# 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 factorEdit

**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 weightingEdit

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

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## Primary variable switchingEdit

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 and , the upper and lower limits for variable substitution (see Equation 3.99) can be changed using the following commands:

## Upper limitEdit

**switch_t**Upper limit of the variable substitution approach,*tol*in Equation 3.99. The default value is 0.99._{f}

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## Lower limitEdit

**switch_f**Lower limit of the variable substitution approach,*tol*in Equation 3.99. The default value is 0.89._{b}

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