Saturated Flow:Global parameters

Global parameters for 2-D saturated flow modeling are specified by toggling on Subsurface in the GSSHA Job Control Parameters dialog. This produces the Subsurface Parameters dialog box where the following parameters may be specified.

• Time-step – groundwater model time-step (s). Typically the groundwater time-step may be much larger than the overall model time-step, typical values are 300 - 1,200 sec. The groundwater time-step must be equal to or larger than the overall model time-step, and the overall model time-step must be integer divisible into the groundwater model time-step. During rainfall events, the groundwater model time-step is temporarily changed to the overall model time-step, and then changed back at the end of channel routing and overland flow routing. This switching is handled internally.
• LSOR direction – The solution technique currently used to solve the 2-D groundwater free surface flow equations is line successive over relaxation (LSOR) (for example Tannehill, Anderson, and Pletcher 1997). When LSOR is applied, the solution is either by rows or by columns. The solution should be aligned with the principal direction of saturated groundwater flow. The options are either Vertical or Horizontal, which indicates that your principal direction of flow is in the vertical direction, along the y axis, or in the horizontal direction, along the x axis.
• LSOR convergence – LSOR is an iterative method. The user must supply a convergence criteria. The criterion is in meters of groundwater head or depth. The default value, 10-5, is typically sufficient for a good solution. Stricter criteria may lead to slower solutions, or nonconvergence.
• Relaxation – The relaxation coefficient is used to project the current solution out into the future in an attempt to speed up convergence. A value of 1.0, the default, indicates no projection. Values greater than 1.0 indicate that projections into the future are desired. Values up to 1.5 may speed the groundwater model solution. Typically, a value of about 1.2 provides the fastest solution. Increasing the relaxation coefficient to greater than 1.0 can result in nonconvergence of the solution. If this happens, the relaxation coefficient should be reduced. It is sometimes necessary to reduce the relaxation coefficient to less than 1.0 to obtain convergence of the solution.
• Leakage rate – this is the leakage rate (cm/hr) through the bottom of the aquifer. This is a uniform value that is applied to every cell in the grid. The default value is 0.0.
• Define initial moisture – when this option is toggled on, it means you wish to use the values of initial moisture of cells in the unsaturated zone as entered in the mapping table. Otherwise, the default, initial soil moistures in the unsaturated zone are assumed to be the water contents that correspond to soil pressures in equilibrium with the saturated groundwater elevation.

Related Topics

GSSHA Wiki Main Page
Primer Main Page

Saturated Flow

Global parameters

Distributed parameters

Boundary conditions