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