Reevaluation of transit time distributions, mean transit times and their relation to catchment topography

The transit time of water is a fundamental property of catchments, revealing information about the flow pathways, source of water and storage in a single integrated measure. While several studies have investigated the relationship between catchment topography and transit times, few studies expanded the analysis to a wide range of catchment properties and assessed the influence of the selected transfer function (TF) model. We used stable water isotopes from mostly baseflow samples with lumped convolution models of time-invariant TFs to estimate the transit time distributions of 24 meso-scale catchments covering different geomorphic and geologic regions in Switzerland. The sparse network of 13 precipitation isotope sampling sites required the development of a new spatial interpolation method for the monthly isotopic composition of precipitation. A point-energy-balance based snow model was adapted to account for the seasonal water isotope storage in snow dominated catchments. Transit time distributions were estimated with three established TFs (exponential, gamma distribution and two parallel linear reservoirs). While the exponential TF proved to be less suitable to simulate the isotopic signal in most of the catchments, the gamma distribution and the two parallel linear reservoirs transfer function reached similarly good model fits to the fortnightly observed isotopic compositions in discharge, although in many catchments the transit time distributions implied by equally well fitted models differed markedly from each other and in extreme cases, the resulting mean transit time (MTT) differed by orders of magnitude. A more thorough comparison showed that equally suited models corresponded to agreeing values of cumulated transit time distributions only between 3 and 6 months. The short-term (< 30 days) component of the transit time distributions did not play a role because of the limited temporal resolution of the available input data. The long-term component (> 3 years) could hardly be assessed by means of stable water isotopes, resulting in ambiguous MTT and hence questioning the relevance of an MTT determined with stable isotopes. Finally we investigated the relation between MTT estimates based on the three different TF types as well as other transit time properties and a range of topographical catchment characteristics. Depending on the selected transfer model, we found a weak correlation between transit time properties and the ratio between flow path length over the flow gradient, drainage density and the mean discharge. The catchment storage derived from MTTs and mean discharge did not show a clear relation to any catchment properties, indicating that in many studies the mean annual discharge may bias the MTT estimates.