Stormwater Quality Descriptions Using the Three Parameter Lognormal Distribution. In: Effective Modeling of Urban Water Systems

The cumulative probability distribution used to describe the variability of stormwater pollutant concentrations has been a matter of interest in recent years. Many predictive models attempt to estimate appropriate stormwater constituent concentrations based on land use and the amount of impervious area. The most important study that characterized stormwater was the Nationwide Urban Runoff Program (NURP) (EPA 1983). NURP was conducted throughout the U.S. and included about 2300 events from 1978 thru 1982. One of the conclusions of the final NURP report was that the event mean concentrations (EMCs) of stormwater constituents were described by lognormal distributions. This finding has been re-evaluated recently, with the conclusion that not all stormwater constituents were adequately described by lognormal distributions (Van Buren, 1997; Beherra, 2000). Stormwater managers have generally accepted the assumption of lognormality of stormwater constituent concentrations between the 5th and 95th percentiles. Based on this assumption, it is common to use the log-transformed EMC values to evaluate difference