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New Jersey Water Science Center

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Source: SRI 2007-5206: Development of the Hydroecological
Integrity Assessment Process for Determining Environmental Flows for New
Jersey Streams, Appendix 7. Definitions
of the 171 hydrologic indicies.
The following information for the 171 hydrologic
indices is from Olden and Poff, 2003 (see citation at
end of this Help page). USGS revised a limited number of the formula and/or
definitions when deemed appropriate. A USGS Scientific Investigation Report in
preparation will document these changes. The Olden and Poff
(2003) article contains twelve additional references from which the indices
were derived. Two of these articles are referenced here because they provide
examples and additional explanation for complex indices.
Following each
definition, in parentheses, are 1) the units of the index, and 2) the type of
data, temporal or spatial data, from which the upper and lower percentiles
limits (e.g., 75/25) are derived. Temporal data are from a multi-year daily
flow record from a single stream gage. For example, index MA1 - mean for the
entire flow record - uses 365 mean daily flow values for each year in the flow
record to calculate the mean for the entire flow record. Consequently, there
are 365 values for each year to calculate upper and lower percentile limits.
However, formulas for 60 of the indices do not produce a range of values from
which percentile limits can be calculated. MA5 (skewness),
for example, - mean for the entire flow record divided by the median for the
entire record - results in a single value, and thus, upper and lower percentile
limits cannot be calculated. NJHAT uses spatial data, values for each stream
gage for all the streams within a stream type, to compute limits. Upper and
lower percentile limits are calculated from the 31 MA5 values from the 31
stream gages that were identified from the classification analysis as Stream
Type A. Exceedence and percentile are used in the calculation for a number of
indices. Note the difference - a 90% exceedence means
that 90% of the values are equal to or greater than the 90% exceedence
value, while a 90th percentile means that 10% of the values are equal to or
greater than the 90th percentile value.
Note - 1.67 year flood threshold (Poff, 1996) - For indices FH11, DH22, DH23, DH24, TA3, and
TH3 compute the log10 of the peak annual flows. Compute the log10 of the daily
flows for the peak annual flow days. Calculate the coefficients for a linear
regression equation for logs of peak annual flow versus logs of average daily
flow for peak days. Using the log peak flow for the 1.67 year recurrence
interval (60th percentile) as input to the regression equation, predict the
log10 of the average daily flow. The threshold is 10 to the log10 (average
daily flow) power (cubic feet/second).
Note
- 1.67 year flood threshold (Poff, 1996) - For
indices FH11, DH22, DH23, DH24, TA3, and TH3 compute the log10 of the peak
annual flows. Compute the log10 of the daily flows for the peak annual flow
days. Calculate the coefficients for a linear regression equation for logs of
peak annual flow versus logs of average daily flow for peak days. Using the log
peak flow for the 1.67 year recurrence interval (60th percentile) as input to
the regression equation, predict the log10 of the average daily flow. The
threshold is 10 to the log10 (average daily flow) power (cubic feet/second).
Note - 1.67 year flood threshold (Poff, 1996) - For indices FH11, DH22, DH23, DH24, TA3, and
TH3 compute the log10 of the peak annual flows. Compute the log10 of the daily
flows for the peak annual flow days. Calculate the coefficients for a linear
regression equation for logs of peak annual flow versus logs of average daily
flow for peak days. Using the log peak flow for the 1.67 year recurrence
interval (60th percentile) as input to the regression equation, predict the
log10 of the average daily flow. The threshold is 10 to the log10 (average
daily flow) power (cubic feet/second).
Note - 5 year flood threshold (Poff, 1996) - For TL3 and TH3 compute the log10 of the peak
annual flows. Compute the log10 of the daily flows for the peak annual flow
days. Calculate the coefficients for a linear regression equation for logs of
peak annual flow versus logs of average daily flow for peak days. Using the log
peak flow for the 5 year recurrence interval (20th percentile) as input to the
regression equation, predict the log10 of the average daily flow. The threshold
is 10 to the log10 (average daily flow) power (cubic feet per second).
Note - 5 year flood threshold (Poff, 1996) -
For TL3 and TH3 compute the log10 of the peak annual flows. Compute the log10
of the daily flows for the peak annual flow days. Calculate the coefficients
for a linear regression equation for logs of peak annual flow versus logs of average
daily flow for peak days. Using the log peak flow for the 5 year recurrence
interval (20th percentile) as input to the regression equation, predict the
log10 of the average daily flow. The threshold is 10 to the log10 (average
daily flow) power (cubic feet per second).
Colwell R.K. 1974. Predictability, constancy, and contingency of
periodic phenomena. Ecology 55: 1148-1153. Olden, J.D. and
N.L. Poff. 2003. Redundancy
and the choice of hydrologic indices for characterizing streamflow
regimes. River Research and Applications 19:101-121. Poff NL. 1996. A hydrogeography of
unregulated streams in the United States and an examination of scale-dependence
in some hydrological descriptors. Freshwater Biology 36: 71-91. |

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