Example 2. Analysis with Low Outliers – Orestimba Creek near Newman, CA

Example 2 illustrates the computation of a peak flow frequency curve using EMA and Bulletin 17C procedures with an annual maximum series comprised of systematic flood peaks when low outliers (a.k.a. Potentially Influential Low Flows, PILFs) are present. For this example, USGS gage 11274500 Orestimba Creek near Newman, CA is used. Orestimba Creek is a tributary to the San Joaquin River, whose 134 sq. mi. drainage area lies on the eastern slope of the Diablo Range section of the Coast Range Mountains of California. The drainage basin has an average basin elevation of approximately 1,550 feet with peak flows usually occurring in late winter. Orestimba Creek is one of the few tributaries in the area to maintain a definite stream channel from the foothills to the San Joaquin River (England, et al., 2018).

The Orestimba Creek near Newman, CA stream gage has an annual peak record consisting of 82 peaks beginning in 1932 and ending in water year 2013. Of the 82 annual peaks, there are 12 years for which the annual peak is 0 cfs, meaning the stream was dry for the entire year. The annual maximum series is plotted in Figure 1 and tabulated in Table 1.


Table 1. Orestimba Creek near Newman, CA Annual Peak Flow Record.

A Bulletin 17 Analysis using EMA and Bulletin 17C procedures has been developed for this example. To open the analysis, either double-click on the analysis labeled "B17C Example 2" from the Study Explorer or from the Analysis menu select open, then select "B17C Example 2" from the list of available analyses. When "B17C Example 2" is selected, the Bulletin 17 analysis editor will appear as shown in Figure 2. As shown, the Skew option was set to use the Station Skew.
 

No changes to the Options tab are necessary. The EMA Data tab for this example is shown in Figure 3. Since this example uses an annual maximum series consisting entirely of systematic data with a complete record, a single zero – inf perception threshold is adequate. No modifications to the default flow ranges and data types are necessary.

Once all of the General and EMA Data tab settings are set or selected, the user can press the Compute button to perform the analysis. Once the computations have been completed, a message window will open stating Compute Complete. Close this window and then select the Tabular Results tab. The analysis window should resemble Figure 4. 

In addition to the tabular results, a graphical plot of the computed frequency curves can be obtained by pressing the Plot Curve button at the bottom of the analysis window. The Log Pearson Type III distribution fit using EMA to the input annual maximum flow data set, the 5% and 95% confidence limits, and the computed plotting positions are shown in Figure 5.

As shown in Figure 4, the Multiple Grubbs-Beck Test identified a low outlier threshold (critical value) of 782 cfs, which corresponds to the smallest peak that was retained. Thirty annual peak flows were thus identified and censored, as shown in Figure 6. These annual peak flows were then recoded to have a flow interval of zero – 782. The perception threshold for all systematic observations were also adjusted to correspond with the Multiple Grubbs-Beck Test low outlier threshold (i.e. 782 – inf), as shown in Figure 7.