BOARD02442

• Board Meeting Date: 5/11/1998
• Description: South Platte River Basin Issues - South Platte River Basin Aquifer Studies - SB 96-74 and SB 96-153
• Board Meetings - Doc Type: Memo

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Depletion Standard

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This chapter examines the physical impacts to stream systems in the state, as
well as possible legal and administrative implications of de minimis standards for
injury based upon an annual depletions standard. In this report, de minimis
means a small amount of depletion in a calendar year that could be allowed
without requiring the owner of a well causing the depletion, to augment or replace
the water depleted or removed from the stream and aquifer system.

The analyses included a review of types of water uses authorized by the Legislature
where small amounts of un-replaced depletions to tributary water sources were
deemed acceptable such as small capacity wells used for residential and livestock
purposes and water stored in small reservoirs (livestock water tanks).

The State Engineer recommends that, prior to changing the current policy, the
Division of Water Resources solicit and evaluate public comment on a de minimis
standard for depletion from the pumping of a Denver Basin "not non-tributary"
aquifer based on an annual pumping volume of 3 acre-feet per year.

ChaDter 9 - The Effect of Four Percent Replacement and Two Percent
Relinquishment Requirements on Future Water Supplies, Existing Water Rights
and the Need for Replacement of Post Pumping Depletion Resulting from .
Withdrawal of Denver Basin Groundwater (Technical Addendum No.2)

Chapter 9 summarizes Technical Addendum No.2, which is a study conducted by
staff of the State Engineer's Office to evaluate the requirements of SB-85-5 to
replace 4 percent of the pumping volumes for "not non-tributary" aquifers and 2
percent relinquishment for non-tributary aquifers. .

To evaluate the effect of the current replacement (4%) and relinquishment (2%)
requirements on future water supplies, water rights and need for replacement of
post pumping depletions resulting from withdrawal of Denver Basin ground water,
the following is required:

a) updating and improving of existing ground water models,

b) use of the model to evaluate current depletions and predict long term future
depletions to the discharge areas of the South Platte River Basin drainages,
and

c) evaluation of the model results and effect on existing water rights, future water
supplies, and Denver Basin Aquifers.

The model developed for this study uses the USGS MODFLOW computer code.
This code is well accepted in the engineering community and is considered the best .
groundwater modeling code currently available. In computer modeling, the

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