WSP09428

• File Number: 8449.926
• Description: South Metro Water Supply Study
• State: CO
• Basin: South Platte
• Date: 2/1/2004
• Author: SMWSS Board
• Title: South Metro Water Supply Study
• Water Supply Pro - Doc Type: Publication

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were necessary so that the model would simulate 1991-2000 drawdown rates consistent with
observations by the South Metro water providers. To accomplish this, and recalling that the
"pristine head" targets were actually developed from a very sparse and uncertain dataset, the
a and tP weights were reduced to zero and equal weights were assigned to
PSSEH1991, t;SSEDDl991-2000, and roSSEH2000. To further emphasize the importance of
the Arapahoe aquifer, for the overall summer SSE weights were applied to the individual layer
SSEs of I, I, 2, I for the Lower Dawson, Denver, Arapahoe, and Laramie - Fox Hills aquifers,
respectively. The calibration process continued until the new weight error sum was minimized
(Fig. 3.5b).

The last refmements to achieve what was deemed as a fmal calibrated model ready for
application to future pumping scenarios were undertaken to improve the "visual fit" of key flow
features in the 1991 and 2000 water level maps (Reference 3.1, p. 205). For example, among the
flow features assessed was the region of steep hydraulic gradients along the southeast side of the
South Metro area. To achieve close correspondence between the model and data, the following
was completed:

. Reduction of the conductivity for all layers on the eastern side of the basin.
. Insertion ofa 3- to 7-cell thick low-permeability zone from the vicinity of the intersection of
the Arapahoe, Douglas, and Elbert county boundaries on the northeast diagonally trending to
the southwest through Castle Rock and terminating in the vicinity of Dawson Butte.

These adjustments greatly improved the model fit to the 1991-2000 aquifer behavior and they are
consistent with the hydrostratigraphic conceptual model of the Denver Basin being developed by
Raynolds (Reference 3.3).

3.2.2.6 Example Results from Calibrated Model - Figures 3.6 through 3.12 present some
results to illustrate the fit of the final calibrated model to observations.

. Figure 3.6 shows the final calibration recharge distribution
. Figure 3.7 presents the final calibrated hydraulic conductivity fields for: (a) the Lower
Dawson, (b) the Denver, (c) the Upper Arapahoe, and (d) the Laramie - Fox Hills aquifer
units.
. Figure 3.8 provides representative predicted and observed hydrographs for wells completed
in each of the aquifers at several locations across the basin.
. Figure 3.9 presents contour maps of observed and simulated 2000 heads in the Lower
Dawson aquifer. Both of these charts indicate close correspondence between the calibrated
model and field observations for the Dawson aquifer.
. Figures 3.1 0, 3.11, and 3.12 present the observed and predicted 2000 heads for the Denver,
Upper Arapahoe, and Laramie-Fox HiJls aquifers.
. Figure 3.13 shows posting maps of head residuals (data- model) for: (a) the Lower Dawson,
(b) the Denver, (c) the Upper Arapahoe, and (d) the Laramie - Fox Hills aquifers.

3.2.3 Model Application for Future Development Scenarios - Using the calibrated model, a
simulation of future aquifer response to pumping was performed under three separate scenarios.
For completeness, below are listed all five scenarios analyzed in the overall water supply study.

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