Development of a technique to determine adequate sample size using subsampling and return interval estimation

To better manage aquatic and riparian ecosystems, knowledge of streamflow r eturn intervals is essential. This leads hydrologists and planners to often question how many years of record are necessary to adequately estimate str eamflow characteristics. We propose a nonparametric technique that uses sub sampling and return interval estimation to help determine adequate sample s ize, in order to help answer this question. This technique was applied to 5 1 years of streamflow record from an untreated catchment known as East St. Louis Creek in the Fraser Experimental Forest. Subsamples of consecutive st reamflow record ranging in size from 5 to 25 years were used to empirically estimate the 1.5-, 5-, and 15-year return intervals. These subsample estim ates were compared to the 'true' return intervals, which were calculated us ing the entire period of record. Results showed that an ability to estimate these return intervals within 50% of the true return interval levels requi red only 5-10 years of data. Increasing the sample size to 15 years provide d estimates with up to a 25% error rate, and 25 or more years of data were required to provide estimates with less than a 20% error. These are simply the results of a case study, and could be applied to similar catchments. Ho wever, the methodology can be applied to any stream. (C) 2001 Elsevier Scie nce B.V. All rights reserved.