Rainfall Duration Series: What’s The Difference Between Annual Maximum and Partial Duration?

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Leslie Munoz

Stormwater Engineer

Historical time series can be analyzed in two ways: annual maximum series or partial duration series.

Annual maximum series include only the highest values that occur within each year of the period of record, whereas partial duration series include all the values that occur within the period of record as long as they are higher than some threshold value. Using partial-duration series data for frequency analysis typically yields higher values for a given frequency than using annual maximum series data. The difference in values is greater in the more frequent events (i.e. 2-year, 5-year, 10-year) and decreases as the recurrence interval increases. For less frequent events (i.e. 25-year, 50-year, 100-year, 500-year), the difference in values is minimal between the two series.

Using a partial duration series to analyze event frequency increases the sample size by capturing more events of interest, but it requires that each data point used represent an independent event. The value of the threshold value also affects the distribution parameters, so special care should be taken to choose a good threshold. To avoid testing for independence among data points and skewing the data with the threshold value chosen, annual series are used in frequency analysis and the resulting values are often converted to partial values using conversion factors.

There are several sources of rainfall data for Texas. Although they all use the raw data from annual rainfall series, not all studies report the rainfall depths associated with frequency events as annual depths. The table below summarizes the input and output data type of some common resources as well as the correct input for HEC-HMS frequency storm modeling.

Table of input and output types for various rainfall depth sources

When modeling a frequency storm in HEC-HMS, HEC-HMS will require the user to specify the rainfall depth data type that is being used in the analyses (input type) and the desired resulting data type (output type). It will also require the probability of the storm. These three parameters are used by the program to apply conversion factors to the input rainfall depths as needed. If the input and output data type are the same, no conversion factor will be applied by the program.

Choosing the “Other” option under the Probability drop-down menu of the frequency storm window in HEC-HMS prevents the program from applying any conversion factors, regardless of the input and output data type options selected. The “Other” option should not be used to model frequent events (less than 10-year) without first ensuring that the depths used are from a partial-duration series or converting depths from an annual maximum series into partial-duration depths to avoid underestimating flood risk.

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Leslie Munoz, PE, CFM, is a Stormwater Project Manager in Houston.