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

Abstract

Urbanization 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.

Integrated monitoring and modeling to disentangle the complex spatio-temporal dynamics of urbanized streams under drought stress

“…However, such modeling would also need to be integrated with the capabilities of models such as SWMM (Rossman & Simon, 2022 ) and MIKE + (DHI, 2024 ) to parameterize the effects of non-stationarity in the management of the catchment, with increasing use of flood storage retention ponds in sustainable urban drainage, as well as local effects of groundwater pumping. This would also benefit from the storm drainage network being directly parameterized when such information is available (e.g., Ariano & Oswald, 2022 ).…”

Reconceptualizing threshold‐mediated runoff responses: A case study from the Humber River watershed, Ontario, Canada

“…However, Q MAX was reached fastest at 02HC027, where the largest average Q TOT value was also observed. The high‐magnitude and flashy responses of the highly urbanized 02HC027 sub‐watershed (Table 1 and Figure S1) are due to the relatively large impervious area coverage, denser road network and water conveyance by stormwater infrastructure (Ariano & Oswald, 2022a, 2022b). While these factors are often associated with larger runoff ratios (Shuster et al, 2005), this rationale does not explain the observed maximum Q TOT across sub‐watersheds: although the maximum Q TOT at 02HC027 was large, the biggest runoff responses were at 02HC031, which has physical characteristics near Humber River watershed averages (Table 1).…”

Small forested watershed weathers effects of climate change better than a nearby urban watershed in Northern Virginia, USA

“…Mean daily flashiness values were averaged by WY for spatial comparisons and trend analyses. The Richard‐Baker flashiness index is a routinely cited method for assessing streamflow variability in small urban streams (Ariano & Oswald, 2022; Booth & Konrad, 2017; Porter, 2022; Porter et al, 2020).…”