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| Titel |
Reevaluation of transit time distributions, mean transit times and their relation to catchment topography |
| VerfasserIn |
S. Seeger, M. Weiler |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1027-5606
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| Digitales Dokument |
URL |
| Erschienen |
In: Hydrology and Earth System Sciences ; 18, no. 12 ; Nr. 18, no. 12 (2014-12-01), S.4751-4771 |
| Datensatznummer |
250120539
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| Publikation (Nr.) |
 copernicus.org/hess-18-4751-2014.pdf |
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| Zusammenfassung |
| 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. |
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