drainage volume and nitrogen and phosphorus loss concentration of sloping farmland under rainfall events. Use the SWAT model to simulate the changes in drainage volume and nitrogen and phosphorus loss concentration of sloping farmland under different rainfall scenarios, and analyze the relationship between rainfall intensity and drainage volume and nitrogen and phosphorus loss concentration;

[0101] 2) Determine the correlation between rainfall and nitrogen and phosphorus loss concentration on sloping farmland. Construct an empirical formula for the relationship between rainfall and nitrogen and phosphorus loss concentration based on regression analysis (linear or nonlinear), and determine the rainfall and drainage volume corresponding to 80% nitrogen and phosphorus load pollutant concentration;

[0102] 3) Determination of nitrogen and phosphorus loss pollution emission standards. According to relevant farmland drainage pollution control standards, the limit values ​​of nitrogen and phosphorus loss concentrations are determined, such as total nitrogen shall not exceed 15 mg / L and total phosphorus shall not exceed 0.3 mg / L;

[0103] 4) Determination of the nitrogen and phosphorus loss pollution threshold caused by rainfall. Based on the standard concentration of nitrogen and phosphorus loss in sloping farmland, regression analysis of the corresponding rainfall was performed to determine the drainage time corresponding to the rainfall and the threshold of nitrogen and phosphorus loss pollution caused by rainfall in sloping farmland.

[0104] Step 2: Based on the model estimation results, design drainage ditches, rainwater and sewage diversion devices and diversion pipes. The drainage ditches are used to lay out rainwater and sewage diversion devices and diversion pipes, and receive drainage with low nitrogen and phosphorus loss concentrations from sloping farmland, such as in the middle and late stages of heavy rain; the rainwater and sewage diversion devices are used to separate drainage with high nitrogen and phosphorus loss concentrations from sloping farmland into diversion pipes, such as in light rain or the early stages of heavy rain; the diversion pipes are used to lead the diverted water to the vegetation retention pond.

[0105] 2.1 Drainage ditch layout:

[0106] 1) The cross-slope interception ditch is laid to receive drainage from sloping farmland, with a slope ratio greater than 1.2;

[0107] 2) Border drainage ditches are laid out to intercept external water from sloping farmland, with a slope ratio greater than 1.2;

[0108] 3) The main drainage ditch is laid out to receive water from the cross slope interception ditch and the boundary drainage ditch, with a slope ratio greater than 1.2. The width of the main drainage ditch is determined based on the corresponding drainage volume when the maximum rainfall occurs on the sloping farmland as simulated by the SWAT model in step 1.4. The maximum drainage volume is multiplied by a safety factor, which is generally 1.1 to 1.5, to prevent overflow of the drainage ditch due to extreme rainfall. The overall layout is shown in Figure 1 .

[0109] 2.2 Design of rainwater and sewage diversion device:

[0110] 1) Rainwater and sewage diversion devices are arranged at the outlets of the cross slope interception ditch and the boundary drainage ditch, including upper and lower diversion ports, which can be made of stainless steel and set at the end of the canal. The cross-sectional form and size can be flexibly set according to local conditions as needed ( Figure 2 ). The rear water channel can be made of concrete structure or other forms, and the rear of the lower water outlet is connected to the diversion pipe. The two double-link mechanisms are fixed below the partition 1 in the middle of the upper and lower waterways and the upper and rear parts of the baffle 4. Under normal circumstances, they are not exposed to the water flow, which can prevent the entanglement of debris. The partition 1 in the middle of the upper and lower waterways can be made of concrete or metal. The double-link mechanism 2 is made of stainless steel. The double-link mechanism booster spring 3 is protected by a retractable metal sleeve to prevent debris from being caught in the middle of the spring and affecting the reliability of the mechanism. The baffle 4 can be made of metal or plastic. The overall dimensions of the mechanism and the matching springs can be adjusted and configured according to the actual needs of the waterway. Figure 3The baffle shown in the figure is the working position. The mechanism can keep the baffle open at any position. The retention angle is automatically adjusted according to the drainage volume. When the nitrogen and phosphorus concentrations in the drainage are low, the water flow is large and does not enter the vegetation retention pond. The baffle is placed in a fully closed state ( Figure 4 ); When maintenance or equipment is idle, the baffle is placed in the fully open state ( Figure 5 ). The acceptable water flow impact force of the spring is determined based on the 80% nitrogen and phosphorus load pollutant concentration calculated by the SWAT model in step 1.4. When the water flow impact force is above 80% nitrogen and phosphorus load pollutant concentration, the drainage enters the diversion pipe from the lower diversion port; when it is below 80%, the drainage enters the main drainage ditch from the upper diversion port;

[0111] 2) The height and opening of the diversion pipe of the main drainage ditch are determined based on the height of the guardrail. When light rain causes the drainage ditch to discharge small amounts but high nitrogen and phosphorus concentrations, the water enters the diversion pipe and is discharged into the vegetation retention pond; when heavy rain causes the drainage ditch to discharge large amounts but low nitrogen and phosphorus concentrations, the water enters the main drainage ditch and is discharged into the next level drainage ditch or river.

[0112] Step 3: Design a vegetation retention pond based on the runoff of the sloping farmland. The vegetation retention pond effectively reduces the loss of nitrogen and phosphorus from the sloping farmland through plant absorption, sediment filtration and water retention.

[0113] 3.1 Selection of location for vegetation retention pond: The location is determined based on the drainage collection area of ​​sloping farmland in step 1. The layout principle is not to affect agricultural operations, and it is generally built on the edge of the farmland.

[0114] 3.2 Layout size of vegetation retention pond: Design the size of the vegetation retention pond based on the drainage time and nitrogen and phosphorus loss pollution threshold of the sloping farmland catchment area in step 1.4. The depth should be greater than 1.7 meters and the area should be 1-2% of the sloping farmland partition.

[0115] 3.3 Layout of vegetation retention pond treatment system ( Figure 6 ):

[0116] 1) Fiber material layer, thickness 2-3cm, made of plant fiber cloth, natural fiber mat. Function: intercept larger particles, reduce the impact of water flow, and perform preliminary filtration. Reduction rate: nitrogen 0%, phosphorus 0%;

[0117] 2) Organic covering layer, thickness 4-6cm, made of fresh straw, rice straw, and shredded bark. Function: Create and protect the growth environment of microorganisms, intercept some heavy metals and organic pollutants. Reduction rate: 6-8% for nitrogen, 6-8% for phosphorus;

[0118] 3) Biological filter layer, thickness 4-6cm, made of activated carbon, natural minerals, bentonite. Function: absorb nitrogen and phosphorus pollutants in drainage, provide attachment surface for microorganisms, and promote microbial degradation of pollutants. Reduction rate: 8-10% for nitrogen, 8-10% for phosphorus;

[0119] 4) Soil layer, 80-120cm thick, made of a mixture of sand, compost and loam, planted with grasses (alfalfa and foxtail grass), legumes (red clover and alfalfa), shrubs (lilac and acacia), and wetland plants (reed and cattail). Function: further absorb nitrogen and phosphorus pollutants, provide physical support, and prevent soil erosion. Reduction rate: 38-40% for nitrogen and 44-46% for phosphorus;

[0120] 5) Coarse sand layer, thickness 40-60cm, made of sand with particle size less than 5mm. Function: prevent planting soil from entering the gravel layer, and filter and purify the upper layer of water. Reduction rate: nitrogen 10-12%, phosphorus 12-14%;

[0121] 6) Gravel layer, thickness is 40-60cm, made of 5-10mm gravel. Function: supporting layer, collecting water and temporarily storing part of rainwater. Reduction rate: 8-10% for nitrogen, 10-12% for phosphorus.

[0122] 3.4 Drainage system layout:

[0123] 1) Drainage pipe layout, located at the lowest point of the vegetation retention pond, to ensure that the water flowing out of the upper filter material flows into the river or drainage ditch;

[0124] 2) Slope design. Usually a slope of 2-3% is enough to ensure that water flows toward the drainage pipe and avoid water accumulation;

[0125] 3) Anti-blocking design: Drainage pipes and water collection wells should be designed with anti-blocking systems to avoid poor drainage due to the accumulation of sediments and pollutants;

[0126] 4) Inspection port design: Inspection ports should be set up in drainage pipes or sump pits to facilitate daily maintenance and cleaning.

[0127] According to the simulation results of the nitrogen and phosphorus loss load estimation model, the patent of this invention designs a rainwater and sewage diversion device and a vegetation retention pond, which effectively reduces the loss of nitrogen and phosphorus pollutants in surface runoff from sloping farmland under light rain or pre-heavy rain conditions. This method can be used to reduce nitrogen and phosphorus loss from sloping farmland.

[0128] Table 1 SWAT model calibration verification parameters and initial range

[0129]

[0130]

[0131] It should be understood that those skilled in the art can make improvements or changes based on the above description, and all these improvements and changes should fall within the scope of protection of the appended claims of the present invention.

Claims

1. A method for reducing nitrogen and phosphorus loss from sloping farmland, characterized in that: The following steps are involved: Step 1: Determination of the catchment area of ​​sloping farmland and estimation of nitrogen and phosphorus loss loads using the SWAT model; Step 2: Based on the model estimation results, design drainage ditches, rainwater and sewage diversion devices and diversion pipes; the drainage ditches are used to lay out rainwater and sewage diversion devices and diversion pipes, and receive drainage with low nitrogen and phosphorus loss concentrations from sloping farmland, such as in the middle and late stages of heavy rain; the rainwater and sewage diversion devices are used to separate drainage with high nitrogen and phosphorus loss concentrations from sloping farmland into diversion pipes, such as in light rain or the early stages of heavy rain; the diversion pipes are used to lead the diverted water to the vegetation retention pond; Step 3: Design vegetation retention ponds based on the runoff volume of sloping farmland. Vegetation retention ponds can effectively reduce the loss of nitrogen and phosphorus from sloping farmland through plant absorption, sediment filtration and water retention.

2. The method according to claim 1, characterized in that The step 1 comprises the following steps: Step 1.1 Prepare the database required for the SWAT model of sloping farmland; Step 1.2: Division of water catchment area of ​​sloping farmland; Step 1.3 SWAT model estimates nitrogen and phosphorus loss loads from sloping farmland; Step 1.4 Determine the pollution threshold of nitrogen and phosphorus loss from sloping farmland due to rainfall.

3. The method according to claim 2, characterized in that The step 1.1 comprises the following steps: 1) DEM data is used to generate topographic characteristics of sloping farmland, including drainage network, slope and catchment area; 2) Soil data are used to characterize the physical and chemical properties of soil to simulate the migration and transformation of water and nitrogen and phosphorus pollutants in the soil, including soil layer thickness, soil organic matter content, soil texture (sand, silt, clay content), permeability, and soil water holding capacity; 3) Land use data are used to determine surface runoff, evapotranspiration, and nitrogen and phosphorus pollutant loads; 4) Meteorological data are used to drive the model’s hydrological and nitrogen and phosphorus pollutant process simulations, including daily precipitation, maximum and minimum temperatures, wind speed, relative humidity, and solar radiation; 5) Agricultural management data are used to simulate the impact of different management measures on hydrological processes, soil loss and nitrogen and phosphorus pollutant migration on sloping farmland, including crop planting patterns and rotation cycles, irrigation management, fertilizer and pesticide application, and sloping farmland farming methods.

4. The method according to claim 2, characterized in that: The step 1.2 comprises the following steps: Step 1) DEM data input and processing: DEM data is pre-processed to fill low-lying areas in the terrain to ensure the accuracy of slope and watershed; Step 2) Drainage outlet point selection: When the drainage outlet point is automatically selected, the SWAT model automatically identifies the drainage outlet within the basin based on the slope and flow direction algorithm of the basin; however, in some special terrain or regional conditions, the drainage outlet point needs to be manually selected to ensure accurate drainage path and basin division; Step 3) Adjustment of drainage outlet point: In the actual application of sloping farmland, after the SWAT model selects the outlet point, the selection of drainage outlet point will be affected by farmland layout, irrigation channels and land use patterns, and needs to be appropriately adjusted in the model; Step 4) Calculation of water flow accumulation: The SWAT model uses the water flow accumulation algorithm of DEM data to calculate the water flow path and water volume distribution of each grid; by calculating the flow direction and confluence path of each cell in the area, the water flow direction of the sloping farmland is simulated; Step 5) Delineation of the catchment area: After determining the drainage path, the SWAT model will delineate the catchment area based on the flow direction and slope data; Step 6) Setting the catchment threshold; the catchment threshold refers to the minimum area of ​​water accumulation in each grid unit. Only when this area is reached will it be considered as a valid catchment area. By adjusting the catchment threshold, the division of the catchment area is optimized and the impact of too small catchment areas on model accuracy is reduced; Step 7) Adjust the density of the watershed area. The density of the watershed area directly affects the number of hydrological response units in the area. When the density is high, the model can simulate the water flow process more finely, otherwise it loses sensitivity to local hydrological characteristics. In the adjustment process, it is necessary to make reasonable settings based on the actual terrain and farmland water management characteristics. Step 8) Finally, determine the location of the catchment area based on the zoned water outlet.

5. The method according to claim 2, characterized in that: The step 1.3 comprises the following steps: Step 1) Division of hydrological response units; the hydrological response unit HRU is the basic unit for pollution load estimation. The hydrological characteristics of each hydrological response unit are defined by combining factors such as crop planting type, soil type and slope; Step 2) Data meteorological data input; Step 3) Input of agricultural management data; Step 4) SWAT model operation, calibration verification, and data reading; Step 5) Read the nitrogen and phosphorus loss load data of sloping farmland, FLOW_OUT is the drainage data, TOT_N is the total nitrogen data, and TOT_P is the total phosphorus data.

6. The method according to claim 2, characterized in that The step 1.4 comprises the following steps: Step 1) Simulate the drainage volume and nitrogen and phosphorus loss concentration of sloping farmland under rainfall events; use the SWAT model to simulate the changes in drainage volume and nitrogen and phosphorus loss concentration of sloping farmland under different rainfall scenarios, and analyze the relationship between rainfall intensity and drainage volume and nitrogen and phosphorus loss concentration; Step 2) determining the correlation between rainfall and nitrogen and phosphorus loss concentration on sloping farmland; constructing an empirical formula for the relationship between rainfall and nitrogen and phosphorus loss concentration based on regression analysis, and determining the rainfall and drainage volume corresponding to 80% nitrogen and phosphorus load pollutant concentration; Step 3) Determine the emission standards for nitrogen and phosphorus loss pollution: According to the relevant farmland drainage pollution control standards, determine the limit values ​​of nitrogen and phosphorus loss concentrations, such as total nitrogen shall not exceed 15 mg / L and total phosphorus shall not exceed 0.3 mg / L; Step 4) Determine the threshold value of nitrogen and phosphorus loss pollution caused by rainfall; according to the standard concentration of nitrogen and phosphorus loss in sloping farmland, regression analysis is performed on the corresponding rainfall to determine the drainage time corresponding to the rainfall and the threshold value of nitrogen and phosphorus loss pollution caused by rainfall in sloping farmland.

7. The method according to claim 6, characterized in that The step 2 comprises the following steps: Step 2.1 Drainage ditch layout: Step 1) Laying out interception ditches on the cross slopes to receive drainage from sloping farmland, with a slope ratio greater than 1.2; Step 2) Border drainage ditches are laid to intercept external water from sloping farmland, with a slope ratio greater than 1.2; Step 3) The main drainage ditch is laid out to receive water from the cross slope interception ditch and the boundary drainage ditch, with a slope ratio greater than 1.2; the width of the main drainage ditch is determined according to the drainage volume corresponding to the maximum rainfall on the sloping farmland simulated by the SWAT model, and the maximum drainage volume is multiplied by a safety factor of 1.1 to 1.5; Step 2.2 Design of rainwater and sewage diversion device: Step 1) Rainwater and sewage diversion devices are arranged at the outlets of the cross slope interception ditch and the boundary drainage ditch, including an upper diversion port and a lower diversion port; the rear part of the lower diversion port is connected to a diversion pipe; an intermediate partition is arranged between the upper diversion port and the lower diversion port, and the intermediate partition and the baffle are connected by two double-link mechanisms; the opening angle of the baffle stays at any position, and the stay angle is automatically adjusted according to the drainage volume; when the nitrogen and phosphorus concentrations in the drainage are low, the water flow is large, the baffle is placed in a fully closed state, and the water flow does not enter the vegetation retention tank; when maintenance or equipment is idle, the baffle is placed in a fully open state; wherein the acceptable water flow impact force of the spring arranged in the double-link mechanism is determined based on the SWAT model to calculate 80% of the nitrogen and phosphorus load pollutant concentration; when the water flow impact force is above 80% of the nitrogen and phosphorus load pollutant concentration, the drainage enters the diversion pipe from the lower diversion port; when it is below 80%, the drainage enters the main drainage ditch from the upper diversion port; Step 2) The height and opening of the diversion pipe of the main drainage ditch are determined based on the height of the guardrail; when light rain causes the drainage ditch to discharge small amounts but high nitrogen and phosphorus concentrations, the water enters the diversion pipe and is discharged into the vegetation retention pond; when heavy rain causes the drainage ditch to discharge large amounts but low nitrogen and phosphorus concentrations, the water enters the main drainage ditch and is discharged into the next-level drainage ditch or river.

8. The method according to claim 6, characterized in that The step 3 comprises the following steps: Step 3.1 Selection of location for vegetation retention pond: The location is determined based on the drainage collection area of ​​the sloping farmland partition in step 1; Step 3.2 Layout size of vegetation retention pond: Design the size of the vegetation retention pond based on the drainage time of the sloping farmland catchment area and the nitrogen and phosphorus loss pollution threshold. The depth should be greater than 1.7 meters and the area should be 1-2% of the sloping farmland partition.

9. The method according to claim 8, characterized in that The vegetation retention pond comprises: Step 1) Fiber material layer, thickness 2-3 cm, made of plant fiber cloth, natural fiber mat; function: intercepting larger particles, reducing the impact of water flow, preliminary filtration; reduction rate: nitrogen 0%, phosphorus 0%; Step 2) Organic covering layer, thickness 4-6 cm, made of fresh straw, rice straw, and shredded bark; Function: create and protect the growth environment of microorganisms, intercept some heavy metals and organic pollutants; Reduction rate: 6-8% for nitrogen, 6-8% for phosphorus; Step 3) biological filter material layer, thickness 4-6 cm, materials are activated carbon, natural minerals, bentonite; function: absorb nitrogen and phosphorus pollutants in drainage, provide attachment surface for microorganisms, and promote microbial degradation of pollutants; reduction rate: nitrogen 8-10%, phosphorus 8-10%; Step 4) Soil layer, thickness 80-120 cm, made of a mixture of sand, compost and loam, with gramineous plants, legumes, shrubs and wetland plants as the planting crops; function: further absorb nitrogen and phosphorus pollutants, provide physical support and prevent soil erosion; reduction rate: 38-40% for nitrogen and 44-46% for phosphorus; Step 5) coarse sand layer, thickness 40-60cm, made of sand with a particle size less than 5mm; function: prevent planting soil from entering the gravel layer, and filter and purify the upper layer of water; reduction rate: nitrogen 10-12%, phosphorus 12-14%; Step 6) Gravel layer, thickness is 40-60cm, made of 5-10mm gravel; function: supporting layer, collecting water and temporarily storing part of rainwater; reduction rate: nitrogen 8-10%, phosphorus 10-12%; Step 3.4 Drainage system layout: Step 1) Drainage pipes are laid out at the lowest point of the vegetation retention pond to ensure that the water flowing out of the upper filter material flows into the river or drainage ditch; Step 2) Slope design, usually a slope of 2-3% is enough to ensure that water flows toward the drainage pipe and avoid water accumulation; Step 3) Anti-blocking design: Drainage pipes and water collection wells should be designed with anti-blocking systems to avoid poor drainage due to the accumulation of sediments and pollutants; Step 4) Inspection port design: Set up inspection ports in drainage pipes or sump pits.

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