Stormwater Forecasting—A New Indicator to Manage Basin-Scale Urban Runoff Volume

Since 2001, the Metropolitan North Georgia Water Planning District (District) has coordinated strategies to protect watershed conditions and manage stormwater in conjunction with existing regulatory requirements. However, despite these efforts, urban stormwater runoff remains a leading cause of nonpoint source pollution and flooding in the District, leaving watershed managers with ongoing challenges related to water quality, streambank erosion, and nuisance flooding. Historically, managers have primarily focused on strategies to address water quality rather than water quantity, even though they are fundamentally linked. For these reasons, the District has developed a novel water quantity-based indicator, called the Stormwater Forecast (Forecast), as part of the integrated 2022 Water Resources Management Plan. The Forecast is a planning-level estimate of the total potential runoff management volume from development, calculated at the basin scale using site-scale post-construction stormwater performance standards. With this approach, the Forecast represents a full accounting of the runoff volume from developed lands that have the potential to be managed by stormwater control measures (SCMs), for example, traditional stormwater ponds or bioretention basins, if current post-construction stormwater management standards were fully in place. Future use of the Forecast is intended to complement and support the implementation of existing water quality and flood reduction goals in the District.