Fundamentals of Engineering Design
The 2-year recurrence interval was used as an index value by which watersheds could be
compared, and by which discharges computed for one location within a watershed could be transferred
to other locations within the same watershed.
5.1.5.1.3 Flow Duration Curve
A flow duration curve is a cumulative distribution function of discharges, as shown in Figure 5.3.
A cumulative distribution diagram is prepared by dividing the discharge data into equal width classes. A
count of the number of discharges in each class is made to make a histogram, and then adding each bar of
a histogram to construct the cumulative distribution function.
Hotopha Creek
10000
1000
100
10
1
0.1
1.0
10.0
100.0
Percentage of time equaled or exceeded
Figure 5.3
Cumulative Distribution Function of Discharge for
Hotopha Creek, Mean Daily Data
The USGS flow duration procedure divides the data into 35 classes. The lowest class is zero, with
a class width of 0 to 0. The next class width is 0 to the minimum discharge value. The remaining 33 classes
are determined by subtracting the logarithm of the minimum discharge from the logarithm of the maximum
value, and dividing by 33 to form equal logarithmic class widths. The upper interval must include the
greatest measured discharge. After the class widths are set, a spreadsheet can be utilized to develop class
counts for each year of the data and histogram values for equal classes can be directly added to develop
the histogram for the total period of record. For example, histograms for 35 years of record may be
developed in 5-year increments and can be added to form the total data set histogram. Equal width
arithmetic classes can also be used to develop the flow duration relationship. Although these equal width
classes can give better definition or the higher discharge values, representation or the low discharges will
be masked by the relatively larger class intervals at the low discharge portion of the histogram. Arithmetic
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