A layout optimization method for a buried pipe salt drainage system in a frozen soil region

Abstract

This invention discloses a method for optimizing the layout of a subsurface drainage system for permafrost regions in the field of agricultural water and soil engineering. The method includes: acquiring the soil temperature field and pore water pressure field of the target area; inputting the soil temperature field and pore water pressure field of the target area into a pre-processed improved freeze-thaw coupling model to obtain the spatiotemporal distribution of field moisture, temperature, and salinity; and determining the key layout parameters of the subsurface drainage system based on the spatiotemporal distribution of field salinity. The key layout parameters include the spacing, burial depth, and pipe diameter of the subsurface drainage system. This invention accurately simulates the physically realistic water-heat-salt coupled transport process in seasonally frozen soil regions, providing a direct basis for the quantitative optimization of key engineering parameters such as subsurface spacing and burial depth, and providing an effective theoretical model and calculation tool for the precise design of subsurface drainage projects in cold saline-alkali lands.

Description

Technical Field

[0001] This invention relates to a method for optimizing the layout of a buried pipe salt drainage system in permafrost regions, belonging to the field of agricultural water and soil engineering technology. Background Technology

[0002] In arid and semi-arid regions of northern and northwestern my country, secondary soil salinization is a core bottleneck restricting sustainable agricultural development. Subsurface drainage technology, utilizing groundwater level control and soil leaching principles, is an effective engineering measure for treating saline-alkali land. However, in these regions, long and harsh winters cause the soil to undergo intense freeze-thaw cycles. The water and salt transport mechanisms during this process differ fundamentally from those during the non-freezing period, posing significant challenges to the design and operation of subsurface drainage projects.

[0003] Current underground pipe engineering designs often suffer from a "winter-to-spring" phenomenon, meaning that even a well-designed system may be less effective during the spring snowmelt and salt removal season. The root cause of this weakened effectiveness lies in the fact that existing computational models are severely detached from physical reality, and their limitations are mainly reflected in the following aspects:

[0004] (1) Distortion in the simulation of freeze-thaw coupling mechanism: Traditional soil water and salt transport models (such as HYDRUS, SHAW, etc.) often treat ice as a static pore filler when dealing with permafrost problems, only considering its impediment effect on permeability. However, in physical reality, the phase change of water at the freezing front generates a strong suction force (i.e., freeze-suction effect), driving water in unfrozen areas to migrate and accumulate towards the freezing front. Existing models often ignore or have difficulty quantifying this effect on a macroscopic scale, resulting in serious deviations from reality in the prediction of groundwater level fluctuations and salt accumulation on the surface during the freezing period.

[0005] (2) Oversimplification of boundary conditions for underground pipes: In existing numerical simulations, underground pipes are often simplified to constant head or constant flow boundary conditions. This simplification fails to reflect the "intermittent operation" characteristic of underground pipes during freeze-thaw cycles—that is, when the soil freezes, the underground pipes are essentially stagnant due to ice blockage or cessation of infiltration recharge; while in the early stages of thawing, the drainage volume of underground pipes increases explosively as the ice melts and generates a large amount of infiltration water. Simple boundary conditions cannot capture this dynamic response, resulting in an inability to accurately assess the impact of underground pipe spacing and burial depth on the concentrated salt removal effect during the thawing period.

[0006] (3) Lack of theoretical support for engineering design: Due to the lack of calculation tools that can accurately describe the interaction of "water-heat-salt-cave pipe" during the freeze-thaw period, the current design of cave pipe projects often relies on empirical formulas or parameters during the non-freezing period, which leads to an underestimation of the amount of salt return during the freezing period after the project is completed, resulting in a decrease in efficiency during the spring meltwater salt discharge period, or investment waste due to overly conservative design. Summary of the Invention

[0007] The technical problem to be solved by this invention is to provide a method for optimizing the layout of a buried pipe salt drainage system in permafrost areas. This method addresses the problem that existing water and salt transport models ignore the unique freezing and suction effect in permafrost areas, leading to a significantly lower prediction of surface salt accumulation during the spring snowmelt period. This, in turn, misleads designers into using excessively large buried pipe spacing, resulting in a weakened salt drainage effect in the project.

[0008] To achieve the above objectives, the present invention employs the following technical solution:

[0009] This invention proposes a method for optimizing the layout of a buried pipe salt drainage system in permafrost regions, comprising:

[0010] Obtain the soil temperature field and pore water pressure field of the target area;

[0011] The soil temperature field and pore water pressure field of the target area are input into a pre-processed improved freeze-thaw coupling model to obtain the spatiotemporal distribution of field moisture, temperature and salinity.

[0012] Based on the spatiotemporal distribution of salinity in the field, the key layout parameters of the underground pipe salt drainage system are determined; the key layout parameters include the spacing, burial depth and pipe diameter of the underground pipe salt drainage system.

[0013] The processing procedure of the improved freeze-thaw coupling model includes:

[0014] Set the boundary conditions for the model;

[0015] The soil temperature field and pore water pressure field of the target area are obtained. Based on the soil temperature field, pore water pressure field and the pre-constructed dual constraint judgment criterion of liquid water saturation, the liquid water saturation of each point in the target area is calculated. Based on the pre-obtained total water saturation and the liquid water saturation, the ice saturation of each point in the target area is calculated.

[0016] Based on the liquid water saturation and ice saturation, the thermal conductivity, effective heat capacity and relative permeability of the soil are calculated.

[0017] The fluid source and sink terms are calculated based on the pre-constructed fluid source and sink terms, an ice storage space mechanism, and the ice saturation.

[0018] Based on the liquid water saturation, ice saturation, soil thermal conductivity, effective heat capacity and relative permeability, and fluid source and sink terms, the groundwater flow equation, energy transfer equation and solute transfer equation are solved in a coupled manner to obtain the spatiotemporal distribution of field moisture, temperature and salinity.

[0019] The boundary conditions for setting the model include:

[0020] The upper boundary is set as a first-class temperature boundary to reflect changes in atmospheric temperature;

[0021] Set the lower boundary as a flow-free boundary;

[0022] Set the boundary of the concealed pipe drainage as a dynamic seepage boundary.

[0023] The pressure value of the dynamic seepage boundary A constant value is set, either atmospheric pressure or the control head for drainage inside the concealed pipe; the pore water pressure at the soil nodes adjacent to the dynamic seepage boundary within the calculation domain. At this time, liquid water seeps out to the boundary under the drive of the pressure gradient, and its flux is determined by Darcy's law;

[0024] At the dynamic seepage boundary, a zero concentration gradient is set for solute transport, allowing salts to be freely discharged with the seepage flow. The formula is as follows:

[0025] ;

[0026] in, This refers to the solute concentration. For the boundary normal.

[0027] The method, based on the dual constraint criterion of the soil temperature field, pore water pressure field, and pre-constructed liquid water saturation, calculates the liquid water saturation at each point within the target area, and calculates the ice saturation at each point within the target area based on the pre-acquired total water saturation and the liquid water saturation, including:

[0028] When the porous medium is fully saturated, the liquid water saturation is determined by the upper limit of the saturated liquid water content, which is only affected by temperature, and the formula is as follows:

[0029] ;

[0030] in, For liquid water saturation, The maximum liquid water saturation that is only affected by temperature. For soil temperature field;

[0031] When the porous medium is in an unsaturated state, the liquid water saturation is constrained by both the maximum liquid water saturation affected only by temperature and the total water saturation affected only by pore water pressure. The formula is as follows:

[0032] ;

[0033] in, The total water saturation is affected only by pore water pressure. The pore water pressure field;

[0034] Ice saturation is the difference between total water saturation and liquid water saturation. Total water saturation is only related to the pressure at that point, and its formula is:

[0035] ;

[0036] ;

[0037] in, For ice saturation, Total water saturation Calculated using the van Genuchten model;

[0038] The formula for calculating the maximum liquid water saturation that is only affected by temperature is:

[0039] ;

[0040] or:

[0041] ;

[0042] in, The saturation level of the residual liquid water during the freezing process; This is the freezing temperature of liquid water. = ; These are the fitting parameters for the exponential function; The slope of the piecewise linear function; The temperature at which residual liquid water saturation occurs; (x) is an exponential function with the natural constant e as the base, where x is the exponent.

[0043] The calculation of the thermal conductivity and effective heat capacity of the soil includes:

[0044] Soil is generalized as a mixture of soil particles, liquid water, and ice. The thermal conductivity of the soil is calculated by taking the weighted average of the thermal conductivity of these three components, using the following formula:

[0045] ;

[0046] in, The thermal conductivity of the soil. Porosity For liquid water saturation, The thermal conductivity of liquid water, For ice saturation, Let be the thermal conductivity of ice. The thermal conductivity of soil particles;

[0047] The effective heat capacity of the soil is calculated by taking the weighted average of the heat capacities of the soil particles, liquid water, and ice, and considering the latent heat during the phase transition. The formula is as follows:

[0048] ;

[0049] in, For the effective heat capacity of the soil, The density of liquid water, The density of ice, The density of soil particles, The specific heat capacity of liquid water, The specific heat capacity of ice. This refers to the specific heat capacity of soil particles. For fusion latent heat, For soil temperature field.

[0050] The calculation of the relative permeability includes:

[0051] When the porous medium is in an unsaturated state, the relative permeability is calculated based on the saturation of liquid water, using the following formula:

[0052] ;

[0053] ;

[0054] in, This refers to relative penetration rate. To preserve parameters for water, For liquid water saturation, The saturation level of the residual liquid water during the freezing process;

[0055] When the porous medium is fully saturated, the minimum possible value of the relative permeability is set, expressed as: when season in, This represents the minimum possible value of the relative permeability.

[0056] The fluid source and sink terms are calculated based on a pre-constructed fluid source and sink term, introducing an ice storage space mechanism and the ice saturation, including:

[0057] The formula for setting the ice storage space for each unsaturated soil node is as follows:

[0058] ;

[0059] in, It is an ice storage space, the size of which is equal to the gas phase size of the unsaturated soil node. Total water saturation;

[0060] During the freezing process, newly formed ice is moved from the freezing nodes of the unsaturated soil into the ice storage space via a fluid conduit, using the following formula:

[0061] ;

[0062] in, It is a fluid source / sink. It is the time step. Porosity Ice saturation;

[0063] When the ice storage space of the freezing node is completely filled, or when the relative permeability of the freezing node drops to a preset minimum possible value, the movement of ice into the ice storage space is stopped, and the remaining ice is retained in the system.

[0064] Before the frozen node thaws, the ice in the ice storage space is reintroduced into the model as a fluid source term, with the following formula:

[0065] ;

[0066] in, It refers to the ice saturation that exists in the ice storage space.

[0067] The groundwater flow equation, energy transport equation, and solute transport equation are solved using a sequential non-iterative method.

[0068] The groundwater flow equation is:

[0069] ;

[0070] ;

[0071] in, For time, For Darcy speed, For inherent penetration rate, The relative permeability is unsaturated. For fluid viscosity, For the pore water pressure field, It is the acceleration due to gravity;

[0072] The energy transfer equation is as follows:

[0073] ;

[0074] in, It is an identity matrix. It is the temperature of the fluid source / sink;

[0075] The solute transport equation is as follows:

[0076] ;

[0077] in, For pore water salinity, The apparent molecular diffusivity of the solute. For the mechanical dispersion tensor, The concentration of the solute in the fluid source.

[0078] The determination of key layout parameters for the subsurface drainage system based on the spatiotemporal distribution of salinity in the field includes:

[0079] Based on the spatiotemporal distribution of salinity in the field, the spatial location and salinity dynamics of high-salinity accumulation zones in the root zone soil are identified.

[0080] Based on the spatial location and salt dynamics of the high-salt accumulation area, the optimal subsurface pipe layout parameters for salt drainage in the field area were quantitatively determined through multi-scenario simulation and comparison.

[0081] The beneficial effects achieved by this invention are as follows:

[0082] (1) By introducing a dual-constraint liquid water saturation determination criterion, the coordinated control of the water phase state by the temperature field and the pore water pressure field is realized;

[0083] (2) An ice storage space mechanism was innovatively introduced, which dynamically simulates the migration of ice between the storage space and the system by defining the space size and the corresponding source / sink terms;

[0084] (3) By applying sink and source terms respectively during the freezing and thawing stages, the pore water pressure change process driven by water-ice phase transition in unsaturated soil was accurately reproduced. Attached Figure Description

[0085] Figure 1 A flowchart illustrating a method for optimizing the layout of a subsurface salt drainage system in permafrost regions, provided by this invention.

[0086] Figure 2 This is a flowchart of the calculation process of the present invention;

[0087] Figure 3 This is a schematic diagram of the model's geometry;

[0088] Figure 4 The liquid water saturation characteristic curve of the soil;

[0089] Figure 5 This is a schematic diagram of the freezing suction effect and ice storage space.

[0090] Figure 6 This is a flowchart illustrating the mechanism of the cryosuction effect. Detailed Implementation

[0091] The present invention will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and should not be used to limit the scope of protection of the present invention.

[0092] Example 1, as Figure 1 As shown, this invention discloses a method for optimizing the layout of a buried pipe salt drainage system in permafrost areas, comprising:

[0093] Obtain the soil temperature field and pore water pressure field of the target area;

[0094] The soil temperature field and pore water pressure field of the target area are input into a pre-processed improved freeze-thaw coupling model to obtain the spatiotemporal distribution of field moisture, temperature and salinity.

[0095] Based on the spatiotemporal distribution of salinity in the field, the key layout parameters of the underground pipe salt drainage system are determined; the key layout parameters include the spacing, burial depth and pipe diameter of the underground pipe salt drainage system.

[0096] The processing procedure of the improved freeze-thaw coupling model includes:

[0097] Set the boundary conditions for the model;

[0098] The soil temperature field and pore water pressure field of the target area are obtained. Based on the soil temperature field, pore water pressure field and the pre-constructed dual constraint judgment criterion of liquid water saturation, the liquid water saturation of each point in the target area is calculated. Based on the pre-obtained total water saturation and the liquid water saturation, the ice saturation of each point in the target area is calculated.

[0099] Based on the liquid water saturation and ice saturation, the thermal conductivity, effective heat capacity and relative permeability of the soil are calculated.

[0100] The fluid source and sink terms are calculated based on the pre-constructed fluid source and sink terms, an ice storage space mechanism, and the ice saturation.

[0101] Based on the liquid water saturation, ice saturation, soil thermal conductivity, effective heat capacity, relative permeability, and fluid source-sink terms, the groundwater flow equation, energy transfer equation, and solute transfer equation are solved in a coupled manner to obtain the spatiotemporal distribution of field moisture, temperature, and salinity. Based on the groundwater flow equation, energy transfer equation, and solute transfer equation, a well-constructed improved freeze-thaw coupling model is generated.

[0102] The boundary conditions for setting the model include:

[0103] The upper boundary is set as a first-class temperature boundary to reflect changes in atmospheric temperature;

[0104] Set the lower boundary as a flow-free boundary;

[0105] Set the boundary of the concealed pipe drainage as a dynamic seepage boundary.

[0106] The pressure value of the dynamic seepage boundary A constant value is set, either atmospheric pressure or the control head for drainage inside the concealed pipe; the pore water pressure at the soil nodes adjacent to the dynamic seepage boundary within the calculation domain. At this time, liquid water seeps out to the boundary under the drive of the pressure gradient, and its flux is determined by Darcy's law;

[0107] At the dynamic seepage boundary, a zero concentration gradient is set for solute transport, allowing salts to be freely discharged with the seepage flow. The formula is as follows:

[0108] ;

[0109] in, This refers to the solute concentration. For the boundary normal.

[0110] The spacing, burial depth, and diameter parameters of the underground pipes can directly affect the simulated distribution of the two-dimensional pore water pressure field, velocity field, and salinity field within the computational domain.

[0111] The method, based on the dual constraint criterion of the soil temperature field, pore water pressure field, and pre-constructed liquid water saturation, calculates the liquid water saturation at each point within the target area, and calculates the ice saturation at each point within the target area based on the pre-acquired total water saturation and the liquid water saturation, including:

[0112] When the porous medium is fully saturated, the liquid water saturation is determined by the upper limit of the saturated liquid water content, which is only affected by temperature, and the formula is as follows:

[0113] ;

[0114] in, For liquid water saturation, The maximum liquid water saturation that is only affected by temperature. For soil temperature field;

[0115] When the porous medium is in an unsaturated state, the liquid water saturation is constrained by both the maximum liquid water saturation affected only by temperature and the total water saturation affected only by pore water pressure. The formula is as follows:

[0116] ;

[0117] in, The total water saturation is affected only by pore water pressure. The pore water pressure field;

[0118] Ice saturation is the difference between total water saturation and liquid water saturation. Total water saturation is only related to the pressure at that point, and its formula is:

[0119] ;

[0120] ;

[0121] in, For ice saturation, Total water saturation Calculated using the van Genuchten model;

[0122] The formula for calculating the maximum liquid water saturation that is only affected by temperature is:

[0123] ;

[0124] or:

[0125] ;

[0126] in, The saturation level of the residual liquid water during the freezing process; This is the freezing temperature of liquid water. = ; These are the fitting parameters for the exponential function; The slope of the piecewise linear function; The temperature at which residual liquid water saturation occurs; (x) is an exponential function with the natural constant e as the base, where x is the exponent.

[0127] The calculation of the thermal conductivity and effective heat capacity of the soil includes:

[0128] Soil is generalized as a mixture of soil particles, liquid water, and ice. The thermal conductivity of the soil is calculated by taking the weighted average of the thermal conductivity of these three components, using the following formula:

[0129] ;

[0130] in, The thermal conductivity of the soil. Porosity For liquid water saturation, The thermal conductivity of liquid water, For ice saturation, Let be the thermal conductivity of ice. The thermal conductivity of soil particles;

[0131] The effective heat capacity of the soil is calculated by taking the weighted average of the heat capacities of the soil particles, liquid water, and ice, and considering the latent heat during the phase transition. The formula is as follows:

[0132] ;

[0133] in, For the effective heat capacity of the soil, The density of liquid water, The density of ice, The density of soil particles, The specific heat capacity of liquid water, The specific heat capacity of ice. This refers to the specific heat capacity of soil particles. For fusion latent heat, For soil temperature field.

[0134] The calculation of the relative permeability includes:

[0135] When the porous medium is in an unsaturated state, the relative permeability is calculated based on the saturation of liquid water, using the following formula:

[0136] ;

[0137] ;

[0138] in, This refers to relative penetration rate. To preserve parameters for water, For liquid water saturation, The saturation level of the residual liquid water during the freezing process;

[0139] When the porous medium is fully saturated, the minimum possible value of the relative permeability is set, expressed as: when season in, This represents the minimum possible value of the relative permeability.

[0140] The step of introducing an ice storage space mechanism based on a pre-constructed fluid source-sink term and calculating the fluid source-sink term based on the ice saturation includes:

[0141] The formula for setting the ice storage space for each unsaturated soil node is as follows:

[0142] ;

[0143] in, It is an ice storage space, the size of which is equal to the gas phase size of the unsaturated soil node. Total water saturation;

[0144] During the freezing process, newly formed ice is moved from the freezing nodes of the unsaturated soil into the ice storage space via a fluid conduit, using the following formula:

[0145] ;

[0146] in, It is a fluid source / sink. It is the time step. Porosity Ice saturation;

[0147] When the ice storage space of the freezing node is completely filled, or when the relative permeability of the freezing node drops to a preset minimum possible value, the movement of ice into the ice storage space is stopped, and the remaining ice is retained in the system.

[0148] Before the frozen node thaws, the ice in the ice storage space is reintroduced into the model as a fluid source term, with the following formula:

[0149] ;

[0150] in, It refers to the ice saturation that exists in the ice storage space.

[0151] The groundwater flow equation, energy transport equation, and solute transport equation are solved using a sequential non-iterative method.

[0152] The groundwater flow equation is:

[0153] ;

[0154] ;

[0155] in, For time, For Darcy speed, For inherent penetration rate, The relative permeability is unsaturated. For fluid viscosity, For the pore water pressure field, It is the acceleration due to gravity;

[0156] The energy transfer equation is as follows:

[0157] ;

[0158] in, It is an identity matrix. It is the temperature of the fluid source / sink;

[0159] The solute transport equation is as follows:

[0160] ;

[0161] in, For pore water salinity, The apparent molecular diffusivity of the solute. For the mechanical dispersion tensor, The concentration of the solute in the fluid source.

[0162] The determination of key layout parameters for the subsurface drainage system based on the spatiotemporal distribution of salinity in the field includes:

[0163] Based on the spatiotemporal distribution of salinity in the field, the spatial location and salinity dynamics of high-salinity accumulation zones in the root zone soil are identified.

[0164] Based on the spatial location and salt dynamics of the high-salt accumulation area, the optimal subsurface pipe layout parameters for salt drainage in the field area were quantitatively determined through multi-scenario simulation and comparison.

[0165] Example 2, based on the same inventive concept as Example 1, provides a method for optimizing the layout of a buried pipe salt drainage system in permafrost regions. The calculation process is as follows: Figure 2 As shown, by introducing the ice storage space mechanism and dynamic seepage boundary, the process of deep salt migration driven by water-ice phase change and the burst drainage characteristics of underground pipes are accurately reproduced in numerical simulation, thus providing accurate quantitative design basis for saline-alkali land management. The steps are as follows.

[0166] Step 1: Establish an improved freeze-thaw coupling model to reasonably reconstruct the three parameters of soil: thermal properties, freezing function, and permeability.

[0167] S1: Set the boundary conditions for the model.

[0168] like Figure 3 As shown, the computational domain of the freeze-thaw coupling model is a two-dimensional profile, and its boundary conditions are set as follows:

[0169] (1) The upper boundary is set as a first-class temperature boundary to reflect changes in atmospheric temperature;

[0170] (2) Set the lower boundary as a flow-free boundary;

[0171] (3) The boundary of the concealed pipe drainage is set as a dynamic seepage boundary, and the boundary pressure value is... A constant value is set, either atmospheric pressure or the control head for drainage inside the concealed pipe. This is the pore water pressure at soil nodes adjacent to the seepage boundary within the computational domain. At this point, liquid water seeps towards the boundary driven by the pressure gradient, and its flux is determined by Darcy's law; at this boundary, a zero concentration gradient condition is applied to solute transport, allowing salts to be freely discharged with the seeping water flow, i.e. ,in, This refers to the solute concentration. For the boundary normal.

[0172] S2: Based on thermal conductivity To measure the thermal conductivity of soil, soil is generalized as a mixture of solid matrix, water, and ice. The overall thermal conductivity is calculated as a weighted average of these three components, using the following formula:

[0173] ;

[0174] in, The thermal conductivity of the soil. Porosity The saturation level of liquid water. The saturation level of ice. The thermal conductivity of liquid water, Let be the thermal conductivity of ice. denoted as , where is the thermal conductivity of the soil particles.

[0175] Through effective heat capacity The amount of heat required to raise the temperature of soil by one unit is calculated as the weighted arithmetic mean of the heat capacities of the matrix components (liquid water, ice, and solid particles), taking into account the latent heat during phase transition. The formula is as follows:

[0176] ;

[0177] in, For the effective heat capacity of the soil, The density of liquid water, The density of ice, The density of soil particles, The specific heat capacity of liquid water, The specific heat capacity of ice. This refers to the specific heat capacity of soil particles. It is the latent heat of fusion. For soil temperature field.

[0178] S3: When the porous medium is fully saturated, the saturation of liquid water and ice is determined by the upper limit of the saturated liquid water content, which is only affected by temperature. The formula is as follows:

[0179] ;

[0180] in, This represents the maximum liquid water saturation that is only affected by temperature.

[0181] In practical use, The calculation formula is:

[0182] ;

[0183] or:

[0184] ;

[0185] in, The saturation level of the residual liquid water during the freezing process; This is the freezing temperature of liquid water. = ; These are the fitting parameters for the exponential function; The slope of the piecewise linear function; The temperature at which residual liquid water saturation occurs.

[0186] For any point in the model, its total water saturation (the sum of liquid water and ice saturation) is only related to the pressure at that point, and its formula is:

[0187] ;

[0188] in, Total water saturation The total water saturation is affected only by pore water pressure. The pore water pressure field; Calculated using the classic van Genuchten model.

[0189] When the porous medium is in an unsaturated state, the liquid water saturation is subject to dual constraints: the maximum liquid water saturation, which is only affected by temperature, and the total water saturation. That is, according to... Calculate, while not exceeding the total water saturation. The formula is to take the smaller of the maximum liquid water saturation determined by temperature and the total water saturation determined by pressure, and the formula is as follows:

[0190] ;

[0191] The liquid water saturation characteristic curve of the soil is as follows: Figure 4 As shown.

[0192] Ice saturation is the total water saturation minus the liquid water saturation, and the formula is:

[0193] ;

[0194] in, For ice saturation; this formula provides dynamic logical judgments at each unit and time step to determine the upper limit of liquid water content, coupled with the temperature field. and pressure field The combined effects on the aqueous phase.

[0195] S4: In an unsaturated frozen groundwater system, the relative permeability depends on the saturation of the liquid water, and the formula is:

[0196] ;

[0197] ;

[0198] in, This refers to relative penetration rate. Parameters are retained for water;

[0199] Even if the soil is completely frozen, the model still allows for extremely small amounts of water migration, therefore a minimum possible value for relative permeability needs to be set to avoid [unclear]. Its formula is:

[0200] ;

[0201] in, This represents the minimum possible value of the relative permeability.

[0202] Step 2: In the freeze-thaw coupling model, an ice storage space mechanism is introduced to simulate the freeze-thaw effect.

[0203] like Figure 5 As shown, during freeze-thaw cycles, the water-ice phase transition significantly affects pore water pressure, driving nearby liquid water to migrate towards the freezing front—a phenomenon known as the freeze-thaw effect. In unsaturated soils, the presence of a gas phase in the pore space provides conditions for water redistribution during the phase transition process. To accurately capture this crucial physical process, this invention introduces a phase transition source / sink term to simulate the dynamic changes in pore water pressure.

[0204] like Figure 6 As shown, the ice storage space for each unsaturated soil node is first defined. Its formula is:

[0205] ;

[0206] in, It is an ice storage space, the size of which is the size of the gas phase at that node.

[0207] During the freezing process, newly formed ice is moved from the freezing nodes into the ice storage space through a sinking process. This operation reduces the total water saturation of the nodes, thus accurately reflecting the decrease in pore water pressure and simulating the freezing absorption effect. The formula is as follows:

[0208] ;

[0209] in, It is a fluid source / sink. It is the time step;

[0210] When the ice storage space of a node is completely filled, or when the relative permeability of a node drops to a preset minimum possible value, the movement of ice into the ice storage space stops, and the remaining ice is retained in the system.

[0211] Before the nodes are thawed, the ice in the ice storage space is reintroduced into the model as a source term, and the formula is as follows:

[0212] ;

[0213] in, It refers to the ice saturation that exists in the ice storage space.

[0214] Ice does not undergo a phase change when entering or leaving the ice storage space. Its specific heat capacity and thermal conductivity still participate in the energy balance calculation of the node, but do not participate in the solute exchange process.

[0215] Step 3: Couple the solution of the groundwater flow equation, energy transport equation and solute transport equation to simulate the spatiotemporal distribution of field moisture, temperature and salinity;

[0216] S1: The improved equation governing groundwater flow is:

[0217] ;

[0218] ;

[0219] in, For time, For Darcy speed, For inherent penetration rate, The relative permeability is unsaturated. For fluid viscosity, This is the acceleration due to gravity.

[0220] S2: The improved control energy transfer equation is:

[0221] ;

[0222] in, It is an identity matrix. It is the temperature of the fluid source / sink.

[0223] S3: The improved equation for controlling solute transport is:

[0224] ;

[0225] in, For pore water salinity, The apparent molecular diffusivity of the solute. For the mechanical dispersion tensor, The concentration of the solute in the fluid source.

[0226] When the water-ice phase transition occurs, the solute present remains in the liquid component, while ice is assumed to be free of solute. This characteristic is achieved through… This is achieved through the item.

[0227] Step 4: Based on the simulated spatiotemporal distribution of salinity in the field, determine the key layout parameters for the underground pipe project. First, obtain the spatiotemporal distribution data of salinity in the target area obtained from Step 3 simulation within one or more complete freeze-thaw cycles. Then, based on the salinity spatiotemporal distribution data, identify the high-salt accumulation zone in the root zone and the dynamic change pattern of salinity. Next, according to the spatial location and salinity dynamics of the high-salt accumulation zone, select the boundary conditions and operating conditions corresponding to the improved freeze-thaw coupling model. Finally, based on the selected model operating conditions, quantitatively determine the underground pipe layout parameters for optimal salt drainage effect in the field area through multi-scenario simulation comparison. The layout parameters include underground pipe spacing, burial depth, and pipe diameter.

[0228] Compared with existing technologies, this invention addresses the core deficiency of existing models such as SUTRA 4.0 in simulating freeze-thaw cycles by neglecting the freezing suction effect. It first introduces a dual-constraint liquid water saturation criterion. This achieved coordinated control of the water phase state by the temperature field and the pore water pressure field; then, an innovative ice storage space mechanism was introduced, by defining the size of the space. The model dynamically simulates the migration of ice between the storage space and the system, along with corresponding source / sink terms. Finally, by applying sink and source terms during the freezing and thawing stages respectively, it accurately reproduces the pore water pressure change process driven by the water-ice phase transition in unsaturated soil. Furthermore, unlike existing models that typically simplify underground pipes to fixed flow rates or fixed head boundaries, this invention, for the first time, employs dynamic seepage boundary conditions in a freeze-thaw coupling model to physically characterize the drainage and salt removal functions of underground pipes. This innovative approach allows: the influence of the two-dimensional engineering layout characteristics of underground pipes (such as burial depth and spacing) to be directly reflected in the model calculation results; the drainage capacity of underground pipes to dynamically respond to changes in soil hydraulic properties during freeze-thaw processes, making the simulation more physically realistic; and the model to accurately calculate the water and salt flux discharged through underground pipes, providing a direct basis for the quantitative optimization of key engineering parameters such as pipe spacing and burial depth.

[0229] In summary, the present invention can accurately simulate the physically realistic water-heat-salt coupled transport process in seasonally frozen soil regions, providing an effective theoretical model and computational tool for the precise design and optimization of underground pipe salt drainage projects in cold saline-alkali lands. It also provides crucial technical support for farmland water and salt regulation and ecological environment management during freeze-thaw cycles, contributing to improved agricultural water resource utilization efficiency and food security. The improved mechanism proposed in this invention is universally applicable, not only to underground pipe salt drainage simulation in agriculture but also extendable to numerical simulation studies of all unsaturated frozen soil water-heat coupled processes, including cold-region hydrogeology, frozen soil engineering, and pollutant migration.

[0230] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0231] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0232] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0233] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0234] The embodiments of the present invention have been described above with reference to the accompanying drawings. However, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many other forms under the guidance of the present invention without departing from the spirit and scope of the claims. All of these forms are within the protection scope of the present invention.

Claims

1. A method for optimizing the layout of a buried pipe salt drainage system in permafrost regions, characterized in that, include: Obtain the soil temperature field and pore water pressure field of the target area; The soil temperature field and pore water pressure field of the target area are input into a pre-processed improved freeze-thaw coupling model to obtain the spatiotemporal distribution of field moisture, temperature and salinity. Based on the spatiotemporal distribution of salinity in the field, the key layout parameters of the underground pipe salt drainage system are determined; the key layout parameters include the spacing, burial depth and pipe diameter of the underground pipe salt drainage system. The processing procedure of the improved freeze-thaw coupling model includes: Set the boundary conditions for the model; The soil temperature field and pore water pressure field of the target area are obtained. Based on the soil temperature field, pore water pressure field and the pre-constructed dual constraint judgment criterion of liquid water saturation, the liquid water saturation of each point in the target area is calculated. Based on the pre-obtained total water saturation and the liquid water saturation, the ice saturation of each point in the target area is calculated. Based on the liquid water saturation and ice saturation, the thermal conductivity, effective heat capacity and relative permeability of the soil are calculated. The fluid source and sink terms are calculated based on the pre-constructed fluid source and sink terms, an ice storage space mechanism, and the ice saturation. Based on the liquid water saturation, ice saturation, soil thermal conductivity, effective heat capacity and relative permeability, and fluid source and sink terms, the groundwater flow equation, energy transfer equation and solute transfer equation are solved in a coupled manner to obtain the spatiotemporal distribution of field moisture, temperature and salinity.

2. The method according to claim 1, wherein, The boundary conditions for setting the model include: The upper boundary is set as a first-class temperature boundary to reflect changes in atmospheric temperature; Set the lower boundary as a flow-free boundary; Set the boundary of the concealed pipe drainage as a dynamic seepage boundary.

3. The method for optimizing the layout of a buried pipe salt drainage system in permafrost areas according to claim 2, characterized in that, The pressure value of the dynamic seepage boundary A constant value is set, either atmospheric pressure or the control head for drainage inside the concealed pipe; when the pore water pressure of the soil nodes adjacent to the dynamic seepage boundary within the calculation domain... At this time, liquid water seeps out to the boundary under the drive of the pressure gradient, and its flux is determined by Darcy's law; At the dynamic seepage boundary, a zero concentration gradient is set for solute transport, allowing salts to be freely discharged with the seepage flow. The formula is as follows: ； wherein is the solute concentration, is the boundary normal.

4. The method of layout optimization of subsurface drainage system in permafrost regions for salt removal according to claim 1, wherein, The method, based on the dual constraint criterion of the soil temperature field, pore water pressure field, and pre-constructed liquid water saturation, calculates the liquid water saturation at each point within the target area, and calculates the ice saturation at each point within the target area based on the pre-acquired total water saturation and the liquid water saturation, including: When the porous medium is fully saturated, the liquid water saturation is determined by the upper limit of the saturated liquid water content, which is only affected by temperature, and the formula is as follows: ； wherein, is the liquid water saturation, is the maximum liquid water saturation influenced by temperature only, is the soil temperature field; When the porous medium is in an unsaturated state, the liquid water saturation is constrained by both the maximum liquid water saturation affected only by temperature and the total water saturation affected only by pore water pressure. The formula is as follows: ； wherein, S0is the total water saturation affected by only the pore water pressure, PWPis the pore water pressure field; Ice saturation is the difference between total water saturation and liquid water saturation. Total water saturation is only related to the pressure at that point, and its formula is: ； ； where, is ice saturation, is total water saturation, is calculated from the van Genuchten model.

5. The method according to claim 4, wherein, The formula for calculating the maximum liquid water saturation that is only affected by temperature is: ； or: ； in, The saturation level of the residual liquid water during the freezing process; This is the freezing temperature of liquid water. = ; These are the fitting parameters for the exponential function; The slope of the piecewise linear function; The temperature at which residual liquid water saturation occurs; (x) is an exponential function with the natural constant e as the base, where x is the exponent.

6. The method for optimizing the layout of a buried pipe salt drainage system in permafrost areas according to claim 1, characterized in that, The calculation of the thermal conductivity and effective heat capacity of the soil includes: Soil is generalized as a mixture of soil particles, liquid water, and ice. The thermal conductivity of the soil is calculated by taking the weighted average of the thermal conductivity of these three components, using the following formula: ； in, The thermal conductivity of the soil. Porosity For liquid water saturation, The thermal conductivity of liquid water is... For ice saturation, Let be the thermal conductivity of ice. The thermal conductivity of soil particles; The effective heat capacity of the soil is calculated by taking the weighted average of the heat capacities of the soil particles, liquid water, and ice, and considering the latent heat during the phase transition. The formula is as follows: ； in, For the effective heat capacity of the soil, The density of liquid water, The density of ice, The density of soil particles, The specific heat capacity of liquid water, The specific heat capacity of ice. This refers to the specific heat capacity of soil particles. For fusion latent heat, For soil temperature field.

7. The method for optimizing the layout of a buried pipe salt drainage system in permafrost areas according to claim 1, characterized in that, The calculation of the relative permeability includes: When the porous medium is in an unsaturated state, the relative permeability is calculated based on the saturation of liquid water, using the following formula: ； ； in, This refers to relative penetration rate. To preserve parameters for water, For liquid water saturation, The saturation level of the residual liquid water during the freezing process; When the porous medium is fully saturated, the minimum possible value of the relative permeability is set, expressed as: when season in, This represents the minimum possible value of the relative permeability.

8. The method for optimizing the layout of a buried pipe salt drainage system in permafrost areas according to claim 1, characterized in that, The step of introducing an ice storage space mechanism based on a pre-constructed fluid source-sink term and calculating the fluid source-sink term based on the ice saturation includes: The formula for setting the ice storage space for each unsaturated soil node is as follows: ； in, It is an ice storage space, the size of which is equal to the gas phase size of the unsaturated soil node. Total water saturation; During the freezing process, newly formed ice is moved from the freezing nodes of the unsaturated soil into the ice storage space via a fluid conduit, using the following formula: ； in, It is a fluid source / sink. It is the time step. Porosity Ice saturation; When the ice storage space of the freezing node is completely filled, or when the relative permeability of the freezing node drops to a preset minimum possible value, the movement of ice into the ice storage space is stopped, and the remaining ice is retained in the system. Before the frozen node thaws, the ice in the ice storage space is reintroduced into the model as a fluid source term, with the following formula: ； in, It refers to the ice saturation that exists in the ice storage space.

9. The method for optimizing the layout of a buried pipe salt drainage system in permafrost areas according to claim 6, characterized in that, The groundwater flow equation, energy transport equation, and solute transport equation are solved using a sequential non-iterative method. The groundwater flow equation is: ； ； in, For time, For Darcy speed, For inherent penetration rate, The relative permeability is unsaturated. For fluid viscosity, For the pore water pressure field, It is the acceleration due to gravity; The energy transfer equation is as follows: ； in, It is an identity matrix. It is the temperature of the fluid source / sink; The solute transport equation is as follows: ； in, For pore water salinity, The apparent molecular diffusivity of the solute. For the mechanical dispersion tensor, The concentration of the solute in the fluid source.

10. The method for optimizing the layout of a buried pipe salt drainage system in permafrost areas according to claim 1, characterized in that, The determination of key layout parameters for the subsurface drainage system based on the spatiotemporal distribution of salinity in the field includes: Based on the spatiotemporal distribution of salinity in the field, the spatial location and salinity dynamics of high-salinity accumulation zones in the root zone soil are identified. Based on the spatial location and salt dynamics of the high-salt accumulation area, the optimal subsurface pipe layout parameters for salt drainage in the field area were quantitatively determined through multi-scenario simulation and comparison.

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