The Statistics of Stormwater

Southern California watershed managers rely on watershed modeling to decide how and where to implement stormwater control measures to reduce pollution reaching the coastal ocean. Managers then use these models to make informed decisions about how to allocate their limited resources to achieve maximum water-quality improvement, including compliance with regulatory programs and informing multi-billion-dollar, multi-decade capital improvement programs. Unfortunately, the watershed modeling input used to make consequential management decisions is more than two decades out of date. Specifically, Southern California watershed models rely on a type of data input known as land use Event Mean Concentrations (EMCs). EMCs, originally generated by the Southern California Coastal Water Research Project, are a fundamental metric in stormwater monitoring as they provide a practical way to quantify the levels and types of pollution that wash off various urban and non-urban land uses. These EMCs are currently in use throughout Southern California and were generated over 20 years ago, a major shortcoming given that land-use-specific EMCs have undoubtedly changed over the last 20 years due to changes in pollutant sources, building codes, and treatment options.

USC Sea Grant is funding this project to take advantage of the latest advanced statistical techniques to generate updated EMCs for use in Southern California watershed modeling. These EMCs will be immediately transferred to the end-user stormwater management community through the Southern California Stormwater Monitoring Coalition, a collaborative research organization made up of the region’s 18 largest stormwater management agencies spanning the regulated and regulatory sectors. The Coalition’s member agencies have already committed to immediately using the updated EMCs to update their watershed models. The updated modeling, in turn, will enable managers to abandon their investments in stormwater control measures that are not necessary or useful – an anticipated realignment of resources that could save potentially billions of dollars in ineffective capital improvement projects and hasten other effective pollutant reduction measures.