318 - Rainfall Frequency Analysis from the Point to the Watershed Scale

Studies of rainfall intensity-duration-frequency (IDF) have predominantly concentrated on point-scale rainfall. This emphasis arises from the availability of rain gauge data, along with its direct relevance to small-scale hydrological infrastructure. Conversely, rainfall IDF at larger scales, such as that of a stormwater management system or watershed, have received notably less scrutiny. This is despite the pivotal role that the relationships between rainfall and drainage spatial scales play in shaping both the severity and timing of flooding events. The expanding dimensions of stormwater collection systems, heightened storm frequency due to atmospheric warming, and the growing significance of proactive management techniques like pumping stations, dynamic storage solutions, and stormwater treatment facilities underscore the imperative for a more comprehensive consideration of rainfall frequency beyond the point scale. In this study, we delve into the characteristics of extreme rainfall within our southeastern Michigan study area, spanning from the level of individual rain gauges to encompass the whole 2,800 km2 service area of the Great Lakes Water Authority (GLWA), one of the nation's largest water and wastewater utilities. We find that recent storms, such as those occurring in August 2014, can demonstrate highly varied recurrence intervals contingent upon the spatial scale at which they are evaluated. This highlights the need for more sophisticated approaches in planning, design, and public communication strategies regarding extreme rainfall, not only for GLWA but also for other major water management organizations.