Abstract
Abstract. Permeable sedimentary bedrock overlain by glacial till leads to large storage capacities and complex subsurface flow pathways in the Canadian Rocky Mountain region. While some inferences on the storage and release of water can be drawn from conceptualizations of runoff generation (e.g., runoff thresholds and hydrologic connectivity) in physically similar watersheds, relatively little research has been conducted in snow-dominated watersheds with multilayered permeable substrates that are characteristic of the Canadian Rocky Mountains. Stream water and source water (rain, snowmelt, soil water, hillslope groundwater, till groundwater, and bedrock groundwater) were sampled in four sub-watersheds (Star West Lower, Star West Upper, Star East Lower, and Star East Upper) in Star Creek, SW Alberta, to characterize the spatial and temporal variation in source water contributions to streamflow in upper and lower reaches of this watershed. Principal component analysis was used to determine the relative dominance and timing of source water contributions to streamflow over the 2014 and 2015 hydrologic seasons. An initial displacement of water stored in the hillslope over winter (reacted water rather than unreacted snowmelt and rainfall) occurred at the onset of snowmelt before stream discharge responded significantly. This was followed by a dilution effect as snowmelt saturated the landscape, recharged groundwater, and connected the hillslopes to the stream. Fall baseflows were dominated by either riparian water or hillslope groundwater in Star West. Conversely, in Star East, the composition of stream water was similar to hillslope water in August but plotted outside the boundary of the measured sources in September and October. The chemical composition of groundwater seeps followed the same temporal trend as stream water, but the consistently cold temperatures of the seeps suggested deep groundwater was likely the source of this late fall streamflow. Temperature and chemical signatures of groundwater seeps also suggest highly complex subsurface flow pathways. The insights gained from this research help improve our understanding of the processes by which water is stored and released from watersheds with multilayered subsurface structures.
Highlights
Rather than calculating the mixing ratios or percent contribution of sources to stream water on the basis of an unmixing routine in end-member mixing analysis (EMMA), trends in stream water distribution were described in relation to source water dynamics and runoff processes
Twice monthly stream water and source water samples collected in Star Creek, from April to October in 2014 and 2015, have been used here to conceptualize runoff generation in Alberta’s Rocky Mountains
Results from this study allow for a detailed examination of temporal patterns in source water chemistry and a qualitative description of source water contributions to stream water
Summary
Forest disturbance from wildfire, pathogens, or forest harvesting removes the forest canopy, increasing the total precipitation that reaches the forest floor (Williams et al, 2019; Burles and Boon, 2011; Boon, 2012; Pugh and Small, 2012; Varhola et al, 2010), often altering the dominant flow pathways, increasing streamflow quantity, and changing the timing of flows in forested watersheds (Stednick, 1996; Scott, 1993; Bearup et al, 2014; Winkler et al, 2017). Large variability has been observed in streamflow responses following disturbance due to differences in disturbance type, vegetation type, precipitation regimes, and soil moisture storage (Brown et al, 2005; Stednick, 1996). Some studies in Alberta’s Rocky Mountains have reported little, if any, change in streamflow following disturbance It has been suggested that watersheds exhibiting a lack of change in streamflow following disturbance might be associated with a large storage capacity and complex subsurface flow pathways (Harder et al, 2015), but the higher-order controls regulating these muted responses remain unclear
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