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Titel |
Keeping the secret: Insights from repeated catchment-scale tracer experiments under transient conditions |
VerfasserIn |
Christina Bogner, Michael Hauhs, Holger Lange |
Konferenz |
EGU General Assembly 2016
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Medientyp |
Artikel
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Sprache |
en
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 18 (2016) |
Datensatznummer |
250131113
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Publikation (Nr.) |
EGU/EGU2016-11476.pdf |
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Zusammenfassung |
Catchment-level tracer experiments are generally performed to identify site-specific
hydrological response functions of the catchment. The existence and uniqueness of these
response functions are hardly ever questioned.
Here, we report on a series of replicated tracer experiments in two small first-order
catchments, G1 (0.6 ha, roofed) and F4 (2.3 ha, without roof) at Gårdsjön in SW Sweden.
The soils in both catchments are shallow (< 50 cm) with the bedrock partly visible at the
surface. In G1 (irrigated area approximately 1000 m2), tracer experiments were conducted
under a roof between 1993 and 2003 during steady state flow conditions. In contrast, in F4
(irrigated area approximately 500 m2) the experiments were done without a roof mostly at
transient conditions.
The catchment F4 was equipped with a sprinkler system with a watering capacity of
around 38–45 m3 day−1. Natural rainfall comes in addition. A bromide tracer solution was
injected to groundwater at a single location about 40 m upstream the weir over a period of
less than an hour, and was monitored using a set of groundwater tubes and the weir at the
outlet over the following 4 days. In addition, discharge was measured. The experiments were
repeated each summer from 2007 to 2015.
While steady state conditions were guaranteed in G1, steady runoff has been
achieved only four times in F4. We investigated tracer recovery rates against cumulated
runoff since tracer application. Substantially different transit times and qualitatively
different behaviour of the breakthrough curves were observed, even under steady
state conditions. In G1, no single system response function could be identified in 5
replicates. Similarly, the catchment response functions in F4 under steady state
differed between experiments. However, they remained in a similar range as in
G1.
Based on these results, we question the identifiability of flow paths and system properties,
such as saturated water content or hydrologic transmissivity, at the catchment scale using
tracer experiments. Rather, the series demonstrate the utter importance of the initial and
boundary conditions which largely determine the response of the system to inert tracer pulses. |
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