Hydrologic coupling of slopes, riparian zones and streams: an example from the Canadian Shield

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Streamflow generation at the basin scale is dictated by the differential ability of landscape elements to generate runoff and the degree of hydrologic integration of these elements. There is increasing use of hydrological models that employ surface topographical properties to represent hydrologic linkages between morphological units in a drainage basin. We show that these models may provide a limited, and possibly incorrect, view of streamflow generation processes, based on measurements in a small forested basin on the Canadian Shield in south-central Ontario. Variability of soil thickness was a first-order control on the ability of slope elements to deliver runoff to the basin's riparian zone, such that slopes with average soil thicknesses <0.2 m consistently produced runoff once depression storage on the bedrock surface was satisfied. The interaction between event size, depth of groundwater held in bedrock depressions, and the degree of hydrologic coupling between these groundwater pockets controlled runoff contributions from slope elements with thicker soil cover as well as groundwater dynamics in the riparian zone. The latter dictated whether slope runoff contributed to basin-scale stormflow. The basin exhibits a relatively minor divergence between surface and bedrock topography; nevertheless, the major control that spatial variations in soil thickness exert on streamflow generation means that such variations should be considered in hydroecological monitoring and modeling studies in this landscape.