Drought Spell Statistics SOW

• Applicant: AMEC Earth & Environmental Inc
• Grant Type: Severance Tax
• Fiscal Year ‎(i.e. 2008)‎: 2010
• Project Name: Spell Statistics Enhancement for Drought Vulnerability Assessment
• CWCB Section: Water Conservation & Drought Planning
• Contract/PO #: OE PDA 11-65
• Grants - Doc Type: Scope of Work

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Memorandum
To: Taryn Hutchins - Cabibi, Veva Deheza,
Water Conservation and Drought Planning Section
Colorado Water Conservation Board
cc:
From: Ben Harding
Subject: Scope of Work for Refined Drought Statistics
Date: August 18, 2010
This memorandum presents a scope of work and cost estimate for enhancement of the
spell statistics used in the CWCB Drought Vulnerability Assessment. These
enhancements will involve 1) calculating spell statistics for projected climate- impacted
flows against the mean flow from the historic record, 1950 — 2005, and 2) calculating
spell statistics and return periods from a single, long synthetic trace.
Task 1: Calculate spell statistics against historic mean.
Spell statistics that were calculated for the CWCB Drought Vulnerability Assessment
were calculated against the mean flow of each individual trace. In order to put projected
future spells in the context of recent historic conditions, the threshold used to define a
spell will be modified to be the mean of the historic natural flow trace. This task will
involve modification of the computer codes used to calculate the basic spell statistics,
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revisions to the codes that calculate the statistics presented in the Vulnerability
Assessment,
revisions to the codes that produce the box plots of spell statistics, and
testing and validation. The modified codes will be run against each of the ten projected
climate cases to produce a separate estimate of spell statistics for each case.
Task 2: Calculate spell statistics against a single long trace
Spell statistics calculated for the Colorado River Water Availability Study and used in the
Drought Vulnerability Assessment were calculated on the basis of a set of 100 56 -year
traces of natural flows. This was done so that projected future droughts could be put in
the context of the drought of record over the 56 -year period from 1950 through 2005. An
alternative approach is to calculate the statistics of spells with a giv return i nterval. In l
order to calculate statistics of spells with long return intervals this analysis should_be `
done against a long synthetic trace. This task will involve modifying the computer codes
that implement the stochastic model used to generate streamflow traces so as to
produce a single long trace, developing a new code to calculate the basic spell statistics
from a single long trace and to report those statistics along with return intervals, and
testing and validation. These codes will be used to generate long traces for each of the
ten projected climate cases and to develop separate estimates of spell statistics for each
of those cases.
AMEC Earth & Environmental
Boulder Office
1002 Walnut Street, Suite 200
Boulder, CO 80302 www.amec.com
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