Irrigation salt control effect evaluation method in soil freezing and thawing process

Abstract

The invention discloses a method for evaluating an irrigation salt control effect in a soil freezing and thawing process. The method comprises the following steps: sampling on site to obtain a dielectric constant, conductivity and temperature of soil; the conductivity is converted into solute concentration, the dielectric constant is converted into volumetric moisture content and input into a numerical model, and the temperature is directly input into the numerical model for calculation; the numerical model calculates the collected data and then outputs corresponding numerical values of the concentrations and the volumetric water contents at different time and different depths; the output numerical value is substituted into STI for calculation, and when the STI value is positive, it is indicated that irrigation can effectively control and leach salt; when the STI value is negative, it is indicated that the salt leaching capacity of irrigation is insufficient, salt accumulation is caused, and compared with a single salt control evaluation index, STI can reflect the salt leaching and salt control effect of irrigation measures in the long-term implementation process in an all-around and multi-angle mode, so that an irrigation strategy is accurately formulated, and the optimal salt control effect is achieved.

Description

Technical Field

[0001] This invention relates to the field of soil salinization prevention and control technology, specifically a method for evaluating the effect of irrigation on salt control during soil freeze-thaw cycles. Background Technology

[0002] Soil salinization is one of the key obstacles restricting the sustainable development of agriculture in arid and semi-arid regions of northern my country. Especially in cold regions, the water and salt migration process in soils is very complex during the freeze-thaw cycle. To inhibit surface salt accumulation and prevent secondary salinization, winter irrigation and spring irrigation have become important salt control measures commonly used in cold-region agriculture. The principle is to utilize the movement of water before freezing or in the early stages of thawing to drive salt to migrate to deeper layers or be discharged with runoff.

[0003] Among existing salt control technologies, winter irrigation and spring irrigation are important measures commonly used in agriculture in cold northern regions. However, the salt control effect of these two types of irrigation methods is currently characterized by static concentration indicators such as electrical conductivity (EC), total dissolved solids (TDS), and sodium adsorption ratio (SAR). These single indicators can only reflect the salt status of a certain moment or soil layer, and are difficult to reveal the whole process of salt migration, deposition, and redeposition with water throughout the entire freeze-thaw period. Summary of the Invention

[0004] The purpose of this invention is to provide a method for evaluating the salt control effect of irrigation during soil freeze-thaw cycles, which can comprehensively and from multiple perspectives reflect the leaching of salt and the salt control effectiveness of irrigation measures during long-term implementation.

[0005] The technical solution of this invention is:

[0006] A method for evaluating the effect of irrigation on salt control during soil freeze-thaw cycles includes the following steps:

[0007] The dielectric constant, conductivity, and temperature of the soil were obtained through on-site sampling; the dielectric constant and conductivity were measured by sensors, while the temperature was measured by independent thermistors, thermocouples, or infrared sensors.

[0008] The dielectric constant was converted into volumetric water content, and the conductivity was converted into solute concentration and input into the numerical model. The temperature was directly input into the numerical model for calculation. The modified values ​​of the other parameters are given in Table 1.

[0009] The steps to convert conductivity to concentration are as follows: measure the conductivity and consult the molar conductivity table for the corresponding temperature, then apply the formula c = κ / Λm. Here, k refers to the conductivity, and Λm is the molar conductivity at the corresponding temperature.

[0010] The formula for converting dielectric constant to volumetric water content is:

[0011] In the formula, a and b are correction parameters, θ is volumetric water content, and ε is dielectric constant. It is worth noting that different soil types have different correction parameters. The correction parameters are obtained from the "Empirical Study on the Dielectric Constant and Volumetric Water Content of Different Soil Types".

[0012] The numerical model processes the collected data and outputs the corresponding concentration and volumetric water content values ​​at different times and depths; the volumetric water content is used to calculate the salt mass in the STI. Salt mass = 0.321 × concentration × volumetric water content.

[0013] The concentrations at different times and depths are converted into salt migration mass and incorporated into the STI index for calculation. The STI index comprehensively considers salt migration mass, solute concentration, and the duration of salt control, providing a direct reflection of the actual performance of irrigation measures in long-term salt control. By establishing a multi-dimensional comprehensive index—the Salt Time Index (STI)—that integrates salt migration, concentration changes, and timeliness, the limitations of single indicators are overcome, and the problem of insufficient comprehensive evaluation of salt control effects in existing irrigation salt control technologies is solved, enabling precise evaluation of salt control effects.

[0014]

[0015] STI stands for Salt-Time Index, which reflects the actual performance of irrigation measures in long-term salt control. s The actual mass (kg) of salt migrated / leached through irrigation; T e To ensure the effective duration (days) of controlling the solute concentration of salt below a predetermined threshold, the concentration model results at different times can be directly output, with the start of irrigation as the initial time and the point where the concentration exceeds the threshold as the final time; this period is the duration. i The solute concentration at any given time (mol / m 3 C0 is the solute threshold concentration (mol / m³). 3 When the STI value is positive, it indicates that irrigation can effectively control and leach salts, and the larger the STI value, the better the desalination effect. Conversely, when the STI value is negative, it indicates that irrigation is not strong enough to leach salts, leading to salt accumulation.

[0016] Because different soil textures respond significantly differently to water and salt transport, groundwater level changes have a crucial impact on salt control pathways. Traditional models often neglect freeze-thaw coupling, the water retention effect of soil pore ice, and key processes in salt phase change dynamics, leading to insufficient understanding of the timeliness of salt control, salt accumulation layer distribution, and deep-seated migration trends, making it difficult to provide a reliable basis for refined irrigation management. The numerical model described above is a water-heat-salt three-field coupled model based on various assumptions. Addressing the limitations of existing models in boundary condition handling and parameter value universality, it includes temperature field equations, water field equations, and salt field equations.

[0017] Furthermore, the various assumptions include: the soil is an isotropic and uniformly distributed porous medium; the change in mass caused by soil deformation is considered; the soil particles are incompressible, and changes in porosity will cause soil deformation; water migration is mainly in liquid form, with salt migration accompanying water migration, and the migration of ice crystals and salt crystals is not considered.

[0018] Furthermore, the temperature field equation, taking into account the water phase change, salt phase change, and convective heat transfer, is as follows:

[0019]

[0020] In the formula, C(θ) is the volumetric heat capacity; θ is the volumetric water content; T is the transient temperature of the soil (°C); λ is the thermal conductivity (W / (m·K)); L w The latent heat of phase change of water (kJ / kg); ρ i The density of ice (kg / m³) 3 );θ i The volume fraction of ice; L c The latent heat of phase transition of salt (kJ / kg); ρ c The density of crystalline salt (kg / m³) 3 );m c Salt crystal content; C w ρ represents the volume occupied by water per unit volume of porous medium. w The density of water (kg / m³) 3 ), where t is the time of the transient temperature.

[0021] Furthermore, to improve computational efficiency, an equivalent thermal conductivity λ is introduced. * :

[0022]

[0023] λ=λ f +(λ u -λ f )·H(T);

[0024] In the formula, λ *λ is the equivalent thermal conductivity (W / (m·K)); L is the thermal conductivity (W / (m·K)); w The latent heat of phase change of water (kJ / kg); λ f λ is the thermal conductivity of frozen soil, taken as 1.28 W / (m·K); u The thermal conductivity of unfrozen soil is taken as 1.25 W / (m·K); H(T) is a two-dimensional step smoothing function; c1 is the molar concentration of the solution (ions) (mol / m³). 3 ), where D is the liquid diffusion coefficient; D u This represents the diffusion coefficient of water in the untouched soil.

[0025] The formula for calculating the volumetric heat capacity C(θ) of soil during the freezing and thawing process is as follows:

[0026]

[0027] C(θ)=C f +(C u -C f )·H(T);

[0028] In the formula, C * L is the equivalent volumetric heat capacity. w The latent heat of phase change of water (kJ / kg); ρ w The density of water (kg / m³) 3 );m u C represents the unfrozen water content in the soil. f The specific heat capacity of frozen soil is taken as 989 J / (kg·K); C u The specific heat capacity before soil movement is taken as 1240 J / (kg·K); H(T) is a two-dimensional step smoothing function.

[0029] Furthermore, the moisture field equation is constructed based on Darcy's law and the law of conservation of mass, according to the Richards equation, and considering the hindering effect of pore ice on the migration of unfrozen water. The moisture field equation is a differential equation for the migration of unfrozen water in unsaturated permafrost, which is:

[0030]

[0031] In the formula, D(θ) u The diffusivity of water in frozen soil; θ u k is the volume content of unfrozen water in frozen soil. g It represents the permeability coefficient of unsaturated soil in the direction of gravitational acceleration.

[0032] Since salt lowers the freezing temperature of soil, under the same sub-zero conditions, soils with higher salt content also have higher unfrozen water content. Considering the effect of salt on the unfrozen water volume content θ... uThe effect is shown in the following formula.

[0033]

[0034] In the formula, n N Equivalent salt content; C1 and C2 are the salt solution concentrations before and after cooling, respectively; θ u1 θ u2 These represent the unfrozen water volume content in the soil before and after cooling; θ0 represents the initial unfrozen water volume content; 180(C1θ) u1 -C2θ u2 +Δn N ) represents the volume of unfrozen water that has crystallized into ice; H(T1) is a two-dimensional step smoothing function.

[0035] Equivalent salt content n N The calculation formula is as follows:

[0036]

[0037] ρ c θ represents the concentration of the crystalline salt. c M represents the volume content of crystalline salts. c The content of crystalline salt molecules; C c This refers to the salt solution concentration;

[0038] The water diffusivity D(θ) in the frozen soil u The calculation formula is as follows:

[0039]

[0040] In the formula, k(θ) u ) represents the permeability (m / s) of unsaturated soil; c(θ) u ) represents the specific water capacity (1 / m³), determined by the perched water model; I is the impedance factor, indicating that the migration and transport of unfrozen water in the soil is hindered by pore ice in the solid state, and its expression is:

[0041] The permeability of unsaturated soil is expressed as:

[0042] k(θ u )=k s ·S l [1-(1-S lm ) m ] 2 ;

[0043] The change in water content caused by the change in matrix potential is expressed as:

[0044] c(θ u ) = a o m / (1-m)·S lm (1-Slm ) m ;

[0045] In unsaturated seepage analysis, relative saturation S is a key parameter describing the proportion of liquid water occupying the pores in soil. In the Van Genuchten (VG) permeability model, the relative saturation S of frozen soil is defined as follows:

[0046]

[0047] In the formula, k(θ) u ) represents the permeability (m / s) of the unsaturated soil; a0 and m are parameters in the VG model; θ r and θ s These represent the residual moisture content and saturated moisture content of the soil, respectively.

[0048] Furthermore, the salt field equation is:

[0049]

[0050] In the formula, n N This is the equivalent salt content; q lx Let D be the water flow rate in the x-direction (g / cm·s). sh (v,θ u D is the hydrodynamic dispersion coefficient of the solute under the influence of a concentration gradient. st θ is the hydrodynamic dispersion coefficient of the solute under the temperature gradient; c J represents the volume content of the crystalline salt, where C is the concentration of the solute; x Let x be the component of salt flux in the x-direction; x is the spatial coordinate.

[0051] Furthermore, the temperature and moisture fields contain three unknowns: temperature, pore ice volume content, and unfrozen water volume content. Therefore, the solid-liquid ratio B is introduced. i B is used to connect the moisture field equation and the temperature field equation to solve the hydrothermal coupling model. i The B represents the ratio of the volume of pore ice to the volume of unfrozen water in frozen soil. i The expression is:

[0052]

[0053] The solid-liquid ratio B i Given a piecewise function with respect to temperature T, we obtain θ. i , T, θ u The relationship between the three is as follows:

[0054] θ i =B i (T)·θ u ;

[0055] In the formula, A is a parameter variable, and θ0 is the initial total water content (volume fraction before freezing).

[0056] In porous media, the movement of fluids such as groundwater and soil water is often accompanied by solute transport. The hydrodynamic dispersion coefficient D... sh The formula is as follows:

[0057] D sh =D h +D d ;

[0058] D h =R s ql / θ u ;

[0059]

[0060] In the formula, D d The molecular diffusion coefficient in soil (m 2 / s); D0 is the diffusion coefficient of the solute in water at 0℃; γ is a constant reflecting the pore curvature of the soil; γ and R S D is the tortuosity coefficient. h R is the mechanical dispersion coefficient. s Let q be the dispersion scale constant. l For Darcy's water flow, T k This refers to absolute temperature.

[0061] Compared with the prior art, the beneficial effects of the present invention are:

[0062] This invention proposes a multi-dimensional comprehensive index—the Salt Time Index (STI)—that integrates salt migration, concentration control, and timeliness. Compared to previous single salt control assessment indicators, the STI can comprehensively and from multiple perspectives reflect the leaching of salt and the effectiveness of salt control measures during long-term implementation, effectively overcoming the limitations of traditional assessment methods in terms of comprehensiveness and accuracy. This allows for the precise formulation of irrigation strategies to achieve optimal salt control results, providing a more scientific, reasonable, and practical quantitative assessment tool for the prevention and control of saline-alkali land.

[0063] Furthermore, the numerical model of this invention establishes temperature field equations, moisture field equations, and salinity field equations to address the limitations of existing models in terms of boundary condition handling and parameter value universality. By improving the thermal conductivity, permeability, and saturated water content in the numerical model, the model can more accurately simulate the changes in salinity migration under different soil types, irrigation amounts, and groundwater levels during freeze-thaw processes. Attached Figure Description

[0064] Figure 1 This is a schematic diagram of the steps of the present invention.

[0065] Figure 2 The irrigation volume of this invention is 50m³. 3 A comparison of STI and concentration variation contour lines per acre, showing the groundwater level at -2.0m and the soil type as loam. Figure 2 (a) and (b) are winter irrigation examples. Figure 2 (c) and (d) refer to spring irrigation.

[0066] Figure 3 This is a comparison of contour lines showing the STI and concentration changes in clay loam soil according to the present invention, where the irrigation amount is 100m³. 3 / mu, groundwater level is -2.0m, Figure 3 (a) and (b) are winter irrigation examples. Figure 3 (c) and (d) refer to spring irrigation.

[0067] Figure 4 This is a comparison map of groundwater level -0.5mSTI and concentration variation contour lines according to the present invention, wherein the irrigation amount is 100m. 3 / mu, soil type is loam. Figure 4 (a) and (b) are winter irrigation examples. Figure 4 (c) and (d) refer to spring irrigation. Detailed Implementation

[0068] The following is combined with Figures 1 to 4 The specific embodiments of the present invention will be described in detail below.

[0069] Example

[0070] like Figure 1 As shown, a method for evaluating the effect of irrigation on salt control during soil freeze-thaw cycles includes the following steps:

[0071] The dielectric constant, conductivity, and temperature of the soil were obtained through on-site sampling; the dielectric constant and conductivity were measured by sensors, while the temperature was measured by independent thermistors, thermocouples, or infrared sensors.

[0072] The dielectric constant was converted into volumetric water content, and the conductivity was converted into solute concentration and input into the numerical model. The temperature was directly input into the numerical model for calculation. The modified values ​​of the other parameters are given in Table 1.

[0073] The steps to convert conductivity to concentration are as follows: measure the conductivity and consult the molar conductivity table for the corresponding temperature, then apply the formula c = κ / Λm. Here, k refers to the conductivity, and Λm is the molar conductivity at the corresponding temperature.

[0074] The formula for converting dielectric constant to volumetric water content is:

[0075] In the formula, a and b are correction parameters, θ is volumetric water content, and ε is dielectric constant. It is worth noting that different soil types have different correction parameters. The correction parameters are obtained from the "Empirical Study on the Dielectric Constant and Volumetric Water Content of Different Soil Types".

[0076] The numerical model processes the collected data and outputs the corresponding concentration and volumetric water content values ​​at different times and depths; the volumetric water content is used to calculate the salt mass in the STI. Salt mass = 0.321 × concentration × volumetric water content.

[0077] The concentrations at different times and depths are converted into salt migration mass and incorporated into the STI index for calculation. The STI index comprehensively considers salt migration mass, solute concentration, and the duration of salt control, providing a direct reflection of the actual performance of irrigation measures in long-term salt control. By establishing a multi-dimensional comprehensive index—the Salt Time Index (STI)—that integrates salt migration, concentration changes, and timeliness, the limitations of single indicators are overcome, and the problem of insufficient comprehensive evaluation of salt control effects in existing irrigation salt control technologies is solved, enabling precise evaluation of salt control effects.

[0078]

[0079] STI stands for Salt-Time Index, which reflects the actual performance of irrigation measures in long-term salt control. s The actual mass (kg) of salt migrated / leached through irrigation; T e To ensure the effective duration (days) of controlling the solute concentration of salt below a predetermined threshold, the concentration model results at different times can be directly output, with the start of irrigation as the initial time and the point where the concentration exceeds the threshold as the final time; this period is the duration. i The solute concentration at any given time (mol / m) 3 C0 is the solute threshold concentration (mol / m³). 3 When the STI value is positive, it indicates that irrigation can effectively control and leach salts, and the larger the STI value, the better the desalination effect. Conversely, when the STI value is negative, it indicates that irrigation is not strong enough to leach salts, leading to salt accumulation.

[0080] To address the limitations of existing models in handling boundary conditions and the universality of parameter values, a numerical model is established based on multiple assumptions to create a water-heat-salt three-field coupled model, which includes temperature field equations, moisture field equations, and salt field equations.

[0081] The above-mentioned assumptions include:

[0082] The soil is an isotropic and uniformly distributed porous medium;

[0083] Consider the mass change caused by soil deformation;

[0084] Soil particles are incompressible, and changes in porosity will cause soil deformation.

[0085] Water migration is mainly in liquid form, with salt migration accompanying water migration. The migration of ice crystals and salt crystals is not considered.

[0086] The temperature field equation, considering the water phase change, salt phase change, and convective heat transfer, is as follows:

[0087]

[0088] In the formula, C(θ) is the volumetric heat capacity; θ is the volumetric water content; T is the transient temperature of the soil (°C); λ is the thermal conductivity (W / (m·K)); L w The latent heat of phase change of water (kJ / kg); ρ i The density of ice (kg / m³) 3 );θ i The volume fraction of ice; L c The latent heat of phase transition of salt (kJ / kg); ρ c The density of crystalline salt (kg / m³) 3 );m c Salt crystal content; C w ρ represents the volume occupied by water per unit volume of porous medium. w The density of water (kg / m³) 3 ), where t is the time of the transient temperature.

[0089] To improve computational efficiency, an equivalent thermal conductivity λ is introduced. * :

[0090]

[0091] λ=λ f +(λ u -λ f )·H(T);

[0092] In the formula, λ * λ is the equivalent thermal conductivity (W / (m·K)); L is the thermal conductivity (W / (m·K)); w D is the latent heat of phase change of water (kJ / kg); D is the liquid diffusion coefficient; D u λ is the diffusion coefficient of water in the untouched soil. f λ is the thermal conductivity of frozen soil, taken as 1.28 W / (m·K); u The thermal conductivity of unfrozen soil is taken as 1.25 W / (m·K); H(T) is a two-dimensional step smoothing function; c1 is the molar concentration of the solution (ions) (mol / m³).3 ).

[0093] The formula for calculating the volumetric heat capacity C(θ) of soil during the freezing and thawing process is as follows:

[0094]

[0095] C(θ)=C f +(C u -C f )·H(T);

[0096] In the formula, C * L is the equivalent volumetric heat capacity. w The latent heat of phase change of water (kJ / kg); ρ w The density of water (kg / m³) 3 );m u C represents the unfrozen water content in the soil. f The specific heat capacity of frozen soil is taken as 989 J / (kg·K); C u The specific heat capacity before soil movement is taken as 1240 J / (kg·K); H(T) is a two-dimensional step smoothing function.

[0097] The moisture field equation is constructed based on Darcy's law and the law of conservation of mass, according to the Richards equation, and considering the hindering effect of pore ice on the migration of unfrozen water. The moisture field equation is the differential equation for the migration of unfrozen water in unsaturated permafrost, which is:

[0098]

[0099] In the formula, D(θ) u The diffusivity of water in frozen soil; θ u k is the volume content of unfrozen water in frozen soil. g It represents the permeability coefficient of unsaturated soil in the direction of gravitational acceleration.

[0100] Since salt lowers the freezing temperature of soil, under the same sub-zero conditions, soils with higher salt content also have higher unfrozen water content. Considering the effect of salt on the unfrozen water volume content θ... u The effect is shown in the following formula.

[0101]

[0102] In the formula, n N Equivalent salt content; C1 and C2 are the salt solution concentrations before and after cooling, respectively; θ u1 θ u2 These represent the unfrozen water volume content in the soil before and after cooling; θ0 represents the initial unfrozen water volume content; 180(C1θ) u1 -C2θ u2+Δn N ) represents the volume of unfrozen water that has crystallized into ice; H(T1) is a two-dimensional step smoothing function.

[0103] Equivalent salt content n N The calculation formula is as follows:

[0104]

[0105] The diffusivity of water in frozen soil, D(θ) u The calculation formula is as follows:

[0106]

[0107] In the formula, k(θ) u ) represents the permeability (m / s) of unsaturated soil; c(θ) u ) represents the specific water capacity (1 / m³), determined by the perched water model; I is the impedance factor, indicating that the migration and transport of unfrozen water in the soil is hindered by pore ice in the solid state, and its expression is:

[0108] The permeability of unsaturated soil is expressed as:

[0109] k(θ u )=k s ·S l [1-(1-S lm ) m ] 2 ;

[0110] The change in water content caused by the change in matrix potential is expressed as:

[0111] c(θ u ) = a o m / (1-m)·S lm (1-S lm ) m ;

[0112] In unsaturated seepage analysis, relative saturation S is a key parameter describing the proportion of liquid water occupying the pores in soil. In the Van Genuchten (VG) permeability model, the relative saturation S of frozen soil is defined as follows:

[0113]

[0114] In the formula, k(θ) u ) represents the permeability (m / s) of the unsaturated soil; a0 and m are parameters in the VG model; θ r and θ s These represent the residual moisture content and saturated moisture content of the soil, respectively.

[0115] The salt field equation is:

[0116]

[0117] In the formula, n N This is the equivalent salt content; q lx Let J be the water flux in the x-direction (g / cm·s); C be the solute concentration; J be the solute concentration. x Let C be the component of salt flux in the x-direction; x is the spatial coordinate; C c ρ is the concentration of the salt solution. c θ represents the concentration of the crystalline salt. c M represents the volume content of crystalline salts. c The content of crystalline salt molecules; D sh (v,θ u D is the hydrodynamic dispersion coefficient of the solute under the influence of a concentration gradient. st θ is the hydrodynamic dispersion coefficient of the solute under the temperature gradient; c This represents the volume content of the crystalline salt.

[0118] The temperature and moisture fields contain three unknowns: temperature, pore ice volume content, and unfrozen water volume content. Therefore, the solid-liquid ratio B is introduced. i B is used to connect the moisture field equation and the temperature field equation to solve the hydrothermal coupling model. i B represents the ratio of the volume of pore ice to the volume of unfrozen water in permafrost. i The expression is:

[0119]

[0120] Solid-liquid ratio B i Given a piecewise function with respect to temperature T, we obtain θ. i , T, θ u The relationship between the three is as follows:

[0121] θ i =B i (T)·θ u ;

[0122] In the formula, A is a parameter variable, and θ i θ represents the volume fraction of ice; u ρ represents the volumetric content of unfrozen water in frozen soil. w ρ is the density of water. i Let θ be the density of ice, T be the transient temperature of the soil, and θ0 be the initial total water content.

[0123] In porous media, the movement of fluids (such as groundwater and soil water) is often accompanied by solute transport. The hydrodynamic dispersion coefficient D... sh The formula is as follows:

[0124] D sh =D h+D d ;

[0125] D h =R s ql / θ u ;

[0126]

[0127] In the formula, D h R is the mechanical dispersion coefficient. s Let q be the dispersion scale constant. l For Darcy's flow rate, θ u D represents the volumetric content of unfrozen water in frozen soil. d The molecular diffusion coefficient in soil (m 2 / s); D sh γ is the hydrodynamic dispersion coefficient; D0 is the solute diffusion coefficient in water at 0℃; γ is a constant reflecting the pore curvature of the soil; γ and R S T is the tortuosity coefficient. k This refers to absolute temperature.

[0128] The parameters and important variables for model improvement are set as shown in Table 1.

[0129] Table 1 Parameters and Variables

[0130]

[0131]

[0132] Figure 2 , Figure 3 and Figure 4 Two-dimensional contour maps of STI and salt concentration under different conditions were created. Figure 2 , Figure 3 and Figure 4 The thick solid line STI represents 0. When STI > 0, it indicates desalination; the larger the STI value, the better the desalination effect. When STI < 0, it indicates salt accumulation. The evaluation results of the evaluation method in this embodiment can comprehensively reflect the salt control effect of irrigation, breaking through the limitations of traditional single-point indicators, thereby accurately formulating irrigation strategies, including determining reasonable irrigation volume and irrigation time, to achieve the best salt control effect.

[0133] The above-disclosed embodiments are merely preferred embodiments of the present invention. However, the embodiments of the present invention are not limited thereto, and any variations that can be conceived by those skilled in the art should fall within the protection scope of the present invention.

Claims

1. A method for evaluating the effect of irrigation on salt control during soil freeze-thaw cycles, characterized in that, Includes the following steps: The dielectric constant, conductivity, and temperature of the soil were obtained through on-site sampling. The dielectric constant is converted into volumetric water content and input into the numerical model, the electrical conductivity is converted into solute concentration and input into the numerical model, and the temperature is directly input into the numerical model for calculation. The numerical model is a water-heat-salt three-field coupled model based on multiple assumptions, and has temperature field equation, water field equation and salt field equation. The numerical model processes the collected data and outputs the corresponding values ​​of concentration and volumetric water content at different times and depths. The concentrations at different times and depths are converted into salt migration mass and then fed into STI for calculation. STI stands for Salinity Time Index, which reflects the actual performance of irrigation measures in long-term salt control. s The mass of salt actually migrated / leached through irrigation; T e To ensure the effective duration for which the solute concentration of salt is controlled below a predetermined threshold, C i C0 represents the solute concentration at any given time; C0 represents the solute threshold concentration. When the STI value is positive, it indicates that irrigation can effectively control and leach salts, and the larger the STI value, the better the desalination effect. When the STI value is negative, it indicates that the irrigation has insufficient ability to leach salts, leading to salt accumulation.

2. The method for evaluating the effect of irrigation on salt control during soil freeze-thaw cycles according to claim 1, characterized in that, The various assumptions include: the soil is an isotropic and uniformly distributed porous medium; the change in mass caused by soil deformation is considered; the soil particles are incompressible, and changes in porosity will cause soil deformation; water migration is mainly in liquid form, with salt migration accompanying water migration, and the migration of ice crystals and salt crystals is not considered.

3. The method for evaluating the effect of irrigation on salt control during soil freeze-thaw cycles according to claim 1, characterized in that, The temperature field equation is: In the formula, C(θ) is the volumetric heat capacity; θ is the volumetric water content; T is the transient temperature of the soil; λ is the thermal conductivity; L w The latent heat of phase transition of water; ρ i θ is the density of ice; i The volume fraction of ice; L c The latent heat of the salt phase transition; ρ c The density of the crystalline salt; m c Salt crystal content; C w ρ represents the volume of water per unit volume of porous medium. w Let be the density of water, and t be the time of the transient temperature.

4. The method for evaluating the effect of irrigation on salt control during soil freeze-thaw cycles according to claim 3, characterized in that, To improve computational efficiency, an equivalent thermal conductivity λ is introduced. * : λ=λ f +(λ u -l f )·H(T); In the formula, λ * λ is the equivalent thermal conductivity; L is the thermal conductivity; w The latent heat of phase transition of water; ρ w D is the density of water; D is the liquid diffusion coefficient; D u λ is the diffusion coefficient of water in the untouched soil. f λ is the thermal conductivity of frozen soil; T is the transient temperature of the soil; u is the thermal conductivity of unfrozen soil; H(T) is the two-dimensional step smoothing function; c1 is the molar concentration of the solution.

5. The method for evaluating the effect of irrigation on salt control during soil freeze-thaw cycles according to claim 3, characterized in that, The moisture field equation is the differential equation for the migration of unfrozen water in unsaturated frozen soil. The differential equation for the migration of unfrozen water in unsaturated frozen soil is: In the formula, D(θ) u The diffusivity of water in frozen soil; t is the time of the transient temperature change; θ u k is the volume content of unfrozen water in frozen soil. g ρ is the unsaturated soil permeability coefficient in the direction of gravitational acceleration. i ρ is the density of ice. w θ is the density of water; i This represents the volumetric content of ice.

6. The method for evaluating the effect of irrigation on salt control during soil freeze-thaw cycles according to claim 1, characterized in that, The salt field equation is: In the formula, n N This is the equivalent salt content; q lx Let J be the water flow rate in the x-direction; C be the solute concentration; J be the solute concentration. x D represents the component of salt flux in the x-direction; x is the spatial coordinate; D sh (v,θ u D is the hydrodynamic dispersion coefficient of the solute under the influence of a concentration gradient; st is the hydrodynamic dispersion coefficient of the solute under a temperature gradient.

7. The method for evaluating the effect of irrigation on salt control during soil freeze-thaw cycles according to claim 5, characterized in that, Introducing solid-liquid ratio B i B is used to connect the moisture field equation and the temperature field equation to solve the hydrothermal coupling model. i The B represents the ratio of the volume of pore ice to the volume of unfrozen water in frozen soil. i The expression is: The solid-liquid ratio B i Given a piecewise function with respect to temperature T, we obtain θ. i , T, θ u The relationship between the three is as follows: i i =B i (T)·θ u ; In the formula, A is a parameter variable, and θ i θ represents the volume fraction of ice; u ρ represents the volumetric content of unfrozen water in frozen soil. w ρ is the density of water. i Let θ be the density of ice, T be the transient temperature of the soil, and θ0 be the initial total water content.

8. The method for evaluating the effect of irrigation on salt control during soil freeze-thaw cycles according to claim 6, characterized in that, Hydrodynamic dispersion coefficient D sh The formula is as follows: D sh =D h +D d ; D h =R s ql / θ u ; In the formula, D h R is the mechanical dispersion coefficient. s Let q be the dispersion scale constant. l For Darcy's water flow, θ u D represents the volumetric content of unfrozen water in frozen soil. d D is the molecular diffusion coefficient in soil. sh γ is the hydrodynamic dispersion coefficient; D0 is the solute diffusion coefficient in water at 0℃; γ is a constant reflecting the pore curvature of the soil; γ and R S T is the tortuosity coefficient. k This refers to absolute temperature.

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