# Richards Equation:Global parameters

The global parameters for Richards’ equation are set from the GSSHA Job Control Parameters dialog under the Infiltration options. With Richard’s infiltration toggled on, hit the Edit Parameters... button to access the global parameter list. The global parameters are:

• Weight – Fraction between 0.0 and 1.0 (dimensionless). This is the weighting factor used to calculate inter-cell hydraulic conductivities when using an arithmetic mean to calculate the inter-cell hydraulic conductivities. Use 1.0 for forward weighting, 0.0 for backwards, and 0.5 for central. The default value is 1.0 Figure 30. GSSHA representation of the unsaturated zone
• DTHETA Max – Fraction between 0.0 and the porosity of the soil (m^3/m^3), the maximum allowable water content change in any finite difference cell during a single time-step. Typical range is between 0.002 and 0.030 (Belmans, Wesseling, and Feddes 1983). The default value is 0.025.
• C OptionBrooks or Havercamp sets the curves used to define the relationships between water content and soil suction, pressure, and water content and hydraulic conductivity. An example of the curves for a typical clay and sand are shown in Figure 22. The user is referred to the GSSHA User’s Manual for more detailed information on this topic. The default value is Brooks.
• K Option – (Arithmetic or Geometric) method used to calculate inter-cell hydraulic conductivities.
• Upper Option – method used to determine the hydraulic conductivity at the soil surface during ponded water conditions; the options are Normal, Green Ampt, and Average. Normal specifies that the normal cell-centered value of hydraulic conductivity be used, Green Ampt specifies that the saturated hydraulic conductivity of the soil in the cell be used, and Average specifies that an average of the two be used. The default value is Normal. For more information, see the GSSHA User’s Manual.
• Max Iteration – the maximum number of iterations each time-step to determine water capacity and hydraulic conductivity. The typical range is 1 to 10 iterations. The default value is 1.
• Max Num – integer value, maximum number of cells in any unsaturated soil column.

## Related Topics

Richards' Equation
Global parameters
Distributed parameters
Soil depth and discretization