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

Abstract. δ 18 O measurements in precipitation and stream waters were used to investigate hydrological flow paths and residence times at nested spatial scales in the mesoscale (233 km 2 ) River Feugh catchment in the northeast of Scotland over the 2001-2002 hydrological year. Precipitation δ 18 O exhibited strong seasonal variation, which although significantly damped within the catchment, was reflected in stream water at six sampling sites. This allowed δ 18 O variations to be used to infer the relative influence of soil-derived storm flows with a seasonally variable isotopic signature, and groundwater of apparently more constant isotopic composition. Periodic regression analysis was then used to examine the sub-catchment difference using an exponential flow model to provide indicative estimates of mean stream water residence times, which varied between approximately 3 and 14 months. This showed that the effects of increasing scale on estimated mean stream water residence time was minimal beyond that of the smallest (ca. 1 km 2 ) headwater catchment scale. Instead, the interaction of catchment soil cover and topography appeared to be the dominant controlling influence. Where sub-catchments had extensive peat coverage, responsive hydrological pathways produced seasonally variable δ 18 O signatures in runoff with short mean residence times (ca. 3 months). In contrast, areas dominated by steeper slopes, more freely draining soils and larger groundwater storage in shallow valley-bottom aquifers, deeper flow paths allow for more effective mixing and damping of δ 18 O indicating longer residence times (>12 months). These insights from δ 18 O measurements extend the hydrological understanding of the Feugh catchment gained from previous geochemical tracer studies, and demonstrate the utility of isotope tracers in investigating the interaction of hydrological processes and catchment characteristics at larger spatial scales.