Using stable isotope tracers to identify hydrological flow paths, residence times and landscape controls in a mesoscale catchment

Abstract

Δ18O tracer measurements of precipitation and stream waters were used to investigate hydrological flow paths and residence times at nested spatial scales in the mesoscale (233km2) River Feugh catchment in the northeast of Scotland over the 20012002 5 hydrological year. Precipitation δ18O exhibited strong seasonal variation, which al- though significantly damped by catchment mixing processes, was reflected in stream water outputs at six sampling sites. This allowed δ18O variations to be used to in- fer the relative influence of soil-derived storm flows with a seasonally variable isotopic signature, and groundwater of more constant isotopic composition. Periodic regres- 1 sion analysis was then used to examine the sub-catchment differences in the mixing of 0 these two main hydrological sources processes more quantitatively, using an exponen- tial flow model to provide preliminary estimates of mean stream water residence times, which varied between 0.42.9 years. This showed that the effects of increasing scale on estimated mean stream water residence time was minimal beyond the smallest (ca. 1 1km2) headwater catchment scale. Instead, the interaction of catchment soil cover and 5 topography acted as the dominant influence. Responsive hydrological pathways, asso- ciated with peat soils in the headwater sub-catchments, produced seasonally variable δ18O signatures in runoff with short mean residence times (0.40.8 years). In con- trast, areas dominated by more freely draining soils and larger groundwater storage in 2 shallow aquifers appear to provide effective mixing and damping of variable precipita- 0 tion inputs implying longer residence times (1.42.9 years). These insights from δ18O 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 2 at the mesoscale. 5 2