7812

• UCREFRP Catalog Number: 7812
• Author: Many
• Title: Rivers, Studies in the Science, Environmental Policy, and Law of Instream Flow
• USFW Year: 1990
• USFW - Doc Type: Rivers
• Copyright Material: YES

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CFSM VALUES
FIGURE 3. Frequency distribution of cfsm values computed from 48 USGS gages used to derive the
New England ABF recommendation (USFWS 1981).
further shows that August flow data are
generally non-symmetrical, with low-range
flow values occurring more often than mid-
or high-range values. The use of a mean
flow value in this situation is not appro-
priate. The median cfsm value determined
using the New England ABF method for
each of the 48 gages based on this method
is subject to this same flaw and therefore
is not the best approximation of August
flow.
The range of 48 August cfsm values de-
rived from the 48 gages used by the USFWS
forms a non-symmetrical frequency distri-
bution (Figure 3). For the same reason as
stated above, the 48 August cfsm values
should not be averaged together as the New
England ABF policy does, since the mea-
sures of central tendency differ widely.
Flow duration data are commonly ap-
plied in hydrologic analyses and are em-
ployed to determine the likelihood of oc-
currence of a flow event (Bovee 1982). Flow
duration data are considered to be precise
portrayals of flow variability at a point on
a stream (Dingman 1978; Bovee 1982). The
flow value that resides at the 50th percen-
tile on a flow duration curve is the statis-
tical median monthly flow.
The flow duration method used in this
paper determines the median August flow
for any given gage by pooling all daily
August flow values across years of record
of interest. For each gage, all daily values
for a given month are pooled for all years
of record and arranged in numerical order.
The median value from this array is the
flow value residing at the 50th percentile.
For a given gage, the median value can
then be expressed as a cfsm value by di-
viding the median value by the drainage
area of that gage.
In order to compare the flow duration
method to the New England ABF method,
this study began by using the same 48
stream gages selected by the USFWS. Table
1 compares the August cfsm values as de-
rived by both of the above methods for the
48 individual New England gages selected
by the USFWS. Flow duration method cfsm
values ranged from 0.16 to 0.78 cfsm; New
England ABF method cfsm values ranged
from 0.21 to 0.94 cfsm. The flow duration
method resulted in lower cfsm values for
B. H. Kulik 13 Jr