Impact of stormwater infiltration on rainfall-derived inflow and infiltration: A physically based surface-subsurface urban hydrologic model

The subsurface fate of stormwater infiltration enhanced by green infrastructure (GI) is unknown. GI redirects stormflows from the surface into the subsurface, which can be partitioned to evapotranspiration or baseflow, or be intercepted by buried infrastructure trenches. Stormwater that infiltrates sanitary sewer pipes is referred to as rainfall-derived inflow and infiltration (RDII) and represents a rapid pathway for infiltrated stormwater to reach receiving waters. To quantify the partitioning of infiltrated stormwater between slow and fast flows, a physically based hydrologic model was developed integrating coupled surface-subsurface processes and urban sewer systems and applied to a sewershed near Milwaukee, WI. In this region with shallow groundwater table and leaky sanitary sewer systems, RDII accounted for 73-79% of the sanitary sewer flow volume and 21% of the sewershed water balance. Sensitivity analysis indicated that the surface direct inflow was controlled more by precipitation/ evapotranspiration ratio and the subsurface infiltration was controlled more by groundwater table depth and sanitary sewer defect density. The largest effect of GI was to shift surface runoff to evapotranspiration and reduce peak flow in urban sewer systems. The volume of RDII was relatively less sensitive to GI infiltration because reductions in directly connected surface runoff were offset by increases in groundwater inflow. A moderate level of GI implementation (5-10% replacement of impervious area) reduced RDII volume by 10%, however, these changes were less than 1% of the overall water balance. On the other hand, higher levels of GI implementation (~20% replacement of impervious area) resulted in no change to RDII volume as more infiltrated stormwater was routed to the sewer system through the subsurface. This study highlights the necessity of considering the full hydrologic context of GI and balancing the runoff reduction by GI and groundwater infiltration into sewers.

Automated summary

The study investigates the subsurface fate of stormwater infiltration enhanced by green infrastructure (GI) and its impact on the overall water balance. A physically based hydrologic model is developed and applied to a sewershed near Milwaukee, WI, to quantify the partitioning of infiltrated stormwater. The findings show that GI implementation can reduce surface runoff and peak flow in urban sewer systems, but its impact on rainfall-derived inflow and infiltration (RDII) is relatively small.