WSPC01394

• File Number: 8460.500
• Description: Platte River Basin - Endangered Species Issues - South Platte Recovery Program
• State: CO
• Basin: South Platte
• Water Division: 1
• Date: 10/1/1996
• Title: South Platte Agreement - Irrigation Water Conservation - Opportunities and Limitations in Colorado - Report of the Agricultural Water Conservation Task Force
• Water Supply Pro - Doc Type: Report/Study

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Summary

The Agricultural Water Conservation Task Force was assembled by the Colorado Water Resources
Research Institute to consider issues related to irrigation water conservation in Colorado, The overall
objective of the task force was to provide information describing the nature of irrigation water use in relation
to other uses and the implications of various conservation strategies, The group, which was composed of
academic faculty from Colorado State University and representatives of water agencies from throughout the
state, assembled relevant information and conducted monthly discussion sessions in the process of producing
a thorough report, The resulting document presents the findings of this task force and attempts to clarify the
complex issues involved, It should not be interpreted as a legal document or an endorsement of specific
public policy recommendations,

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Water use in Colorado is structured within the institutional framework of the prior appropriation
doctrine, which serves as the legal basis for water rights administration, Under the priority system, the timing
of the appropriation determines the user's priority to apply surface and tributary ground water to beneficial
use, The prior appropriation system also allows for the permanent transfer of water rights between different
users and uses, subject to the protection of other water right holders.

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Water rights are quantified based on the rate of diversion from the stream system or the volume stored,
The consumptive use portion of a water right is specifically defined as water that is no longer available within
a stream or aquifer system after it has been applied to beneficial use, For crop production systems, plant
transpiration and evaporation from soil and crop surfaces accounts for almost all the water that is
consumptively used. With most irrigation water distribution and application systems, some water is applied
in excess of the soil water holding capacity and actual use, The excess water either percolates through the
soil and becomes ground water that reenters the stream or river or flows overland through surface channels or
drainages to the stream or river, These returning waters, whiCh are in excess of water actually used or held in
storage within the crop root zone, are called return flows,

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Beneficial uses of return flows from irrigation result in an increase in the overall efficiency of agricultural
water use in several of Colorado's river basins, They allow for multiple uses of water within basin systems
and increase the water supply for downstream users in the latter part of the irrigation season, In some
instances, the benefit of greater water use efficiency derived from return flows can be partially offset by
degrildation of water quality resulting from repeated use of the same water,

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In Colorado, irrigation water use (measured as withdrawals and deliveries) accounts for more than 90%
of total used for all purposes on a state-wide basis, Although irrigation water use comprises a much lower
percentage of total use in the South Platte and Arkansas basins, where population densities are greater and
water use by the municipal and industrial sectors is greater, it accounts for more than all other uses cOmbined.

The sources of water used for irrigation vary among the state's major river basins, Within the South
Platte and Arkansas basins, most of the irrigation water demand is furnished by diversion of streamflow and
ground water pumping from shallow alluvial aquifers, Segments of the eastern plains overlie nontributary
deep aquifers that are part of the larger ground water system known as the Ogallala Aquifer, This
nontributary ground water is the source of most of the irrigation water applied to crops in these areas.
Irrigation water supplies in the Rio Grande basin are obtained from direct surface diversions and pumping
from unconfined and confined ground water aquifers, Throughout most of the Colorado River tributary basin
and the North Platte River basin, surface water diversion is the primary source of irrigation water.

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