How do we decide how long the monitoring period needs to be?

An I/I study needs sufficient data to characterize both dry weather and wet weather performance at each flow monitor location, and in the United States and Canada, it is often the number of storm events that drive the length of the study period. Statistically, the more storm events the better, and practically, we look for 6 to 12 storm events where a meaningful wet weather response is observed in the sewers. For this reason, a minimum 90-day study period is often needed.