Annual Peak Discharge of Floyd River

This validation test is similar to that for Fishkill Creek, except in this case the observed annual peak discharges need to be adjusted to account for a high outlier.  Here the results from the HydroTools LogPearson function with automatic outlier omission are compared the the published analysis of the data in "Guidelines for Determining Flood Flow Frequency" (Bulletin 17B of the Hydrology Committee, U.S. Geological Survey, Reston, VA).  The annual peak flow data from 1935 to 1973 are given below.  Following the published example a skew coefficient of 0.1 is used.

When the outlier discharge from the year 1953 is not omitted from the analyses, the estimated 100-year discharge is 46700 cfs.  Using the LogPearson() function with automatic outlier omission the estimated 100-year discharge is 33400 cfs, which is identical the estimated 100-year discharge when the outlier discharge from 1953 is manually omitted from the record. Omitting the outlier also narrows the confidence limits.  The 95% confidence limits without omitting the outlier are (28800 - 91600 cfs).  Omitting the outlier the confidence limits become (21500 - 61800 cfs). This demonstrates that the LogPearson() function correctly omits the outlier value automatically. 

For comparison, if the full record is used (1935-2011), the LogPearson() function with automatic outlier omission using a skew coefficient of 0.1 estimates the 100-year discharge to be 36400 cfs with 95% confidence limits of (26600 - 54200 cfs).  This is similar to the estimate of the 100-year discharge (38700 cfs) when the statistics are adjusted to account for the length of the historic record, as described in the "Guidelines for Determining Flood Flow Frequency".  If the outlier is retained, the estimated 100-year discharge is 91800 cfs with 95% confidence limits of (58888 - 164900 cfs).  This demonstrates the importance of accounting for outliers.

Year      Annual Peak Discharge (cfs)
1935     1460
1936     4050
1937     3570
1938     2060
1939     1300

1940     1390
1941     1720
1942     6280
1943     1360
1944     7440

1945     5320
1946     1400
1947     3240
1948     2710
1949     4520

1950     4840
1951     8320
1952    13900
1953    71500
1954     6250

1955     2260
1956      318
1957     1330
1958      970
1959     1920

1960    15100
1961     2870
1962    20600
1963     3810
1964      726

1965     7500
1966     7170
1967     2000
1968      829
1969    17300

1970     4740
1971    13400
1972     2940
1973     5660

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