A method for reducing nitrogen and phosphorus loss from sloping farmland

Abstract

The application discloses a method for reducing nitrogen and phosphorus loss of slope farmland, which comprises the following steps: step 1, determining a slope farmland catchment area and estimating nitrogen and phosphorus loss load of a SWAT model; step 2, designing a drainage ditch, a rain and sewage diversion device and a diversion pipe according to the estimation result of the model; and step 3, designing a vegetation retention pool according to the runoff of the slope farmland; the vegetation retention pool effectively reduces the loss of nitrogen and phosphorus of the slope farmland through the absorption of plants, the filtration of sediments and the retention effect of water flow. The method can realize diversion treatment of small rain and high-concentration nitrogen and phosphorus runoff in the early stage of heavy rain, and the nitrogen reduction rate of the slope farmland drainage reaches 70-80%, and the phosphorus reduction rate reaches 80-90%.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of environmental protection and agricultural environment, and particularly relates to a method for reducing nitrogen and phosphorus loss of slope farmland. BACKGROUND

[0002] Slope farmland is an important land type in agricultural production. The topographic conditions of slope farmland make the soil nutrients such as nitrogen and phosphorus susceptible to surface runoff caused by rainfall, resulting in significant loss. The nitrogen and phosphorus loss of slope farmland is significantly correlated with rainfall. Under the condition of light rain or early stage of heavy rain, the contact time between soil and water is longer, resulting in higher concentration of nitrogen and phosphorus in surface runoff, which needs to be treated by purification measures; while under the condition of late stage of heavy rain, the concentration of nitrogen and phosphorus in surface runoff is lower, and no reduction treatment is needed. At present, the biological retention facility is a key measure to reduce nitrogen and phosphorus wastewater discharge from farmland, and the removal rate of nitrogen can reach 30%-70%, and the removal rate of phosphorus is 60%-90%. However, the existing biological retention facility technology has some disadvantages in the application of slope farmland. In construction, due to the influence of water and soil loss and heavy rainfall, the construction of biological retention facility on slope farmland needs to consider the complex topography, hydrological conditions and soil characteristics, and needs regular maintenance. In rainwater and sewage diversion, traditional facilities such as intercepting ditch and sedimentation tank usually receive all farmland drainage, and when the concentration of drainage is low, the system cannot run efficiently, which limits its treatment capacity for low-concentration nitrogen and phosphorus drainage. Overall, there are two problems in the practical application of traditional biological retention facility technology: 1) large amount of engineering, which is not conducive to popularization and application; 2) difficult to realize automatic separation and treatment of low-concentration nitrogen and phosphorus drainage. SUMMARY

[0003] The technical problem to be solved by the present application is to provide a method for reducing nitrogen and phosphorus loss of slope farmland.

[0004] The technical scheme of the present application is as follows:

[0005] A method for reducing nitrogen and phosphorus loss of slope farmland, comprising the following steps:

[0006] Step 1: determination of slope farmland catchment area and estimation of nitrogen and phosphorus loss load by SWAT model;

[0007] Step 2: according to the model estimation results, design drainage ditch, rainwater and sewage diversion device and diversion pipe; wherein the drainage ditch is used to arrange the rainwater and sewage diversion device and diversion pipe, and receives the drainage with low concentration of nitrogen and phosphorus loss of slope farmland, such as late stage of heavy rain; the rainwater and sewage diversion device is used to separate the drainage with high concentration of nitrogen and phosphorus loss of slope farmland into the diversion pipe, such as light rain or early stage of heavy rain; the diversion pipe is used to guide the separated water to the vegetation retention pool;

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

[0009] The method, the first step 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 the catchment area of sloping farmland;

[0012] Step 1.3 Estimation of nitrogen and phosphorus loss load of sloping farmland by SWAT model;

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

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

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

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

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

[0018] 4) Meteorological data is used to drive the model of hydrological and nitrogen and phosphorus pollutant process simulation, including daily precipitation, maximum temperature and minimum temperature, wind speed, relative humidity, solar radiation;

[0019] 5) Farm management data is used to simulate the influence of different management measures on the hydrological process, soil loss and nitrogen and phosphorus pollutant migration of sloping farmland, including crop planting pattern and rotation period, irrigation management, fertilizer and pesticide application, and slope farmland tillage method.

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

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

[0022] Step 2) Selection of drainage outlet point; when automatically selecting the drainage outlet point, the SWAT model automatically identifies the drainage outlet within the watershed based on the slope and flow direction algorithm; however, in some special topographic or regional conditions, the drainage outlet point needs to be manually selected to ensure accurate drainage path and watershed division;

[0023] Step 3) Adjustment of drainage outlet point; in the actual application of slope farmland, the selection of drainage outlet point will be affected by farmland layout, irrigation channels and land use patterns after the SWAT model selects the outlet point, and appropriate adjustment needs to be made 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 distribution of each grid; by calculating the flow direction and confluence path of each cell in the region, the water flow direction of slope farmland is simulated;

[0025] Step 5) Division of catchment area; after determining the drainage path, the SWAT model will divide the catchment area according to the water flow direction and slope data;

[0026] Step 6) Setting of catchment threshold; the catchment threshold refers to the minimum area of water flow accumulation of each grid cell, and only when the area reaches the threshold will it be considered as an effective catchment area; by adjusting the catchment threshold, the division of catchment area is optimized, and the influence of small catchment area on model accuracy is reduced;

[0027] Step 7) Adjustment of catchment area density; the density of catchment area directly affects the number of hydrological response units in the region; when the density is high, the model can simulate the water flow process more finely, otherwise it will lose sensitivity to local hydrological characteristics; during the adjustment process, reasonable setting needs to be made in combination with actual topography and farmland water management characteristics;

[0028] Step 8) Finally, determine the catchment area position according to the partition outlet.

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

[0030] Step 1) Division of hydrological response unit; hydrological response unit (HRU) is the basic unit for pollution load estimation, which defines the hydrological characteristics of each hydrological response unit by combining crop planting types, soil types and slope, etc;

[0031] Step 2) Input of meteorological data;

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

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

[0034] Step 5) read the data of nitrogen and phosphorus loss load of slope 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, the step 1.4 comprises the following steps:

[0036] Step 1) simulate the drainage amount and nitrogen and phosphorus loss concentration of slope farmland in a rainfall event; the SWAT model is used to simulate the change of the drainage amount and nitrogen and phosphorus loss concentration of slope farmland under different rainfall amount scenarios, and the relationship between rainfall intensity and the drainage amount and nitrogen and phosphorus loss concentration is analyzed;

[0037] Step 2) determine the correlation between rainfall amount and nitrogen and phosphorus loss concentration; based on regression analysis, an empirical formula of the relationship between rainfall amount and nitrogen and phosphorus loss concentration is constructed, and the rainfall amount and drainage amount corresponding to 80% of the nitrogen and phosphorus load pollutant concentration are determined;

[0038] Step 3) determine the nitrogen and phosphorus loss pollution emission standard; according to the relevant farmland drainage pollution control standard, the limit value of the nitrogen and phosphorus loss concentration is determined, for example, the total nitrogen should not exceed 15 mg / L, and the total phosphorus should not exceed 0.3 mg / L;

[0039] Step 4) determine the rainfall-induced nitrogen and phosphorus loss pollution threshold; according to the standard concentration of nitrogen and phosphorus loss of slope farmland, the corresponding rainfall amount is determined by regression analysis, and the drainage time corresponding to the rainfall amount and the threshold of the rainfall amount leading to the nitrogen and phosphorus loss pollution of slope farmland are determined.

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

[0041] Step 2.1 drainage ditch layout:

[0042] Step 1) transverse slope interception ditch layout, receiving the drainage of slope farmland, and the slope gradient ratio is greater than 1.2;

[0043] Step 2) boundary drainage ditch layout, intercepting external water of slope farmland, and the slope gradient ratio is greater than 1.2;

[0044] Step 3) main drainage ditch layout, receiving the water of transverse slope interception ditch and boundary drainage ditch, and the slope gradient ratio is greater than 1.2; the width of the main drainage ditch is determined according to the drainage amount corresponding to the maximum rainfall amount on the slope farmland in step 1.4, and the maximum drainage amount is multiplied by a safety factor, generally 1.1-1.5;

[0045] Step 2.2 rainwater and sewage separation device design:

[0046] Step 1) Rain and sewage shunt device is laid at the outlet of cross-slope intercepting ditch and boundary drainage ditch, including upper and lower water distribution ports; the lower water distribution port is connected with a shunt pipe at the back; an intermediate partition plate is arranged between the upper and lower water distribution ports, and the intermediate partition plate and the baffle are connected through two double-link mechanisms; the baffle is opened at an angle and 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 relatively low, the water flow is large, the baffle is in a fully closed state, and the water flow does not enter the vegetation retention pool; when the equipment is idle or under maintenance, the baffle is in a fully open state; wherein the spring arranged in the double-link mechanism can accept the water flow impact force, and the water flow impact force is determined based on the 80% nitrogen and phosphorus load pollutant concentration calculated in step 1.4 through the SWAT model; when the water flow impact force is above the 80% nitrogen and phosphorus load pollutant concentration, the drainage enters the shunt pipe from the lower water distribution port; when it is below 80%, the drainage enters the main drainage ditch from the upper water distribution port;

[0047] Step 2) The shunt pipe of the main drainage ditch is laid, and the height and opening are determined based on the height of the fence plate; when the small rain causes small drainage volume of the drainage ditch but high nitrogen and phosphorus concentration, it enters the shunt pipe and is discharged into the vegetation retention pool; when the heavy rain causes large drainage volume of the drainage ditch but low nitrogen and phosphorus concentration, it enters the main drainage ditch and is discharged into the next level drainage ditch or river.

[0048] The method, the third step includes the following steps:

[0049] Step 3.1 Selection of vegetation retention pool layout position: the position is determined based on the slope field partition drainage collection area in step 1.

[0050] Step 3.2 Vegetation retention pool layout size: the size of the vegetation retention pool is designed according to the drainage time of the slope field catchment area and the nitrogen and phosphorus loss pollution threshold in step 1.4, the depth is greater than 1.7 meters, and the area is 1-2% of the slope field partition.

[0051] The method, the vegetation retention pool includes:

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

[0053] Step 2) Organic cover layer, thickness 4-6 cm, material is fresh straw, straw and tree bark; function: creating and protecting the growth environment of microorganisms, intercepting part of heavy metals and organic pollutants; reduction rate: nitrogen 6-8%, phosphorus 6-8%;

[0054] Step 3) Biological filter material layer, thickness 4-6 cm, material is activated carbon, natural mineral and bentonite; function: absorbing nitrogen and phosphorus pollutants in drainage, providing attachment surface for microorganisms, promoting microbial degradation of pollutants; reduction rate: nitrogen 8-10%, phosphorus 8-10%;

[0055] Step 4) Soil layer, thickness 80-120 cm, material is a mixture of sand, compost and loam, planted crops are grasses, legumes, shrubs and wetland plants; function: further absorption of nitrogen and phosphorus pollutants, provide physical support, prevent soil erosion; reduction rate: nitrogen 38-40%, phosphorus 44-46%;

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

[0057] Step 6) Gravel layer, thickness 40-60 cm, material is gravel with particle size 5-10 mm; function: support layer, collect and temporarily store part of the rainwater; reduction rate: nitrogen 8-10%, phosphorus 10-12%;

[0058] Step 3.4 Drainage system layout:

[0059] Step 1) Drainage pipe layout, located at the lowest point of the planted retention pond, water flowing from the upper layer of filter material ensures that it flows into the river or drainage ditch;

[0060] Step 2) Slope design, a slope of 2-3% is usually sufficient to ensure water flow towards the drainage pipe and avoid water accumulation;

[0061] Step 3) Anti-blocking design: the drainage pipe and water collection well should be designed with an anti-blocking system to prevent poor drainage caused by sediment and pollutant accumulation;

[0062] Step 4) Access hole design: access holes are provided at the drainage pipe or water collection well.

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

[0064] 1. Effective reduction of nitrogen and phosphorus loss. The present application is based on the SWAT model to estimate the nitrogen and phosphorus loss load of slope farmland, and combines the relationship between rainfall intensity and nitrogen and phosphorus concentration to propose the design of rain and sewage separation device. By realizing the separation and treatment of small rain and high concentration of nitrogen and phosphorus runoff in the early stage of heavy rain, the nitrogen reduction rate of slope farmland drainage reaches 70-80%, and the phosphorus reduction rate reaches 80-90%. Among them, the fiber material layer, the organic mulch layer, the biological filter layer, the soil layer, the coarse sand layer and the gravel layer.

[0065] 2. Rain and sewage unpowered diversion. The rain and sewage diversion device fully considers the dynamic change of runoff nitrogen and phosphorus concentration under rainfall conditions, can automatically separate high concentration and low concentration runoff according to rainfall intensity, does not need manual intervention, solves the problem that the traditional facilities are difficult to realize automatic separation and treatment, and reduces the infrastructure demand and long-term maintenance cost. BRIEF DESCRIPTION OF DRAWINGS

[0066] Figure 1 It is the overall layout of the application;

[0067] Among them: 10 is the boundary drainage ditch, 11 is the rain and sewage diversion device, 12 is the horizontal slope interception ditch, 13 is the rain and sewage diversion pipe, 14 is the main drainage ditch, 15 is the vegetation retention pool;

[0068] Figure 2 It is the rain and sewage diversion device design and waterway up and down water distribution structure schematic diagram of the application;

[0069] Figure 3 It is the double connecting rod mechanism schematic diagram of the baffle water control self-closing function (the position of the baffle working state);

[0070] Among them: the middle partition plate in the upper and lower waterways 1, the double connecting rod mechanism 2, the double connecting rod mechanism booster spring 3, the baffle 4;

[0071] Figure 4 It is the double connecting rod mechanism schematic diagram of the baffle water control self-closing function (the position of the baffle full closing state);

[0072] Figure 5 It is the double connecting rod mechanism schematic diagram of the baffle water control self-closing function (the position of the baffle full opening state);

[0073] Figure 6 It is the vegetation retention pool structure of the application;

[0074] Among them: 151 is the fiber material layer, 152 is the organic cover layer, 153 is the biological filter material layer, 154 is the soil layer, 155 is the coarse sand layer, and 156 is the broken stone layer; DETAILED DESCRIPTION

[0075] The application will be described in detail below in combination with specific embodiments.

[0076] A method for reducing nitrogen and phosphorus loss on sloping farmland, comprising the following steps:

[0077] Step 1: Determine the slope farmland catchment area and the soil and water assessment tool (SWAT) model nitrogen and phosphorus loss load estimation

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

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

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

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

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

[0083] 5) Farm management data is used to simulate the impact of different management measures (such as irrigation, fertilization, tillage, etc.) on the hydrological process of sloping farmland, soil erosion and nitrogen and phosphorus pollutant migration, including crop planting patterns and rotation period, irrigation management (time, frequency and water volume), chemical fertilizer and pesticide application (type, amount and time), slope farmland tillage method.

[0084] 1.2 Division of catchment area of sloping farmland:

[0085] 1) DEM data input and processing. The DEM data is preprocessed to fill in the low-lying areas of the terrain to ensure the accuracy of the slope and watershed;

[0086] 2) Selection of drainage outlet point. When automatically selecting the drainage outlet point, the SWAT model automatically identifies the drainage outlet within the watershed based on the slope and watershed flow direction algorithm. However, in some special terrain or regional conditions, manual selection of the drainage outlet point is required to ensure accurate drainage path and watershed division;

[0087] 3) Adjustment of drainage outlet point. In the practical application of sloping farmland, the selection of the drainage outlet point will be affected by the layout of farmland, irrigation channels and land use methods after the SWAT model selects the outlet point, 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 distribution of each grid. By calculating the flow direction and confluence path of each cell in the region, the water flow direction of sloping farmland is simulated;

[0089] 5) Division of catchment area. After determining the drainage path, the SWAT model will divide the catchment area according to the water flow direction and slope data;

[0090] 6) The threshold of catchment area. The threshold of catchment area is the minimum area of water accumulation in each grid cell, only when the area reaches the threshold, it is considered as a valid catchment area. By adjusting the threshold of catchment area, the division of catchment area is optimized, and the influence of small catchment area on the accuracy of the model is reduced;

[0091] 7) The density of catchment area. The density of catchment 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 will lose sensitivity to local hydrological characteristics. During the adjustment process, the actual topography and the characteristics of farmland water management should be considered for reasonable setting;

[0092] 8) Finally, the location of the catchment area is determined according to the partition outlet.

[0093] 1.3 Estimation of nitrogen and phosphorus loss load on slope farmland by SWAT model:

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

[0095] 2) Input of meteorological data;

[0096] 3) Input of agricultural management data;

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

[0098] 5) Reading of nitrogen and phosphorus loss load data on slope 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 threshold of nitrogen and phosphorus loss pollution caused by rainfall on slope farmland:

[0100] 1) Simulation of slope farmland drainage and nitrogen and phosphorus loss concentration under different rainfall scenarios. The relationship between rainfall intensity and drainage and nitrogen and phosphorus loss concentration is analyzed by using SWAT model to simulate the changes of slope farmland drainage and nitrogen and phosphorus loss concentration under different rainfall scenarios;

[0101] 2) Determination of the correlation between rainfall and nitrogen and phosphorus loss concentration. Based on regression analysis (linear or nonlinear), an empirical formula is constructed to determine the relationship between rainfall and nitrogen and phosphorus loss concentration, and the corresponding rainfall and drainage of 80% nitrogen and phosphorus load pollutant concentration is determined;

[0102] 3) Determination of nitrogen and phosphorus loss pollution emission standard. According to the relevant farmland drainage pollution control standard, the limit value of nitrogen and phosphorus loss concentration is determined, such as total nitrogen not exceeding 15 mg / L and total phosphorus not exceeding 0.3 mg / L;

[0103] 4) The threshold value of rainfall leading to nitrogen and phosphorus loss pollution is determined. According to the standard concentration of nitrogen and phosphorus loss of slope farmland, the corresponding rainfall is analyzed by regression analysis, and the drainage time corresponding to the rainfall and the threshold value of rainfall leading to nitrogen and phosphorus loss pollution of slope farmland are determined.

[0104] Step 2: According to the estimation results of the model, the drainage ditch, rain and sewage separation device and diversion pipe are designed. Among them, the drainage ditch is used to arrange the rain and sewage separation device and diversion pipe, and receives the drainage with low concentration of nitrogen and phosphorus loss of slope farmland, such as the later period of heavy rain; the rain and sewage separation device is used to separate the drainage with high concentration of nitrogen and phosphorus loss of slope farmland into the diversion pipe, such as the early stage of heavy rain; the diversion pipe is used to guide the separated water to the vegetation retention pool.

[0105] 2.1 Drainage ditch arrangement:

[0106] 1) Arrangement of horizontal slope interception ditch to receive drainage of slope farmland with slope ratio greater than 1.2;

[0107] 2) Arrangement of boundary drainage ditch to intercept external water of slope farmland with slope ratio greater than 1.2;

[0108] 3) Arrangement of main drainage ditch to receive water from horizontal slope interception ditch and boundary drainage ditch with slope ratio greater than 1.2. The width of main drainage ditch is determined according to the corresponding drainage volume of maximum rainfall on slope farmland simulated by SWAT model in step 1.4, and a safety factor is multiplied on the basis of maximum drainage volume, generally 1.1-1.5, to prevent overflow of drainage ditch caused by extreme rainfall. The overall layout is shown in Figure 1 .

[0109] 2.2 Rain and sewage separation device design:

[0110] 1) Rain and sewage separation device is arranged at the outlet of horizontal slope interception ditch and boundary drainage ditch, including upper and lower water inlets, which can be made of stainless steel, and is arranged at the end of water channel. Its section form and size can be set flexibly according to the actual situation Figure 2 ). The rear water channel can be made of concrete structure or other forms, and the lower water inlet is connected with the diversion pipe. Two double-link mechanisms are fixed below the intermediate partition plate 1 and above the rear of the baffle 4 in the upper and lower waterways, which generally do not contact water flow, so as to avoid entanglement of sundries. The intermediate partition plate 1 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 spring 3 has a telescopic metal sleeve to protect it from being jammed by sundries, which affects the reliability of the mechanism. The baffle 4 can be made of metal or plastic. The size of the mechanism and the matching spring can be adjusted and configured according to the actual waterway requirements. Figure 3The baffle shown in the middle is the working position. The mechanism can make the baffle open at any angle. The angle is automatically adjusted according to the water discharge. When the nitrogen and phosphorus concentrations in the drainage are low, the water flow is large, and the baffle is in the fully closed state Figure 4 ); when it is maintained or idle, the baffle is in the fully open state Figure 5 ). The spring can withstand the impact of the water flow based on the 80% nitrogen and phosphorus load pollutant concentration calculated in step 1.4. When the water flow impact is above 80% of the nitrogen and phosphorus load pollutant concentration, the drainage enters the shunt pipe from the lower water outlet; when it is below 80%, the drainage enters the main drainage ditch from the upper water outlet;

[0111] 2) The height and opening of the main drainage ditch shunt pipe are determined based on the height of the baffle. When it rains, the drainage capacity of the drainage ditch is small, but the nitrogen and phosphorus concentrations are high, and it enters the shunt pipe and is discharged into the vegetation retention pool; when it rains heavily, the drainage capacity of the drainage ditch is large, but the nitrogen and phosphorus concentrations are low, and it enters the main drainage ditch and is discharged into the next level of drainage ditch or river.

[0112] Step 3: Design the vegetation retention pool based on the runoff of the slope farmland. The vegetation retention pool effectively reduces the loss of nitrogen and phosphorus from slope farmland through the absorption of plants, filtration of sediments, and retention of water flow.

[0113] 3.1 Vegetation retention pool layout position selection: The position is determined based on the slope farmland zoning drainage collection area in step 1. The layout principle is not to affect agricultural operations, and it is generally built at the edge of the farmland.

[0114] 3.2 Vegetation retention pool layout size: The size of the vegetation retention pool is designed based on the drainage time of the slope farmland collection area and the nitrogen and phosphorus loss pollution threshold in step 1.4. The depth is greater than 1.7 meters, and the area is 1-2% of the slope farmland zoning.

[0115] 3.3 Vegetation retention pool treatment system layout Figure 6 ):

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

[0117] 2) Organic mulch layer, thickness 4-6 cm, material fresh straw, straw, and tree bark. Function: create and protect microbial growth environment, intercept part of heavy metals and organic pollutants. Reduction rate: nitrogen 6-8%, phosphorus 6-8%;

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

[0119] 4) Soil layer, thickness 80-120 cm, material mixed from sand, compost and loam, planted crops are grasses (alfalfa and couchgrass), legumes (red clover and alfalfa), shrubs (syringa and locust), wetland plants (reed and cattail). Function: further absorb nitrogen and phosphorus pollutants, provide physical support, prevent soil erosion. Reduction rate: 38-40% of nitrogen, 44-46% of phosphorus;

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

[0121] 6) Gravel layer, thickness 40-60 cm, material composed of 5-10 mm gravel. Function: support layer, collect effluent and temporarily store part of the rainwater. Reduction rate: 8-10% of nitrogen, 10-12% of phosphorus.

[0122] 3.4 Drainage system layout:

[0123] 1) Drainage pipe layout, located at the lowest point of the planted retention pond, water flowing from the upper layer of filter material, ensuring flow to the river or drainage ditch;

[0124] 2) Slope design, a slope of 2-3% is usually sufficient to ensure water flow towards the drainage pipe, avoiding water accumulation;

[0125] 3) Anti-blocking design: drainage pipes and water collection wells should be designed with anti-blocking systems to prevent poor drainage due to sediment and pollutant accumulation;

[0126] 4) Access port design: access ports are provided at the drainage pipe or water collection well for daily maintenance and cleaning.

[0127] According to the simulation results of the nitrogen and phosphorus loss load estimation model, the rainwater and sewage separation device and the planted retention pond are designed, effectively realizing the reduction of nitrogen and phosphorus pollutants in surface runoff under the conditions of small rain or early stage of heavy rain on slope farmland. This method can be used for the reduction of nitrogen and phosphorus loss on slope farmland.

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

[0129]

[0130]

[0131] It is to be understood that the above description is intended to be illustrative and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the application should, therefore, be determined not with reference to the above description, but instead with reference to the appended claims, along with their full scope of equivalents.

Claims

1. A method for reducing the loss of nitrogen and phosphorus from sloping farmland, characterized by, Comprising the following steps: Step 1: Determining the catchment area of slope farmland and estimating the nitrogen and phosphorus loss load of SWAT model; Said Step 1 comprises the following steps: Step 1.1 Preparing the database required by the SWAT model of slope farmland; Said Step 1.1 comprises the following steps: 1) DEM data is used to generate the topographic features of slope farmland, including drainage network, slope and catchment area; 2) Soil data is used to characterize the physical and chemical properties of soil to simulate the migration and transformation of water and nitrogen and phosphorus pollutants in soil, including soil layer thickness, soil organic matter content, soil texture, permeability, soil water holding capacity; 3) Land use data is used to determine the surface runoff, evapotranspiration and nitrogen and phosphorus pollutant load; 4) Meteorological data is used to drive the hydrological and nitrogen and phosphorus pollutant process simulation of the model, including daily precipitation, maximum temperature and minimum temperature, wind speed, relative humidity, solar radiation; 5) Farm management data is used to simulate the influence of different management measures on the hydrological process, soil erosion and nitrogen and phosphorus pollutant migration of slope farmland, including crop planting pattern and rotation period, irrigation management, fertilizer and pesticide application, slope farmland tillage method; Step 1.2 Division of slope farmland catchment area; Said Step 1.2 comprises the following steps: Step 1) DEM data input and processing; DEM data is preprocessed to fill in the low-lying areas in the terrain to ensure the accuracy of slope and watershed; Step 2) Selection of drainage outlet point; When automatically selecting the drainage outlet point, the SWAT model automatically identifies the drainage outlet in the watershed based on the slope and watershed flow direction algorithm; However, in some special terrain or regional conditions, the drainage outlet point needs to be manually selected to ensure accurate drainage path and watershed division; Step 3) Adjustment of drainage outlet point; In the actual application of slope farmland, after the SWAT model selects the outlet point, the selection of the drainage outlet point will be affected by the farmland layout, irrigation channels and land use methods, which need to be 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 distribution of each grid; By calculating the flow direction and confluence path of each cell in the region, the water flow direction of slope farmland is simulated; Step 5) Division of catchment area; After determining the drainage path, the SWAT model will divide the catchment area according to the water flow direction and slope data; Step 6) Setting of catchment threshold; The catchment threshold refers to the minimum area of water flow accumulation of each grid cell, only when the area reaches the threshold, it will be considered as an effective catchment area; By adjusting the catchment threshold, the division of the catchment area is optimized, and the influence of small catchment area on the accuracy of the model is reduced; Step 7) Adjustment of catchment area density; The density of catchment area directly affects the number of hydrological response units in the region, when the density is high, the model can simulate the water flow process more finely, otherwise it will lose the sensitivity to local hydrological characteristics; In the adjustment process, reasonable setting needs to be combined with the actual terrain and farmland water management characteristics; Step 8) Finally, determine the position of the catchment area according to the partition outlet; Step 1.3 Estimation of nitrogen and phosphorus loss load of slope farmland by SWAT model; Said Step 1.3 comprises the following steps: Step 1) division of hydrological response unit; the hydrological response unit HRU is a basic unit for estimating pollution load, and the hydrological characteristics of each hydrological response unit are defined by combining crop planting types, soil types and slope factors; Step 2) meteorological data input; Step 3) input of agricultural management data; Step 4) running of the SWAT model, calibration and verification, and data reading; Step 5) reading of the nitrogen and phosphorus loss load data of the slope farmland, FLOW_OUT being the drainage data, TOT_N being the total nitrogen data, and TOT_P being the total phosphorus data; Step 1.4 determination of the slope farmland nitrogen and phosphorus loss pollution threshold caused by rainfall; The step 1.4 comprises the following steps: Step 1) simulation of the slope farmland drainage amount and nitrogen and phosphorus loss concentration in a rainfall event; the SWAT model is used to simulate the changes of the slope farmland drainage amount and nitrogen and phosphorus loss concentration under different rainfall amount scenarios, and the relationship between the rainfall intensity and the drainage amount and nitrogen and phosphorus loss concentration is analyzed; Step 2) determination of the correlation between the slope farmland rainfall amount and the nitrogen and phosphorus loss concentration; a regression analysis is conducted to build an empirical formula of the relationship between the rainfall amount and the nitrogen and phosphorus loss concentration, and the rainfall amount and the drainage amount corresponding to the 80% nitrogen and phosphorus load pollutant concentration are determined; Step 3) determination of the nitrogen and phosphorus loss pollution emission standard; the limit value of the nitrogen and phosphorus loss concentration is determined according to the relevant farmland drainage pollution control standard; Step 4) determination of the slope farmland nitrogen and phosphorus loss pollution threshold caused by rainfall; the drainage time corresponding to the rainfall amount and the threshold of the slope farmland nitrogen and phosphorus loss pollution caused by the rainfall amount are determined according to the standard concentration of the slope farmland nitrogen and phosphorus loss and the regression analysis of the corresponding rainfall amount; Step 2: according to the model estimation results, design the drainage ditch, rainwater and sewage separation device and diversion pipe; wherein the drainage ditch is used to arrange the rainwater and sewage separation device and diversion pipe, and receives the drainage with low nitrogen and phosphorus loss concentration of the slope farmland in the later period of heavy rain; the rainwater and sewage separation device is used to separate the drainage with high nitrogen and phosphorus loss concentration of the slope farmland into the diversion pipe in the early stage of light rain or heavy rain; the diversion pipe is used to guide the separated water to the vegetation retention pool; The step 2 comprises the following steps: Step 2.1 drainage ditch arrangement: Step 1) arrangement of the transverse slope interception ditch, which receives the drainage of the slope farmland, and the slope gradient ratio is greater than 1.2; Step 2) arrangement of the boundary drainage ditch, which intercepts the external water of the slope farmland, and the slope gradient ratio is greater than 1.2; Step 3) arrangement of the main drainage ditch, which receives the water of the transverse slope interception ditch and the boundary drainage ditch, and the slope gradient ratio is greater than 1.2; the width of the main drainage ditch is determined according to the drainage amount corresponding to the maximum rainfall amount on the slope farmland simulated by the SWAT model, and is multiplied by a safety factor of 1.1~1.5 on the basis of the maximum drainage amount; Step 2.2 rainwater and sewage separation device design: Step 1) Rain and sewage shunt device is laid out at the outlet of cross-slope intercepting ditch and boundary drainage ditch, including upper and lower water distribution ports; the lower water distribution port is connected with a shunt pipe at the back; an intermediate partition is arranged between the upper and lower water distribution ports, and the intermediate partition and the baffle are connected through two double-link mechanisms; the baffle can stay at any position when it is opened, and the stay angle is automatically adjusted according to the drainage volume; when the nitrogen and phosphorus concentrations in the drainage are relatively low, the water flow is large, and the baffle is in a fully closed state, so that the water flow does not enter the plant retention pool; when the equipment is idle or needs to be maintained, the baffle is in a fully open state; the spring arranged in the double-link mechanism can accept the water flow impact force, and the acceptable water flow impact force is determined based on the 80% nitrogen and phosphorus load pollutant concentration calculated by the SWAT model; when the water flow impact force is above the 80% nitrogen and phosphorus load pollutant concentration, the drainage enters the shunt pipe from the lower water distribution port; when it is below 80%, the drainage enters the main drainage ditch from the upper water distribution port; Step 2) The shunt pipe of the main drainage ditch is laid out, and the height and opening are determined based on the height of the fence; when the small rain causes small drainage volume of the drainage ditch but high nitrogen and phosphorus concentration, the drainage enters the shunt pipe and is discharged into the plant retention pool; when the heavy rain causes large drainage volume of the drainage ditch but low nitrogen and phosphorus concentration, the drainage enters the main drainage ditch and is discharged into the next level drainage ditch or river; Step 3: The plant retention pool is designed according to the runoff of the slope farmland; the plant retention pool effectively reduces the loss of nitrogen and phosphorus in the slope farmland through the absorption of plants, the filtration of sediments and the retention of water flow; The third step includes the following steps: Step 3.1: The plant retention pool is laid out at a position determined based on the slope farmland partition drainage collection area; Step 3.2: The size of the plant retention pool is designed according to the drainage time of the slope farmland catchment area and the nitrogen and phosphorus loss pollution threshold, and the depth is greater than 1.7 meters, and the area is 1-2% of the slope farmland partition.

2. The method of claim 1, wherein, Steps 3.3 and 3.4 are also included: Step 3.3: The plant retention pool system is laid out: Step 1) Fiber material layer, thickness 2-3 cm, material is plant fiber cloth, natural fiber pad; Effect: intercept larger particles, reduce water flow impact, preliminary filtration; reduction rate: nitrogen 0%, phosphorus 0%; Step 2) Organic cover layer, thickness 4-6 cm, material is fresh straw, straw, and tree bark; Effect: create and protect the growth environment of microorganisms, intercept part of heavy metals and organic pollutants; Reduction rate: nitrogen 6-8%, phosphorus 6-8%; Step 3) Biological filter layer, thickness 4-6 cm, material is activated carbon, natural mineral, bentonite; Effect: absorb nitrogen and phosphorus pollutants in drainage, provide attachment surface for microorganisms, promote microbial degradation of pollutants; Reduction rate: nitrogen 8-10%, phosphorus 8-10%; Step 4) Soil layer, thickness 80-120 cm, material is a mixture of sand, compost and loamy soil, and crops are planted, such as gramineous plants, leguminous plants, shrubs, and wetland plants; effect: further absorb nitrogen and phosphorus pollutants, provide physical support, prevent soil erosion; reduction rate: nitrogen 38-40%, phosphorus 44-46%; Step 5) Coarse sand layer, thickness 40-60 cm, material is sand with a particle size less than 5 mm; Function: Prevents the planting soil from entering the gravel layer and filters and purifies the water from the upper layer; Reduction rate: 10-12% of nitrogen and 12-14% of phosphorus; Step 6) Gravel layer, thickness of 40-60 cm, material of 5-10 mm gravel; Function : Support layer, collects and temporarily stores part of the rainwater; Reduction rate: 8-10% of nitrogen and 10-12% of phosphorus; Step 3.4 Drainage system layout: Step 1) Drainage pipe layout, located at the lowest point of the planting retention pond, the water flowing from the upper layer of filter material ensures that it flows into the river or drainage ditch; Step 2) Slope design, a slope of 2-3% is usually sufficient to ensure water flow towards the drainage pipe, avoiding water accumulation; Step 3) Anti-blocking design: The drainage pipe and water collection well should be designed with an anti-blocking system to prevent poor drainage caused by the accumulation of sediments and pollutants; Step 4) Access hole design: Access holes are provided at the drainage pipe or water collection well.

3. The method of claim 1, wherein, The limit value of the determined nitrogen and phosphorus loss concentration is that the total nitrogen should not exceed 15 mg / L and the total phosphorus should not exceed 0.3 mg / L.

Images