How Many Events Do You Need?

Understanding how a specific stormwater BMP performs relative to planning level or compliance expectations can direct operation and maintenance efforts to minimize underperformance. Municipal operations are increasingly expanding to include field monitoring programs to measure BMP performance for planning and/or verification purposes. BMP performance monitoring typically involves sampling and measuring runoff into and out of a BMP, which is a resource intensive activity. Robust performance characterization depends on sampling over a range of operational conditions that might be induced by storm/climate characteristics, pollutant sources, or maintenance status, among others. The design of most monitoring programs thus introduces competing objectives to enabling robust BMP characterization while minimizing monitoring costs. The current investigation explores a scientific approach to determine the number of storm events that should be sampled to accurately represent the performance of a BMP.