HomeMy WebLinkAbout5 - Update on Boulder's water yields, operations models and reliability criteria
CITYOFBOULDER
WATER RESOURCES ADVISORY BOARD
AGENDA ITEM
MEETING DATE: December 21, 2009
AGENDA TITLE: Information Item: Update on Boulder's water yields, operations
models and reliability criteria
PRESENTERS: Carol Ellinghouse, Water Resources Coordinator
Lee Rozaklis, AMEC Earth and Environmental, Inc.
EXECUTIVE SUMMARY:
Since the late 1980s, Boulder has used the Boulder Creek Watershed Model (BCWM) to
estimate the yields of its water rights for general planning purposes. The BCWM was
developed for the city by Hydrosphere Resource Consultants (now AMEC Earth and
Environmental) as part of the city's 1988 RWMP and has been refined on an on-going
basis. Lee Rozaklis with AMEC Earth and Environmental will provide a presentation to
WRAB on the BCWM and how it is used to model the city's water supply system.
Fiscal or Other Impacts: None-information item only
Staff recommendation: None-information item only
Analysis:
BCWM is a network model that represents the surface water hydrology, stream segments,
water rights, water diversion and storage facilities, water demands, and return flows in
the Boulder Creek basin as a system of connected links and nodes. The model uses
collections of variables associated with quarter-month periods, called time steps.
Historical data is input into the model for each time step and can be modified to analyze
various planning scenarios. Primary inputs are natural streamflows at gage sites, climate
variables and demands associated with the water rights in the Boulder Creek basin. The
model calculates values for variables, such as streamflow, return flows and diversions at
each network node for each step in time. These time series of data can be analyzed to
determine information such as projected water rights yield or minimum reservoir levels
reached under various assumed operating conditions for the city's water system.
Calls from downstream water rights on the South Platte River are represented based upon
historical South Platte call records obtained from the Colorado Division of Water
Resources covering the years 1940-2006. For years prior to 1940 and for tree ring-based
hydrology for a longer historical period, South Platte calls are generated based upon the
historical relationship between overall South Platte water supply conditions and call
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patterns. Natural flows in Boulder Creek are highly correlated to overall South Platte
water supply conditions and are used as a proxy.
The primary use of BCWM is for assessing the reliability of Boulder's water supply
system. However, it is also used for a wide range of other purposes, including
environmental analysis (hydrologic and water quality impacts), reservoir operation
studies, cooperative planning, water rights analysis and short-term forecasting.
BCWM simulates the operation of all major water rights and water diversion/storage
facilities in the Boulder Creek basin (including Boulder's water supply system) under
varying hydrologic conditions. It also includes a representation of the CBT and Windy
Gap project systems for the purpose of estimating Boulder's annual allotments from those
projects. Natural streamflow time series data have been developed for the years 1907-
2006 based upon stream gage data and for the years 1566 to 2002 based upon tree ring
data to depict historical conditions.
In any given model run, demands associated with water rights reflect fixed assumptions
regarding municipal service area land use, populations and agricultural irrigated acreages.
However, demands differ annually according to variations in irrigation season
temperature and precipitation. Time series of natural streamflows, temperature and
precipitation, and downstream South Platte calls represent climatic and hydrologic
variability based upon historical data (gage records, tree rings) or climate change
modeling, depending on the modeling scenario. Within the model, demands associated
with Boulder's water supply system are reduced in times of significant drought in
accordance with Boulder's drought response triggers as depicted in Boulder's Drought
Plan. For any given BCWM modeled scenario, the simulated performance of Boulder's
water supply system is assessed to determine if the number of years with water shortages
exceed the number allowed under Boulder's water supply reliability criteria.
In 1989, the City Council adopted reliability criteria for operation of the municipal water
supply system. The criteria defined the levels of reduction in water service that would be
acceptable during drought or emergency conditions. Since that time, staff has been
working to assure that the water system operation meets the criteria. The criteria as
adopted by City Council in 1989 are as follows:
"The City's policy regarding water supply reliability states that the City should
maintain control over water supplies sufficient to meet the forecasted build-out water
demands of the Boulder Valley, with three levels of specified reliability,
1) water needed for essential uses should be assured against droughts with 1000
year recurrence intervals or less,
2) water needed to sustain landscaping should be assured against droughts with
100 year recurrence intervals or less,
3) water sufficient to meet total demand should be assured against droughts with
20 year recurrence intervals or less."
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