DHIJWASv Software FEFLOW 6.1

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conductivity. This residual water depth can be defined
on the Free Surface page in Problem Class.
Groundwater recharge is applied on the top of the
model in phreatic mode and therefore has to pass the
partially saturated/dry elements before reaching the
water table.
The phreatic mode avoids all slice movement and
related parameter interpolation and is therefore applicable
to water tables with steep gradients that extend over
multiple layers. On the other hand, dry elements with
low conductivity values can lead to strong contrasts in
the model, making the solution more difficult. The
default low residual water depth might cause difficulties
for the infiltration of recharge into dry soil, especially
in cases with time-varying groundwater
recharge.
In phreatic mode, the unconfined storage term is
always only applied to the slice set to ’phreatic’, i.e.,
usually to the top slice. The values for drain-/fillable
porosity, however, are correctly derived from the layer
where the water table is located at a given time. While
this simplification has no negative consequences for
typical regional models with significant horizontal
flow components, it makes the phreatic mode less suitable
for problems in which vertical flow is important,
such as simulations of drainage of a soil column from
the bottom.
Source and sink parameters assigned within
phreatic layers can be scaled by the local pseudosaturation
if the option Scale sources/sinks by pseudosaturation
is set active. This will result in reduced source/sink
values wherever the hydraulic head falls below the element
top elevation (elements are not fully saturated).
By default, the scaling of source/sink values is enabled
to maintain compatibility with older FEFLOW versions
not providing the two options.
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To allow combining of different methods of
phreatic-surface handling, the method is set for each
slice separately.
Typically, the first slice is set to either phreatic or
free, while all the slices below are set to dependent
except for the bottom slice which is always set to fixed.
Dependent slices are defined by the first slice above
that is non-dependent, e.g., dependent slices below a
free slice can move if necessary.
Layers whose top slice is set to confined are treated
as fully saturated, no matter whether the hydraulic head
is above or below the layer. In models using the
free&movable approach, fixed is especially useful to
avoid slice movement (and possible material-property
interpolation) in layers that are known to be saturated
during the entire simulation.
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In models with a phreatic surface, two particular
cases may occur:
• All layers can be dry in a certain location.
• The water level can exceed the top of the model.
FEFLOW provides two options for dealing with a
dry model bottom:
• Unconstrained head (default):
Hydraulic-head values lower than the model bottom
are tolerated. The saturated thickness is considered
equal to the residual water depth at these
locations.
• Constrained head:
FEFLOW will prevent hydraulic-head values
below the model bottom. For this purpose, first