8195

• UCREFRP Catalog Number: 8195
• Author: Richter, B. D., J. V. Baumgartner, J. Powell and D. Braun
• Title: A Method for Assessing Hydrologic Alteration within Ecosytems
• USFW Year: 1996
• USFW - Doc Type: Conservation Biology
• Copyright Material: YES

1166 Is cs,siu, 111'diolq;ir.Werulion
system) or current conditions to simulated results
based on models of future modification to a system.
The data we use in estimating all attribute values are
daily mean water conditions (e.g., levels, heads, flow
rates). The same computational strategies will work
with any regular-interval hydrologic data, such as
monthly means, but the sensitivity of the IRA method
for detecting hydrologic alteration is increasingly com-
promised with time intervals longer than a day. Detec-
tion of certain types of hydrologic impacts, such as the
rapid flow fluctuations associated with hydropower.gen-
eration at dams, may require data from even shorter in-
tervals (e.g., hourly).
Hydrologic Attributes
Hydrologic conditions can vary in four dimensions
within an ecosystem (three spatial dimensions and
time). If the spatial domain is restricted to a specific
point within a hydrologic system, however, (such as a
measurement point in a river, a lake, or an aquifer), the
hydrologic regime can be defined in terms of one tem-
poral and one spatial dimension: changes in water con-
ditions (e.g., levels, heads, rates) at a single location over
time. Such temporal changes in water conditions are
commonly portrayed as plots of water condition against
time, or hydrographs (Fig. 1).
. Our goal is to characterize the temporal variation of
hydrologic conditions using attributes that are biologi-
cally relevant yet sensitive to human influences such as
reservoir operations, ground water pumping, and agri-
cultural diversions. Many attributes of hydrologic re-
gimes can be used to characterize the "physical habitat
templates" (Southwood 1977, 1988; Poff & Ward 1990;
Townsend & Hildrew 1994) or "environmental filters"
(Keddy 1992) that shape the biotic composition of
aquatic, wetland, and riparian ecosystems. The IRA
method is based on 32 biologically relevant hydrologic
parameters divided into five major groups to statistically
characterize intra-annual hydrologic variation (Table 1).
These 32 parameters are based upon five fundamental
characteristics of hydrologic regimes:
(1) The magnitude of the water condition at any
given time is a measure of the availability or suit-
ability of habitat and defines such habitat attributes
as wetted area or habitat volume, or the position of
a water table relative to wetland or riparian plant
rooting zones.
(2) The tinning of occurrence of particular water con-
ditions can determine whether certain life-cycle re-
quirements are met or can influence the degree of
stress or mortality associated with extreme water
conditions such as floods or droughts.
(3) The frequency of occurrence of specific water
conditions such as droughts or floods may be tied
kichlc'rl sir.
1600
1400
1200
1000
?E
Soo
E
W
600
400
200
0
1600
1400
1200
1000
800
E
600
400
200
0
Month
Figure 1. Two hydrographs for the Roanoke River at
Roanoke Rapids in North Carolina can be cbaracter-
ized by the five general features of a hydrologic re-
gime: magnitude, frequency, duration, tinning, and
rate of change. These regime features can be altered by
human influences sucli as dams, as illustrated by a
comparison of the.upper pre-danz 15ydrograph for
1942 with the lower post-danz hj?drograph for 1975
(cins = cubic meters per second = 35.315 cubic feet
per second).
to reproduction or mortality events for various
species, thereby influencing population dynamics.
(4) The duration of time over which a specific water
condition exists may determine whether a particu-
lar life-cycle phase can be completed or the degree
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