The Richards’ Equation (dl) interface (), found under the Porous Media and Subsurface Flow branch () when adding a physics interface, is used to analyze flow in variably saturated porous media. The analytic formulas of van Genuchten, Brooks, and Corey are available for modeling variably saturated flow, where hydraulic properties change as fluids move through the porous medium, filling some pores and draining others. The physics interface can be used for stationary and time-dependent analysis.

When this physics interface is added, these default nodes are also added to the Model Builder — Richards’ Equation: Unsaturated Porous Medium (which adds the equation for the pressure and provides an interface for defining the fluid material and its properties, including the retention model), No Flow (the default boundary condition), and Initial Values. Then, from the Physics toolbar, add other nodes that implement, for example, boundary conditions and mass sources. You can also right-click Richards’ Equation to select physics features from the context menu.

The Label is the default physics interface name.

The Name is used primarily as a scope prefix for variables defined by the physics interface. Refer to such physics interface variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the Name field. The first character must be a letter.

The default Name (for the first physics interface in the model) is dl.

Enter a Reference pressure level pref (SI unit: Pa). The default value is 1[atm].

Gravity effects are active by default. Clear the check box Include gravity to deactivate the acceleration of gravity.

Under Acceleration of gravity, enter the value for the acceleration of gravity (SI unit m/s2). The default value is taken from the predefined physical constant g_const, which is the standard acceleration of gravity on earth.

The dependent variable (field variable) is for the Pressure. The name can be changed but the names of fields and dependent variables must be unique within a model.

You can choose the order of the shape functions used for the pressure variable solved by the Richards’ Equation interface. The default shape functions are Quadratic Lagrange.

To display all settings available in this section, click the Show button () and select Advanced Physics Options.