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| Titel |
How old is upland catchment water? |
| VerfasserIn |
Harald Hofmann, Ian Cartwright, Uwe Morgenstern, Benjamin Gilfedder |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250090504
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| Publikation (Nr.) |
 EGU/EGU2014-4744.pdf |
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| Zusammenfassung |
| Understanding the dynamics of water supply catchments is an essential part of water
management. Upland catchments provide a continuous, reliable source of high quality water
not only for some of the world’s biggest cities, but also for agriculture and industry.
Headwater streams control river flow in lowland agricultural basins as the majority of river
discharge emerges from upland catchments. Many rivers are perennial and flow throughout
the year, even during droughts. However, it is still unclear how reliable and continuous upland
catchment water resources really are. Despite many efforts in upland catchment research,
there is still little known about where the water is stored and how long it takes to travel
through upper catchments. Resolving these questions is crucial to ensure that this resource is
protected from changing land use and to estimate potential impacts from a changing
climate.
Previous research in this important area has been limited by existing measurement
techniques. Knowledge to date has relied heavily on the use of variation in stable isotope
signals to estimate the age and origin of water from upland catchments. The problem
with relying on these measures is that as the water residence time increases, the
variation in the stable isotope signal decreases. After a maximum period of four
years, no variation can be detected This means that to date, the residence time in
upland catchments is likely to have been vastly underestimated. Consequently, the
proportion of water flow out of upland river catchments to the total river flow is also
underestimated.
Tritium (3H) combines directly with water molecules and enters the flow paths with the
infiltrating water. Its half-life (12.32 years) makes it ideal to describe residence times in upper
catchment reservoirs as it can theoretically measure water up to about 150 years old. The
bomb pulse peak in the southern hemisphere was several orders of magnitude lower than in
the northern hemisphere. Hence the Tritium activities in the southern hemisphere have long
decayed down the natural background levels, which allows unique ages to be determined by
single measurements.
In this study major ion chemistry, stable isotopes and Tritium were determined at 2
locations and various stages of discharge (18 Tritium samples in between April 2013 and
January 2014) in a first-order perennial stream draining a 7.3 km2 catchment in the
Dandenong National Park, Melbourne, Australia. Even during major discharge event major
ions and stable isotope data have little variation and Tritium activities remain low (1.4 to
1.8 TU) in comparison to local rainfall of ~ 3TU. Age estimations based on an
exponential flow model are 15 to 25 years indicating that water draining from upland
catchments is much older than we have previously estimated using stable isotopes. |
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