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

are great uncertainties concerning the magnitude of the pumping stresses. These stresses have a
direct impact on the resulting head distribution: the drawdown is linearly proportional to the
quantity of water pumped.

Historical (1880 - 1995) pumping locations and magnitudes were obtained from those used in
the SB-74 model. These pumping stresses were applied to our historical model runs used in
model calibration. Future pumping estimates were obtained from two sources. For the South
Metro region, current and future (2000 and beyond) pumping estimates were derived from
current demand information and projected growth provided by each of the then twelve (l2)
participating water providers (parker Water and Sanitation has since withdrawn from the study).
Given the uncertainty as to where future wells would be installed, the projected pumping for each
of the water providers was uniformly distributed across all the gridblocks that lie within each of
their respective service areas. For areas outside the South Metro water provider service
boundaries, the current and future pumping conditions (locations and magnitudes) associated
with the BASE scenario for the SB-74 model were applied with minor adjustments associated
with new demand patterns in the South Metro area. For all future development scenarios
considered (see Section 3.2.3), Figure 3.4 shows the future pumping demands specified for the
entire basin from the historical period through 2050. Table 3.1 presents the future pumping
estimates for the three ground water scenarios modeled for key South Metro water providers.
The pumping volumes presented in this table have been refmed in the study process and do not
match exactly the volumes modeled. However, these minor revisions which are less than ot 5%
have little effect on the study results.

3.2.2 Model Calibration - Model calibration involves adjusting one or more of the model
parameters described in Section 3.2.1 to minimize the differences between model predictions and
historical observations of aquifer response. "Aquifer response" refers to some history of how the
ground water system responds to stress, and these historical data are referred to as calibration
targets.

After providing a broad discussion of model calibration targets, error measures, and general
considerations related to steady-state and transient conditions, a chronological summl!rY of the
calibration process is provided.

3.2.2.1 Calibration Targets - In our calibration process, there were five data sets used as
calibration targets:

. Pre-development heads, reflecting steady-state conditions in the bedrock aquifers prior to
installation of wells in the basin,
. Historical heads in 1991,
. Historical drawdown rates from pre-development through 1991,
. Historical heads in 2000, and
· Drawdown rates between 1991 and 2000.

It is generally recognized that calibration of a steady-state model is more likely to converge on a
unique solution if flow targets are used in addition to head targets (see for example, Reference
3.1). Unfortunately, there is essentially no available high quality data that characterizes
interactions between the bedrock aquifers and streams and/or the alluvial that could be used for
flow targets during calibration of the steady-state model.

Page 3-10

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