Hydrometric and Isotopic Investigation of Streamflow Generation in Urban Watersheds

Numerous studies have investigated hydrological processes, rainfall-runoff regimes, and water ages in natural forested catchments. These studies have contributed to enhancements in process-based understanding of rainfall-runoff dynamics, water storage and source water partitioning in these landscapes. There are fewer hydrology studies that investigate these processes in human dominated landscapes. There is a common assumption that as watersheds urbanize their hydrologic functioning becomes less variable. This is attributed to high levels of impervious surface cover and dense urban drainage networks that facilitate delivery of rainfall and meltwater to streams producing rapid hydrologic response and degraded water quality. Research on urban stormwater is often conducted in single catchments and understanding of the impact of urbanization on water storage-discharge relationships and streamwater quality will be limited until a greater understanding of hydrologic functioning in a diverse range of urban watersheds are investigated. The overarching goal of this thesis was to improve our conceptual model of urban streamflow generation by examining broad-scale hydrologic response and water age patterns to contribute to a greater understanding of urban catchment functioning over various spatial and temporal scales. First, the spatial variability in hydrologic response for urban and urbanizing watersheds (n=152) and the role of impervious surface cover was investigated using metrics of catchment functioning and urbanization. Second, the extent to which impervious surface cover explained inter-watershed variability in bulk hydrologic transport across 8 heavily urbanized watersheds, as represented by several common transit time metrics was examined. Finally, to improve understanding of water age dynamics at the event scale, examination of stormflow partitioning into old and new water was conducted for three heavily urban catchments. This work demonstrated that using impervious cover to solely predict runoff responses and mean transit times is limited, and that urban catchments do not partition water as previously thought. Notably, these findings challenge hydrologists to consider a wider range of explanatory variables to untangle urban catchment functioning other than just impervious surface cover. Other factors such as the role of urban greenspaces, functioning of storm sewer networks, the role of stormwater control measures, urban catchment storage, and event characteristics on urban streamflow generation should be investigated across scales and cities that have different development histories, geology, and climate.