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

Richter et al.
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Figure 4. A comparison of the annual series of annual
maximum 1-day values (Group 2: magnitude and du-
ration of annual extreme conditions) for the pre- and
post-impact periods on the Roanoke River. Broken
lines indicate values of the mean (dashes) and SD
(dots) for each period.
tions about the size of detectable changes and their po-
tential biological importance. Accordingly, the results of
the IRA method are most usefully presented in terms of
the magnitude of the differences in central tendency (or
dispersion) between the pre- and post-impact periods
(Figs. 4-7), along with confidence limits for this differ-
ence, rather than asp values for the null hypotheses that
the central tendencies are the same. Hypothesis testing
may be valuable for specific cases in which biologically
relevant thresholds to hydrologic change can be identi-
fied. In these cases an equivalence test (McBride et al.
1993) can be used to test the null hypothesis that the ob-
6U Pre-dam: 1913-1949
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Post-dam: 19S6-1991
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1915 1925 1935 1945 1955 1965 1975 1985
1920 1930 19,40 1950 1%0 1970 1980 1990
Year
Figure S. A comparison of the annual series of annual
low pulse counts (Group 4: frequency and duration of
high and low pulses) for the pre- and post-impact peri-
ods on the Roanoke River. Broken lines indicate val-
tces of the mean (dashes) and SD (dots) for each pe-
riod.
Assessing 1(ydrologic Alteration 1169
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Pre-dam: 19134949 Post-dam: 1956-1991

1915 1925 1935 1945 1955 1965 1975 1985
1920 1930 1940 1950 1%0 1970 1980 1990
Year
Figure 6 A comparison of the annual series of values
for the timing Qulian date) of annual minimum
1-day values (Group 3: timing of annual extreme con-
ditions) for the pre- and post-impact periods on the
Roanoke River. Dashed lines indicate values of the
mean for each period. -.10.
served difference is greater than some user-identified bi-
ologically significant value.
When adequate hydrologic records are available for
both the pre-impact and post-impact time periods, appli-
cation of the IHA method will be relatively straightfor-
ward by means of the statistical procedures described.
When pre- or post-impact records are nonexistent, in-
clude data gaps, or are inadequate in length, however,
various data reconstruction or estimation procedures
will be needed. Examples of such procedures include
the techniques for hydrologic record extension de-
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140
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100
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60
40
20
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Pre-dam: 1913-1949 Post-dam: 1956-1991
Figure 7. A comparison of the annual series of values
for annual average rates of hydrograph rise (Group 5:
rate and frequency of change in conditions) for the
pre- and post-impact periods on the Roanoke River.
Broken lines indicate values of the mean (dashes) and
SD (dots) for each period.
tarn>cnalien Ri?d?„_c
0
1915 1925 1935 1945 1955 1965 1975 1985
1920 1930 1940 1950 1960 . 1970 1980 1990
Year
1915 1925 1935 1945 r9» - - -
1920 1930 1940 1950 1%0 1970 1980 1990
Year
Pre-dam: 1913-1949 Past-dam: 1956-1991