S7_11-15-04

• Basin: Statewide
• Title: SWSI Phase 1 Report - Section 7 Availability of Existing Water Supplies
• Date: 11/15/2004
• Author: CWCB
• SWSI - Doc Type: Final Report

Section 7
Availability of Existing Water Supplies
Luis Valley in south central Colorado within the Rio
Grande Basin. Of the many aquifer systems located in
bedrock deposits, the most significant of these are the
aquifers of the Denver Basin, located east of the Front
Range, and the Ogallala (High Plains) aquifer located in
eastern Colorado.
7.2.1 Definition of Groundwater
Resources
Groundwater is administered by the State DWR to
regulate and manage its use. Section 4 provides
additional information on water rights as it affects
groundwater resources. To reiterate, Colorado
recognizes four types of groundwater and has separate
sets of rules for each. These are based on interaction
with surface water and/or on geographic location:
^ Tri~~~.~v~ - groundwater that is hydrologically
connected to a natural stream.
^ No~-tribut~ry - groundwater located outside of a
designated basin, the withdrawal of which will not,
within 100 years, deplete the flow of a stream at an
annual rate greater than one-tenth of 1 percent of the
annual rate of withdrawal.
^ Design~ted ~~sin - groundwater in areas not
adjacent to a continuously flowing stream or required
to fulfill decreed surface water rights, and located
within the boundaries of a designated basin as
defined by the legislature.
^ Der~v~r Ba~~r~ - groundwater located outside of a
designated basin and located within the boundaries of
the Denver basin aquifers as defined in 1985.
Tributary and non-tributary groundwater supplies are
located throughout the state, while Denver Basin and
designated basin groundwater are located in specified
areas in eastern Colorado.
Tributary groundwater occurs in the shallow alluvial
aquifers adjacent to streams. This type of groundwater is
administered under the Prior Appropriation System of
water rights as are surface water supplies. In most
basins, groundwater use is junior to surface water and so
its use is allowed only if augmentation plans have been
filed with the State Engineer that describe how the
predicted depletions of stream flow due to the
groundwater usage are offset.
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Non-tributary groundwater occurs in deeper bedrock
aquifers. This type of groundwater is administered based
on ownership of the land overlying the aquifer,
independent of the Prior Appropriation System. Permits
limit annual usage to depleting a certain percentage of
the computed aquifer volume, usually 1 percent.
In many cases the groundwater supplies are limited
either by their physical or legal availability. The physical
availability is the amount of water an aquifer can
produce. The legal availability is the amount of
groundwater that can be extracted from an aquifer under
the water rights system that is present for the specific
groundwater basin.
The amount of groundwater that each of these aquifers
can produce is difficult to determine. This is due to
several factors including uncertainty about the
transmissivity, porosity, thickness of an aquifer, its
extent, and locally, the effects of pumping that draws
down the groundwater supply.
The transmissivity of an aquifer describes its potential to
provide water. An aquifer with high transmissivity can
provide a large amount of water per foot of aquifer
drawdown. Transmissivity is a product of the aquifer
saturated thickness and its water-bearing properties.
Both of these aspects vary naturally throughout an
aquifer. The aquifer saturated thickness and the extent of
an aquifer usually are estimated based on a review of
driller's logs of the subsurface and mapping of the
permeable aquifer zones. An aquifer is composed mostly
of soil or rock particles, with the groundwater existing in
the porous void spaces in between. Soil and rock strata
of both aquifer and non-aquifer materials change in
composition due to how the strata were deposited, so the
void spaces also vary. The water-bearing properties of
an aquifer, defined as its hydraulic conductivity, are
related to the size, number, and interconnectedness of
the void spaces. It can vary by several orders of
magnitude due to natural variations in the aquifer
materials. Estimates of hydraulic conductivity can be
made from the aquifer grain size and from aquifer
pumping tests. The natural variation in porosity affects
the ability to accurately estimate the amount of
groundwater in storage in an aquifer. The range in
porosity also can be up to several orders of magnitude
for consolidated bedrock deposits and by a factor of 2 or
3 and for unconsolidated deposits. Due to the natural
variations of these aquifer properties, any estimates of
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