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

Items I through 5 relate to setting up the model, and item 6 is necessary for model calibration.
Each of these items is described below.

3.2.1 Model Setup - In this section, the approach to setup of the model is described in terms of
model geometry, parameter distributions, boundary conditions, aquifer stress, and observation
data used as calibration targets. Since this model was developed as a modification to the State
Engineer Office (SEO) SB-74 Denver Basin MODFLOW model, particular attention will be
focused on the modifications to the original SB-74 model. To provide a robust tool for assessing
impacts of ground water development in the South Metro region, this ground water model needs
to extend the capabilities of the existing SB-74 model in at least two important ways:

. It should incorporate the best available up-to-date data and information, and
. It should be capable of projecting regional drawdowns and water elevations to support the
local (well and well-field) scale analyses for estimating the costs of adding supplemental and
alternate points of diversion as needed to maintain production through the aquifers' life.

3.2.1.1 Model Domain and Geometry - The model domain in the plan view encompasses the
entire areal coverage of each of the aquifer units within the Denver Basin (Figure 3.1). In the
plan view, the model grid is oriented parallel to the state of Colorado's township-range cadastral
system, with 1 mile by I mile grid cells. Each grid cell can be mapped to a section.

The model domain was discretized vertically to simulate each of the important water producing
units, or aquifers, which supply water for various uses in the basin (Figure 3.2). Those aquifers
are, from the top of the stratigraphic column to the bottom: the Dawson, the Denver, the
Arapahoe, and the Laramie-Fox Hills. Each aquifer was simulated as a separate layer except the
Dawson and the Arapahoe, which were further discretized into upper and lower units. The top
and bottom elevations of each unit were derived from the top and bottom elevations of the units
as inferred from well logs. Given that there was not a well in every grid cell, aquifer top and
bottom elevation maps were developed based on interpolation of the measured values. Our
model aquifer top and bottom elevation maps deviate slightly from the SB-74 model in that we
incorporated all data from the South Metro area available through 2001, whereas the SB-74
model incorporated no data collected after 1985. To illustrate typical differences between the
SB-74 and the updated South Metro aquifer top and bottom elevations, Figure 3.3 shows the
aquifer top configuration for the Upper Arapahoe aquifer for both models. More details on this,
as well as on the hydrological parameter distributions discussed in the next section, can be found
in Appendix 3B.

3.2.1.2 Hydrological Parameter Distributions - To simulate aquifer behavior, hydrologic
parameters (e.g., transmissivity, storage coefficient, leakance, ...) must be specified for each of
the model units. The starting point for each parameter was the distribution specified in the SB-
74 model, and these values were updated based on data collected since 1985. In the calibration
process, some of these values were adjusted as required to improve model fit to observations.
Below each of the hydrologic parameters is discussed.

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