Ecodifference_trial_balloon

Ecological Flow Science Advisory Board
April 16, 2013

Chris Goudreau, N.C. Wildlife Resources Commission
Eco-Deficit/Surplus
Vogel et al. 2007
Create a median flow duration curve of unaltered flows on an
annual (seasonal, monthly) basis
Do the same for altered flows
Difference between the curves is termed “eco-surplus” or
“eco-deficit”
It is really median flow surplus or deficit
The focus is on changes in flow magnitude
Monthly curves also reflect aspects of timing
Duration, frequency and rate of change are not fully considered
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Eco-Deficit/Surplus
“Eco-difference” is absolute value of “eco-surplus” or
“eco-deficit”
When calculated on an overall percentage basis,
eco-difference gives measure of relative change from the
unaltered condition
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Sustainability Boundaries
Richter et al. 2011
Presumptive standard concept
Deviation from inflow on a daily basis
Moderate protection: ±11-20%
High protection: ±0-10%
Maintains inter- and intra-annual flow variability
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Combining Concepts
Median flow duration curves – because based on multiple data
points, one can create a band of variability around the median
This is similar to the sustainability boundary
The width of the band should consider biological and
management aspects
In following example, using 15%
The Susquehanna Basin report (TNC 2010) used a similar
approach

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Use as a Planning Tool
Various ways to “trip the red flag”
1.Compare median altered to median unaltered (no band)
If eco-difference >15%
This is comparing the overall difference between median FDCs
It does not trip if certain parts of the FDC exceed the threshold
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Use as a Planning Tool
Various ways to “trip the red flag”
2.Compare median altered FDC to the unaltered with band
If any part outside of band
This is more stringent than option 1
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