Richards’ Equation
Begin by specifying the properties for the bottom soil layer in the default Unsaturated Porous Medium node, then duplicate this node and modify the domain selection and properties to match the top layer.
Unsaturated Porous Medium1
1
In the Model Builder under the Richards’ Equation interface , click the Unsaturated Porous Medium 1 node.
2
Locate the Porous Medium Section and from the Storage model list select User Defined.
3
In the Sp text field, type Sp_1.
The Fluid subnode under the Unsaturated Porous Medium node uses default values for the material properties. The material properties will be defined later.
4
Go to the Porous Matrix subnode and locate the Matrix Properties section:
-
From the Permeability model list, select Hydraulic conductivity. In the Ks text field, enter Ks_1.
5
Under Retention Model:
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Keep the default van Genuchten model in the Retention model list.
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In the α text field, enter alpha_1.
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In the n text field, enter n_1.
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Leave the default value 0.5 in the l text field.
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Enter thetar_1 in the θr text field for the residual liquid volume fraction.
Unsaturated Porous Medium 2
1
In the Model Builder right-click Unsaturated Porous Medium 1. Select Duplicate .
2
In the Settings window verify that the Domain Selection consists of domains 2 and 4 only (the upper layer). Select 1 and 3 in the list and click on the Remove from Selection  button.
3
Locate the Porous Medium section. In the Sp text field, type Sp_2.
4
Select the Porous Matrix subnode and locate the Matrix Properties section:
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In the Ks text field, enter Ks_2.
5
Under Retention Model:
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Keep the default van Genuchten model in the Retention model list.
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In the α text field, enter alpha_2.
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In the n text field, enter n_2.
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Keep the default value 0.5 in the l field.
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Enter thetar_2 in the θr text field for the residual liquid volume fraction.
Gravity 1
For the Richards’ Equation interface Gravity is active by default.
1
Go to the Gravity 1 node under Richards’ Equation (dl).
2
Locate the Gravity section, and choose Elevation from the Specify list.
Initial Values 1
1
Select the Initial Values 1 node under Richards’ Equation (dl).
2
Locate the Initial Values section, and click the Pressure head button.
3
In the Hp text field, enter -(z+1.2) to set the water table to 1.2 m below the surface.
Initial Values 2
1
Right-click the Initial Values 1 node and choose Duplicate .
2
In the Settings window for Initial Values 2 under Domain Selection, select Domains 2 and 4 (upper layer) only.
3
Locate the Initial Values section. In the Hp text field, enter the expression -(z+1.2)-0.2*(z+0.4).
Next, define the boundary conditions for the Richards’ Equation interface.
Pressure Head 1
1
In the Model Builder click Richards’ Equation (dl) . In the Physics toolbar click Boundaries  and select Pressure Head. Alternatively, right-click on Richards’ Equation (dl) and select Hydraulics > Pressure Head from the menu.
2
In the Boundary Selection window, select Boundary 5 only.
3
Under Pressure Head, enter Hp0 in the Hp0 text field.
Pervious Layer 1
4
In the Physics toolbar click Boundaries  and select Pervious Layer. Alternatively, right-click on Richards’ Equation (dl) and select Hydraulics > Pervious Layer.
5
In the Boundary Selection window, select Boundaries 2 and 8 only. These are the boundaries at the bottom.
6
Find the Pervious Layer section, under External hydraulic head enter -2 in the Hb text field, and under Conductance enter 1/5[d] in the Rb text field.
This concludes the setup of the Richards’ Equation interface.