Controls on hillslope-stream connectivity and threshold catchment response (Invited)

Abstract:

Streamflow is often a threshold function of precipitation, antecedent soil moisture conditions, or antecedent soil moisture plus precipitation, with runoff ratios or streamflow volumes being relatively small during small events or dry conditions and increasing significantly during large events or wet conditions (Detty and McGuire 2010a; Penna et al., 2011; Fu et al., 2013). These streamflow thresholds are frequently attributed to the establishment of connected transient saturated areas between the hillslopes and the stream network and the contribution of subsurface stormflow from the hillslopes to streamflow during large events or wet conditions (Detty and McGuire 2010a; Penna et al., 2011). This functional or process connectivity (Bracken et al., 2013) may be spatially variable and dependent on the contributing area of the hillslope (Jencso and McGlynn 2011), the bedrock topography (Tromp-van Meerveld and McDonnell 2006), or vegetation (Bachmair and Weiler 2014; Emanuel et al., 2014). Understanding threshold streamflow responses in small to intermediate catchments thus requires that we understand how hillslopes connect to the stream network (or the riparian zone) and what controls this connectivity.

Several studies have shown that there is a large spatial variability in shallow groundwater responses to rainfall (Seibert et al., 2003; Bachmair and Weiler 2012; Penna et al. 2014), with riparian sites reacting differently than hillslope sites (Haught and van Meerveld 2011) and footslopes reacting differently than shoulder slopes (Detty and McGuire 2010b). Other studies have shown that the difference in groundwater response timing on hillslopes is related to surface topography, bedrock topography or soil depth (Tromp-van Meerveld and McDonnell 2006; Rodhe and Seibert 2011; Penna et al. 2014). This presentation will provide an overview of the controls on hillslope-stream connectivity and the effects of hillslope-stream connectivity on threshold streamflow responses at several study sites in Europe and North America. The focus will be on the competing influence of surface and bedrock topography on transient saturation and groundwater responses. It will be shown that early during an event, when groundwater levels are relatively low, bedrock topography may dominate the flow directions and the connectivity of transient saturated zones but that later in an event, when water levels are high, surface topography exerts a larger control on flow directions and subsurface flow. Similarly, in hillslopes where water levels are high, transient saturation and hillslope-stream connectivity are influenced less by the bedrock topography and more by the surface topography. These different controls on hillslope connectivity suggest that soil type, variability in soil depth, and surface and bedrock topography may lead to different functional hillslope types and that a hillslope classification based on the differences in connectivity may be useful.

References

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