Farmland water and salt regulation and control method for recycling irrigation area recession water

Abstract

The invention discloses a farmland water and salt regulation and control method for recycling irrigation area recession water, and relates to the technical field of efficient utilization of water resources and agricultural water saving. The method comprises the following steps: by dynamically monitoring the water quantity and salinity of a farmland and a drainage ditch wetland system, establishing a discharge ratio between water outlet and water inlet of the drainage ditch wetland and a water quantity exchange ratio and a salinity exchange ratio between the drainage ditch system and the farmland; and forming a quantitative calculation method for the water-salt exchange relation between the farmland and the drainage ditch system under the condition of recession recycling. Meanwhile, a two-dimensional water and salt migration model is constructed based on HYDRUS software, the processes of drainage from the farmland to the drainage ditch system and reverse osmosis supply from the drainage ditch system to the farmland are simulated, and dynamic simulation of the farmland soil water and salt migration rule is achieved. By means of the method, the influence on farmland soil salinity under the condition of recession water recycling can be scientifically evaluated, drainage regulation and control strategies of external water in different areas are obtained, and therefore on the premise that farmland production safety is guaranteed, the utilization efficiency of water resources is improved, and salt accumulation is reduced.

Description

Technical Field

[0001] This application relates to the fields of efficient water resource utilization and agricultural water-saving technology, and in particular to a method for regulating farmland water and salt through the reuse of irrigation runoff. Background Technology

[0002] In irrigation districts of arid and semi-arid regions, water scarcity and soil salinization are prevalent problems, especially downstream. Due to the distance from water sources, irrigation water distribution is difficult to guarantee, leading to large areas of low-yield or even abandoned farmland. Meanwhile, runoff from upstream irrigation and some rainy season runoff (from outside the district) often temporarily accumulate and stagnate in downstream drainage ditches, forming reusable water resources. For example, the average annual runoff volume in the Jiaokou-Weihe irrigation district reaches 59.82 million cubic meters. 3 The average annual water discharge in the Jingdian Irrigation District of Gansu Province reaches as high as 150 million cubic meters. 3 When this part of the external water accumulates in the downstream drainage ditch, the water level in the drainage ditch is often higher than the groundwater level in the farmland. This part of the water replenishes the shallow groundwater in the farmland through reverse osmosis, thus possessing a certain potential for crop utilization.

[0003] However, the utilization of this type of "external water" carries certain risks: on the one hand, when the salinity of the external water is high, the salt will enter the soil through reverse osmosis, causing salt accumulation in the topsoil and threatening crop growth; on the other hand, when the replenishment is too large, it will raise the groundwater level, hindering farmland drainage and salt removal, disrupting the original water-salt balance, and even leading to secondary salinization. Existing drainage and irrigation management measures are mostly aimed at a one-way "water intake-drainage" approach, using irrigation water volumes that exceed the crop's water requirements by a certain proportion (leaching requirements) to achieve dynamic control of root zone salinity. This parameter is based on the steady-state assumption. However, the magnitude and distribution of soil salinity are the result of interactions between irrigation, evapotranspiration, leaching, and drainage. These interactions involve crop root water absorption and yield, soil salt accumulation and leaching, and are relatively complex. Their mechanisms are not yet fully understood or quantified. This leads to the following problems when using a leaching ratio based on the steady-state assumption to guide field salinity control: a. Leaching requirements are based on the maximum crop yield requirement, but maximum yield is often not the most economical yield. Relying solely on this parameter to guide agricultural irrigation leaching is insufficient; b. Neither the steady-state assumption nor the leaching parameter considers rainfall, salt precipitation and reaction, nor the leaching effect of preferential flow; c. Different crops and different growth conditions... In the long term, the salt tolerance limit varies at different growth stages. A uniform leaching ratio and steady-state assumption obviously ignore the different salt leaching requirements of crop growth patterns. Not every irrigation requires leaching. Furthermore, the degree of soil salinization and control requirements vary greatly in different irrigation districts or different locations within the same irrigation district. For example, upstream of an irrigation district generally has better natural drainage and abundant water resources. After several years of irrigation leaching and drainage management, salt levels can easily be controlled within the crop's salt tolerance limit. Only a small amount of salt needs to be leached from the soil profile each year to meet crop growth needs. Drainage systems and irrigation regimes designed according to the steady-state assumption still operate according to the uniform standards set at the beginning of the design, and some parts of the irrigation district clearly exceed the leaching requirements. Therefore, the previous use of a uniform "drainage-to-intake ratio" or "leaching ratio" value for the entire irrigation district has significant limitations. Moreover, this "leaching ratio" model assumes that salt only comes from irrigation water, neglecting the utilization of shallow groundwater and the salt brought into the soil surface by capillary action during shallow groundwater utilization. This is clearly inconsistent with the reality that saline-alkali land is low-lying and has shallow groundwater. Therefore, drainage management based on the "leaching ratio" and targeting "water diversion and drainage" lacks a systematic consideration of the two-way water-salt exchange mechanism in the process of wastewater reuse. The steady-state assumption also ignores the dynamic characteristics of farmland water and salt and neglects the value of wastewater reuse, resulting in water waste and making it difficult to simultaneously achieve efficient water resource utilization and safe soil salinity control.

[0004] Interception or artificial recharge in drainage ditches is the main means of achieving in-situ reuse of runoff. Appropriate drainage ditch depth management strategies are a prerequisite for ensuring the full utilization of water from outside the area. However, current theoretical research has not established effective quantitative water and salt analysis indicators, nor has it established the interrelationships among runoff utilization, farmland-drainage ditch water-salt interaction, drainage intensity, and drainage control depth. Furthermore, it lacks research on the dynamic laws governing soil water and salt transport under runoff reuse. Current technologies and drainage management strategies cannot accurately assess the impact of runoff utilization on soil salinity under different conditions, posing an uncontrollable risk of secondary soil salinization. This makes it difficult to apply current drainage ditch regulation research results to practical production.

[0005] In summary, existing theories and technologies lack an understanding of the bidirectional water-salt exchange patterns between farmland and drainage ditches, making it difficult to formulate drainage regulation and control solutions that both meet the requirements of efficient water resource utilization and ensure safe farmland production based on the amount of water flowing into different areas. Summary of the Invention

[0006] Therefore, it is necessary to provide a method for regulating farmland water and salt through the reuse of irrigation runoff, addressing the aforementioned technical problems.

[0007] The following technical solution is adopted in this specification: This manual provides a method for regulating farmland water and salt levels in the reuse of irrigation runoff, including: To obtain the inflow rate and salinity concentration of the drainage ditch system; to obtain the groundwater depth and soil salinity concentration of farmland adjacent to the drainage ditch system; and to obtain the drainage volume of the drainage ditch system. Based on the inflow and outflow of water from farmland and drainage ditch systems, the discharge ratio and water level depth of the drainage ditch system are obtained; based on the inflow salt concentration and the original water salt concentration of the drainage ditch system, the water salt concentration of the drainage ditch system after water inflow is obtained. Based on the groundwater depth in farmland, soil salinity in farmland, water and salt concentration in drainage ditch system, discharge ratio of drainage ditch system, and water level depth in drainage ditch system, a two-dimensional water and salt transport simulation model was established using HYDRUS to simulate the water and salt changes in farmland and drainage ditch system, and to obtain the farmland drainage volume and salt discharge volume, as well as the water volume and salt volume that seep back into the farmland from the drainage ditch system. Based on the amount of farmland drainage and salt discharge, as well as the amount of water and salt that seep back into the farmland from the drainage ditch system, the water exchange ratio and salt exchange ratio between the farmland and the drainage ditch system are obtained. Based on the discharge ratio, water exchange ratio, and salt exchange ratio, the proportion of wastewater reused from outside the farmland and drainage ditch system and the farmland water and salt regulation strategy were determined.

[0008] Furthermore, the discharge ratio of the drainage system includes: The formula for calculating the discharge ratio, used to characterize the drainage intensity of a drainage system, is as follows: ; in, The discharge ratio of the drainage ditch system; W in The amount of water entering the drainage system; Q out The amount of water flowing out of the drainage ditch system.

[0009] Furthermore, the calculation process for the water level depth of the drainage ditch system is as follows: Based on the inflow and outflow ratio of the drainage ditch system, the amount of water stored in the drainage ditch system and the change in water volume per unit length of the drainage ditch system are obtained. △w ; Based on the change in water volume within a unit length drainage ditch system △w The water level depth of the drainage ditch system is obtained; The change in water volume within the drainage ditch system per unit length △w The calculation formula is: ; in, △w This represents the change in water volume per unit length of the drainage ditch system. △h This refers to the change in water level in the drainage ditch system. H The depth of the drainage ditch system; i The slope coefficient of the drainage ditch system; This refers to the width of the bottom of the drainage ditch system; The formula for calculating the water level depth of the drainage ditch system is as follows: ; in, This indicates the water level depth of the drainage ditch system after water storage; This indicates the water level depth of the drainage ditch system before water storage.

[0010] Furthermore, the water exchange ratio between the farmland and the drainage system specifically includes: The ratio of farmland drainage water entering the drainage ditch system to the amount of water input into the farmland via reverse osmosis from the drainage ditch system is calculated using the following formula: ; in, γ w This refers to the water exchange ratio. W ag = D × A ag It is the amount of groundwater drained from farmland; A agIt refers to the area of ​​farmland; D This refers to the depth of underground drainage in farmland. W s = w s × A ag This refers to the amount of reverse osmosis replenishment for the drainage ditch system. w s The depth of the reverse osmosis replenishment water for the drainage ditch system.

[0011] Furthermore, the salt exchange ratio specifically includes: The ratio of salt discharge from farmland to salt input into farmland via reverse osmosis from the drainage system is calculated using the following formula: ; in, γ s This refers to the salt exchange ratio; c d The salt concentration in farmland groundwater drainage; c water This refers to the salt concentration of the water in the drainage system. salinity of water in drainage ditch system c water The calculation formula is: ; in, c r This represents the salt concentration in surface runoff. c q This refers to the salt concentration of the influent to the drainage ditch system. c water The salt concentration in the drainage ditch system and wetland system; W s (t) This refers to the water that seeps back into the groundwater of farmland from the drainage system; S p Salt uptake rate of plants per unit area; H s The equivalent depth for soil salt storage in drainage ditch systems; c s This represents the initial salt concentration of the soil in the drainage ditch system. H w (t) The depth of the water within the wetland.

[0012] Furthermore, the two-dimensional water-salt transport simulation model includes simulations of drainage from farmland to the drainage ditch system and simulations of reverse osmosis replenishment from the drainage ditch system to the farmland.

[0013] Furthermore, the drainage ditch system includes main ditches, branch ditches, sub-ditches, farm ditches, and furrow ditches, which are interconnected through channels to form a drainage ditch system.

[0014] Furthermore, the proportion of out-of-area water return and reuse in the farmland and drainage ditch system is related to the farmland water and salt regulation strategy. Based on the amount of water flowing into farmland and drainage systems from outside the area, a salt exchange ratio of 1 is used as the critical value: When the salt exchange ratio of farmland is equal to 1, the salt concentration of farmland reverse osmosis is equal to the actual concentration of groundwater in the field, and the water from outside the area in the drainage ditch will not change the salt concentration of groundwater in the field. When the salt exchange ratio is greater than 1, the salt concentration of farmland reverse osmosis is less than the concentration of groundwater in the field. The water from outside the area in the drainage ditch plays a role in diluting the salt concentration of groundwater, and the upstream runoff in the drainage ditch can be fully utilized. When the salt exchange ratio is less than 1, the salt concentration of reverse osmosis is greater than that of groundwater in the field. The reverse osmosis recharge effect of water from outside the area will have an adverse effect on the accumulation of salt in the farmland. When the accumulation reaches the salt tolerance limit of the crop root zone, leaching measures need to be taken for the farmland.

[0015] The above-mentioned technical solutions adopted in this specification can achieve the following beneficial effects: This specification provides a method for regulating farmland water and salt levels during the reuse of irrigation runoff. By proposing three quantitative indicators—"water exchange ratio," "salt exchange ratio," and "discharge ratio"—for farmland and drainage ditch systems, it achieves a quantitative characterization of the water and salt exchange relationship between farmland and drainage ditch systems during the reuse of farmland runoff under different external water inflow conditions. Under the premise of ensuring farmland safety, it realizes the reuse of runoff and rainfall runoff, reducing dependence on irrigation water sources.

[0016] Furthermore, by quantitatively analyzing the dynamic changes in soil salinity under different water inflow conditions, a suitable and operable strategy for controlling the burial depth of wastewater reuse was formulated to avoid secondary salinization caused by blindly utilizing wastewater and to ensure the yield of farmland crops. Attached Figure Description

[0017] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings:

[0018] Figure 1 This specification provides a simplified diagram and flowchart of a method for regulating soil water and salt in farmland through the reuse of irrigation runoff. Figure 2 This document provides a flowchart of a HYDRUS model simulation. Figure 3This specification provides a HYDRUS model simulation boundary condition diagram; Figure 4 This is a cross-sectional view of a drainage ditch system provided in this specification; Figure 5 This is a specific embodiment diagram provided in this specification; Figure 6 This is a diagram showing the water level depth of a drainage ditch system corresponding to the discharge ratio of a specific embodiment 1 provided in this specification. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of this specification clearer, the technical solutions of this application will be clearly and completely described below in conjunction with specific embodiments and corresponding drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments in this specification without creative effort are within the scope of protection of this application.

[0020] The following shortcomings exist in the existing technology for the reuse of irrigation runoff: (8) The lack of understanding of the two-way exchange pattern of water and salt between farmland and drainage ditches makes it impossible to scientifically assess the dynamic changes of farmland soil water and salt under the conditions of water use from outside the area. (9) Lack of quantitative characterization indicators means that existing studies cannot effectively describe the balance between external water supply to farmland and farmland drainage and salt removal needs; (10) The management strategy lacks specificity and it is difficult to formulate a drainage control plan that meets the requirements of efficient water resource utilization and ensures safe farmland production based on the amount of water coming from different areas.

[0021] The purpose of this invention is to propose a method for efficient utilization of runoff resources based on the water-salt exchange relationship of farmland-drainage ditch system, establish drainage management strategies adapted to different external water conditions, and improve the utilization rate of farmland water resources while controlling the risk of soil salinization.

[0022] The technical solutions provided by the various embodiments of this application are described in detail below with reference to the accompanying drawings.

[0023] Figure 1 This document provides a simplified overview and flowchart of the farmland soil water and salt regulation method for the reuse of irrigation runoff in this specification, which specifically includes the following steps: S101: Water from outside the monitoring area.

[0024] At the upstream inlet of the drainage system, water measuring weirs and water-salt sensors are installed to monitor the amount of water and salt concentration of water coming from outside the area, as well as the discharge water from the upstream irrigation area and the runoff from rainfall.

[0025] S102: Propose a key parameter system.

[0026] (1) Exchange ratio: including water exchange ratio (the ratio of farmland drainage into the ditch to the amount of water input into the farmland through reverse osmosis in the drainage ditch) and salt exchange ratio (the ratio of farmland salt discharge to the amount of salt input into the farmland through reverse osmosis in the drainage ditch), used to characterize the amount of water used from outside the area and the farmland soil drainage demand; Water exchange ratio: ; Salt exchange ratio: ; in, W ag It is the amount of groundwater drained from farmland, in meters. 3 , W ag = D × A ag D represents the depth of underground drainage in farmland, in meters. W s The reverse osmosis recharge rate of the drainage ditch, in m 3 , W s = w s × A ag ; w s The depth of the reverse osmosis replenishment water for the drainage ditch, in meters; c d Salt concentration in farmland groundwater drainage, g / L -1 ; c water The salinity concentration of the water in the drainage ditch, in g / L. -1 .

[0027] (2) Discharge ratio: refers to the ratio of the amount of water discharged from the drainage system to the amount of water entering the drainage system, used to characterize the drainage intensity of the drainage system. ; in, W in The volume of water entering the drainage system, in m 3 ; Q out The volume of water flowing out of the drainage system, m 3 .

[0028] S103: Establish water-salt exchange relationships.

[0029] (1) Establish the relationship between the inflow and outflow ratio of water from outside the area and the water level depth of the drainage ditch. Based on the inflow and outflow ratio of water from outside the area, the amount of water stored in the drainage ditch and the change in water volume per unit length (1 m) of the drainage ditch can be determined. wIt can be calculated as follows:

[0030] ; Among them, △ w The change in water volume per unit length (1 m) of the drainage ditch is expressed in m³; △ h △ represents the change in water level in the drainage ditch. h = h 2- h 1, m, the negative sign indicates that when the water level in the drainage ditch rises, the total water volume increases, and the change is positive; conversely, the change in water volume is negative.

[0031] The change in water level depth and water volume in the drainage ditch △w Relationship: ; (2) Construct a dynamic simulation model of water-salt exchange between farmland and drainage ditch under external water inflow. Based on the HYDRUS model, the groundwater depth of the drainage ditch is used as the input value of the HYDRUS model (variable head boundary) to simulate the dynamic water-salt exchange between farmland and drainage ditch.

[0032] S104: Optimize drainage management strategies.

[0033] Model simulations were used to derive the relationship between external water inflow, discharge ratio, exchange ratio, and control burial depth. When the salt exchange ratio equals 1, the salt concentration from reverse osmosis equals the actual groundwater concentration, having no effect on increasing the salt concentration of groundwater in the field. When the salt exchange ratio is greater than 1, the salt concentration from reverse osmosis is less than the concentration of groundwater in the field, thus diluting the groundwater salt concentration. When the salt exchange ratio is less than 1, the salt concentration from reverse osmosis is greater than the concentration of groundwater in the field. In this case, reverse osmosis recharge has an adverse effect on salt accumulation in farmland, requiring leaching when the accumulation reaches the salt tolerance limit of the crop root zone. Using a salt exchange ratio of 1 as the critical value, the critical discharge ratio under different external water inflow rates was determined, thereby determining the control drainage burial depth to achieve efficient utilization of irrigation runoff and coordinated regulation of farmland.

[0034] S105: Engineering support measures.

[0035] Drainage is controlled by installing controllable gates at the regional drainage outlets and controlling the drainage depth based on the critical discharge ratio; intelligent regulation is achieved in conjunction with water level sensors.

[0036] Example 1: Monitoring and Modeling of Typical Farmland and Drainage Ditch Wetland Systems refer to Figure 5The study area was Lupotan, which receives runoff from the Jiaokou-Chouwei Irrigation District and the Erhuang Irrigation District. Typical farmland and adjacent drainage ditches were used as the research objects. Runoff from the upstream irrigation district entered the study area from the westernmost point through the M ditch. Due to the higher terrain east of M11 and the obstructed drainage outlet, when the inflow was large, it could not be discharged in time. Some of the water flowed back into the N ditch, P ditch, and farm ditch, resulting in reverse seepage replenishment of the farmland.

[0037] 1. Monitoring in the study area: Collect inflows such as irrigation runoff and rainy season runoff in the upstream area and conduct quantitative monitoring.

[0038] (1) Monitoring of drainage inflow and outflow: The discharge ratio β under the current drainage mode is 0.582.

[0039] (2) Groundwater depth monitoring: Through observation well monitoring, the groundwater level depth in farmland is between 1.5 and 2.3 m, with an average value of 2.1 m; the average groundwater level in ditches is 1.97 m; (3) Soil salinity monitoring: Soil conductivity was measured using a Hach HQ14d conductivity meter and converted into soil salinity concentration. The average salinity of farmland soil was 2.75 g kg. -1 This meets the salt tolerance requirements of crops (4.72 g kg). -1 ) 2. Calculation of water level depth in drainage ditch: Based on the amount of water coming from outside the area and the drainage pattern of the irrigation area, calculate the amount of water that can be stored in the drainage ditch and the water level depth of the drainage ditch after receiving water from outside the area. According to Formula 5, the burial depth of drainage ditch water level under different discharge ratios was calculated. The burial depth of drainage ditch water level under different discharge ratios in this study area is shown in [reference needed]. Figure 6 .

[0040] 3. Calculation of water-salt exchange ratio between farmland and drainage ditch: Based on the burial depth of the drainage ditch, the soil salinity and drainage salt discharge of farmland are simulated and analyzed, and the exchange ratio is calculated.

[0041] Based on such Figure 2 The HYDRUS software shown is used to create typical farmland and drainage ditch cross-sections (such as...). Figure 4 A two-dimensional water-salt transport model (as shown) Figure 3 As shown in the figure, the simulation includes: Part 1: the process of farmland draining into the drainage ditch; Part 2: the process of drainage ditch recharge into farmland via reverse osmosis. The simulation yields the farmland groundwater drainage volume under different external water inflow rates. D Reverse seepage recharge of drainage ditch W s The water-salt exchange ratio is calculated according to formulas (1) and (2).

[0042] 4. Calculation of critical discharge ratio: Based on the relationship between the salt exchange ratio and discharge ratio under different external water inflow scenarios, the critical discharge ratio is determined.

[0043] Using the critical salt exchange ratio (CRR) that does not increase soil salinity as the standard, and based on the relationship between the CRR and the discharge ratio, the critical discharge ratio values ​​for different external water inflow amounts were obtained. When the external water inflow is less than 20 mm, the critical discharge ratio... β The critical discharge ratio is 0.25 when the inflow from outside the area is 20-30 mm. β The critical discharge ratio is 0.31 when the inflow from outside the area is 30-40 mm. β The critical discharge ratio is 0.56 when the inflow from outside the area is 40-50 mm. β It is 0.67.

[0044] 5. Drainage engineering measures: Based on the critical discharge ratio, determine the appropriate drainage control depth and propose corresponding drainage engineering measures.

[0045] According to the groundwater level depth corresponding to the critical discharge ratio (e.g.) Figure 6 As shown in the figure, the groundwater control depth in the study area under different inflow conditions can be determined. When the inflow from outside the area is less than 20 mm, the control depth is 1.35 m; when the inflow is 20-30 mm, the control depth is 1.45 m; when the inflow is 30-40 mm, the control depth is 1.90 m; and when the inflow is 40-50 mm, the control depth is 2.20 m. Water level regulation at the drainage outlet is achieved by adding control facilities such as gates at the outlet, and intelligent control can be realized with the help of sensors.

[0046] The results verified the feasibility and effectiveness of wastewater reuse in water and salt regulation. Analysis showed that this method can control farmland salinity below the crop salt tolerance threshold while improving water resource utilization efficiency by approximately 20%.

[0047] Compared with the prior art, the present invention has the following advantages and effects: 1. Innovative parameter construction: Two quantitative indicators, "exchange ratio" and "discharge ratio", are proposed, which for the first time realize the quantitative characterization of the water-salt exchange relationship between farmland and drainage ditches in the process of reusing water from outside the area, and can quantitatively analyze the internal water-salt exchange process of farmland and drainage system; 2. Improve water resource utilization: Water and salt regulation can be achieved by controlling drainage measures, eliminating the need to measure drainage volume and salinity. Under the premise of ensuring farmland safety, the reuse of runoff and rainfall can be realized, reducing dependence on irrigation water sources. 3. Preventing soil salinization risk: By quantitatively analyzing soil salinity changes under different water inflow conditions, secondary salinization caused by blindly utilizing runoff can be avoided, thus ensuring crop yields in farmland. 4. High scalability: This method has good versatility. Drainage control can be achieved by adding control facilities such as gates or baffles at the downstream drainage outlet of the irrigation area. It can be widely applied to the water and salt management engineering practice of irrigation areas in arid and semi-arid regions.

[0048] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

Claims

1. A method for regulating farmland water and salt levels through the reuse of irrigation runoff, characterized in that, include: Obtain the inflow rate and salinity concentration of the drainage system; Obtain the groundwater depth and soil salinity concentration of farmland adjacent to the drainage system; And obtain the drainage volume of the drainage ditch system; Based on the inflow and outflow of water from farmland and drainage ditch systems, the discharge ratio and water level depth of the drainage ditch system are obtained; based on the inflow salt concentration and the original water salt concentration of the drainage ditch system, the water salt concentration of the drainage ditch system after water inflow is obtained. Based on the groundwater depth in farmland, soil salinity in farmland, water and salt concentration in drainage ditch system, discharge ratio of drainage ditch system, and water level depth in drainage ditch system, a two-dimensional water and salt transport simulation model was established using HYDRUS to simulate the water and salt changes in farmland and drainage ditch system, and to obtain the farmland drainage volume and salt discharge volume, as well as the water volume and salt volume that seep back into the farmland from the drainage ditch system. Based on the amount of farmland drainage and salt discharge, as well as the amount of water and salt that seep back into the farmland from the drainage ditch system, the water exchange ratio and salt exchange ratio between the farmland and the drainage ditch system are obtained. Based on the discharge ratio, water exchange ratio, and salt exchange ratio, the proportion of wastewater reused from outside the farmland and drainage ditch system and the farmland water and salt regulation strategy were determined.

2. The method for regulating farmland water and salt levels by reusing irrigation runoff as described in claim 1, characterized in that, The discharge ratio of the drainage ditch system includes: The formula for calculating the discharge ratio, used to characterize the drainage intensity of a drainage system, is as follows: ； in, W is the discharge ratio of the drainage ditch system. in Q represents the volume of water entering the drainage system. out The amount of water flowing out of the drainage ditch system.

3. The method for regulating farmland water and salt levels by reusing irrigation runoff as described in claim 1, characterized in that, The calculation process for the water level depth of the drainage ditch system is as follows: Based on the inflow and outflow ratio of the drainage ditch system, the amount of water stored in the drainage ditch system and the change in water volume per unit length of the drainage ditch system, Δw, are obtained. The water level depth of the drainage ditch system can be obtained by calculating the change in water volume Δw within the drainage ditch system per unit length. The formula for calculating the change in water volume Δw within the unit length drainage ditch system is as follows: ； Where △w is the change in water volume per unit length of the drainage ditch system; △h is the change in water level of the drainage ditch system; H is the depth of the drainage ditch system; and i is the slope coefficient of the drainage ditch system. This refers to the width of the bottom of the drainage ditch system; The formula for calculating the water level depth of the drainage ditch system is as follows: ； in, This indicates the water level depth of the drainage ditch system after water storage; This indicates the water level depth of the drainage ditch system before water storage.

4. A method for regulating farmland water and salt levels by reusing irrigation runoff as described in claim 1, characterized in that, The water exchange ratio between the farmland and the drainage ditch system specifically includes: The ratio of farmland drainage water entering the drainage ditch system to the amount of water input into the farmland via reverse osmosis from the drainage ditch system is calculated using the following formula: ； Where, γ w Water exchange ratio; W ag =D×A ag It is the amount of groundwater drained from farmland; A ag D is the area of ​​farmland; W is the depth of underground drainage in farmland. s =w s ×A ag For the reverse osmosis recharge of the drainage ditch system; w s The depth of the reverse osmosis replenishment water for the drainage ditch system.

5. A method for regulating farmland water and salt levels by reusing irrigation runoff as described in claim 1, characterized in that, The salt exchange ratio specifically includes: The ratio of salt discharge from farmland to salt input into farmland via reverse osmosis from the drainage system is calculated using the following formula: ； Where, γ s c is the salt exchange ratio. d The salt concentration of groundwater drainage from farmland; c water This refers to the salt concentration of the water in the drainage system. salinity concentration c in the drainage ditch system water The calculation formula is: ； Among them, c r c represents the salt concentration in surface runoff. q c is the salt concentration of the influent to the drainage system. water The salinity concentration of the drainage ditch system and wetland system; W s (t) represents the water that seeps back into the farmland groundwater from the drainage system; S p The salt uptake rate of plants per unit area; H s The equivalent depth for soil salt storage in drainage ditch systems; c s The initial salinity concentration of the soil in the drainage ditch system; H w (t) represents the water depth within the wetland.

6. A method for regulating farmland water and salt levels by reusing irrigation runoff as described in claim 1, characterized in that, The two-dimensional water and salt transport simulation model includes simulations of drainage from farmland to the drainage ditch system and simulations of reverse osmosis replenishment from the drainage ditch system to farmland.

7. A method for regulating farmland water and salt levels by reusing irrigation runoff as described in claim 1, characterized in that, The drainage ditch system includes main ditches, branch ditches, sub-ditches, farm ditches, and furrow ditches, which are interconnected through channels to form a drainage ditch system.

8. A method for regulating farmland water and salt levels by reusing irrigation runoff as described in claim 1, characterized in that, The proportion of out-of-area water return and reuse in farmland and drainage ditch systems and the farmland water and salt regulation strategy. Based on the amount of water flowing into farmland and drainage systems from outside the area, a salt exchange ratio of 1 is used as the critical value: When the salt exchange ratio of farmland is equal to 1, the salt concentration of farmland reverse osmosis is equal to the actual concentration of groundwater in the field, and the water from outside the area in the drainage ditch will not change the salt concentration of groundwater in the field. When the salt exchange ratio is greater than 1, the salt concentration of farmland reverse osmosis is less than the concentration of groundwater in the field. The water from outside the area in the drainage ditch plays a role in diluting the salt concentration of groundwater, and the upstream runoff in the drainage ditch can be fully utilized. When the salt exchange ratio is less than 1, the salt concentration of reverse osmosis is greater than that of groundwater in the field. The reverse osmosis recharge effect of water from outside the area will have an adverse effect on the accumulation of salt in the farmland. When the accumulation reaches the salt tolerance limit of the crop root zone, leaching measures need to be taken for the farmland.

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