The Role of Inflow and Infiltration (I/I) in Urban Water Balances and Streamflow Regimes: A Hydrograph Analysis along the Sewershed‐Watershed Continuum

Urbanization alters subsurface flow pathways through expansion of sanitary collection and conveyance infrastructure. Inflow and infiltration (I/I) into sewers redistributes slow subsurface flows to fast within‐sewer flows. Acting in concert with connected surface water, redistribution through I/I complicates the net impact of urbanization on streamflow. Elucidation of these processes is key to the characterization and prediction of urban hydrologic cycles. In this study, we collected sanitary sewer flow and streamflow data from 17 sewersheds and 18 watersheds in and around Milwaukee, WI, USA. We compared flow duration curves and baseflow recession characteristics of I/I and streamflow in urban and reference watersheds. Median depth‐normalized I/I (296 mm) was nearly 35% of mean annual precipitation (867 mm), and thereby a major component of the urban drainage budget. I/I flowrates were similar to urban streamflow during high flow events (10th percentile) and larger during intermediate (50th percentile) and low (90th percentile) flow events. Further, I/I recession was slower and more nonlinear in shape than urban streamflow. Increased imperviousness and sewer density were associated with increased high flows, decreased intermediate flows, and quicker streamflow recessions. Sewer density explained more variability in intermediate flows (35%) and baseflow recession rate (49%) than imperviousness (24% and 19% respectively). Based on our findings, I/I takes up valuable volume capacity in sewers, which leads to more frequent overflows and alters streamflow regimes with ecological implications. Thus, I/I should be better considered in the prediction of urban hydrologic fluxes, and the characteristics and hydro‐ecologic impact of I/I should be more thoroughly explored.