A season-specific, multi-site calibration strategy to study the hydrological cycle and the impact of extreme-flow events along an urban-to-agricultural gradient

Abstract Extreme-flow events have been a central point of interest in hydrology and recent developments suggest that their role could be critical in the overall functioning of watershed systems. The development of modelling techniques that can effectively reproduce the hydrological dynamics of watersheds during extreme events is one of the challenges of the contemporary modelling practice. In this study, we present a season-specific, multi-site calibration framework that accommodates the variability in the hydrological responses induced by the agricultural landscape changes during different periods of the year. Our case study is the Hamilton Harbour watershed in southern Ontario, Canada, where discharge data from three main tributaries and six gauging stations were available to constrain the Soil and Water Assessment Tool (SWAT) model. Two distinct values are assigned to the Curve Number for antecedent runoff condition II (CN2), relating surface runoff to precipitation as a function of land uses and soil characteristics, in order to reproduce flow patterns during the growing and dormant seasons. Overall, notwithstanding the model performance decline in the most urbanized catchment of Redhill Creek, our season-specific calibration strategy improved the predictive capacity of the model in the predominantly agricultural catchments of Grindstone Creek and Spencer Creek. Counter to our season-specific process characterization, an alternative calibration strategy postulating a differential watershed response, depending on the magnitude of individual precipitation events, appears to fair better in urbanized settings. The water budget displays significant disparities among the different land uses in the Hamilton Harbour watershed. Our analysis also suggests that the runoff generated from urban sites during the growing season (May–October), i.e., the period when the receiving waterbody is most sensitive to eutrophication, is six times higher than the average contribution from pasture, forest, and cropland areas. The potential implications of the on-going land-use and climate-change trends for the water cycle at both local and catchment scales are also discussed.