Broad scale assessment of key drivers of streamflow generation in urban and urbanizing rivers

Abstract

AbstractUrbanization is characterized by increased impervious cover, artificial drainage via sewers, soil compaction, and vegetation removal, which fundamentally alters how water moves across landscapes. Less rainwater and snowmelt infiltrate the subsurface and stream responses to precipitation are faster and larger compared to natural environments. While the impacts of catchment imperviousness on hydrologic processes have been widely investigated, the relationship between stream hydrologic response and infrastructure‐mediated hydrologic connectivity across a wide range of urban watersheds has not. This study examines the relative magnitude of control that catchment characteristics, total impervious area (TIA), and sewer‐corrected TIA have on broad‐scale spatial variability in hydrologic response. Sewer‐corrected TIA accounts for inter‐catchment water transfer via storm and/or combined sewer pipes and is feasible to estimate at the watershed‐scale using urban drainage asset data. Daily stream discharge data were used to calculate the Richards‐Baker Flashiness Index for 96 watersheds located across southern Ontario, Canada. For a subset of watersheds (n = 39) with suitable stream discharge and rainfall data, watershed average event runoff ratio (RR) was calculated, and watersheds with spatial sewer network data (n = 24), were used to assess if sewer‐corrected TIA is a better predictor of hydrologic response than TIA. As expected, stream flashiness and RR increased linearly as a function of TIA and sewer‐corrected TIA, however, sewer‐corrected TIA did not explain any additional variability in the response variables. Likewise, the relative control of catchment characteristics on the spatial variability in streamflow generation were negligible. Despite sewer‐corrected TIA not improving our understanding of broad scale variation in runoff response, it could be a useful metric to consider when assessing inter‐event variability in runoff response of, heavily urbanized sub‐catchments for improved understanding of flooding and water quality.