MEMORANDUM

TO:

Mayor and City Council

FROM:

Phil Overeynder, Utilities Engineer-Special Projects

THRU:

Dave Hornbacher, Director Utilities and Environmental Initiatives

DATE OF MEMO: June 1, 2016
MEETING DATE: June 6, 2016
RE:

2016 Water Supply Availability Study Update

REQUEST OF COUNCIL: Council is requested to adopt as a planning document an updated
study regarding the availability of raw water supplies to serve Aspen’s future water needs.
Further Council is requested to adopt the Section 7.0 Recommendations for the monitoring
program that will inform future decisions regarding the City’s water supply programs.
PREVIOUS COUNCIL ACTION: The Water Supply Availability study was previously
updated in April 2000 with additional analysis performed in 2009 in connection with an update
to the Aspen Area Community Plan (AACP). Information contained in the availability study was
incorporated into the “Existing Conditions” section of the AACP. The Climate Change
Resiliency Plan adopted by Council 2015 contained a water supply element which encouraged
development of more specific tools related to assessing a shift in climate that will affect the
adequacy of future water supplies.
BACKGROUND: An analysis of raw water availability has been performed at approximately
10 year intervals beginning in 1984. The purpose of this analysis is to provide an estimate of long
term water supply needs and compare those needs to available supplies in order to identify
potential gaps. Previous studies identified projects or other actions were then implemented to
address any expected gaps in water supply and the potential timing of projects, within the
planning horizon. These studies were incorporated into a Comprehensive Water Management
Program (CWMP) including the Asset Management Plan (updated annually) which provides
financial resources to address specific water supply needs. Updates to the availability study were
provided in 1994 and 2000 and in 2009, as noted above, in connection within the AACP Update.
Previous analyses have considered the adequacy of Aspen’s raw water supplies under various
growth scenarios using historical hydrology (estimate of available streamflow’s at supply points,
based on past streamflow conditions). Those analyses did not address climate change
impacts.

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 DISCUSSION: The 2016 analysis extends the planning horizon to a 50-year period while
evaluating a range of impacts to water supplies that are expected to result from several climate
change models. Notably, climate change models predict an earlier initiation of runoff with more
variability between wet and dry periods. Extended periods of low flow conditions in local
streams are expected to result from the earlier commencement of runoff, whether or not the
climate becomes wetter or drier overall.
The identified water supply gaps depend on a number of key variables and assumptions that
provide the basis for the study. First, the analyses assume that the City’s policy (reflected in
numerous documents including the ecological bill of rights as well as Resolution 93-5) to protect
in-stream flow will be maintained even during the driest periods. Second the analyses include a
number of different water demand scenarios reflecting three different levels of development are
forecast over a 50-year planning horizon. In addition, five climate change scenarios were
analyzed that reflect at least 80% of the variability between hundreds of versions of climate
models describing future hydrologic conditions for Colorado. Finally, the extent that existing
water rights that are junior to Aspen’s rights and to the instream flow rights are administered in
priority order (historical practice vs. strict administration) are also analyzed in the study.
One conclusion of the study is that no supply gaps would exist with any of the projected water
needs if historical hydrologic conditions were maintained. However, considering climate shifts,
the study identifies potential gaps and compares the “tool box” of water supply programs that are
currently available to address the expected gaps. Because the supply forecast is analyzed over a
50-year period and because the shift in climate is also projected over a similar period, the report
recommends that Aspen should monitor some key factors to determine which scenario best
describes actual trends over time. These factors identified in Section 7.0 Recommendations are:
1) Date of peak snowpack measured at the Independence Pass and Schofield Pass SNOTEL
Sites.
2) Date of peak snowmelt runoff measured at the Maroon Creek and Castle Creek Intake
Structures
3) Monthly rainfall at the City Water Treatment Plants, and
4) Diversions by other in-basin water users including the Herrick Ditch.
The majority of the data to be collected under these recommendations is already being collected
at various monitoring sites within the Roaring Fork valley, but it will be necessary to compile
that information to provide an accurate tracking of shifts in hydrology in the Aspen area resulting
from climate change. This information will inform further updates and refinement of the Water
Supply Availability Study, and project planning decisions.
FINANCIAL/BUDGET IMPACTS: The water supply projects and programs that have been
identified in order to address supply gaps in the mid-term (10-- 20 year planning horizon) are
already contained in the City’s Asset Management Plan. There will be some additional costs
associated with compiling the meteorological and hydrologic data needed to track hydrological
shifts. Prior to additional actions by City Council specific to elements in this study, staff will
provide a more detailed estimate of these costs.

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 ENVIRONMENTAL IMPACTS: The water supply availability study is intended to support
the Climate Change Resiliency Plan as described above. Projects and activities necessary to
address mid-term water supply gaps are already underway. How specific future projects will meet
longer term water supply needs will need to be addressed as more specific water supply projects
are identified, and as more detailed local data on climate change impacts becomes available.
RECOMMENDED ACTION: Staff recommends that Council adopt the 2016 Water Supply
Availability Study Update as a planning document and that it direct the City Manager to
implement the recommended monitoring program contained in the study.
RECOMMENDED MOTION: I move to approve Resolution Number 81, Series of 2016.
CITY MANAGER COMMENTS:

ATTACHMENTS:
Attachment A--Water Supply Availability Study 2016 Update
Attachment B—Resolution Number 81, Series 2016

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