Methods for development of planning-level estimates of stormflow at unmonitored stream sites in the conterminous United States

This report documents methods for data compilation and analysis of statistics for stormflows that meet data-quality
objectives for order-of-magnitude planning-level water-quality estimates at unmonitored sites in the conterminous
United States. Statistics for prestorm streamflow, precipitation, and runoff coefficients are used to model stormflows
for use with the Stochastic Empirical Loading and Dilution Model (SELDM), which is a highway-runoff model.
SELDM is designed to better quantify the risk of exceeding water-quality criteria as precipitation, discharge, ambient
water quality, and highway-runoff quality vary from storm to storm. Summary statistics also may be used to help
estimate annual-average water-quality loads. Streamflow statistics are used to estimate prestorm flows. Streamflow
statistics are estimated by analysis of data from 2,873 U.S. Geological Survey streamgages in the conterminous
United States with drainage areas ranging from 10 to 500 square miles and at least 24 years of record during the
period 1960−2004. Streamflow statistics are regionalized using U.S. Environmental Protection Agency Level III
nutrient ecoregions. Storm-event precipitation statistics are estimated by analysis of data from 2,610 National Oceanic
and Atmospheric Administration hourly-precipitation data stations in the conterminous United States with at least 25
years of data during the 1965−2006 period. Storm-event precipitation statistics are regionalized using U.S.
Environmental Protection Agency rain zones. Statistics to characterize volumetric runoff coefficients are estimated
using data from 6,142 storm events at 306 study sites. Runoff coefficient statistics are not regionalized, but are
organized by total impervious area. All of the geographic information system files, computer programs, data files, and
regression results developed for this study are included on the CD−ROM accompanying this report.