Simulation system and method for farmland freeze-thaw soil hydrothermal coupling simulation effect

Abstract

The invention provides a simulation system and method for a farmland freeze-thaw soil hydrothermal coupling simulation effect, and relates to the technical field of freeze-thaw soil hydrothermal coupling mechanisms, the freeze-thaw peak surface of frozen soil is analyzed to obtain a frozen soil melting state index and a frozen soil phase change state index, and the melting state and the phase change state of the frozen soil are determined; selecting a Richard equation as a moisture migration equation, taking the phase change state of the frozen soil as a phase change dynamic regulation factor, and introducing the phase change dynamic regulation factor into the Richard equation to construct a final moisture migration equation; selecting a one-dimensional thermal diffusion equation as a heat conduction equation, introducing the melting state of the frozen soil into the heat conduction equation to construct a final heat conduction equation, and combining the final moisture migration equation and the final heat conduction equation to form a hydrothermal coupling model; and simulating the constructed hydrothermal coupling model by using HYDRUS software, and outputting a simulation result of the hydrothermal coupling model.

Description

Technical Field

[0001] The present invention relates to the technical field of freeze-thaw soil water-heat coupling mechanism, and in particular to a simulation system and method for simulating the water-heat coupling effect of freeze-thaw soil in farmland. Background Art

[0002] In cold-region agricultural ecosystems, freeze-thaw interactions significantly alter soil water and heat transport mechanisms. Traditional coupled water-heat models typically employ fixed-parameter water transport and heat conduction equations, failing to effectively characterize the combined effects of soil pore phase transitions and inter-layer thermal property differences during the dynamic evolution of the freeze-thaw front. Especially when the freeze-thaw front migrates to the soil interface, complex physical processes such as differences in specific heat capacity caused by heterogeneous organic matter distribution and sudden changes in pore connectivity caused by ice-water phase transitions can lead to deviations in traditional models' simulations of water redistribution and heat conduction rates. The freeze-thaw front, the interface between the frozen and thawed states of soil, may disappear during a freeze-thaw cycle. Simulating only the melting phase of a freeze-thaw cycle typically assumes the freeze-thaw front persists throughout the melting process, and simulations often assume its constant presence.

[0003] Existing methods for quantifying the freeze-thaw retardation effect mostly rely on empirical thresholds and lack a dynamic parameter adjustment mechanism based on pore-scale phase transition probability. This makes it difficult to accurately characterize the nonlinear response characteristics of water and heat transfer when the freeze-thaw front is blocked, which restricts the accuracy of soil water and heat process prediction in cold regions.

[0004] The above information disclosed in this Background section is only for enhancement of understanding of the background of the present disclosure and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art. Summary of the Invention

[0005] The purpose of the present invention is to provide a simulation system and method for simulating the water-heat coupling effect of farmland freeze-thaw soil, so as to solve the problems raised in the above background technology.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A simulation system for simulating the coupled water-heat effect of freeze-thaw soil in farmland, specifically including:

[0008] The freeze-thaw analysis module is used to analyze the freeze-thaw peak surface of frozen soil, extract the frozen soil thawing state index that characterizes the soil thawing process, and the frozen soil phase change state index that characterizes the change in the soil physical state, and determine the thawing state and phase change state of frozen soil;

[0009] The water selection module selects the Richard equation as the water migration equation to simulate the vertical migration of soil water in the unsaturated range under freeze-thaw conditions. The phase change state of frozen soil is used as a dynamic adjustment factor for phase change and introduced into the Richard equation. The response relationship between soil hydraulic conductivity and moisture content is adjusted by the dynamic adjustment factor for phase change to construct the final water migration equation.

[0010] The heat selection module is used to select the one-dimensional heat diffusion equation as the heat conduction equation to describe the conduction and changes of temperature within the soil profile. The thawing state of permafrost is introduced into the heat conduction equation to construct a final heat conduction equation that reflects the impact of the thawing state of permafrost on heat conduction. The final water migration equation and the final heat conduction equation are combined to form a water-heat coupling model that simulates the co-evolution of water and heat during the freeze-thaw period.

[0011] The result simulation module is used to simulate the constructed hydrothermal coupling model using HYDRUS software and output the simulation results of the hydrothermal coupling model.

[0012] Furthermore, the thawing state of the frozen soil includes a normal state and a blocked state, which are determined by a frozen soil thawing state index. Specifically, the area located on one side of the freeze-thaw peak surface and in a frozen state is defined as a frozen area, and the area located on one side of the freeze-thaw peak surface and in a melted state is defined as a melted area. The soil organic matter content and soil thermal conductivity of the frozen area and the melted area are obtained. The frozen soil thawing state index is calculated based on the difference in soil organic matter content and soil thermal conductivity between the frozen area and the melted area. A threshold value of the frozen soil thawing state index is preset. If the frozen soil thawing state index is less than or equal to the threshold value of the frozen soil thawing state index, the frozen soil thawing state is a normal state. If the frozen soil thawing state index is greater than the threshold value of the frozen soil thawing state index, the frozen soil thawing state is a blocked state. The specific formula for calculating the frozen soil thawing state index is:

[0013]

[0014] Among them, Rh is the permafrost thawing state indicator, OM D is the soil organic matter content in the frozen area, OM R is the soil organic matter content in the melting area, λ D is the thermal conductivity of the soil in the frozen area, λ R is the thermal conductivity of the soil in the melting area, max(OM D ,OM R ) and max(λ D ,λ R ) are OM D ,OM R The maximum value and λ in D ,λ R The maximum value among , used for normalization.

[0015] Furthermore, the phase transition state of the frozen soil includes a normal thawing state and a blocked thawing state, which are determined by the frozen soil phase transition state index. Specifically, a frozen soil pore thawing probability model is constructed based on the volume, water content, and ice content of the pores on the freeze-thaw peak surface, which is specifically expressed as:

[0016]

[0017] Among them, RP i is the melting probability of the i-th pore on the freeze-thaw peak surface, V i is the volume of the i-th pore on the freeze-thaw peak surface, θ wi is the water content of the i-th pore on the freeze-thaw peak surface, θ ci is the ice content of the ith pore on the freeze-thaw peak surface, T is the temperature of the freeze-thaw peak surface, T f is the freezing temperature, i is the index of the pore on the freeze-thaw peak surface, and k is the proportional constant;

[0018] The probability of frozen soil thawing of each pore on the freeze-thaw peak surface is calculated according to the frozen soil pore thawing probability model. A random number U is set for the frozen soil thawing probability of each pore. i , and U i ∈[0,1], if U i <RP i , then the i-th pore on the freeze-thaw peak is judged to be melted, the melting of the pores on the freeze-thaw peak is counted, and the frozen soil phase change state index is generated according to the melting of the pores on the freeze-thaw peak. The frozen soil phase change state index averages the frozen soil melting probability and the thaw-freeze ratio. The specific formula is:

[0019]

[0020]

[0021] in, is the average probability of frozen soil thawing, DR is the thaw-freeze ratio, N is the total number of pores on the freeze-thaw peak surface, and N D is the number of melting pores on the freeze-thaw peak surface;

[0022] If any one of the normal thawing conditions is met, the permafrost thawing state is judged to be the normal thawing state; otherwise, the permafrost thawing state is judged to be the obstructed thawing state; the normal thawing conditions include normal thawing condition 1 and normal thawing condition 2. Normal thawing condition 1 is that the thaw-freeze ratio is greater than 2; normal thawing condition 2 is that the thaw-freeze ratio is less than 2 but greater than 1.5, and the average permafrost thawing probability is greater than 0.6.

[0023] Furthermore, the final moisture migration equation is expressed as:

[0024]

[0025] where δ is the dynamic regulation factor of phase change, θ is the volumetric water content, t is time, z is the freeze-thaw depth, h is the water potential, K(θ) is the soil hydraulic conductivity, Ks is the saturated hydraulic conductivity, θs is the saturated water content, θr is the residual water content, l is the pore connectivity parameter, and m is the curve shape parameter, m∈(0,1).

[0026] Furthermore, the final heat conduction equation is expressed as:

[0027]

[0028] Where ε is the melting state adjustment factor, ρ is the soil density, c is the specific heat capacity of the soil, T is the soil temperature, and λ is the soil thermal conductivity.

[0029] Furthermore, the simulation results of the output water-heat coupling model include temperature distribution, moisture distribution, and water-heat coupling relationship.

[0030] The present invention further provides a simulation method for simulating the hydrothermal coupling effect of farmland freeze-thaw soil. The method is executed to implement the simulation system for simulating the hydrothermal coupling effect of farmland freeze-thaw soil. The specific steps include:

[0031] Step 1: Analyze the freeze-thaw peak surface of the frozen soil, extract the frozen soil thawing state index that characterizes the soil thawing process, and the frozen soil phase change state index that characterizes the change in the soil physical state, and determine the thawing state and phase change state of the frozen soil;

[0032] Step 2: Select the Richard equation as the water migration equation to simulate the vertical migration process of soil moisture in the unsaturated range under freeze-thaw conditions. The phase change state of frozen soil is used as a phase change dynamic adjustment factor and introduced into the Richard equation. The response relationship between soil hydraulic conductivity and moisture content is adjusted by the phase change dynamic adjustment factor to construct the final water migration equation.

[0033] Step 3: Select the one-dimensional heat diffusion equation as the heat conduction equation to describe the conduction and changes of temperature within the soil profile. Introduce the thawing state of permafrost into the heat conduction equation to construct a final heat conduction equation that reflects the effect of the thawing state of permafrost on heat conduction. Combine the final water migration equation with the final heat conduction equation to form a hydrothermal coupling model that simulates the co-evolution of water and heat during the freeze-thaw period.

[0034] Step 4: Use HYDRUS software to simulate the constructed hydrothermal coupling model and output the simulation results of the hydrothermal coupling model.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] The present invention has achieved for the first time a multi-dimensional quantitative characterization of the freeze-thaw front obstruction mechanism by constructing a dual diagnostic system of permafrost thawing state indicators and phase change state indicators. Among them, the freeze-thaw state discrimination based on the difference in organic matter content and thermal conductivity effectively captures the blocking effect of thermal property mutations at the soil interface on heat conduction; and the phase change probability model that integrates the pore water-ice ratio and volume parameters reveals the dynamic regulation mechanism of the ice-water phase change process on hydraulic conductivity from a microscopic scale. By converting the above indicators into dynamic adjustment factors of the hydrothermal equation, an innovative nonlinear mapping relationship between the freeze-thaw front migration obstruction state and macroscopic hydrothermal parameters was established, which significantly improved the model's simulation accuracy for complex phenomena such as water infiltration hysteresis and heat conduction attenuation in the freeze-thaw alternating zone. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 Schematic diagram of the overall system structure of the present invention;

[0038] Figure 2 This is a diagram showing changes in soil temperature and moisture content along with freeze-thaw peaks simulated by the present invention;

[0039] Figure 3 The figure is a diagram showing the movement speed of the freeze-thaw peak surface simulated by the present invention along with the freeze-thaw change of the freeze-thaw peak surface;

[0040] Figure 4 Schematic diagram of the overall method of the present invention. DETAILED DESCRIPTION

[0041] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to specific embodiments.

[0042] It should be noted that, unless otherwise defined, the technical or scientific terms used in the present invention should have the usual meanings understood by people with ordinary skills in the field to which the present invention belongs. The words "first", "finally" and similar words used in the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. Words such as "include" or "comprise" mean that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects. Words such as "connect" or "connected" are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used to indicate relative position relationships. When the absolute position of the object being described changes, the relative position relationship may also change accordingly.

[0043] Example:

[0044] See also Figure 1 -,3 The invention provides a technical solution:

[0045] A simulation system for simulating the coupled water-heat effect of freeze-thaw soil in farmland, comprising the following steps:

[0046] The freeze-thaw analysis module is used to analyze the freeze-thaw peak surface of frozen soil, extract the frozen soil thawing state index that characterizes the soil thawing process and the frozen soil phase change state index that characterizes the change in the soil physical state, and determine the thawing state and phase change state of frozen soil;

[0047] The thawing state of the frozen soil includes a normal state and a blocked state, which is determined by a frozen soil thawing state index. Specifically, the area located on one side of the freeze-thaw peak surface and in a frozen state is defined as a frozen area, and the area located on one side of the freeze-thaw peak surface and in a melted state is defined as a melted area. The soil organic matter content and soil thermal conductivity of the frozen area and the melted area are obtained. The frozen soil thawing state index is calculated based on the difference in soil organic matter content and soil thermal conductivity between the frozen area and the melted area. A threshold value of the frozen soil thawing state index is preset. If the frozen soil thawing state index is less than or equal to the threshold value of the frozen soil thawing state index, the frozen soil thawing state is a normal state. If the frozen soil thawing state index is greater than the threshold value of the frozen soil thawing state index, the frozen soil thawing state is a blocked state. The specific formula for calculating the frozen soil thawing state index is:

[0048]

[0049] Among them, Rh is the permafrost thawing state indicator, OM D is the soil organic matter content in the frozen area, OM R is the soil organic matter content in the melting area, λ D is the thermal conductivity of the soil in the frozen area, λ R is the thermal conductivity of the soil in the melting area, max(OM D ,OM R ) and max(λ D ,λ R ) are OM D ,OM R The maximum value and λ in D ,λ R The maximum value among , used for normalization.

[0050] The construction of this index is based on measuring the difference between two physical properties of frozen soil thawing state (organic matter content and thermal conductivity). Among them, organic matter content is one of the important factors affecting the specific heat capacity of frozen soil. Organic matter content can be regarded as the difference in specific heat capacity. By comparing the soil organic matter content OM on both sides of the freeze-thaw peak, the soil organic matter content OM D and OM R and thermal conductivity λ D and λ R, can quantify the extent to which the freeze-thaw front of frozen soil is hindered by the different thermal properties between soil layers. |OM D -OM R | reflects the difference in soil organic matter content on both sides of the freeze-thaw peak. The soil organic matter content directly affects the specific heat capacity of the soil, which can directly affect the soil's thermal conductivity, and thus directly affect the movement of the freeze-thaw peak. The larger its value, the greater the difference in thermal conductivity on both sides of the freeze-thaw peak, the less conducive it is to heat conduction, and at the same time, it will lead to the blockage of the freeze-thaw peak. |λ D -λ R |Reflects the difference in thermal conductivity of the soil on both sides of the freeze-thaw peak. The larger the value, the greater the difference in heat conduction on both sides of the freeze-thaw peak, which is less conducive to heat conduction. At the same time, it will lead to blockage of the freeze-thaw peak. Due to the different soil properties between different soil layers, when the freeze-thaw peak moves to the boundary between the soil layers, the movement may be hindered due to this difference in properties.

[0051] The phase transition state of the frozen soil includes the normal thawing state and the hindered thawing state of the frozen soil, which is determined by the frozen soil phase transition state index. Specifically, a frozen soil pore thawing probability model is constructed based on the volume, water content, and ice content of the pores on the freeze-thaw peak surface, which is specifically expressed as:

[0052]

[0053] Among them, RP i is the melting probability of the i-th pore on the freeze-thaw peak surface, V i is the volume of the i-th pore on the freeze-thaw peak surface, θ wi is the water content of the i-th pore on the freeze-thaw peak surface, θ ci is the ice content of the ith pore on the freeze-thaw peak surface, T is the temperature of the freeze-thaw peak surface, T f is the freezing temperature, i is the index of the pores on the freeze-thaw peak surface, and k is the proportional constant, which is used to adjust the influence of temperature difference and volume on the freezing probability. Experts in this field can be invited to calibrate it according to the actual soil environment;

[0054] Among them, the volume of the pores has been normalized to the maximum and minimum, which is the existing technology and will not be described in detail here. The freezing temperature is the temperature required for the soil to freeze, which is usually 0°C.

[0055] in, It is one of the classic forms of the cumulative distribution function probability model of the exponential distribution. The present invention introduces the volume, water content, and ice content of the pores on the freeze-thaw peak surface to express the melting probability of the pores on the freeze-thaw peak surface, where V i The larger the size, the larger the pores, and the harder it is to melt during the melting process. is the water-to-ice ratio in the pores. Generally, the more water in the pores, the easier it is to melt.

[0056] The probability of frozen soil thawing of each pore on the freeze-thaw peak surface is calculated according to the frozen soil pore thawing probability model. A random number U is set for the frozen soil thawing probability of each pore. i , and U i ∈[0,1], if U i <RP i , then the i-th pore on the freeze-thaw peak is judged to be melted, the melting of the pores on the freeze-thaw peak is counted, and the frozen soil phase change state index is generated according to the melting of the pores on the freeze-thaw peak. The frozen soil phase change state index averages the frozen soil melting probability and the thaw-freeze ratio. The specific formula is:

[0057]

[0058] in, is the average probability of frozen soil thawing, DR is the thaw-freeze ratio, N is the total number of pores on the freeze-thaw peak surface, and N D is the number of melting pores on the freeze-thaw peak surface;

[0059] The average permafrost thaw probability indicates the overall likelihood of pore thaw on the freeze-thaw peak. Larger values ​​indicate a greater likelihood of thawing, indicating easier movement of the freeze-thaw peak. Conversely, lower values ​​indicate a lower likelihood of thawing and greater resistance to movement. This describes the relative magnitude of permafrost thaw relative to its frozen state. Higher values ​​indicate a greater proportion of thawed pores compared to frozen pores, indicating easier movement of the freeze-thaw peak. If these two indicators are too low on the freeze-thaw peak, movement is likely to be hindered. This resistance is likely due to inconsistencies in soil properties between layers.

[0060] If any one of the normal thawing conditions is met, the permafrost thawing state is judged to be the normal thawing state; otherwise, the permafrost thawing state is judged to be the obstructed thawing state; the normal thawing conditions include normal thawing condition 1 and normal thawing condition 2. Normal thawing condition 1 is that the thaw-freeze ratio is greater than 2; normal thawing condition 2 is that the thaw-freeze ratio is less than 2 but greater than 1.5, and the average permafrost thawing probability is greater than 0.6.

[0061] The water selection module selects the Richard equation as the water migration equation to simulate the vertical migration of soil water in the unsaturated range under freeze-thaw conditions. The phase change state of frozen soil is used as a dynamic adjustment factor for phase change and introduced into the Richard equation. The response relationship between soil hydraulic conductivity and moisture content is adjusted by the dynamic adjustment factor for phase change to construct the final water migration equation.

[0062] The final moisture migration equation is expressed as:

[0063]

[0064] where δ is the dynamic regulation factor of phase change, θ is the volumetric water content, t is time, z is the freeze-thaw depth, h is the water potential, K(θ) is the soil hydraulic conductivity, Ks is the saturated hydraulic conductivity, θs is the saturated water content, θr is the residual water content, l is the pore connectivity parameter, and m is the curve shape parameter, m∈(0,1).

[0065] The phase change state of the soil directly affects the connectivity of the pores. If the soil is in a normal melting phase change state, the connectivity of the soil pores is good. If the soil is in a state of obstructed permafrost thawing, the degree of soil melting phase change is insufficient, the pores are occupied by ice, the connectivity of the pores will decrease, and the pore connectivity will also become smaller.

[0066] The heat selection module is used to select the one-dimensional heat diffusion equation as the heat conduction equation to describe the conduction and changes of temperature within the soil profile. The thawing state of permafrost is introduced into the heat conduction equation to construct a final heat conduction equation that reflects the impact of the thawing state of permafrost on heat conduction. The final water migration equation and the final heat conduction equation are combined to form a water-heat coupling model that simulates the co-evolution of water and heat during the freeze-thaw period.

[0067] The final heat conduction equation is expressed as:

[0068]

[0069] Where ε is the melting state adjustment factor, ρ is the soil density, c is the specific heat capacity of the soil, T is the soil temperature, and λ is the soil thermal conductivity.

[0070] The melting state of the soil is determined by the organic matter content and thermal conductivity, which determine the specific heat capacity of the soil, so ε is used to adjust If the soil is in a melting state, no adjustment is required. If the soil is in a blocking state, the temperature should be lowered. To indicate the blocking state of the soil.

[0071] The result simulation module is used to simulate the constructed hydrothermal coupling model using HYDRUS software and output the simulation results of the hydrothermal coupling model.

[0072] The hydrothermal coupling model established in steps 2-3 is input into the HYDRUS software. The hydrothermal coupling migration of the soil profile during the freeze-thaw process is simulated through numerical calculations, and the final output is a visual simulation result including temperature distribution, moisture distribution and their interaction relationship.

[0073] Specifically, based on actual farmland soil profile data, initial conditions were set: Layer parameters were set: initial moisture content, temperature, organic matter content, thermal conductivity, density, specific heat capacity, and water potential for each soil layer. The initial freezing front position was designed.

[0074] Set boundary conditions: Moisture boundary: rainfall / evaporation flux (meteorological data or experimental set values ​​must be input). Thermal boundary: air temperature change curve (daily / seasonal temperature fluctuations). Free drainage conditions (moisture) and constant ground temperature (heat conduction). Set the total simulation time (such as a freeze-thaw cycle: 30 days), and use an adaptive time step (initial step size 1 minute, maximum step size 1 hour) to ensure dynamic capture of the freeze-thaw front. Update the freeze-thaw status (melting state index, freezing state index, freeze-thaw peak position) in real time. HYDRUS uses the finite element method (FEM) to discretize the soil profile and solve the coupled partial differential equations. These are all existing technologies and will not be described in detail here.

[0075] The simulation results of the output water-heat coupling model include temperature distribution, moisture distribution, and water-heat coupling relationship.

[0076] See also Figure 4 , provides a simulation method for simulating the hydrothermal coupling effect of farmland freeze-thaw soil, the method being executed to implement the simulation system for simulating the hydrothermal coupling effect of farmland freeze-thaw soil, the specific steps comprising:

[0077] Step 1: Analyze the freeze-thaw peak surface of the frozen soil, extract the frozen soil thawing state index that characterizes the soil thawing process, and the frozen soil phase change state index that characterizes the change in the soil physical state, and determine the thawing state and phase change state of the frozen soil;

[0078] Step 2: Select the Richard equation as the water migration equation to simulate the vertical migration process of soil moisture in the unsaturated range under freeze-thaw conditions. The phase change state of frozen soil is used as a phase change dynamic adjustment factor and introduced into the Richard equation. The response relationship between soil hydraulic conductivity and moisture content is adjusted by the phase change dynamic adjustment factor to construct the final water migration equation.

[0079] Step 3: Select the one-dimensional heat diffusion equation as the heat conduction equation to describe the conduction and changes of temperature within the soil profile. Introduce the thawing state of permafrost into the heat conduction equation to construct a final heat conduction equation that reflects the effect of the thawing state of permafrost on heat conduction. Combine the final water migration equation with the final heat conduction equation to form a hydrothermal coupling model that simulates the co-evolution of water and heat during the freeze-thaw period.

[0080] Step 4: Use HYDRUS software to simulate the constructed hydrothermal coupling model and output the simulation results of the hydrothermal coupling model.

[0081] The above formulas are all dimensionless and numerical calculations. The formulas are obtained by collecting a large amount of data and performing software simulation to obtain the most recent real situation. The preset parameters in the formulas are set by technicians in this field according to actual conditions.

[0082] The above embodiments can be implemented in whole or in part by software, hardware, firmware, or any other combination thereof. When implemented using software, the above embodiments can be implemented in whole or in part in the form of a computer program product. Those skilled in the art will appreciate that the units and algorithm steps of each example described in conjunction with the embodiments disclosed herein can be implemented by electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are performed by hardware or software depends on the specific application and design constraints of the technical solution.

[0083] The units described as separate components may or may not be physically separate, and the components shown as units may or may not be physical units, and may be located in one place or distributed across multiple network units. Some or all of these units may be selected to achieve the purpose of this embodiment as needed.

[0084] The above is only a specific implementation method of the present application, but the scope of protection of the present application is not limited thereto. Any technical personnel familiar with this technical field can easily think of changes or replacements within the technical scope disclosed in this application, and they should all be covered by the scope of protection of the present application.

Claims

1. A simulation method for the hydrothermal coupling simulation effect of farmland freeze-thaw soil, characterized in that: Specifically include: The freeze-thaw analysis module is used to analyze the freeze-thaw peak surface of frozen soil, extract the frozen soil thawing state index that characterizes the soil thawing process, and the frozen soil phase change state index that characterizes the change in the soil physical state, and determine the thawing state and phase change state of frozen soil; The water selection module is used to select the Richard equation as the water migration equation to simulate the vertical migration process of soil water in the unsaturated range under freeze-thaw conditions. The phase change state of frozen soil is used as a phase change dynamic adjustment factor and introduced into the Richard equation. The response relationship between soil hydraulic conductivity and moisture content is adjusted by the phase change dynamic adjustment factor to construct the final water migration equation. The heat selection module is used to select the one-dimensional heat diffusion equation as the heat conduction equation to describe the conduction and changes of temperature within the soil profile. The thawing state of permafrost is introduced into the heat conduction equation to construct a final heat conduction equation that reflects the impact of the thawing state of permafrost on heat conduction. The final water migration equation and the final heat conduction equation are combined to form a water-heat coupling model that simulates the co-evolution of water and heat during the freeze-thaw period. The result simulation module is used to simulate the constructed hydrothermal coupling model using HYDRUS software and output the simulation results of the hydrothermal coupling model.

2. The simulation system for simulating the coupled water-heat effect of freeze-thaw soil in farmland according to claim 1, characterized in that: The thawing state of the frozen soil includes a normal state and a blocked state, which is determined by a frozen soil thawing state index. Specifically, the area located on one side of the freeze-thaw peak surface and in a frozen state is defined as a frozen area, and the area located on one side of the freeze-thaw peak surface and in a melted state is defined as a melted area. The soil organic matter content and soil thermal conductivity of the frozen area and the melted area are obtained. The frozen soil thawing state index is calculated based on the difference in soil organic matter content and soil thermal conductivity between the frozen area and the melted area. A threshold value of the frozen soil thawing state index is preset. If the frozen soil thawing state index is less than or equal to the threshold value of the frozen soil thawing state index, the frozen soil thawing state is a normal state. If the frozen soil thawing state index is greater than the threshold value of the frozen soil thawing state index, the frozen soil thawing state is a blocked state. The specific formula for calculating the frozen soil thawing state index is: Among them, Rh is the permafrost thawing state indicator, OM D is the soil organic matter content in the frozen area, OM R is the soil organic matter content in the melting area, λ D is the thermal conductivity of the soil in the frozen area, λ R is the thermal conductivity of the soil in the melting area, max(OM D ,OM R ) and max(λ D ,λ R ) are OM D ,OM R The maximum value and λ in D ,λ R The maximum value among , used for normalization.

3. The simulation system for simulating the coupled water-heat effect of farmland freeze-thaw soil according to claim 1, characterized in that: The phase transition state of the frozen soil includes the normal thawing state and the hindered thawing state of the frozen soil, which is determined by the frozen soil phase transition state index. Specifically, a frozen soil pore thawing probability model is constructed based on the volume, water content, and ice content of the pores on the freeze-thaw peak surface, which is specifically expressed as: Among them, RP i is the melting probability of the i-th pore on the freeze-thaw peak surface, V i is the volume of the i-th pore on the freeze-thaw peak surface, θ wi is the water content of the i-th pore on the freeze-thaw peak surface, θ ci is the ice content of the ith pore on the freeze-thaw peak surface, T is the temperature of the freeze-thaw peak surface, T f is the freezing temperature, i is the index of the pore on the freeze-thaw peak surface, and k is the proportional constant; The probability of frozen soil thawing of each pore on the freeze-thaw peak surface is calculated according to the frozen soil pore thawing probability model. A random number U is set for the frozen soil thawing probability of each pore. i , and U i ∈[0,1], if U i <RP i , then the i-th pore on the freeze-thaw peak is judged to be melted, the melting of the pores on the freeze-thaw peak is counted, and the frozen soil phase change state index is generated according to the melting of the pores on the freeze-thaw peak. The frozen soil phase change state index averages the frozen soil melting probability and the thaw-freeze ratio. The specific formula is: in, is the average probability of frozen soil thawing, DR is the thaw-freeze ratio, N is the total number of pores on the freeze-thaw peak surface, and N D is the number of melting pores on the freeze-thaw peak surface; If any one of the normal thawing conditions is met, the permafrost thawing state is judged to be the normal thawing state; otherwise, the permafrost thawing state is judged to be the obstructed thawing state; the normal thawing conditions include normal thawing condition 1 and normal thawing condition 2. Normal thawing condition 1 is that the thaw-freeze ratio is greater than 2; normal thawing condition 2 is that the thaw-freeze ratio is less than 2 but greater than 1.5, and the average permafrost thawing probability is greater than 0.

6.

4. The simulation system for simulating the coupled water-heat effect of freeze-thaw soil in farmland according to claim 2, characterized in that: The final moisture migration equation is expressed as: where δ is the dynamic regulation factor of phase change, θ is the volumetric water content, t is time, z is the freeze-thaw depth, h is the water potential, K(θ) is the soil hydraulic conductivity, Ks is the saturated hydraulic conductivity, θs is the saturated water content, θr is the residual water content, l is the pore connectivity parameter, and m is the curve shape parameter, m∈(0,1).

5. The simulation system for simulating the coupled water-heat effect of freeze-thaw soil in farmland according to claim 2, characterized in that: The final heat conduction equation is expressed as: Where ε is the melting state adjustment factor, ρ is the soil density, c is the specific heat capacity of the soil, T is the soil temperature, and λ is the soil thermal conductivity.

6. The simulation system for simulating the coupled water-heat effect of freeze-thaw soil in farmland according to claim 1, characterized in that: The simulation results of the output water-heat coupling model include temperature distribution, moisture distribution, and water-heat coupling relationship.

7. The simulation method for simulating the water-heat coupling effect of farmland freeze-thaw soil according to claim 1, characterized in that: The method is executed to realize the simulation system for simulating the hydrothermal coupling effect of farmland freeze-thaw soil according to claims 1-6, and the specific steps include: Step 1: Analyze the freeze-thaw peak surface of the frozen soil, extract the frozen soil thawing state index that characterizes the soil thawing process, and the frozen soil phase change state index that characterizes the change in the soil physical state, and determine the thawing state and phase change state of the frozen soil; Step 2: Select the Richard equation as the water migration equation to simulate the vertical migration process of soil moisture in the unsaturated range under freeze-thaw conditions. The phase change state of frozen soil is used as a phase change dynamic adjustment factor and introduced into the Richard equation. The response relationship between soil hydraulic conductivity and moisture content is adjusted by the phase change dynamic adjustment factor to construct the final water migration equation. Step 3: Select the one-dimensional heat diffusion equation as the heat conduction equation to describe the conduction and changes of temperature within the soil profile. Introduce the thawing state of permafrost into the heat conduction equation to construct a final heat conduction equation that reflects the effect of the thawing state of permafrost on heat conduction. Combine the final water migration equation with the final heat conduction equation to form a hydrothermal coupling model that simulates the co-evolution of water and heat during the freeze-thaw period. Step 4: Use HYDRUS software to simulate the constructed hydrothermal coupling model and output the simulation results of the hydrothermal coupling model.

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