NorthPlatteBasinWaterSupplyandNeedsReport

• Basin Roundtable: North Platte
• Title: Water Supply & Needs Report for the North Platte Basin
• Date: 6/1/2006
• Author: CDM
• Basin Roundtables - Doc Type: Needs Assessment Documents

Section 7
Availability of Existing Water Supplies in the North Platte Basin
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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

CONI

7-2

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
the amount of groundwater in storage and its availability
will have a larger amount of uncertainty associated with
them than will estimates of surface water availability.

The groundwater resources in each basin have been
characterized based on published reports and data for
the major aquifer systems.

7.3 Available Surface Water and

Alluvial Groundwater Supply

Historical flows at key gages in all river basins are
monitored by the State Engineer's Office (SEO). This
map, commonly referred to as the "Snake Diagram" is a
useful tool for illustrating the volume of flows throughout
the state. The snake diagram is shown in Figure 7-1. It is
important to note that the snake diagram does not
include consideration of Colorado's commitments under
compacts and decrees. Therefore, only a portion of the
flows that are shown are available to Colorado.

There are numerous factors that may affect the physical
and/or legal availability of surface water supplies. Some
of the factors that are specific to individual basins are
listed in the basin subsections below. General factors
that must be considered when evaluating the availability
of supply are listed in Table 7-1. As can be seen in the
table, it is difficult to characterize supply availability
without stating which factors have or have not been
included in some fashion in the analysis.

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