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Hydrology in urban areas is substantially altered by urbanization processes. Specifically, replacement of vegetated surfaces with impervious surfaces reduces infiltration and evapotranspiration losses and increases overland flow, concentrating and transforming slow environmental flows into fast stormflows (Finkenbine et al., 2000;Walsh et al., 2005). At the same time, urbanization involves intensive alteration to subsurface hydrology as pipes are constructed for efficient sanitary sewage and stormwater collection and conveyance (Oswald et al., 2023). A

Basement backups and sewer overflows from rainfall‐derived inflow and infiltration (RDII) are a significant threat to human and environmental health; however, reducing inflow and infiltration is a challenge for municipalities due to the difficulty and resources required to accurately identify source areas. This case study seeks to address this challenge by evaluating the influence that sewershed characteristics have on inflow and infiltration into sanitary sewer systems. To do so, we used 4.5 years of monitoring data from 19 sanitary sewer locations in Milwaukee, Wisconsin to explore the relationships between RDII defined using the RTK unit hydrograph method and sewershed physical, land cover, and pipe characteristics. Results demonstrate that inflow, or fast direct flows into the system, is positively correlated to pipe length per acre, number of parcels, and medium intensity land use. Infiltration, or slow inputs from groundwater sources, is negatively correlated with imperviousness, pipe length per acre, low intensity, and medium intensity land use. Multivariable linear regression using these parameters explained between 55% and 72% of the variance in normalized inflow and infiltration. These findings demonstrate a way in which collection system managers may be able to narrow the search areas for RDII sources within their sanitary sewer systems by evaluating sewershed characteristics.