Notes About Habitat Modeling Analysis

Habitat Modeling Analysis Notes

Habitat Modeling

Over the past 25-30 years, the NC Division of Water Resources (DWR) has conducted or been
involved in numerous site specific studies to evaluate the effect of water resource projects on
stream flows and aquatic habitat. The types of project proposals have included federal
hydropower relicensing, water supply reservoirs, new or expanded water supply withdrawals,
and water resource planning studies.

Each study involved the collection of site specific data for the stream channel: cross-section
profiles, depths, velocities, and bottom substrate and cover objects. Individual cross-sections
(transects) were selected to represent the range of habitat types available at each site. Data was
collected under at least three different stream flow conditions, and a series of models (known as
PHABSIM – Physical Habitat Simulation) was calibrated using this data to allow simulation of
the physical conditions over a wide range of flows.

Each guild or species being modeled has a set of habitat suitability indices that represents how
that organism responds to different stream velocities, depths, bottom substrates, and cover
objects. These suitability indices are sometimes referred to as preference curves. When the
preference curves are applied to the physical conditions the PHABSIM model simulates at
different flows, the habitat model produces a relationship between the amount of habitat and
stream flow (cubic feet per second) – with each guild or species having its own habitat vs. flow
relationship for each study site.

A map of existing habitat studies performed in North Carolina can be seen here. A more detailed
presentation on habitat modeling studies can be seen here.

Time Series Analysis

This entails converting a record of daily stream flows into a record of daily habitat amounts. The
river basin hydrologic models being developed by DWR can be used to produce an 80-year
record of daily stream flows at the site of interest under unaltered or various “with-project”
conditions. The habitat versus flow relationships modeled at that site are then used to convert
cubic feet per second (cfs) into weighted usable area (WUA) units of habitat. The record of daily
WUA can then be analyzed to compare the effects of various alternate flow regimes, in contrast
to the unaltered flows.

Index B

This is a metric calculated during time series analysis to quantify and compare the effects of
different flow regimes on aquatic habitat. It is calculated as the average of all daily habitat
values between the 10% and 90% exceedance levels. For example, 100 daily values would be
rank ordered and the lowest and highest 10 would be dropped before calculating the average.
Index B values are calculated for each guild or species on a monthly basis and for each different
flow scenario. The ratio of a particular flow scenario’s Index B value to the Index B determined
for the unaltered flow record is used to compare alternatives. DWR has used 80% of the
unaltered habitat index as a target level of aquatic habitat to maintain when evaluating flow
alternatives. The Ecological Flows Science Advisory Board decided to use the 80% of
unregulated Index B value as a lower threshold for the ratio, but was also interested in evaluating
what flow scenarios resulted in an Index B ratio that was greater than 120% of the unregulated
value. The reasoning was that “enhancing” habitat levels for a particular guild or species by
more than 20% above unaltered conditions represented a significant change and might also have
unknown or unintended consequences for the overall aquatic ecosystem.

Index B ratios that are less than 80% or more than 120% of the value calculated for unaltered
flows are tallied separately and presented in separate graphs and tables. In addition, the output
graphs also indicate separate results that are noted as “small” or “low” WUA. These represent a
separate tally of those guilds or species for which the unregulated Index B values are less than
1000. In general, this is a relatively low value for the habitat index compared to the values for
other guilds or species. While not wanting to ignore or totally discount the guilds/species with
low Index B values, we do want to be aware of them. Since it is the denominator used in
calculating percentages, relatively small changes in habitat amounts resulting from an alternative
flow scenario can result in disproportionately large percentage differences when the unregulated
Index B value is small. Note that on the graphs the solid and striped bars of the same color
represent the tally for guilds/species with Index B values for unaltered flows that are greater than
or less than 1000, respectively.

Calculations and Graphs

The Index B ratio is calculated as Index B for an alternate flow scenario divided by the Index B
value for the unaltered flow scenario. If the ratio is <80% or >120% for a guild/species, it goes
into a separate tally for each of the two thresholds.

Individual study site results (see example graph below):
1. First, the tally of guilds/species that are above or below the 80% and 120% thresholds is
divided by the total number of guilds/species evaluated to determine the percentage
affected. In this case the total number of guilds or species is 19, and this is the
denominator in the percentage calculation. This is done separately for each of the four
seasons.
2. To combine results for all four seasons onto a single graph, the percentage for each
season is divided by 4 – giving each season equal weight – and all four seasons are
shown on a stacked bar graph with a different color for each season.
3. Each individual season result is further broken down on the stacked bar graph into:
the percentage that represents guilds with relatively low amounts of available habitat –
Index B values < 1000 for the unaltered flow regime (the striped portions of the bar);
and those with Index B values >= 1000 for the unaltered flow regime (the solid portions
of the bar).

Study Sites Combined:

A description of the different study sites included in this analysis is included in a separate
document.

Box Plots
1. The results are divided into 8 guilds that are deep/slow, deep/fast or golden redhorse; and
11 guilds that are shallow/slow, shallow/fast, or benthic macro-invertebrates (mayflies,
stoneflies, and caddis flies).
2. Tallies of guilds and species for which the Index B ratio is < 80% or > 120% of the value
for the unaltered flow regime are determined separately for each site, and further
subdivided into the group of 8 or 11 guilds. Percentages are recalculated using either 8 or
11 as the denominator, instead of the total of 19 guilds/species.
3. The results for all sites are shown as box/whisker plots showing the entire range
(whisker), quartiles (blue and red boxes) and mean (black diamonds). See the example
below.
0.0%
5.0%
10.0%
15.0%
20.0%
25.0%
30.0%
35.0%
40.0%
45.0%
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65.0%
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95.0%
100.0%
Ann.
7Q10
Mon.
7Q10
Sep.
Med.
Mon.
Med.
10%20%30%40%50%60%70%75%80%85%90%
Flow Scenario
Tar River, Louisburg site - Percentage (weighted equally for each season)
of 19 Guilds/Species with Less Than 80% of Unregulated Index B Value
WINTER small WUA
WINTER
FALL small WUA
FALL
SUMMER small WUA
SUMMER
SPRING small WUA
SPRING
-- Percentage of Inflow as Flow-by --- --- Percentage Average Flow as Min. Flow---
NOTE: On this chart, the stacked bar percentage for a given season is NOT the the same as the percentage
for that season alone. Instead they have been weighted to put ALL SEASONS COMBINED on a scale of 0 to 100%.

Results are also depicted with separate symbols for each study site, plus the mean (see below)

0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Ann.
7Q10
Mon.
7Q10
Sep.
Med.
Mon.
Med.
10%20%30%40%50%60%70%75%80%85%90%
FALL - Percentage of 8 Deep Guilds & Golden Redhorse
with Less Than 80% of Unregulated Index B Value
25%-50%50%-75%MEAN
-- Percentage of Inflow as Flow-by--

0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Ann.
7Q10
Mon.
7Q10
Sep.
Med.
Mon.
Med.
10%20%30%40%50%60%70%75%80%85%90%
FALL - Percentage of 8 Deep Guilds & Golden Redhorse
with Less Than 80% of Unregulated Index B Value
Eno State Park Buckhorn Creek West Fork Eno Tar River MEAN
First Broad, Upper First Broad, Middle First Broad, Lower Buffalo Creek
-- Percentage of Inflow as Flow-by--
Seasonal Results for All Sites Combined
1. The tallies of species/guilds for which the Index B ratio is < 80% of the unaltered flow
Index B value are added together for all sites.
2. The tally for the Index B ratio >120% was not evaluated, because other results indicated
that this was less valuable for differentiating between flow alternatives.
3. As before, results were divided into two groups of guilds/species representing 8 deep and
11 shallow types.
4. As before, all graphs also use stacked solid and striped bars to differentiate between
relatively low amounts of available habitat – Index B values < 1000 for the unaltered
flow regime (the striped portion of the bar); and those with Index B values >= 1000 for
the unaltered flow regime (the solid portion of the bar).
5. Percentages were calculated by dividing the total tally for each season by the total
number of guilds/species multiplied by the number of sites. For example, in the first
graph below of all sites combined, the denominator is 64 – 8 deep guilds x 8 sites.
6. The second graph below shows the same results as the first, but further divided into the
portions of the total tally represented by each of the guilds/species. As with the first
graph, the denominator in this example is 64.
7. Another analysis and graph were completed which further subdivides the results by
stream classification of the sites. Four of the sites were small flashy and four were small
stable. For the last graph below, the percentages are calculated with a denominator of 32
– 8 deep guilds x 4 sites of each type. Because the percentages are calculated separately
for each class, adding the percentages for the two classes together does not produce the
same result as the single percentage for all classes combined
.

0%
5%
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15%
20%
25%
30%
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40%
45%
50%
55%
60%
65%
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80%
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100%
Ann.
7Q10
Mon.
7Q10
Sep.
Med.
Mon.
Med.
10%20%30%40%50%60%70%75%80%85%90%
Flow Scenario
ALL SITES COMBINED - % of 8 Deep & Golden Redhorse Guilds
with Less Than 80% of Unregulated Index B Value, at all 8 sites combined
FALL small WUA
FALL
-- Percentage of Inflow as Flow-by -- -- Percentage Average Flow as Min. Flow --
FALL

0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
50%
55%
60%
65%
70%
75%
80%
85%
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95%
100%
Ann.
7Q10
Mon.
7Q10
Sep.
Med.
Mon.
Med.
10%20%30%40%50%60%70%75%80%85%90%
Flow Scenario
ALL SITES COMBINED - % of 8 Deep & Golden Redhorse Guilds
with Less Than 80% of Unregulated Index B Value, at all 8 sites combined
GORHJ low WUA GORHJ
GORHA low WUA GORHA
DFCOARSE low WUA DFCOARSE
DFGRCOB low WUA DFGRCOB
DFFINE low WUA DFFINE
DSNC low WUA DSNC
DSCOV2 low WUA DSCOV2
DSCOV low WUA DSCOV
-- Percentage of Inflow as Flow-by-- -- Percentage Average Flow as Min. Flow ---
FALL
0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
50%
55%
60%
65%
70%
75%
80%
85%
90%
95%
100%
Flow Scenario
ALL SITES COMBINED - % of 8 Deep & Golden Redhorse Guilds
with Less Than 80% of Unregulated Index B Value, at 8 sites sorted by class
D (low habitat)
D small flashy
B (low habitat)
B small stable
-- Percentage of Inflow as Flow-by-- -- Percentage Average Flow as Min. Flow--
FALL
Percentages are calculated separately for each class,
NOT as a portion of the percentage determined for all classes combined.
Guilds and Species Modeled

A table of abbreviations and descriptions of the guilds and species modeled is shown below. The
11 highlighted in yellow are the shallow guilds/species and the 8 highlighted in pale orange are
the deep guilds/species. Those that are not highlighted may have been originally modeled for
some sites but are not included in the consistent set of 19 guilds and species being used to
analyze all sites.

abbreviation organism based on
SSYOY shallow slow, young of year guild bluehead chub YOY
SSVEG shallow slow, aquatic vegetation cover guild
notch lip (formerly silver) redhorse YOY,
veg cover
SSWOOD shallow slow, woody debris cover guild
notch lip (formerly silver) redhorse YOY,
woody cover
SSCOARSE shallow slow, coarse substrate generic
SSFINENC shallow slow, fine substrate, no cover guild redbreast sunfish spawning
SFLOWVEL shallow fast lower velocity guild margined madtom adult
SFMODVEL shallow fast moderate velocity guild generic
SFHIVEL shallow fast higher velocity guild fantail darter adult
DSCOV deep slow, cover guild generic
DSCOV2 deep slow, cover guild version 2 redbreast sunfish adult
DSNC deep slow, no cover guild generic
DFFINE deep fast, fine substrate guild notch lip (formerly silver) redhorse adult
DFGRCOB deep fast, gravel/cobble substrate guild white bass spawning
DFCOARSE deep fast, coarse substrate guild shorthead redhorse adult
AMSS2 American shad spawning 2 Stier & Crance 1985
ROBRHS Robust redhorse spawning
GORHA Golden redhorse adult surrogate for Carolina redhorse
GORHJ golden redhorse juvenile surrogate for Carolina redhorse
AMSS4 American shad spawning 4 Hightower et al 2011
EPHEM Mayfly nymph Jim Gore
PLECO stonefly nymph Jim Gore
TRIC1 caddis fly larvae Jim Gore
MACLR Macro-invertebrate diversity, large river Jim Gore