Using stable isotope tracers to assess hydrological flow paths, residence times and landscape influences in a nested mesoscale catchment

摘要

δ18O measurements in precipitation and stream waters were used toinvestigate hydrological flow paths and residence times at nested spatialscales in the mesoscale (233 km2) River Feugh catchment in the northeastof Scotland over the 2001-2002 hydrological year. Precipitation δ18O exhibited strong seasonal variation, which although significantlydamped within the catchment, was reflected in stream water at six samplingsites. This allowed δ18O variations to be used to infer therelative influence of soil-derived storm flows with a seasonally variableisotopic signature, and groundwater of apparently more constant isotopiccomposition. Periodic regression analysis was then used to examine thesub-catchment difference using an exponential flow model to provideindicative estimates of mean stream water residence times, which variedbetween approximately 3 and 14 months. This showed that the effects ofincreasing scale on estimated mean stream water residence time was minimalbeyond that of the smallest (ca. 1 km2) headwater catchment scale.Instead, the interaction of catchment soil cover and topography appeared tobe the dominant controlling influence. Where sub-catchments had extensivepeat coverage, responsive hydrological pathways produced seasonally variableδ18O signatures in runoff with short mean residence times (ca.3 months). In contrast, areas dominated by steeper slopes, more freelydraining soils and larger groundwater storage in shallow valley-bottomaquifers, deeper flow paths allow for more effective mixing and damping ofδ18O indicating longer residence times (>12 months). Theseinsights from δ18O measurements extend the hydrologicalunderstanding of the Feugh catchment gained from previous geochemical tracerstudies, and demonstrate the utility of isotope tracers in investigating theinteraction of hydrological processes and catchment characteristics atlarger spatial scales.