The Role of Inflow and Infiltration (I/I) in Urban Water Balances and Streamflow Regimes

Urbanization includes wastewater collection and conveyance infrastructure, which alters water budgets and streamflow hydrographs. One unique component of the urban water cycle is inflow and infiltration (I/I), where there are inputs to the wastewater collection system from rainfall and runoff, and exchange between underground wastewater pipes and soil water and groundwater. We examined and compared characteristics of I/I and streamflow in urbanized and minimally impacted reference watersheds in Milwaukee, Wisconsin, USA. We found that depth-normalized I/I was 35% of mean annual precipitation and, with much of this in the wet winter and spring seasons. Although the amount of impervious area in a watershed is major contributor to inflows, we found that sewerage density had a comparable or even stronger influence on streamflow alteration. Based on our findings, I/I takes up valuable volume capacity in sewer systems, which can lead to more frequent overflows, and alters streamflow regimes with ecological implications.

Annual depths of inflow and infiltration (I/I) and streamflow in urban and reference watersheds relative to precipitation. Each dot represents the annual depths of I/I or streamflow over the period in a sewershed or watershed. Q1 and Q3 refer to the 25th and 75th percentiles of the data. IQR refers to interquartile range (Q3–Q1). The outliers are values that are located outside 1.5 times IQR above Q3 or below Q1.

Further information is available from: Zhang, K., Sebo, S., McDonald, W., Bhaskar, A., Shuster, W., Stewart, R. D., & Parolari, A. J. (2023). The role of inflow and infiltration (I/I) in urban water balances and streamflow regimes: A hydrograph analysis along the sewershed-watershed continuum. Water Resources Research, 59, e2022WR032529. https://doi.org/10.1029/2022WR032529