Research papersEmpirical assessment of effects of urbanization on event flow hydrology in watersheds of Canada's Great Lakes-St Lawrence basin

- High temporal resolution hydrologic records for 27 watersheds sized 38-800 km2.
- Empirical event-scale hydrologic trends modelled for urban cover 0.1-88%.
- Detectable effects on total runoff at ∼4% urbanized watershed.
- Event flow acceleration increased with increasing urban cover.
- Loss of hydrologic stationarity during a period with no trend in rainfall pattern.

We conducted an empirical hydrological analysis of high-temporal resolution streamflow records for 27 watersheds within 11 river systems in the Greater Toronto Region of the Canadian Great Lakes basin. Our objectives were to model the event-scale flow response of watersheds to urbanization and to test for scale and threshold effects. Watershed areas ranged from 37.5 km2 to 806 km2 and urban percent land cover ranged from less than 0.1-87.6%. Flow records had a resolution of 15-min increments and were available over a 42-year period, allowing for detailed assessment of changes in event-scale flow response with increasing urban land use during the post-freshet period (May 26 to November 15). Empirical statistical models were developed for flow characteristics including total runoff, runoff coefficient, eightieth and ninety-fifth percentile rising limb event runoff and mean rising limb event acceleration. Changes in some of these runoff metrics began at very low urban land use (<4%). Urban land use had a very strong influence on total runoff and event-scale hydrologic characteristics, with the exception of 80th percentile flows, which had a curvilinear relationship with urban cover. Event flow acceleration increased with increasing urban cover, thus causing 80th percentile runoff depths to be reached sooner. These results indicate the potential for compromised water balance when cumulative changes are considered at the watershed scale. No abrupt or threshold changes in hydrologic characteristics were identified along the urban land use gradient. A positive interaction of urban percent land use and watershed size indicated a scale effect on total runoff. Overall, the results document compromised hydrologic stability attributable to urbanization during a period with no detectable change in rainfall patterns. They also corroborate literature recommendations for spatially distributed low impact urban development techniques; measures would be needed throughout the urbanized area of a watershed to dampen event-scale hydrologic responses to urbanization. Additional research is warranted into event-scale hydrologic trends with urbanization in other regions, in particular rising limb event flow accelerations.

Graphical AbstractFitted empirical statistical model for the effect of percent urban land use on rising limb event flow acceleration. Rising limb event-scale flow acceleration was estimated using empirical hydrologic data with 15-min time resolution for post-freshet seasonal periods between May 26th and November 15th inclusive. The database included 27 watersheds in 11 river systems of Ontario's Great Lakes Basin. Flow acceleration was modelled using independent variables for total rainfall, watershed area, urban percent land use and baseflow index. The fitted model, on a log scale, is plotted in a solid black line with shaded 95% confidence intervals for mean database values. Actual data for study watersheds are plotted in grey points.65