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.

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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