Alberta_approach

Ecological Flow Science Advisory Board
July 16, 2013

Chris Goudreau, N.C. Wildlife Resources Commission
Background
Locke and Paul 2011
Based on results of site-specific studies in Alberta and
literature review of other studies (in Canada, US and world)
and riverine ecology
Essentially a version of the Presumptive Standard Approach
(Richter et al. 2012)

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Simple Concept
Natural hydrograph should be followed as template to capture
the five aspects of flows
Magnitude, timing, duration, frequency and rate of change
Percent-of-flow is easiest way to maintain all five aspects,
including intra- and inter-annual variability
Easier to understand than frequency-based standards or
statistical targets
Sustainable Boundary Approach
Allows for some deviation from the natural hydrograph
<10% for high level of ecological protection
<20% for moderate level of ecological protection
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Alberta Method
Requires only hydrologic data
Recommendation consists of two components
Percent-of-flow (POF) – same as percent flow-by concept we’ve
already discussed
Ecosystem Base Flow (EBF) – a low-flow cut-off during low flow
periods
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Percent of Flow
A reduction of the flow for a given time step
Reduction is the cumulative reduction at the point of interest
Reduction is from the natural flow
Time step would be daily for most planning models
For peaking hydro, time step might be hours or minutes

Alberta uses 15% reduction (i.e., 85% flow-by)

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Ecosystem Base Flow
During low flow periods, ecosystems are stressed
Water withdrawals further stress the system by increasing the
magnitude and duration of the bottleneck

Alberta uses 20th-percentile flow (80% exceedence flow)
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Combining Both Components
Flow is the greater of either:
a) the 85% flow by or,
b) the lesser of either the natural flow or the 20 percentile
flow
Do this for each period of interest (i.e., month)

The following graphs make it easier to understand
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1. Calculate POF and EBF
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2. Apply EBF to Critical Period
Without the EBF, the 20th-percentile
flow would occur 33% of the time
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3. Combining POF and EBF