Colorado Water April 2005

• Year: 2005
• Title: Colorado Water
• Author: Water Center of Colorado State University
• Description: April 2005
• Publications - Doc Type: Newsletter

Figure 3. Example of an SDF map (after Jenkins and Taylor, 1974).
cal model; the difference is really a question of where the user
obtains inputs to the model and what effects are accounted for
by the selected inputs.
Although the modeling improved and streamlined estimates
over purely analytical methods, the SDF method is still an
approximation since the response curve is calibrated to one
point only: the time given by the SDF. By definition, the
SDF method matches complex response curves exactly at
the SDF time. And, in a number of
modeling tests, Jenkins observed that
the method matched with "acceptable
accuracy" in the time range between 1/2
SDF and 2 SDF.
We constructed response curves for
bounded aquifers using the image well
method (e.g., Glover 1978) to illustrate
the SDF method's approximation of
boundary effects (Figure 4). Plot-
ted against nondimensionalized time
(time /SDF), the curves are a function of
the relative well position between the
stream and the aquifer boundary (dis-
tance "a" as a fraction of aquifer width
"W "). The SDF approximation is good
for wells closer to the stream until times
much larger than the SDF (t>2SDF),
but for wells closer to the boundary the
approximation departs from the bounded
curves sooner.
�1
Combining the image method with
the SDF method can significantly
improve estimates, but it raises a
question about over - accounting
for boundary effects since SDF's
already account for boundaries.
However, from our review, it is ap-
parent that SDF's in the stream -half
of the aquifer do not account for
boundary effects. The SDF model-
ing did account for them, but for
these locations the calibration point
is reached before the boundary ef-
fects are significant. Consequently,
these SDF values can be legitimately
combined with the image method.
For the other half of the aquifer, the
boundary effects follow a predict-
able adjustment that can be removed
to allow these mapped values to
be used with images as well. This
conclusion and the small adjustment
were evaluated against the numeri-
cal model of the Tamarack site and
they compared well.
Awareness that the SDF method only partially accounts for
boundaries has prompted some water professionals to use
alternatives. These alternatives have value, but continu-
ing to use the SDF method by combining it with the image
method also has benefits: SDF maps are valuable since they
integrate multiple effects in addition to aquifer boundaries
(e.g., spatially variable transmissivities and irregular bound-
ZIA%
t
our
t�l
Figure 4. Response curves for bounded aquifers.
tie's,