Appendix S5. Factors that affect the applicability of the specified-thickness approximation
MODFLOW does not explicitly simulate dynamics of groundwater flow and storage in the
unsaturated zone unless the Unsaturated-Zone Flow (UZF) Package (Niswonger et al. 2006) or
the UZF capability of the Streamflow-Routing (SFR2) Package (Niswonger and Prudic 2005) is
used. If UZF is invoked by MODFLOW, or if another modeling code that explicitly simulates
the dynamics of the unsaturated zone is being used, then setting the top of the model to coincide
with the water table effectively eliminates an actively simulated portion of the model. In that
case, the modeler should be aware that an additional level of approximation is being introduced.
Likewise, when the top of the model is changed, potential effects on boundary condition
specifications should be taken into account. In the numerical examples in this paper, it was not
necessary to change the way in which boundary conditions were specified when the top elevation
of the model changed, although computed flows at model boundaries were, of course, influenced
by changes in the computed head distribution.
In the transient simulation presented in this paper, the water table remains within the top layer of
cells in the calculated-thickness model, and the top layer consists of a single hydrogeologic unit
at any given (row, column) location. In this case, setting the top elevation to coincide with the
initial water table is fairly straightforward because the top elevations of layers below layer 1 are
unaffected. Also, because specific yield does not change with time as the water table moves, the
fixed value of specific storage effectively assigned at each (row, column) location by the SYTP
capability (Anderman and Hill 2003) is sufficient to allow S seff  S y z to be satisfied to a close
approximation as long as relative changes in saturated thickness remain small.
Application of the specified-thickness approximation to cases in which the transient water table
moves vertically through multiple model layers requires further study. One possible approach is
to consolidate the multiple water-table layers into one layer, which would reduce the problem
size and, in the case of HUF Package in MODFLOW, allow for the straightforward application
of the existing SYTP capability.
References
Anderman, E.R. and M.C. Hill. 2003. MODFLOW-2000, the U.S. Geological Survey modular
ground-water model — three additions to the hydrogeologic-unit flow (HUF) package:
Alternative storage for the uppermost active cells (SYTP parameter type), flows in
hydrogeologic units, and the hydraulic-conductivity depth-dependence (KDEP) capability.
U.S. Geological Survey Open-File Report 03-347, 36 p. Reston, Virginia.
Niswonger, R.G., and D.E. Prudic. 2005. Documentation of the Streamflow-Routing (SFR2)
Package to include unsaturated flow beneath streams—A modification to SFR1. U.S.
Geological Survey Techniques and Methods 6-A13, 50 p. Reston, Virginia.
Niswonger, R.G., D.E. Prudic, and R.S. Regan. 2006. Documentation of the Unsaturated-Zone
Flow (UZF1) Package for modeling unsaturated flow between the land surface and the water
table with MODFLOW-2005. U.S. Geological Survey Techniques and Methods 6-A19, 62 p.
Reston, Virginia.
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