Specified Flux: Discharge
(Q) varies with space and time.
No Flow: Discharge
(Q) equals 0.0 across boundary.
Implications:
H will be calculated as the value required to produce
a gradient to yield that flux, given a specified hydraulic conductivity (K).
The resulting head may be above the ground surface in an unconfined aquifer,
or below the base of the aquifer where there is a pumping well; neither of
these cases are desirable.
Implications:
Flow field geometry varies so transmissivity will vary
with head (i.e., this is a nonlinear condition). If the water table is at
the ground surface or higher, water should flow out of the model, as a spring
or river, but the model design may not allow that to occur.
Seepage Surface: The
saturated zone intersects the ground surface at atmospheric pressure and water
discharges as evaporation or as a downhill film of flow.
The location of
the surface is fixed, but its length varies (unknown a priori).
Implications: A
seepage surface is neither a head or flowline. Often seepage faces can be
neglected in large scale models.
You will find these
same boundary conditions referred to by various names including "Type" numbers.
Review the summary table to organize these relationships in your mind.
It is most desirable
to terminate your model at natural geohydrologic boundaries. However, we often
need to limit the extent of the model in order to maintain the desired level
of detail and still have the model execute in a reasonable amount of time.
Consequently models sometimes have artificial boundaries. For example, heads
may be fixed at known water table elevations at a county line, or a flowline
or ground-water divide may be set as a no-flow boundary.
