Method for predicting capillary water flux through capillary water rising height

Abstract

The invention belongs to the field of capillary water rising flux testing, and discloses a novel method for predicting capillary water rising flux based on capillary water rising height, which comprises the following steps: acquiring a soil outgassing value ha, a soil saturated permeability coefficient Ks, a correction coefficient lambda of model capillary water rising height and a fitting parameter beta ', and combining a derived mechanism model to predict the capillary water rising flux. And estimating the rising flux of the capillary water. According to the method, capillary water rising is divided into two stages, based on the Darcy law under the unsaturated condition, a new mechanism model that the capillary water rising flux changes along with the height is deduced and established, the model covers the whole rising process of the water level, and compared with an empirical model, the application range is wider; a model fitting value and an observation value have a good matching degree.

Description

Technical Field

[0001] The invention belongs to the field of capillary water rising flux testing, and specifically provides a method for predicting capillary water rising flux based on capillary water rising height. Background Art

[0002] Soil capillary water refers to liquid water retained and moved in the capillary pores of soil by capillary force. It is one of the main sources of water required for plant growth.

[0003] There are two common prediction models used in existing methods for predicting capillary water flux. One is the mechanism model, which focuses on the rising range of the capillary water fraction. When it exceeds this range, the prediction accuracy is poor. The other is the empirical model, which is applicable to specific conditions. When the actual situation deviates from the conditions on which the model is based, the prediction of the model will be inaccurate. Summary of the invention

[0004] Based on the problems existing in the prior art, the present invention divides the capillary water rise process into two stages with the soil outgassing value as the threshold, and derives a new mechanism model of the capillary water rise flux changing with height according to Darcy's law under unsaturated conditions, which is used to predict the capillary water rise flux at different capillary water rise heights.

[0005] The present invention is achieved by a method for predicting capillary water flux by capillary water rise height, comprising the following steps:

[0006] Step 1: Obtain soil gas emission value h a .

[0007] Step 2: Obtain soil saturated permeability coefficient K s .

[0008] Step 3: Obtain the correction coefficient λ and fitting parameter β' of the capillary water rise height of the model.

[0009] Step 4: Predict the capillary water rise flux based on the above parameters and the derived mechanism model.

[0010] Further, in the step 1, the soil to be tested is taken for a soil moisture absorption experiment. Before the experiment begins, the soil sample to be tested is dried until the soil moisture content is reduced to 0. Next, a certain volume of water is slowly added to the soil, and after it is fully diffused, the volume moisture content and matrix suction in the soil are measured, and then the process of adding water and measuring the volume moisture content and matrix suction is repeated until the volume moisture content reaches saturation and the matrix suction is reduced to 0Kpa, and the soil moisture absorption experiment is completed. The soil matrix suction measured in the experiment is converted into centimeters of water column height, and the matrix potential is used as the x-axis and the volume moisture content is used as the y-axis to draw the moisture absorption curve of the soil moisture characteristic curve.

[0011] The Gardner model (Gardner, 1958) is used as the fitting model of the soil-water characteristic curve:

[0012]

[0013] In the formula, θ, θ S are the average volumetric water content and saturated volumetric water content, respectively; h is the matrix suction, cm; θ r is the residual moisture content; c and m are the model shape parameters.

[0014] Using the soil moisture characteristic curve fitted by the model, draw the tangent line at the saturated volume moisture content and the tangent line at the inflection point, extend the two tangent lines, and the abscissa of their intersection is expressed as the soil gas outlet height h a , unit is cm.

[0015] Furthermore, in step 2, the soil is filled into a columnar container, and the density of the soil in the soil column is restored to the density of the original soil by knocking on the column wall. The soil column is immersed in water. After reaching the saturation time, it is taken out, fixed with an iron stand, a beaker is placed under the soil column, and water is added above the soil column using a Marsh flask to maintain a certain thickness of the water layer and a constant water level.

[0016] The timing starts from the moment the first drop of water appears below the soil column, and the water seepage volume is measured at regular intervals, while the water temperature is recorded. The cumulative water seepage volume V measured each time is set as the y-axis, and the time t spent on the cumulative water seepage volume is set as the x-axis to obtain the curve of the cumulative water seepage volume of the soil over time. The curve of the change of the soil water seepage volume over time gradually shows a linear correlation, indicating that within a certain period of time, the soil infiltration volume is the same and reaches a stable state.

[0017] The permeability coefficient is calculated according to Darcy's formula:

[0018]

[0019] in

[0020] Where: V is the cross-sectional area A (cm2) of the soil column penetrated in time t (h) 2 ) water volume, cm 3 ; I is the hydraulic gradient; K is the permeability coefficient, that is, the infiltration rate when the hydraulic gradient is 1, cm / h; L is the water penetration distance between two specified points in the soil column, cm; Δh is the total head difference between the two specified points, cm.

[0021] Furthermore, in step 3, a new dry soil column is prepared, and a stable groundwater level and sufficient water source are provided by a Martens flask. At the beginning of the experiment, water is supplied by the Martens flask to flow into the soil column. When the water level in the soil column rises to the stable groundwater level, the capillary water rise height at this time is recorded as 0 cm, the rise time is 0 h, and the rise water volume is 0 cm 3 In the first stage of capillary water rise (the capillary water rise height is less than the soil degassing value h a ), every time the capillary water rises to a certain height, record the rising time and the scale of the water tank until the capillary water rises to 1 / 3h a The capillary water rise flux (q 0 ), combined with the capillary water rise height (z T ) and soil gasification value h a and soil saturated permeability coefficient K s , substituting into the following formula, we can get the correction coefficient λ of capillary water rise height:

[0022]

[0023] In formula (3): q 0 is the capillary water flux, cm / h; K s is the saturated permeability coefficient (cm / h); z T is the capillary water rise height, cm; λ is the correction coefficient of the capillary water rise height; h a is the soil gas emission value.

[0024] In the second stage of capillary water rise (the capillary water rise height is greater than the soil gas release value h a ), record the rising time and the scale of the water tank every time the capillary water rises to a certain height, until the capillary water rises to 4 / 3h a The capillary water rise flux is estimated by using the rising water volume and rising time, and then combined with multiple discrete values ​​of the capillary water rise height and the soil outgassing value h a and soil saturated permeability coefficient K s , substituting into the following formula, we can get the model fitting parameter β':

[0025]

[0026] In formula (4), β' is the model fitting parameter.

[0027] Furthermore, in step 4, the model parameters estimated in steps 1 to 3 and formulas (3) and (4) are used to estimate the capillary water rising flux at other heights during the capillary water rising process.

[0028] In summary, the advantages and positive effects of the present invention are:

[0029] The capillary water rise is divided into two stages. Based on Darcy's law under unsaturated conditions, a new mechanism model of capillary water rise flux changing with height is derived and established. This model covers the entire rising process of water level and has a wider range of applications compared to empirical models. The model fitting values ​​have a good match with the observed values. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 Schematic diagram of the method for predicting capillary water flux;

[0031] Figure 2 The first stage of capillary water rise flux (the capillary water rise height is less than h a ) prediction graph;

[0032] Figure 3 The second stage of capillary water rise flux (the capillary water rise height is greater than h a ) prediction graph. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present method is further described in detail below in conjunction with embodiments.

[0034] A method for predicting capillary water flux by capillary water rise height provided in an embodiment of the present invention comprises the following steps:

[0035] Step 1: Obtain soil gas emission value h a .

[0036] Step 2: Obtain soil permeability coefficient K s .

[0037] Step 3: Obtain the correction coefficient λ and fitting parameter β' of the capillary water rise height of the model.

[0038] Step 4: Predict the capillary water rise flux based on the above parameters and the derived mechanism model.

[0039] In the step 1, 0.075 mm quartz sand is taken to carry out the soil moisture absorption experiment. Before the experiment begins, the quartz sand sample needs to be dried until the soil moisture content is reduced to 0. Next, a certain volume of water is slowly added to the soil, and after it is fully diffused, the volume moisture content and matrix suction in the soil are measured, and then the process of adding water and measuring the volume moisture content and matrix suction is repeated until the volume moisture content reaches saturation, the matrix suction is reduced to 0 Kpa, and the soil moisture absorption experiment is completed. The soil matrix suction measured in the experiment is converted into centimeters of water column height, and the matrix potential is used as the x-axis and the volume moisture content is used as the y-axis to draw the moisture absorption curve of the soil moisture characteristic curve.

[0040] The Gardner model (Gardner, 1958) is used as the fitting model of the soil-water characteristic curve:

[0041]

[0042] In the formula, θ, θ S are the average volumetric water content and saturated volumetric water content, respectively; h is the matrix suction, cm; θ r is the residual moisture content; c and m are the model shape parameters.

[0043] Using the soil moisture characteristic curve fitted by the model, the tangent line at the saturated volume moisture content and the tangent line at the inflection point are drawn and extended. The horizontal coordinate of the intersection of the two tangent lines is represented as the soil gasification value height, h a The unit is cm.

[0044] In the step 2, 0.075 mm quartz sand is filled into a columnar container, and the soil density in the soil column is restored to the density of the original soil by knocking on the column wall. The soil column is immersed in water, taken out after reaching the saturation time, and the soil column is fixed by an iron frame. Water is added above the soil column using a Martens flask, and a beaker is placed below to keep the water layer thickness at 5 cm while maintaining a constant water level. The timing starts when the first drop of water appears below the soil column from the start of water supply from the Martens flask, and the water seepage is measured every 5 minutes thereafter, while recording the water temperature. The cumulative water seepage V measured each time is set to the y-axis, and the time t spent by the cumulative water seepage is set to the x-axis to obtain a curve of the cumulative water seepage of the soil over time.

[0045] The permeability coefficient can be calculated according to Darcy's formula to get K s It is 3.23cm / h.

[0046]

[0047] in

[0048] Where: V is the cross-sectional area A (cm2) of the soil column penetrated in time t (h) 2 ) water volume, cm 3 ; I is the hydraulic gradient; K is the permeability coefficient, that is, the infiltration rate when the hydraulic gradient is 1, cm / h; L is the water penetration distance between two specified points in the soil column, cm; Δh is the total head difference between the two specified points, cm.

[0049] In step 3, a new dry soil column is prepared, and a stable groundwater level and sufficient water source are provided by a Martens flask. At the beginning of the experiment, water is supplied by the Martens flask to flow into the soil column. When the water level in the soil column rises to the stable groundwater level, the capillary water rise height at this time is recorded as 0 cm, the rise time is 0 h, and the rise water volume is 0 cm3 In the first stage of capillary water rise (the capillary water rise height is less than the soil degassing value h a ), record the rising time and the scale of the water tank every 2cm, until the capillary water rises to 1 / 3h a The capillary water rise flux (q 0 ), combined with the capillary water rise height (z T ) and soil gasification value h a and soil saturated permeability coefficient K s Substituting into the following formula, the correction coefficient λ of the capillary water rise height is 0.7475.

[0050]

[0051] In the above formula: q 0 is the capillary water flux, cm / h; z T is the capillary water rise height, cm; λ is the correction coefficient of the capillary water rise height; β' is the fitting parameter.

[0052] In the second stage of capillary water rise (the capillary water rise height is greater than the soil gas release value h a ), record the rising time and the scale of the water tank every time the capillary water rises 2cm, until the capillary water rises to 4 / 3h a The capillary water rise flux is estimated by using the rising water volume and rising time, and then combined with multiple discrete values ​​of the capillary water rise height and the soil outgassing value h a and soil saturated permeability coefficient K s , substituting into the following formula, we can get the model fitting parameter β' as 0.086.

[0053]

[0054] In step 4, the model parameters estimated in steps 1 to 3 and formulas (7) and (8) are used to estimate the capillary water rising flux at other heights during the capillary water rising process.

[0055] The above implementation modes are only for illustrating the technical concept and features of the present invention, and their purpose is to enable people familiar with this technology to understand the content of the present invention and implement it, and they cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for predicting capillary water flux by capillary water rise height, characterized in that Follow the steps below: Step 1: Obtain the soil outgassing value h through soil moisture absorption experiment a ; Step 2: Use the soil column experiment to calculate the soil saturated permeability coefficient K according to Darcy's formula s ; Step 3: In the first stage of capillary water rise (0<h a ≤z T ), according to the capillary water rise experimental data and formula (1), the correction coefficient λ of the capillary water rise height of the model is obtained; in the second stage of capillary water rise (h a >z T ), and obtain the model fitting parameter β' according to the capillary water rise experimental data and formula (2); Step 4: Substitute the model parameters h estimated in steps 1 to 3 a , K s , λ, β' are substituted into formulas (1) and (2) to predict the capillary water rise flux. Where q0 represents the capillary water flux in cm / h; h a is the soil gas release value; K s is the saturated permeability coefficient (cm / h); z T is the capillary water rise height (cm); λ is the correction coefficient of the capillary water rise height; β' is the model fitting parameter.

2. The method for predicting capillary water flux by capillary water rise height according to claim 1, characterized in that: In step 1, the soil to be tested is taken for soil moisture absorption experiment. The soil moisture characteristic curve fitted by the Gardner model is used to draw the tangent line at the saturated volume moisture content and the tangent line at the inflection point and extend them. The horizontal coordinate of the intersection of the two tangent lines is represented as the soil gas outlet value height h a , this parameter is used to divide the capillary water rising process into two stages.

3. The method for predicting capillary water flux by capillary water rise height according to claim 1, characterized in that: In step 3, the capillary water rise flux q0 and the capillary water rise height z are obtained by using a Martens flask to conduct an experiment. T Multiple discrete values ​​of, combined with the soil gasification value h a and soil saturated permeability coefficient K s , and substituted into the derived multi-stage capillary water flux mechanism model to obtain the correction coefficient λ of the capillary water rise height and the fitting parameter β'.

4. The method for predicting capillary water flux by capillary water rise height according to claim 1, characterized in that: In step 4, the capillary water rising flux at other heights during the capillary water rising process is estimated using the model parameters estimated in steps 1 to 3 and the derived multi-stage capillary water flux mechanism model.

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