Isotopic and geochemical tracers reveal similarities in transit times in contrasting mesoscale catchments

There is a need for more isotopic tracer studies at the mesoscale to extend our understanding of catchment transit times and their associated controls beyond smaller experimental sites (typically < 10 km(2)). This paper, therefore, examines the isotope hydrology of six mesoscale (10(1)-10(2) km(2)) sub-catchments of the 2000 km(2) basin of the River Dee in northern Scotland. All the catchments were upland in character (mean altitude > 400 m) with similar suites of soil coverage (predominantly regosols, gleys, peats and podzols), although the relative distribution varied, as did the presence of other landscape features such as aquifers in Quaternary drifts and lakes. Input-output relationships of delta O-18 in precipitation and runoff revealed contrasting responses and differential damping which were broadly consistent with catchment characteristics. The mean transit times (MTTs) were estimated using a convolution integral with a Gamma distribution as the transfer function. These varied from 528 days in the most responsive catchments to > 800 days in catchments where the tracer signature was most damped. Shorter MTTs were found in sub-catchments with a higher percentage cover of responsive soils (i.e. regosols, gleys and peats), whilst sub-catchments with longest MTTs had a higher coverage of free-draining podzolic and alluvial soils, as well as significant amount of stored water either in fluvio-glacial aquifers or large lakes. The MTT of all six catchments had the same order of magnitude; this contrasts with studies in the Scottish Highlands with smaller (< 10 km(2)) catchments where MTT has been shown to vary between 60 and 1200 days.