7843

• UCREFRP Catalog Number: 7843
• Author: Stalnaker, C. B., et al.
• Title: The Instream Flow Incremental Methodology, A Primer for IFIM.
• USFW Year: 1995.
• USFW - Doc Type: Biological Report 29,
• Copyright Material: NO

A
Site-specific
microhabitat
data

. .
I I!:: I
! II, ;::;:
II II -I.
I I II III
! ! ~ ~- ~ ~! ~ Velocity
D. D,D,.. ..... "'Depth
C,c,c,....... "'Cover
A, .. ..... . . . . . .. . Area

C
100,000

THE INSTREAM FLOW INCREMENTAL METHOLDOLOGY 31

B

~::Vll OJ
_ 0.8
III
D.4
D2
o
0123401234
Velocity (ft/see) Depth (II)

1'0[]]
Habitat 0.8
sultablHty in D.e
criteria 0.4
0.2

00 0.2 0.40.80.8 1.0
Cover

Seasonal relation
between discharge
and microhabitat
lor each IlIe stage

PHABSIM

i~
1:'"
11

/~\
.'" \.
"
....,
..........
.....
.......

o
o

DisCllarge

100

Fig. 5.3 Conceptualization of how PHABSIM calculates habitat values as a function of discharge. (A) First, depth
(Di), velocity (Vi), cover conditions (Ci), and area (Ai) are measured or simulated for a given discharge. (B)
Suitability index (S1) criteria are used to weight the area of each cell for the discharge. The habitat values for
all cells in the study reach are summed to obtain a single habitat value for the discharge. The procedure is
repeated for a range of discharges to obtain the graph (C). (Adapted from Nestler et al. 1989.)

The hydraulic models have two major steps.
The first is to calculate the water surface elevation
for a specified flow, thus predicting the depth. The
second is to simulate the velocities across the cross
section. Each of these two steps can use tech-
niques based on theory or empirical regression
techniques, depending on the circumstances. The
empirical techniques require much supporting
data; the theoretical techniques much less. Most
applications involve a mix of hydraulic sub-mod-
els to characterize a variety of hydraulic condi-
tions at various simulated flows.
The habitat component weights each stream cell
using indices that assign a relative value between
o and 1 for each habitat attribute (depth, velocity,
substrate material, cover), indicating how suitable
that attribute is for the life stage under considera-
tion. These attribute indices are usually termed
habitat suitability indices and are developed using
direct observations of the attributes used most
often by a life stage, by expert opinion about what
the life requisites are, or by a combination. Various
approaches are taken to factor assorted biases out

of suitability data, but they remain indices that are
used as weights of suitability. In the last step of the
habitat component, the hydraulic estimates of
depth and velocity at different flow levels are com-
bined with the suitability values for those attrib-
utes to weight the area of each cell at the simulated
flows. The weighted values for all cells are
summed-thus the term weighted usable area
(WUA).
There are many variations on the basic ap-
proach outlined above, with specific analyses tai-
lored for different water management phenomena
(such as hydropeaking and unique spawning habi-
tat needs) or for special habitat needs (such as
bottom velocity instead of mean column velocity)
(Milhous et al. 1989). However, the fundamentals
of hydraulic and habitat modeling remain the
same, resulting in a WUA versus discharge func-
tion (Fig. 5.3c). This function should be combined
with water availability to develop an idea of what
life stages are impacted by a loss or gain of avail-
able habitat at what time of the year. Time series
analysis plays this role and also factors in any