# HydroGeoSphere/Variably-Saturated Flow

By default, HydroGeoSphere uses the upstream weighting scheme (weighted harmonic mean) for relative permeability, with an upstream weighting factor value of 1.0. The default behaviour can be changed using the following commands:

## Upstream weighting factor

1. upwfactor Upstream weighting factor.

Assigns a new value for the upstream weighting factor, which defaults to 1.0. This value should be in the range from 0.5 (central weighting) to 1.0 (upstream weighting).

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## Central weighting

Causes HydroGeoSphere to use central weighting (weighted arithmetic mean) instead of upstream weighting.

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## Primary variable switching

Causes HydroGeoSphere to use the primary variable substitution technique outlined in Section 3.13.3.

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If desired, the default values of ${\displaystyle tol_{f}}$  and ${\displaystyle tol_{b}}$ , the upper and lower limits for variable substitution (see Equation 3.99) can be changed using the following commands:

## Upper limit

1. switch_t Upper limit of the variable substitution approach, tolf in Equation 3.99. The default value is 0.99.
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## Lower limit

1. switch_f Lower limit of the variable substitution approach, tolb in Equation 3.99. The default value is 0.89.
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