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| Titel |
Seasonal tracer cycles, mean travel time, and young water in heterogeneous and nonstationary catchments |
| VerfasserIn |
James Kirchner |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250107481
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| Publikation (Nr.) |
 EGU/EGU2015-7186.pdf |
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| Zusammenfassung |
| Many environmental systems are nonstationary and heterogeneous, but we often analyze
them using theoretical frameworks that assume stationarity and homogeneity, resulting in
aggregation errors that are rarely explored and almost never quantified. Here I explore this
general problem in one specific context: the use of seasonal cycles in chemical or
isotopic tracers, such as Cl-, δ18O, or δ18H to estimate catchment mean travel
times.
Seasonal tracer cycles are widely used to quantify time scales of transport and mixing in
catchments. Any system that transports and mixes fluids will damp sinusoidal cycles of
tracers that those fluids carry. If the system is homogeneous and stationary, one can compare
the amplitudes of the input and output cycles, using simple algebraic formulas, to calculate
the tracer’s (and thus the fluid’s) mean travel time.
Here I show that these calculations will often be wrong by several hundred percent, when
applied to catchments with realistic degrees of heterogeneity and nonstationarity. This
aggregation error arises from the strong nonlinearity in the relationship between tracer cycle
amplitude and mean travel time.
One can show, however, that seasonal tracer cycles reliably estimate the "young water
fraction" in stream flow, defined as the fraction of runoff with travel times of less than
roughly 0.2 years. Numerical experiments show that this young water fraction (not
to be confused with the event-based "new water" in hydrograph separations) is
predicted by seasonal tracer cycle amplitudes within a precision of a few percent,
across the entire range of mean travel times from nearly zero to near infinity. Most
importantly, this relationship is virtually free of aggregation error; that is, the same
relationship also holds (again within a few percent) for runoff from highly heterogeneous
mixtures of strongly contrasting subcatchments, and for runoff from catchments
exhibiting strong nonstationarity. Some implications of these results will be discussed. |
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