Drought & Water Management

• Title: What the Current Drought Means for the Future of Water Management in Colorado
• Date: 1/1/2003
• Description: 2002 Drought Impact Report
• Basin: Statewide
• Drought Mitigation - Doc Type: Reports

IV. Drought Responses of Major Water Management Systems
A. Background
Streamflows in Colorado during the year 2002 were the lowest in several centuries.
While water supply planners have been aware that extremely dry years like 2002 have
occurred in the past and will probably occur again, most major municipal water supply
systems are designed to withstand drought conditions that have occurred in recently
recorded history. In most parts of Colorado, flow measurements for major streams have
been recorded only over the last 50 to 100 years, and precipitation measurements over the
last 20 to 60 years. Most water providers have used the historical drought of 1954 -1956
as the standard against which to design their systems.
Water supply planners have not adopted a universal definition of drought or common
design standards for water supply system reliability. The vulnerability of any particular
water supply system to shortages depends on many factors such as natural variations in
the geographic patterns of drought conditions, types of water supply sources (e.g. surface
water or groundwater), and types of water diversion and storage facilities. In many
instances, legal and institutional factors may be the primary determinant of system
reliability, whereas in other cases the actual availability of physical supply may be the
limiting factor in times of drought.
B. An Example of a Municipal Drought Management Plan: Denver Water
Denver Water operates the largest municipal water supply system in Colorado and has
adopted a conservative approach to planning for drought. Denver bases its estimate of
the amount of demand that can be reliably met on the firm yield of its system. The firm
yield is determined using a computer model to calculate the maximum amount of demand
that could be met during the driest period in a representative hydrologic study period,
1947 through 1991. Three consecutive dry years within that period, 1954 through 1956,
determine the firm yield of the Denver Water system. For the Denver system, water year
1954 yielded the least amount of supply of any year in the hydrologic study period. 1954
was followed by two additional dry years during which the firm yield of the system is
limited by natural inflows to the system and the amount of water in storage reservoirs.
Recognizing the potential for droughts more severe than the 1954 -56 period and other
uncertainties, Denver Water utilizes additional measures to reduce the risk of water
supply shortages. In modeling the water supply system, Denver assumes that water
demands will be higher than average during periods of drought. The firm yield of
Denver's system is thus based upon an unrestricted demand, whereas during an actual
drought under Denver's Drought Response Plan (2002) water use restrictions would be
imposed to reduce demand. In addition, Denver Water reserves 30,000 acre -feet of its
firm yield as a safety factor to help insure the reliability of its system in the event of
severe drought and other risk factors.
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