Abstract
Abstract. The time and geographic sources of streamwater in low-relief watersheds are poorly understood. This is partly due to the difficult combination of low runoff coefficients and often damped streamwater isotopic signals precluding traditional hydrograph separation and convolution integral approaches. Here we present a dual-isotope approach involving 18O and 2H of water in a low-angle forested watershed to determine streamwater source components and then build a conceptual model of streamflow generation. We focus on three headwater lowland sub-catchments draining the Savannah River Site in South Carolina, USA. Our results for a 3-year sampling period show that the slopes of the meteoric water lines/evaporation water lines (MWLs/EWLs) of the catchment water sources can be used to extract information on runoff sources in ways not considered before. Our dual-isotope approach was able to identify unique hillslope, riparian and deep groundwater, and streamflow compositions. The streams showed strong evaporative enrichment compared to the local meteoric water line (δ2H = 7.15 · δ18O +9.28‰) with slopes of 2.52, 2.84, and 2.86. Based on the unique and unambiguous slopes of the EWLs of the different water cycle components and the isotopic time series of the individual components, we were able to show how the riparian zone controls baseflow in this system and how the riparian zone "resets" the stable isotope composition of the observed streams in our low-angle, forested watersheds. Although this approach is limited in terms of quantifying mixing percentages between different end-members, our dual-isotope approach enabled the extraction of hydrologically useful information in a region with little change in individual isotope time series.
Highlights
The spatial and temporal sources of runoff in low-angle, forested headwater watersheds are poorly understood
We examined the source of runoff in a set of lowland forested watersheds in South Carolina, USA
Streamflow was very ephemeral and the time series of the stable isotopic composition of streamwater showed minimal temporal dynamics compared to rainfall
Summary
The spatial and temporal sources of runoff in low-angle, forested headwater watersheds are poorly understood. Much recent work has focused on the threshold sequencing of spatial sources in upland forested watersheds (Sidle et al, 2000; Seibert and McDonnell, 2002), hillslope–riparian connectivity (McGlynn and McDonnell, 2003), and the importance of spatial patterns of hillslope–riparian–stream connectivity (Jencso et al, 2009; Jencso and McGlynn, 2011). Such connectivity may be strongly nonlinear (Buttle et al, 2004; Zehe et al, 2007; Penna et al, 2011). Klaus et al.: Where does streamwater come from in low-relief forested watersheds?
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