FLOOD08081

• County: Statewide
• Basin: Statewide
• Title: Groundwater in Colorado: A Primer
• Date: 10/1/2002
• Prepared For: State of Colorado
• Prepared By: Rock Talk
• Floodplain - Doc Type: Educational/Technical/Reference Information

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. . . .. Residential demolld

entity, owns the righttouse water. .
Many of the water rights in Col,-
orado are administered by the doc-
trine of prior-appropriation or, first
infune, first in right. The person,
or entity, that first puts water to
use has the first right to use the
water. In times of low water, or
hydrological drought; those with
the oldest, most senior, water
rights will continue to be able to
use the water, while those with
newer, more junior; water rights
will have to curtail their water
usage. The system initi\\lly evolved
around diversions of surface water
by early farmers and miners. As
ground-water usage grew, diver-
sions of alluvial ground water were
brought into the same system. This
aspect of water rights, and water
rights administration, plays into
how ground water can be used in
times of drought.
An overview of the different
geologic settings of ground water throughout the state
is presented in the first article in this issue of RockTalk.
In the context of water management and drought, the
aquifers that hold ground water can be seen both as a
primary source of water as well as components of the
water-storage system.
Ground water can be either directly connected to
surface water or not connected with surface water at
all, depending on geological conditions. This distinc-
tion is important to understand, since it affects how
the resource is managed. The connection with surface
water affects the ability of an aquifer to be recharged,
and thus, it affects the sustainability of the resource.
Furthermore, Colorado water law has evolved around
this distinction with ground water beingda~ified as
tributary when it is well connected with surface water
and non-tributary when it is disconnected.
The schematic diagram on the top of the next page
shows typical geologic settings of tributary and non-
tributary ground water in Colorado. Tributary ground
water typically occurs in alluvial aquifers beneath
rivers and streams as well as in fractured-bedrock
aquifers underlying much of the mountain areas.
Non-tributary ground water typically occurs in
deeply buried sedimentary aquifers whereJayers of
impermeable shale or clay separate the aquifer from
tributary aquifers and surface water and where dis-
tances to the outcrop of the aquifer are large.
A good example of a lributary alluvial aquifer is
the valley-fill aquifer system of the South Platte River
and its tributaries. Covering a 4,000 square mile area

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Creek flow

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Plots 01 normalized stream410w in Cherry Creek and residential demand
showing the out-ol-phase timing 01 natural surface-water supply with demand.
This demonstrates both the need lor storage and why many communitities
turn to grouncJ..woter sources. DATA CQUlmSY or'MRKER WATER AND SANf'fATION DISTRICT

For example, the supply/demand curve above illus-
trates changes in water demand in a residential com-
Inunity in the south Denver metropolitan area. Demand
on municipal water begins to increase in the spring
with the start-up of landscape irrigation. Peaking in
July during the long hot summer days, the demand
tapers off with the coming of the typical monsoon rain
pattern and shortening days. In contrast, stream flow
in Cherry Creek peaks in the spring, and has tapered
off before the peak in demand. This "out-of-sync" tim-
ing highlights the necessity of water storage projects,
such as reservoirs, which capture the peak in runoff to
meet the subsequent demand. It also highlights why
many communities rely on ground water.
Extended periods of drought wreak havoc on the
relationship between supply and demand and chal-
lenge the management of water storage to accommo-
date demand. Meteorological and hydrological
drought early in the year suppress the runoff peak
forcing water suppliers to rely on carry-over storage
from previous years. The situation compounds when
the meteorological drought continues into the high
demand season, increasing the demand on the
supply. This has been the case during the 2002
drought cycle, when the monsoon flow pattern, usual-
ly occurring during the peak demand season, arrived
late and contained relatively little moisture. In addi-
tion, the 2002 drought follows several dry years in
some parts of the state.
In Colorado, one does not own water, it is consid-
ered the state's property. Instead, an individual, or

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Colorado Geolosical Survey ROCKTALKVol. 5, No. ./