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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CHAPTER 3

REGIONAL GROUND WATER MODEL
FOR
ANAL YSIS OF FUTURE SCENARIOS

3.1 Regional Ground Water Model

As described in Chapter I, one of the first steps in investigating future water management
scenarios for the South Metro region involved development of a regional ground water flow
model to predict the decline in regional ground water heads in each of the four aquifers of the
Denver Basin over the study period. Water levels in the bedrock aquifers have been steadily
decreasing over the past twenty years. Over the period from 1995 through 2000, water levels in
the Arapahoe Aquifer declined at an average rate of nearly 30 feet per year in the South Metro
area. These falling water levels seriously impact the water supplier's ability to produce water
from the bedrock-ground water resource. Therefore, a major component of this study was to
predict the falling regional head in each of the aquifers over time so that estimates can be made
of well production in the future.

Hydrosphere, Inc., using the USGS computer code MODFLOW2000, has prepared the regional
model. Utilizing the SB-74 ground water model (prepared by the State Engineer Office using
MODFLOW96 in 1998) as a foundation, Hydrosphere provided an updated model that
incorporates the most up-to-date available data. In particular, this model updates the SB-74
model with a significant amount of data from the South Metro Area water providers. The
primary application of the model was to estimate future regional water levels at ten-year
increments over the fifty-year study period. These estimates were then used as input to the local
well analysis.

The local well analysis (Chapter 4) predicts the production level in each well, based upon the
regional head data and estimated well-to-well interference.

3.1.1 Regional Ground Water Model Development Process - In developing the regional
model, Hydrosphere followed a deliberate, step-by-step process:

. Task I: Model Requirements. The model requirements were developed through a fonnal
elicitation process that involved an iterative dialogue (including interviews, questionnaires,
meetings, etc.) between Hydrosphere, the Study Board, and the ground water Peer Review
Committee. Questions and issues addressed in the process included: intended uses of the
regional model, model features (temporal/spatial characteristics), data availability and
requirements, and end users. At the completion of the elicitation process, Hydrosphere
developed a Model Requirements document. lbis report is attached as Appendix 3A.

. Task 2: Model Development Plan. In October 2001, Hydrosphere submitted the Regional
Ground Water Model Development Plan for review by the Peer Review Committee and the
Study Board. The model development plan report provided a comprehensive summary of all
work completed related to the regional ground water model in the first 10 months of the

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