Stable isotope tracers as diagnostic tools in upscaling flow path understanding and residence time estimates in a mountainous mesoscale catchment

υ 18 O measurements of precipitation and stream waters were used as a natural tracer to investigate hydrological pathways and residence times in the River Feshie, a complex mesoscale (231 km 2 ) catchment in the Cairngorm Mountains of Scotland. Precipitation υ 18 O exhibited strong seasonal variation over the 2001–02 hydrological year, ranging from � 6Ð9‰ in the summer, to � 12Ð0‰ during winter snowfalls (mean υ 18 O � 9Ð59‰). Although damped, this seasonality was reflected in stream water outputs at seven sampling sites in the catchment, allowing υ 18 O variations to be used to infer hydrological source areas. Thus, stream water υ 18 O was generally controlled by a seasonally variable storm flow end member, mixing with groundwater of more constant isotopic composition. Periodic regression analysis allowed the differences in this mixing process between monitoring subcatchments to be assessed more quantitatively to provide a preliminary estimate of mean stream water residence time. This demonstrated the importance of responsive hydrological pathways associated with peat and shallow alpine soils in the headwater subcatchments in producing seasonally variable runoff with short mean residence times (33–113 days). In contrast, other tributaries with more freely draining soils and larger groundwater storage in shallow aquifers provided more effective mixing of variable precipitation inputs, resulting in longer residence time estimates (178–445 days). The mean residence time of runoff leaving the Feshie catchment reflected an integration of these contrasting influences (110–200 days). These insights from υ 18 O measurements extend the hydrological understanding of the Feshie catchment gained from other hydrochemical tracers, and demonstrate the utility of isotope tracers in investigating hydrological processes at the mesoscale. Copyright  2005 John Wiley & Sons, Ltd.