Towards simple approaches for mean residence time estimation in ungauged basins using tracers and soil distributions

Recently the mean residence time (MRT) of water in catchments has been shown to be a useful descriptor of hydrological function and a valuable tool for inter-catchment comparisons. In this paper the conservative tracer chloride and digital soil maps are used to develop simplified approaches to estimating MRTs for 22 montane catchments with areas ranging from 1 km2 to 293 km2. The study was based in sub-catchments of the River Dee in the Cairngorm Mountains of Scotland. Previous work in this geomorphic province used stable isotopes to derive the MRTs of catchments which were found to be predictable from catchment soil characteristics. These relationships formed the basis for developing transferable approaches to estimating MRTs with two simplified methods. First, simple input–output relationships were used as a MRT surrogate based on the ratio of the normalised standard deviation of Cl in streamwater to that of precipitation (MRTCl). Secondly, MRTs were predicted from the percentage coverage of hydrologically responsive soils using catchment soil maps derived from the UK hydrology of soil type (HOST) digital data base (MRTsoil). The two approaches were shown to have broadly comparable results; though the ones derived from Cl data compared better with independently-derived estimates from isotopic image data in five of the study catchments. Predictions were also improved by considering only riparian soil cover, presumably because this better represents the connectivity of responsive soils with channel networks or the ability of free-draining soils to damp the tracer response. The derived MRTs were also strongly correlated with mean catchment slope, indicating that in some geographical locations, topographic maps alone may have utility in predicting residence times in ungauged basins.