Impacts of urban development on runoff event characteristics and unit hydrographs across warm and cold seasons in high latitudes

- Impacts of urbanization on runoff were monitored during construction works.
- The greatest changes in runoff were observed during common minor rainfall events.
- Urbanization caused dissimilar impacts on runoff depending on season.
- Gamma parameters describing unit hydrographs decreased during the construction works.
- For change detection, simultaneous monitoring at control catchments is essential.

SummaryThe impacts of urbanization on catchment hydrology are widely studied by comparing how different urban catchments respond to storm events, but rarely by realizing long-term observations of hydrological changes during the construction process at urbanizing small catchments. In this study, the changes occurring in runoff generation were monitored in a developing catchment under construction and in two urban control catchments. As the imperviousness of the developing catchment increased from 1.5% to 37%, significant increases were observed in event runoff depths and peak flows during rainfall-runoff events. At the same time, the only statistically significant changes that were observed for the cold period runoff events were the shorter duration and smaller runoff depths. The effect of urbanization on event runoff dynamics was studied in terms of changes in the instantaneous unit hydrographs (IUH). Negative trends were detected in the gamma parameters of IUHs, which became more consistent across events and produced a sharper shape of the hydrograph as the construction works progressed. Because urban development caused the greatest relative changes in runoff during frequently occurring minor rainfall events, the study results underlined the importance of small storms in urban runoff management for maintaining the predevelopment water balance. During infrequent major rainfall events and the cold period snowmelt events the impacts of urbanization were less pronounced. The impact of urbanization on runoff was best detected based on peak flow rates, volumetric runoff coefficients, or mean runoff intensities. Control catchments were essential to distinguish the hydrological impact caused by catchment characteristics from those caused by changes in the meteorological conditions or season.