ABSTRACT

Streamflow statistics are used routinely by watershed planners, regulators, engineers, and managers for water-resource projects. Streamflow statistics for a streamgage can change over time as more flow data become available and (or) land use or climate in the watershed changes; consequently, streamflow statistics should be updated periodically. This report, done in cooperation with the Pennsylvania Department of Environmental Protection, presents updated streamflow statistics for 526 U.S. Geological Survey (USGS) continuous-record streamgages in Pennsylvania, New York, New Jersey, Delaware, Maryland, West Virginia, and Ohio using data collected through 2008. The following statistics are presented: low flows (1-day, 10-year; 7-day, 10-year; 7-day, 2-year; 30-day, 10-year; 30-day, 2-year; and 90-day, 10-year low flows), base flows (10-year, 25-year, and 50-year base flows), peak flows (50-, 20-, 10-, 4-, 2-, 1-, and 0.2-percent annual exceedance flood), mean flows (mean annual flow and harmonic mean flow), and flow durations (99-, 95-, 90-, 85-, 80-, 70-, 60-, 50-, 40-, 30-, 20-, 15-, 10-, 5-, and 1-percent exceedance). Streamflow statistics (7-day, 10-year and 30-day, 10-year low flows; harmonic mean; and 95-, 50-, and 5-percent flow duration exceedances) for 72 streamgage locations in Pennsylvania were compared using a Wilcoxon signed-rank test to previously reported statistics. A statistically significant difference was found between the data sets for the 30-day, 10-year low-flow; harmonic mean; 50-percent flow-duration exceedance; and 5-percent flow-duration exceedance statistics. No significant difference was found between the data sets for the 7-day, 10-year low-flow and 95-percent flow-duration exceedance statistics. The lower flow statistics show a decrease since 1996, and the mid- to higher flows show an increase since 1996.

First posted May 3, 2011