Example 9. [0 – value] Perception Threshold – Clark Fork River below Cabinet Gorge Dam, ID

Example 9 exhibits how a [0 – value] perception threshold can be used to compute a peak flow frequency curve using EMA and Bulletin 17C procedures. When using a [0 – value] perception threshold, the corresponding flow range spans [value – inf]. For instance, when using a [0 – 5000 cfs] perception threshold, the corresponding flow range would be [5000 cfs – inf]. This implies that the flow for the specified missing years must have exceeded 5000 cfs. The data for this example comes from the USGS gage 12391950 Clark Fork River below Cabinet Gorge Dam, ID. The annual maximum series consists of unregulated daily average flows. The period of record used for this example contains 77 peaks beginning in 1929 and ending in 2008. Two periods of missing data also exist within this record, 1940-1941 and 1944. The annual maximum series is plotted in Figure 1 and tabulated in Table 1.

Table 1. Clark Fork River below Cabinet Gorge Dam, ID 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 9" from the Study Explorer or from the Analysis menu select open, then select "B17C Example 9" from the list of available analyses. When "B17C Example 9" 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. This example uses an annual maximum series consisting of systematic data with two missing periods. The missing periods include 1940 – 1941 and 1944. Since 17C EMA requires a non zero – inf perception threshold for all periods of missing data, a total of three perception thresholds are required. In this example, both missing periods make use of a perception threshold of 0 – 50000. This perception threshold implies that had a flow less than 50000 cfs occurred, someone would have measured and recorded it. The complementary flow range for this perception threshold is 50000 – inf. Once all three perception thresholds have been entered as shown in Figure 3, click the Apply Thresholds button to assign the complementary flow ranges for the periods of missing data.

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 6, the perception thresholds for the missing years were encoded as 0 – 50000 while the flow ranges were set to 50000 – inf. This causes the EMA-computed standard deviation and skew absolute value to decrease (compared to the use of inf – inf perception thresholds).