WSP08278

• File Number: 8449.926
• Description: South Metro Water Supply Study
• State: CO
• Basin: South Platte
• Date: 2/1/2004
• Author: SMWSS Board
• Title: Technical Appendices - Appendix 3B - Hydrosphere's Model Development Plan
• Water Supply Pro - Doc Type: Publication

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Sou/h 'Metro Groundwater Model Development Report

Oct. II, 2001
Page 3

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2.0 SB-74 MODEL ASSUMPTIONS AND LIMITATIONS

At the direction of the 1996 Colorado legislature, the S8-74 model was originally
developed between 1996 and 1998 by the Colorado SEO to evaluate the effect of the 4%
replacement and 2% relinquishment requirements on future water supplies and on
existing water rights. It is our understanding based on personal communication with
Brian Ahrens (SEO) that one of the constraints on SB-74 model development mandated
in the enabling legislation was that the SB-5 model with no new data be used as the basis
for model development; consequently, no data gathered since 1985 was incorporated into
the development of the SB-74 model. This constraint, together with the lack of
sufficiently detailed data, forced model developers to employ indirect approaches to infer
a number of key model parameters, including: (I) those that describe surface water-
groundwater interactions, as well as (2) aquifer layering characteristics that can strongly
affect model predictions as the bedrock aquifers transition from confined to unconfined
conditions.

2.1 Stream - Alluvial Aquifer I Bedrock Aquifer Interactions
Due to concerns about the adequacy of the SB-74 model's ability to project stream
depletions caused by pumping in the Denver Basin bedrock aquifers, a group of peer
reviewers, called the Technical Scope Committee (TSC), was convened. The charge of
the TSC was to develop a scope of work and cost estimate for additional work needed to
improve the SEO SB-74 model, particularly as it relates to characterization of stream
depletions. It was generally recognized at that time that data limitations severely
constrained our ability to defensibly quantify such depletions. In December 1998, the
TSC recommended a program of seven tasks that included both field and laboratory data
gathering, as well as data analysis and model enhancements. The cost for the program
exceeded $2,000,000 (including the cost of new full-time staff in the SEO) and at least
two years were anticipated to be required for completion of the seven tasks. Since that
time, none of the recommendations have been implemented, although the SEO is
committed to implementing those tasks as part of the SPDSS project (Brian Ahrens, pers.
comm.,4 Oct. 2001).

Given the current status of progress made on implementing the TSC's study plan, there
remains insufficient data available to implement the model enhancements recommended
by the TSC. An alternative approach to estimate subsurface flows between the

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