Uses
of GFLOW
GFLOW is the most efficient
groundwater flow modeling system on the market. It
facilitates a stepwise modeling approach,
allowing you to quickly set up an initial model
and painlessly build up complexity as your
understanding of the groundwater regime grows. For
this purpose, GFLOW has very powerful elements:
e.g., linesinks with bottom resistance, drains,
fully or partially penetrating wells, domains with
differing hydraulic conductivity, bottom
elevation, porosity and recharge. It also supports
fully or partially penetrating slurry walls that
may be open or closed, threedimensional pathline
tracing, flux inspection lines, stream networks
with baseflow, overlandflow and streamflow, etc.
What truly sets it apart are conjunctive
surface water and groundwater solutions
(stream networks) and instant extraction of a
MODFLOW model. The stream network feature is
similar to the MODFLOW Streamflow Package but much
easier to implement. Streamflows offer valuable
calibration targets in addition to the head
targets commonly used in groundwater flow models.
GFLOW lets you design a MODFLOW model inside the
GFLOW graphical user interface of all or part of
the model domain. The MODFLOW model will inherit
all of the model properties and will be
preconditioned with the GFLOW solution. This
feature is often used to create a local detailed
MODFLOW model with boundary conditions on the grid
perimeter derived from the regional analytic
element model.
GFLOW is based on the DOS program
GFLOW, which has been used in academia, government
agencies and consulting firms for more than five
years. The native windows program feels just like
your other windows applications making it easy to
use. GFLOW is an analytic element model similar in
design to the US EPA program WhAEM, but has
all the power of other commercial analytic element
programs. And more!
GFLOW
Stepwise Modeling
Perhaps the most practical advantage
of the analytic element method is its operational
efficiency. In the absence of a mesh or element
network, the hydrologist is concerned only with
entering hydrologic features in the model.
Representing streams by strings of straight line
elements and lakes by polygons is a rather
intuitive task. Also, for initial modeling runs, a
limited set of surface water features may be
introduced. Later, when insight into the
groundwater flow regime increases, more data may
be added to locally refine the modeling. This
stepwise modeling is not new. For example, Ward
applied what he calls a "telescopic mesh
refinement modeling approach" to the ChemDyne
hazardous waste site in southwestern Ohio (Ward et
al., 1987). However, Ward had to use three
different computer models for the three different
scales at which he was modeling. Conditions on the
grid boundary of the "local scale" were obtained
from the "regional scale" modeling results, while
similarly the conditions on the grid boundary of
the "site scale" were obtained from the "local
scale" modeling results. In contrast, the analytic
element method allows these different scales to be
treated within the same model by locally refining
the input data, thus avoiding transfer of
conditions along artificial boundaries from one
model into the other. When necessary, even
threedimensional flow features can be included.
While uniquely suitable for
groundwater flow modeling at different scales,
current generation analytic element models have
some limitations. For instance, both transient
flow and threedimensional flow are only partially
implemented in analytic element models. Gradually
varying aquifer properties cannot be represented
in analytic element models. GFLOW also does not
support multiaquifer flow. Depending on
circumstances and on the purpose of the modeling,
however, these phenomena may be important. The
GFLOW graphical user interface allows the user to
carry the modeling beyond the limitations of the
analytic element method. This is done by
extracting a MODFLOW model out of a GFLOW model,
transferring the internal boundaries (streams,
wells, lakes) and the domains with differing
aquifer properties as defined in GFLOW directly to
the finite difference grid. The GFLOW steadystate
groundwater flow solution is used to define either
head or discharge specified conditions on the grid
perimeter, and to precondition the MODFLOW
solution procedure with heads at each cell center
from the GFLOW solution. This procedure of
extracting a MODFLOW model is quick and easy using
the "grid" menu option in GFLOW. In this manner,
the stepwise modeling procedure within the
analytic element model is extended to modeling
groundwater flow with MODFLOW and, when needed,
contaminant transport with e.g., MT3D.
Analytic
Element Method
The analytic element method was
developed at the end of the seventies by Otto
Strack at the University of Minnesota (Strack and
Haitjema, 1981a). There are two books about the
analytic element method. "Groundwater Mechanics"
by O. D. L. Strack, 1989, contains detailed
mathematical descriptions of the analytic elements
and their numerical implementation. "Analytic
Element Modeling of Groundwater Flow" by H. M.
Haitjema, 1995, provides the basic theoretical
framework for the analytic element method and
focusses on its use.
This new method avoids the
discretization of a groundwater flow domain by
grids or element networks. Instead, only the
surfacewater features in the domain are
discretized, broken up in sections, and entered
into the model as input data. Each of these stream
sections or lake sections are represented by
closed form analytic solutions: the analytic
elements. The comprehensive solution to a complex,
regional groundwater flow problem is obtained by
superposition of all, a few hundred, analytic
elements in the model.
Traditionally, superposition of
analytic functions was considered to be limited to
homogeneous aquifers of constant transmissivity.
However, by formulating the groundwater flow
problem in terms of appropriately chosen discharge
potentials, rather than piezometric heads, the
analytic element method becomes applicable to both
confined and unconfined flow conditions as well as
to heterogeneous aquifers (Strack and Haitjema,
1981b).
The analytic elements are chosen to
best represent certain hydrologic features. For
instance, stream sections and lake boundaries are
represented by line sinks, small lakes or wetlands
may be represented by areal sink distributions.
Areal recharge is modeled by areal source
distributions (areal sinks with a negative
strength). Streams and lakes that are not fully
connected to the aquifer are modeled by line sinks
or area sinks with a bottom resistance.
Discontinuities in aquifer thickness or hydraulic
conductivity are modeled by use of line doublets
(double layers). Specialized analytic elements may
be used for special features such as drains,
cracks, slurry walls, etc. Locally
threedimensional solutions may be added such as a
partially penetrating well (Haitjema, 1985).
GFLOW Output
Graphical output can be sent
directly to the printer. GFLOW also writes DXF
files (AutoCAD) and BLN files (Surfer) with
selected graphical data (analytic elements,
contours, pathlines, and wellhead protection
areas). GFLOW uses background maps ("base maps")
in a special binary format (.bbm) for fast
redraws. These maps can be created from DXF files,
USGS DLG or STDS files. The US EPA is developing a
national coverage of the .bbm files for its public
domain program WhAEM. These .bbm maps can be
selected using a map browser that is part of the
WhAEM and GFLOW program. GFLOW is UCODE and PEST
ready in that it can write ASCII input files for
the GFLOW Solver which can be used by UCODE and
PEST to run the Solver for parameter estimation
purposes.
GFLOW Hardware
Requirements
 32bit Windows operating system
(95/98/2000/NT)
 15 MB of hard disk space
 16 MB RAM
