Agricultural small watershed integrated water and soil loss prevention and control method

Abstract

The invention discloses an agricultural small-watershed integrated water and soil loss prevention and control method, which comprises the following steps of: performing source reduction in a slope cropland area with the gradient of more than 6 degrees to control a slope cropland erosion source so as to treat surface erosion and runoff expansion; process blocking and control are conducted from the middle upstream area to the middle downstream area, and sediment migration is blocked and the confluence speed is adjusted through linkage of the grading ditch system and the sedimentation and storage facility; tail end purification is carried out in a drainage basin outlet area, water quality is purified through ecological engineering measures, and a drainage basin outlet is stabilized; the technical problems that in the prior art, comprehensive prevention and control means starting from the whole drainage basin are lacked, traditional measures have obvious defects in the aspects of function coupling, space coordination and ecological restoration capacity, and the dual requirements of current agricultural green development and ecological environment protection are difficult to meet are solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of agricultural ecological engineering and soil and water conservation, and in particular relates to an integrated soil and water loss prevention and control method for agricultural small watersheds. Background Art

[0002] The Southwest region is a typical high mountain valley landscape with dramatic terrain fluctuations, widespread sloping farmland, and high rainfall intensity and concentration, leading to particularly serious soil erosion problems. Existing soil and water conservation measures primarily focus on localized, point-based management, employing a single approach. These measures are unable to systematically address non-point erosion in small watersheds characterized by complex terrain, high agricultural intensity, and frequent rainfall, and lack comprehensive, basin-wide control measures. Furthermore, traditional measures lack functional coupling, spatial coordination, and ecological resilience, making them unable to meet the dual demands of green agricultural development and ecological and environmental protection. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide an integrated method for soil and water loss prevention and control in agricultural small watersheds, so as to solve the technical problems such as the lack of comprehensive prevention and control measures based on the overall watershed in the existing technology, the obvious deficiencies in functional coupling, spatial coordination and ecological restoration capacity of traditional measures, and the difficulty in meeting the current dual needs of green agricultural development and ecological environmental protection.

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

[0005] An integrated method for soil and water loss prevention and control in agricultural small watersheds includes source reduction in sloping farmland areas with a slope greater than 6° to control the source of sloping farmland erosion, thereby controlling surface erosion and runoff expansion; process control in the upper and middle reaches of the river, by linking a graded ditch system with sedimentation and storage facilities to block sediment migration and regulate the convergence rate; and terminal purification in the watershed outlet area, by purifying water quality and stabilizing the watershed outlet through ecological engineering measures.

[0006] The source reduction methods include:

[0007] In sloping farmland areas with a slope of 6 to 25 degrees, terraces are built along the contour lines;

[0008] On steep slope farmland with a slope of more than 25°, soil conservation tillage and micro-topography transformation are implemented.

[0009] The terraced fields are built with rammed earth or stone ridges, with a ridge height of 20 to 30 cm, a ridge top width of >30 cm, a ridge bottom width of 40 to 60 cm, and a side slope gradient of ≤75°; deep-rooted shrub hedges are planted on the backwater side of the ridge, and the species are selected from Amorpha fruticosa or Daylily, with a plant spacing of 0.3 to 0.5 m to form a biological soil barrier, and at the same time, herbaceous plants are sown, and the species is selected from Sweet Elephant Grass.

[0010] The soil conservation tillage and micro-topography transformation include: contour reverse slope tillage: excavating reverse slope steps along the contour line, with a step width of 80 to 100 cm, a reverse slope angle of 5° to 8°, and a step spacing of 1 to 2 m; furrow and ridge planting: ridge height of 15 to 20 cm, ridge spacing of 40 to 50 cm, and sowing green manure plants in the ridges and furrows, with a sowing amount of 20 to 30 kg per mu; surface covering: after the crops are harvested, the straw is crushed and returned to the field, with a covering thickness of ≥5 cm.

[0011] Process control includes the use of a multi-level drainage and sediment deposition structure through the linkage of a graded ditch system and sedimentation facilities, mainly including: the secondary ditch is set in the main water collection ditch, the tertiary ditch is arranged in the branch ditch, and herbaceous plants are planted in the ditch to stabilize the ditch slope and slow down the flow rate. It is arranged at the downstream intersection to intercept the sediment carried by the upstream based on the quasi-stationary sedimentation principle, reduce the load of the downstream water storage and purification system, and ensure the primary purification of water quality; at the same time, reservoirs at different elevations are arranged along the slope catchment area to realize segmented regulation and storage, irrigation reuse and sediment deposition functions: each reservoir receives the water from above, and effectively deposits sediment through sedimentation and residence time control; the downstream reservoir also serves as an irrigation water source, providing stable water replenishment support for agricultural production in the gentle slope area.

[0012] The ditch system includes:

[0013] Secondary concrete trench: laid in the main trench with slope greater than 6°;

[0014] The third-level grass-planting ditch is an earthen branch ditch connected to the second-level ditch, in which grass is planted, and the grass is bermudagrass or zoysia grass;

[0015] The channel section is designed for a maximum rainfall of 10 minutes in a 5-year return period. The corresponding peak flow is calculated based on the ground slope and catchment area of ​​the channel. The calculation formula is:

[0016] Q m =16.67φqF ​​(1)

[0017] Where: Q m is the peak flow generated by the design frequency; φ is the runoff coefficient; q is the average rainfall intensity within the design recurrence period and rainfall duration; F is the catchment area;

[0018] Then, based on the designed flood peak flow and considering the actual terrain conditions through which the water flows, the cross-sectional dimensions and form of the intercepting ditch are calculated and selected. The calculation formula is:

[0019]

[0020] Where: Q is the design flow rate; A is the cross-sectional area of ​​the water flow; C is the Xie Cai coefficient, Where n is the roughness coefficient; R is the hydraulic radius; χ is the wetted perimeter and i is the gradient.

[0021] Methods for achieving sediment deposition structures include:

[0022] Sedimentation tank design: The design adopts the quasi-static sedimentation method. The total sediment volume of the sedimentation tank is calculated as follows:

[0023] Vs=λ×Ms×F / c (3)

[0024] Where: V s is the total amount of sediment entering the sedimentation basin; λ is the transport ratio; Ms is the average soil erosion modulus; F is the catchment area; c is the sediment bulk density;

[0025] Calculation of sedimentation tank volume:

[0026] V=φ×Ws / n (4)

[0027] Where: V is the volume of the sedimentation tank; φ is the efficiency of the sedimentation tank, which is taken as 75%; n is the number of times the sedimentation tank is cleared;

[0028] Reservoir design

[0029] The capacity of the reservoir is calibrated based on the dual factors of irrigation demand and storm runoff. First, the maximum annual irrigation water volume required for the crop is determined based on the local farmland irrigation water quota. The irrigation water demand is calculated as follows:

[0030] V 需 =M×A / ω (5)

[0031] Where: V 需 is the irrigation water requirement (m 3 ); M is the net irrigation quota (m 3 / mu); A is the irrigation area (mu); ω is the irrigation water utilization coefficient.

[0032] The design frequency of rainstorm runoff in a small watershed with a return period of 10 years and 1 hour (GB 51018-2014) is used to calculate:

[0033] Vw=1000Fh (6)

[0034] Where: Vw is the design frequency rainstorm runoff, m 3 ; F is the rain-bearing area, km 2 ; h is the average runoff depth, mm.

[0035] Calculation of total reservoir capacity:

[0036] V 降 =K(Vw+Vs) (7)

[0037] Where: V is the capacity of the reservoir; Vw is the design frequency of rainstorm runoff; Vs is the cumulative sediment deposition in the design desilting year; K is the safety factor;

[0038] When V 需 >V 降 , adopt the equipment of irrigation and drainage facilities; V 需 ≤V 降 When using V 需 As the volume of the reservoir.

[0039] End-of-pipe purification includes improving the water quality and ecological compensation of the final runoff by constructing a composite ecological purification zone, through ecological drainage ditches and river buffer plant belts; specifically, it includes: setting up gravel layers and biochar layers in the ecological drainage ditch as purification media, and planting submerged plants and emergent plants for water quality bioremediation; arranging multi-layer vegetation communities of trees, shrubs and herbs on the outside of the ditch to form a green buffer zone, enhance water stability and purify residual pollutants.

[0040] The purification layer of the ecological drainage ditch includes:

[0041] Bottom layer: 30-40 cm graded gravel layer, coarse sand: gravel = 1:2, porosity 30-35%;

[0042] Middle layer: 20cm biochar mixed soil layer, biochar accounts for 20-30%;

[0043] Upper layer: Plant submerged and emergent plants, submerged plants: Vallisneria, density 8-10 plants / m 2 Emergent plants: Cattail, density 4-6 plants / m 2 .

[0044] Beneficial effects of the present invention:

[0045] The present invention has the advantages of strong systematicity, high adaptability, and significant ecological benefits. It can effectively deal with the problem of soil erosion in small watersheds under the complex terrain conditions of the high mountain canyon areas in the southwest. The present invention effectively reduces the intensity of slope erosion, reduces the amount of sediment transport, and performs ecological purification treatment on the outlet runoff through a multi-dimensional coordinated zoning management strategy of "source-process-end", realizing a full-process closed-loop management from source control to terminal repair. After implementation, the regional vegetation coverage and land water retention capacity have been significantly improved, and the water resource regulation and ecosystem self-repair functions have been enhanced. In addition, the present invention organically integrates ecological measures with engineering means, has a simple structural design, mature construction technology, good landscape compatibility and ecological stability, and has strong technical promotion value and regional adaptability. It is particularly suitable for promotion and application in soil and water conservation and ecological management projects in agricultural small watersheds in the southwest.

[0046] The present invention is based on a small watershed soil and water loss prevention and control technology system with a three-stage management path of "source reduction - process control - terminal repair", which realizes the coordinated management of the entire process from slope erosion control to terminal runoff purification, and improves the system efficiency of soil and water conservation and the level of ecological environment restoration.

[0047] It solves the technical problems such as the lack of comprehensive prevention and control measures based on the overall basin in existing technologies, the obvious deficiencies in functional coupling, spatial coordination and ecological restoration capabilities of traditional measures, and the difficulty in meeting the current dual needs of green agricultural development and ecological environmental protection. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 This is a schematic diagram of the overall layout of the prevention and control measures of the present invention;

[0049] Figure 2 This is a schematic cross-sectional view of the slope-to-staircase structure of the present invention;

[0050] Figure 3 This is a schematic diagram of the connection between the grit chamber and the water reservoir of the present invention;

[0051] Figure 4 Schematic diagram of the configuration of river buffer vegetation belt. DETAILED DESCRIPTION

[0052] See Figure 1 The present invention proposes an integrated soil and water loss prevention and control method for agricultural small watersheds, which is laid out from top to bottom according to the terrain to form a comprehensive prevention and control network with zoning, classification, and coordinated linkage. First, the topographic map and the watershed slope of the land use type are used to divide the area. The sloping farmland with a slope of 6 to 25 degrees is divided into terraced fields, and the sloping farmland with a slope greater than 25 degrees is divided into micro-topography storage areas. Source reduction: The system is deployed in sloping farmland areas with a slope greater than 6 degrees ( Figure 1 In the middle areas (①, ②, ③, and ④), a variety of measures are used to jointly control surface erosion and runoff expansion, including: constructing compacted slope terraces in areas with slopes of 6–25° (③ and ④), and weakening runoff energy by adjusting the slope gradient and shape; planting shrub hedges on the terrace ridges and backwater sides, which have the functions of root consolidation and runoff interception, and can effectively reduce the output of sediment and pollutants at the source. In areas with slopes greater than 25° (① and ②), reverse slope ridge planting and straw return and field covering are implemented to enhance the water retention and soil conservation functions of the slope. Process control: Relying on the terrain and water flow path, the system constructs a multi-level drainage and sediment deposition structure from the middle and upper reaches to the middle and lower reaches (corresponding to Figure 1The system mainly includes secondary concrete ditches, tertiary earth ditches, sedimentation basins and reservoirs, etc., mainly including: secondary ditches are set in the main drainage channel, tertiary ditches are arranged in the branch ditches, and herbaceous plants are planted in the ditches to stabilize the ditch slope and slow down the flow rate; they are arranged at the downstream intersection to intercept the sediment carried by the upstream based on the principle of quasi-stationary sedimentation, reduce the load on the downstream water storage and purification system, and ensure the primary purification of water quality. At the same time, multiple reservoirs at different elevations are arranged along the important water collection areas of the slope to realize the functions of segmented storage, irrigation reuse and sediment deposition: each reservoir can receive the water from above and effectively deposit sediment through sedimentation and residence time control; the downstream reservoir can also be used as an irrigation water source, providing stable water replenishment support for agricultural production in the gentle slope area and improving the efficiency of water resource recycling. End-of-pipe purification: Located in the basin outlet area (⑤ and ⑥), this system improves the water quality of the final runoff and provides ecological compensation by constructing a composite ecological purification zone. This includes: installing purification media such as gravel and biochar layers within the ecological drainage ditch, and planting submerged and emergent plants for water bioremediation. Outside the ditch, a multi-layered vegetation community of trees, shrubs, and herbs forms a green buffer zone, enhancing water stability and purifying residual pollutants. These three subsystems interconnect and coordinate their operation according to the overarching principle of "source control – process blocking – end-of-pipe purification," forming a comprehensive technical system for the prevention and control of soil and water loss in small watersheds with complementary functions, a rational layout, and significant benefits.

[0053] Source reduction:

[0054] Used to control the source of erosion on sloping farmland, graded control measures are adopted according to the slope differences, including:

[0055] 1) Slope to Staircase Project

[0056] In sloping farmland areas with a slope of 6 to 25°, terraces are built along the contour lines.

[0057] (1) Field ridge structure: Rammed earth or stone ridges are used, with a ridge height of 20-30 cm, a top width of >30 cm, a bottom width of 40-60 cm, and a side slope of ≤75°;

[0058] (2) Plant hedges: Plant deep-rooted shrub hedges on the backside of the field ridge. Choose Amorpha fruticosa or Daylily, with a spacing of 0.3 to 0.5 meters, to form a biological soil barrier. At the same time, herbaceous plants can be sown, and sweet elephant grass can be selected as the species.

[0059] 2) Micro-topography storage unit

[0060] On steep slopes with a slope greater than 25°, soil conservation farming and micro-topography transformation are implemented, characterized by:

[0061] (1) Contour reverse slope farming: dig reverse slope steps along the contour line, with a step width of 80-100 cm, a reverse slope angle of 5°-8°, and a step spacing of 1-2 m;

[0062] (2) Furrow and ridge planting: ridge height 15-20 cm, ridge spacing 40-50 cm, sowing green manure plants in the furrows, sowing rate 20-30 kg / mu;

[0063] (3) Surface cover: After the crops are harvested, the straw is crushed and returned to the field, with a covering thickness of ≥5cm.

[0064] Process control:

[0065] By linking the graded ditch system with the sediment storage facilities, the sediment migration is blocked and the convergence speed is regulated, including: 1) Graded ditch system

[0066] (1) Secondary concrete trench: laid in the main trench with a slope greater than 6°.

[0067] (2) Level 3 grass-planting ditch: an earthen branch ditch connecting the level 2 ditch, with grass (bermuda grass or zoysia grass) planted in the ditch.

[0068] The channel section is designed based on the maximum rainfall of 10 minutes in 5 years (GB 51018-2014). The corresponding peak flow is calculated based on the ground slope and catchment area of ​​the channel. Calculation formula:

[0069] Q m =16.67φqF ​​(1) Where: Q m is the peak flow rate generated by the design frequency, m 3 / s; φ is the runoff coefficient; q is the average rainfall intensity within the design return period and rainfall duration, mm / min; F is the catchment area, km 2 .

[0070] Then, based on the designed flood peak flow and the actual terrain conditions, the cross-sectional dimensions and form of the intercepting ditch are calculated and selected. The calculation formula is:

[0071]

[0072] Where: Q is the design flow rate, m 3 / s; A is the cross-sectional area of ​​water, m 2 ; C is Xie Cai coefficient, Where n is the roughness coefficient; R is the hydraulic radius, m; χ is the wetted perimeter, m; i is the gradient.

[0073] 2) The submerged storage facilities are linked, and the submerged storage facilities adopt a segmented submerged storage structure.

[0074] (1) Grit chamber design

[0075] The design of the sedimentation basin refers to the "Design Specifications for Sedimentation Basins in Water Conservancy and Hydropower Projects" (SL269-2001), and the design adopts the quasi-static sedimentation method. Calculation of the total sediment volume in the sedimentation basin:

[0076] Vs=λ×Ms×F / c (3)

[0077] Where: V s is the total amount of sediment entering the sedimentation tank, m 3 ; λ is the transport ratio; Ms is the average soil erosion modulus (t / km 2 .a); F is the catchment area, km 2 ; c is the bulk density of sediment, t / m 3 .

[0078] Calculation of sedimentation tank volume:

[0079] V=φ×Ws / n (4)

[0080] Where: V is the volume of the sedimentation tank, m 3 ; φ is the efficiency of the sedimentation tank, which is taken as 75%; n is the number of times the sedimentation tank is cleared.

[0081] (2) Design of reservoir

[0082] The capacity of the reservoir is calibrated based on the dual factors of irrigation demand and storm runoff. First, the maximum annual irrigation water volume required for the crop is determined based on the local farmland irrigation water quota. The irrigation water demand is calculated as follows:

[0083] V 需 =M×A / ω (5)

[0084] Where: V 需 is the irrigation water requirement (m 3 ); M is the net irrigation quota (m 3 / mu); A is the irrigation area (mu); ω is the irrigation water utilization coefficient.

[0085] The design frequency of rainstorm runoff in a small watershed with a return period of 10 years and 1 hour (GB 51018-2014) is used to calculate:

[0086] Vw=1000Fh (6)

[0087] Where: Vw is the design frequency rainstorm runoff, m 3 ; F is the rain-bearing area, km 2 ; h is the average runoff depth, mm.

[0088] Calculation of total reservoir capacity:

[0089] V 降 =K(Vw+Vs) (7)

[0090] Where: V is the capacity of the water tank, m 3 ; Vw is the design frequency rainstorm runoff, m 3; Vs is the cumulative sediment deposition volume in the designed desilting year, m 3 ; K is the safety factor.

[0091] When V 需 >V 降 , it is necessary to equip irrigation facilities; V 需 ≤V 降 When it is not necessary to equip irrigation facilities, use V 需 It can be used as the volume of the water reservoir.

[0092] End repair:

[0093] Purify water quality and stabilize watershed outlets through ecological engineering measures, including:

[0094] 1) First-level ecological drainage ditch

[0095] (1) Drainage ditch section design

[0096] The design of drainage ditch section is calculated according to formulas (1) and (2).

[0097] (2) Purification layer

[0098] Bottom layer: 30-40 cm graded gravel layer (coarse sand: gravel = 1:2, porosity 30-35%);

[0099] Middle layer: 20 cm biochar mixed soil layer (biochar accounts for 20-30%);

[0100] Upper layer: Plant submerged and emergent plants (submerged plants: Vallisneria, density 8-10 plants / m 2 Emergent plants: Cattail, density 4-6 plants / m 2 ).

[0101] 2) River buffer plant belt

[0102] (1) Stereoscopic configuration

[0103] Nearshore area: revetment with living wooden piles (willow rafts, pile spacing 0.6m);

[0104] Shallow water area: 50 cm wide gravel infiltration zone (gravel particle size 2 to 5 cm);

[0105] Slope area: shrub-grass composite belt (shrubs: mulberry, density 1 plant / 2m 2 ; Herb: reed, density 15 plants / m 2 );

[0106] Outer area: tree isolation zone (Yunnan willow, plant spacing 3 to 4 meters).

[0107] Step 4: Benefit evaluation:

[0108] (1) Source reduction:

[0109] The reduction in soil loss was estimated using the USLE universal soil loss equation:

[0110] A=R·K·LS·C·P (8)

[0111] ΔA=A 无 -A 有 (9)

[0112] Where A is the annual soil loss (t / ha·yr); R is the rainfall erosivity factor; K is the soil erodibility factor; LS is the slope length and slope factor; C is the surface cover and management factor; P is the soil and water conservation measures factor; A 有 and A 无 are the annual soil loss with and without measures, respectively; ΔA is the reduction.

[0113] (2) Process control:

[0114] The sediment reduction efficiency is estimated by referring to the formula in the "Design Specifications for Sedimentation Basins of Water Conservancy and Hydropower Projects" (SL269-2001):

[0115]

[0116] Where η s For sediment reduction efficiency; is the volume of settled sediment; is the total amount of sediment input.

[0117] Pollution load reduction is estimated using the unit sediment load method (typically TP / TN):

[0118]

[0119] Where Cs is the unit attached pollution concentration of sediment; the total amount of TP or TN reduction (kg) is obtained.

[0120] End repair:

[0121] The empirical value of ecosystem purification efficiency is calculated as follows:

[0122]

[0123] Where η n Nitrogen / phosphorus / SS and other pollutant removal rates (%); C in and C out are the pollutant concentrations in the incoming and outgoing water bodies (mg / L), respectively.

[0124] Total reduction estimates:

[0125] Lremoved =η n ·C in Q m (13)

[0126] Q m is the annual runoff volume (m 3 ), which can be estimated by the storm runoff formula (1).

[0127] The following combination Figure 1 The present invention is further described in detail with examples:

[0128] Table 1 is a statistical table of cultivated land area.

[0129] Table 1 Statistics of cultivated land area

[0130]

[0131] The areas ① and ② adopt contour reverse slope farming and furrow ridge farming, wherein contour reverse slope farming is to excavate reverse slope steps along the contour line, the step width of the steps is 80-100 cm, the reverse slope angle is 5°-8°, and the step spacing is 1-2 m, which is used to weaken the slope runoff potential energy and extend the convergence path; furrow ridge farming is to set up ditch structures on the steps, the ridge height is 15-20 cm, the ridge spacing is 40-50 cm, and the ridges are covered with crushed straw after harvesting crops, with a covering thickness of not less than 5 cm, which is used to enhance the water retention and anti-scouring properties of the soil surface and improve the water and soil conservation capacity of the slope.

[0132] The areas ③ and ④ are converted from slopes to terraces by compacting earth ridges, and the compaction degree of the earth ridges is not less than 90%, which is used to improve the stability of the boundaries and enhance the anti-erosion ability. The structural parameters of the field ridges are: ridge height 20 to 30 cm, ridge top width greater than 30 cm, ridge bottom width 40 to 60 cm, and side slope gradient not greater than 75°. Deep-rooted shrub hedges are planted on the back side of the field ridges. The plant species include Amorpha fruticosa, day lily and daylily, with a plant spacing of 0.3 to 0.5 m, which are used to enhance the stability of the soil structure and suppress non-point source runoff. Optionally, herbaceous plants are sown between the field ridges to improve surface coverage and erosion resistance. The herbaceous plant species are preferably sweet elephant grass.

[0133] The cross-section design of the ditch system is based on the maximum rainfall of 10 minutes in 5 years (GB 51018-2014). The design peak flow is determined according to the slope of the ditch ground and the size of the catchment area. The design peak flow calculation formula is:

[0134] Q m =16.67φqF

[0135] Where: Q m is the peak flow rate generated by the design frequency, m3 / s; φ is the runoff coefficient; q is the average rainfall intensity within the design return period and rainfall duration, mm / min; F is the catchment area, km 2 .

[0136] Then, based on the designed flood peak flow and the actual terrain conditions, the cross-sectional dimensions and form of the intercepting ditch are calculated and selected. The calculation formula is:

[0137]

[0138] Where: Q is the design flow rate, m 3 / s; A is the cross-sectional area of ​​water, m 2 ; C is Xie Cai coefficient, Where n is the roughness coefficient; R is the hydraulic radius, m; χ is the wetted perimeter, m; i is the gradient.

[0139] The rainfall intensity q in the area is 46.52 mm, and the runoff coefficient is 0.80. Based on the above parameters and calculation results, the following ditch cross-section structure was determined: the secondary drainage ditch adopts a trapezoidal cross-section, C20 concrete, with cross-sectional dimensions of 0.50-0.60 m bottom width, 0.5 m height, and a slope ratio of 1:1.5. It is intended for use in the main ditch with steeper slopes and more concentrated water collection. The tertiary earth ditch adopts a trapezoidal cross-section, with cross-sectional dimensions of 0.2-0.3 m bottom width, 0.3 m height, and a slope ratio of 1:1.5. It is deployed in the branch ditch system and planted with herbaceous slope-stabilizing plants within the ditch.

[0140] The design of the regional sedimentation basin is based on the "Design Specifications for Sedimentation Basins for Water Conservancy and Hydropower Projects" (SLT 269-2019), and the design adopts the quasi-static sedimentation method. The total amount of sediment entering the sedimentation basin is calculated as follows:

[0141] Vs=λ×Ms×F / c

[0142] Where: V s is the total amount of sediment entering the sedimentation tank, m 3 ; λ is the transport ratio; Ms is the average soil erosion modulus (t / km 2 .a); F is the catchment area, km 2 ; c is the bulk density of sediment, t / m 3 .

[0143] Calculation of sedimentation tank volume:

[0144] V=φ×Ws / n

[0145] When the average soil erosion modulus of the area is 1520t / km 2a. After calculating the catchment area and sediment bulk density, the structural parameters of the grit chamber were determined as follows: The grit chamber adopts a straight-walled rectangular concrete structure with a bottom width of 1.5m and a height of 1.2-1.5m. An extended buffer zone is installed at the water inlet to reduce flow velocity, and a sedimentation zone partition wall is installed in the middle to extend the retention time and enhance sediment deposition. The grit chamber is equipped with a desilting port, which must be cleared manually or mechanically within 1-2 days after a heavy rainfall event. The cleared sediment can be reused for agricultural soil improvement or field treatment, improving the comprehensive utilization rate of resources.

[0146] The volume of the regional reservoir is calibrated based on the irrigation demand and storm runoff factors. The maximum annual irrigation water volume required for crops is determined based on the local farmland irrigation water quota. The irrigation water requirement is calculated as follows:

[0147] V 需 =M×A / ω

[0148] Where: V 需 is the irrigation water requirement (m 3 ); M is the net irrigation quota (m 3 / mu); A is the irrigation area (mu); ω is the irrigation water utilization coefficient.

[0149] The design frequency of rainstorm runoff in a small watershed with a return period of 10 years and 1 hour (GB 51018-2014) is used to calculate:

[0150] Vw=1000Fh

[0151] Where: Vw is the design frequency rainstorm runoff, m 3 ; F is the rain-bearing area, km 2 ; h is the average runoff depth, mm.

[0152] Calculation of total reservoir capacity:

[0153] V 降 =K(Vw+Vs)

[0154] Where: V is the capacity of the water tank, m 3 ; Vw is the design frequency rainstorm runoff, m 3 ; Vs is the cumulative sediment deposition volume in the designed desilting year, m 3 ; K is the safety factor.

[0155] The V of the region 需 <V 降 , V 需 185m 3 Preferably, two water storage tanks are configured, located at different elevations, with the upstream tank volume being 85m 3 , the volume of the downstream pool is 100m 3. Both pools are built with C20 reinforced concrete materials and are designed as cylindrical structures. The pool height is uniformly 3.0m, the upper pool diameter is 6.7m, and the lower pool diameter is 7.3m. The inner wall and bottom of the reservoir are provided with an anti-seepage layer to prevent leakage. A spillway is set on the top of the pool and connected to the drainage ditch system to discharge excess runoff during heavy rain. The water outlet of the reservoir is connected to the cultivated land irrigation canal below to supply water to the irrigation area; water allocation and interconnection between multiple reservoirs are achieved through the ecological ditch system to build a distributed, multi-node water storage and utilization network.

[0156] Some grit chambers are located in front of the reservoir's inlet to allow for initial sedimentation before runoff enters the reservoir, reducing the rate of sedimentation and extending its lifespan. These chambers are arranged in series with the reservoir, connected by a short diversion channel. This creates an integrated "sedimentation-water storage" process control unit, effectively regulating both sediment and runoff.

[0157] The first-level ecological drainage ditch is set in the outlet area of ​​the basin, and the channel cross-section adopts a trapezoidal structure. The cross-section parameters are calculated and determined according to the above-mentioned hydraulic formula. Preferably, the bottom width of the first-level drainage ditch is 1.2m, the height is 1.2m, and the slope ratio is 1:1.5, which is used to receive terminal runoff and carry out ecological purification treatment. Three layers of purification medium structure are laid in sequence at the bottom of the drainage ditch, including from bottom to top: the bottom layer is a gravel infiltration layer with a thickness of 30 to 40cm, the material ratio is coarse sand: gravel = 1:2, and the porosity is controlled at 30% to 35%, which is used for rapid infiltration and preliminary pollutant interception; the middle layer is a biochar mixed soil layer with a thickness of 20cm, and the mass proportion of biochar is 20% to 30%, which is used to enhance the adsorption capacity of organic pollutants, heavy metals, nitrogen, phosphorus and other elements, and provide a matrix for microbial attachment; the upper layer is planted with a combination of aquatic plants, among which the submerged plant is Vallisneria, and the planting density is 8 to 10 plants / m 2 The emergent plant is cattail, with a planting density of 4 to 6 plants / m 2 , using plant roots to enhance the purification function of the micro-ecosystem. The three-level structure works synergistically to achieve multiple purification functions such as solid-liquid separation, pollutant adsorption and ecological restoration, providing an effective biophysical treatment unit for terminal water bodies.

[0158] The river channel buffer plant belt is set outside the primary ecological drainage ditch, and a multi-layered vegetation structure is constructed in sequence to enhance the end-of-runoff purification capacity and the ecological stability of the ditch bank. The buffer belt is divided into the following areas from the inside out: near the shore: two rows of living wooden piles are set up to protect the bank. The piles are made of willow branches with a pile spacing of 0.6m, forming a flexible anti-scour structure to weaken the flow rate, stabilize the ditch bank, and reduce scour damage; the slope area: a composite vegetation belt of shrubs and grasses is planted, among which mulberry trees are used as shrubs, with a planting density of 1 plant / m 2 The herb is Zoysia japonica, and the sowing rate is 50kg / hm2 , used to increase surface coverage, stabilize soil, and reduce slope runoff intensity; in the peripheral area: a tree isolation forest belt is configured, with the preferred plant species being Salix yunnanensis, with a spacing of 3 to 4 meters, to enhance the ecological landscape, regulate the microclimate, and provide ecological buffering and noise isolation functions. The river buffer plant belt, through the composite configuration of "hydraulic structure + plant purification + ecological isolation", achieves multi-stage purification of terminal runoff, pollutant interception, and ecological restoration, constructing a comprehensive terminal treatment unit that integrates interception, filtration, slope consolidation, and beautification, significantly improving the water quality and ecological stability of the entire small watershed outlet.

[0159] The following calculation method is proposed for the water and soil conservation and pollution purification effects of the three modules of the source reduction, process control and terminal repair system prevention and control technology system:

[0160] The soil loss reduction assessment for the source reduction module is estimated using the USLE universal soil loss equation:

[0161] A=R·K·LS·C·P (8)

[0162] ΔA=A 无 -A 有 (9)

[0163] Where A is the annual soil loss (t / ha·yr); R is the rainfall erosivity factor; K is the soil erodibility factor; LS is the slope length and slope factor; C is the surface cover and management factor; P is the soil and water conservation measures factor; A 有 and A 无 are the annual soil loss with and without measures, respectively; ΔA is the reduction.

[0164] The sediment reduction efficiency of the process control module is estimated by referring to the formula in the "Design Specifications for Sedimentation Basins of Water Conservancy and Hydropower Projects" (SL269-2001):

[0165]

[0166] Where η s For sediment reduction efficiency; is the volume of settled sediment; is the total amount of sediment input.

[0167] The pollution load reduction of the process control module is estimated using the unit sediment load method (typically TP / TN):

[0168]

[0169] Where Cs is the unit attached pollution concentration of sediment; the total amount of TP or TN reduction (kg) is obtained.

[0170] The empirical value of the ecosystem purification efficiency of the terminal repair module is calculated as follows:

[0171]

[0172] Where η n Nitrogen / phosphorus / SS and other pollutant removal rates (%); C in and C out are the pollutant concentrations in the incoming and outgoing water bodies (mg / L), respectively.

[0173] The total reduction of the terminal repair module is estimated as follows:

[0174] L remobed =η n ·C in Q m (13)

[0175] Q m is the annual runoff volume (m 3 ), which can be estimated by the storm runoff formula (1).

[0176] Table 2 Benefit calculation statistics

[0177]

[0178]

[0179] The present invention adopts a three-stage management model of "source reduction - process control - end-of-pipe repair", integrating multiple measures such as terrain transformation, ecological drainage ditches, sand settling facilities and riverbank buffer zones to form a full-process, multi-node linkage management system, aiming to effectively reduce soil erosion, improve the ecological environment of the watershed, and enhance soil and water conservation effects.

Claims

1. An integrated method for preventing and controlling soil and water loss in an agricultural small watershed, characterized by: The method includes: conducting source reduction in sloping farmland areas with a slope greater than 6° to control the source of sloping farmland erosion, so as to control surface erosion and runoff expansion; conducting process control in the middle and upper reaches to the middle and lower reaches, blocking sediment migration and regulating the convergence speed through the linkage of a graded ditch system and sedimentation storage facilities; conducting terminal purification in the basin outlet area, purifying water quality and stabilizing the basin outlet through ecological engineering measures.

2. The method for controlling soil and water loss in an agricultural small watershed according to claim 1, characterized in that: The source reduction methods include: In sloping farmland areas with a slope of 6 to 25 degrees, terraces are built along the contour lines; On steep slope farmland with a slope of more than 25°, soil conservation tillage and micro-topography transformation are implemented.

3. The method for controlling soil and water loss in an agricultural small watershed according to claim 2, characterized in that: The terraced fields are built with rammed earth or stone ridges, with a ridge height of 20 to 30 cm, a ridge top width of >30 cm, a ridge bottom width of 40 to 60 cm, and a side slope gradient of ≤75°; deep-rooted shrub hedges are planted on the backwater side of the ridge, and the species are selected from Amorpha fruticosa or Daylily, with a plant spacing of 0.3 to 0.5 m to form a biological soil barrier, and at the same time, herbaceous plants are sown, and the species is selected from Sweet Elephant Grass.

4. The method for controlling soil and water loss in an integrated agricultural watershed according to claim 2, characterized in that: The soil conservation tillage and micro-topography transformation include: contour reverse slope tillage: excavating reverse slope steps along the contour line, with a step width of 80 to 100 cm, a reverse slope angle of 5° to 8°, and a step spacing of 1 to 2 m; furrow and ridge planting: ridge height of 15 to 20 cm, ridge spacing of 40 to 50 cm, and sowing green manure plants in the ridges and furrows, with a sowing amount of 20 to 30 kg per mu; surface covering: after the crops are harvested, the straw is crushed and returned to the field, with a covering thickness of ≥5 cm.

5. The method for controlling soil and water loss in an integrated agricultural watershed according to claim 1, characterized in that: This is achieved through the linkage of a graded ditch system and sedimentation facilities using a multi-level drainage and sediment deposition structure, which mainly includes: the secondary ditch is set in the main water collection ditch, the tertiary ditch is arranged in the branch ditch, and herbaceous plants are planted in the ditch to stabilize the ditch slope and slow down the flow rate. It is arranged at the downstream intersection to intercept the sediment carried by the upstream based on the quasi-stationary sedimentation principle, reduce the load of the downstream water storage and purification system, and ensure the primary purification of water quality; at the same time, reservoirs at different elevations are arranged along the slope catchment area to realize segmented storage, irrigation reuse and sediment deposition functions: each reservoir receives water from above, and effectively deposits sediment through sedimentation and residence time control; the downstream reservoir also serves as an irrigation water source, providing stable water replenishment support for agricultural production in the gentle slope area.

6. The method for controlling soil and water loss in an integrated agricultural small watershed according to claim 5, characterized in that: The ditch system includes: Secondary concrete trench: laid in the main trench with slope greater than 6°; The third-level grass-planting ditch is an earthen branch ditch connected to the second-level ditch, in which grass is planted, and the grass is bermudagrass or zoysia grass; The channel section is designed for a maximum rainfall of 10 minutes in a 5-year return period. The corresponding peak flow is calculated based on the ground slope and catchment area of ​​the channel. The calculation formula is: Q m =16.67φqF (1) Where: Q m is the peak flow generated by the design frequency; φ is the runoff coefficient; q is the average rainfall intensity within the design recurrence period and rainfall duration; F is the catchment area; Then, based on the designed flood peak flow and considering the actual terrain conditions through which the water flows, the cross-sectional dimensions and form of the intercepting ditch are calculated and selected. The calculation formula is: Where: Q is the design flow rate; A is the cross-sectional area of ​​the water flow; C is the Xie Cai coefficient, Where n is the roughness coefficient; R is the hydraulic radius; χ is the wetted perimeter and i is the gradient.

7. The method for controlling soil and water loss in an integrated agricultural small watershed according to claim 6, characterized in that: Methods for achieving sediment deposition structures include: Sedimentation tank design: The design adopts the quasi-static sedimentation method. The total sediment volume of the sedimentation tank is calculated as follows: Vs=λ×Ms×F / c (3) Where: V s is the total amount of sediment entering the sedimentation basin; λ is the transport ratio; Ms is the average soil erosion modulus; F is the catchment area; c is the sediment bulk density; Calculation of sedimentation tank volume: V=φ×Ws / n (4) Where: V is the volume of the sedimentation tank; φ is the efficiency of the sedimentation tank, which is taken as 75%; n is the number of times the sedimentation tank is cleared; Reservoir design The capacity of the reservoir is calibrated based on the dual factors of irrigation demand and storm runoff. First, the maximum annual irrigation water volume required for the crop is determined based on the local farmland irrigation water quota. The irrigation water demand is calculated as follows: V 需 =M×A / ω (5) Where: V 需 is the irrigation water requirement (m 3 ); M is the net irrigation quota (m 3 / mu); A is the irrigation area (mu); ω is the irrigation water utilization coefficient. The design frequency of rainstorm runoff in a small watershed with a return period of 10 years and 1 hour (GB 51018-2014) is used to calculate: Vw=1000Fh (6) Where: Vw is the design frequency rainstorm runoff, m 3 ; F is the rain-bearing area, km 2 ; h is the average runoff depth, mm. Calculation of total reservoir capacity: V 降 =K(Vw+Vs) (7) Where: V is the capacity of the reservoir; Vw is the design frequency of rainstorm runoff; Vs is the cumulative sediment deposition in the design desilting year; K is the safety factor; When V 需 >V 降 , adopt the equipment of irrigation and drainage facilities; V 需 ≤V 降 When using V 需 As the volume of the reservoir.

8. The method for controlling soil and water loss in an agricultural small watershed according to claim 1, characterized in that: End-of-pipe purification includes improving the water quality and ecological compensation of the final runoff by constructing a composite ecological purification zone, through ecological drainage ditches and river buffer plant belts; specifically, it includes: setting up gravel layers and biochar layers in the ecological drainage ditch as purification media, and planting submerged plants and emergent plants for water quality bioremediation; arranging multi-layer vegetation communities of trees, shrubs and herbs on the outside of the ditch to form a green buffer zone, enhance water stability and purify residual pollutants.

9. The method for controlling soil and water loss in an agricultural small watershed according to claim 1, characterized in that: The purification layer of the ecological drainage ditch includes: Bottom layer: 30-40 cm graded gravel layer, coarse sand: gravel = 1:2, porosity 30-35%; Middle layer: 20cm biochar mixed soil layer, biochar accounts for 20-30%; Upper layer: Plant submerged and emergent plants, submerged plants: Vallisneria, density 8-10 plants / m 2 Emergent plants: Cattail, density 4-6 plants / m 2 .

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