Weighting of Field Monitoring Data with Probability Distributions of Daily Rainfall

Many BMPs undergo field monitoring to assess the ability of the BMP to meet water quality goals. This is typically accomplished using protocols that outline data quality objectives. Some of these objectives include a minimum number of storms, minimum storm depth, and flow rates or water quality volumes that span the operating range of the BMP. Typically during the data analysis, mass weighting is used to calculate performance efficiency on a load basis. However, considerations for the weighting of the storms, relative to the frequency of occurrence is typically not done. This can be problematic in that the collection of too many small storms may not reflect the ability of the BMP to perform for larger storms, or the collection of too many large storms could underestimate the relative performance of the BMP on a long-term basis. This paper investigates a method to weight the storms collected relative to the frequency distribution of the average annual daily rainfall. This method could be used in the development of sampling protocols and methods currently being established by committee through EWRI.