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| Titel |
Viscosity changes of riparian water controls diurnal fluctuations of stream-flow and DOC concentration |
| VerfasserIn |
Michael Schwab, Julian Klaus, Laurent Pfister, Markus Weiler |
| 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 |
250110364
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| Publikation (Nr.) |
 EGU/EGU2015-10350.pdf |
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| Zusammenfassung |
| Diurnal fluctuations in stream-flow are commonly explained as being triggered by the daily
evapotranspiration cycle in the riparian zone, leading to stream flow minima in the afternoon.
While this trigger effect must necessarily be constrained by the extent of the growing season
of vegetation, we here show evidence of daily stream flow maxima in the afternoon in a small
headwater stream during the dormant season. We hypothesize that the afternoon maxima
in stream flow are induced by viscosity changes of riparian water that is caused
by diurnal temperature variations of the near surface groundwater in the riparian
zone.
The patterns were observed in the Weierbach headwater catchment in Luxembourg. The
catchment is covering an area of 0.45 km2, is entirely covered by forest and is dominated by a
schistous substratum. DOC concentration at the outlet of the catchment was measured with
the field deployable UV-Vis spectrometer spectro::lyser (scan Messtechnik GmbH) with a
high frequency of 15 minutes over several months. Discharge was measured with an ISCO
4120 Flow Logger. During the growing season, stream flow shows a frequently
observed diurnal pattern with discharge minima in the afternoon. During the dormant
season, a long dry period with daily air temperature amplitudes of around 10 Ë C
occurred in March and April 2014, with discharge maxima in the afternoon. The
daily air temperature amplitude led to diurnal variations in the water temperature of
the upper 10 cm of the riparian zone. Higher riparian water temperatures cause a
decrease in water viscosity and according to the Hagen-Poiseuille equation, the
volumetric flow rate is inversely proportional to viscosity. Based on the Hagen-Poiseuille
equation and the viscosity changes of water, we calculated higher flow rates of near
surface groundwater through the riparian zone into the stream in the afternoon which
explains the stream flow maxima in the afternoon. With the start of the growing
season, the viscosity induced diurnal effect is overlain by the stronger influence of
evapotranspiration.
Diurnal DOC fluctuations show daily maxima in the afternoon. While daily variations in
DOC concentrations are often explained by faster in-stream biogeochemical processes during
daylight, we here propose that the viscosity effect in the riparian zone could explain the
afternoon peaks in DOC concentrations. Our records show that daily water temperature
variations and therefore viscosity changes only occur in the near surface parts of the riparian
zone, where the DOC concentrations are higher than in deeper parts of the riparian zone. We
calculated, that the viscosity induced higher flow rates from the near surface parts of
the riparian zone can explain the DOC concentration maxima in the afternoon.
As the viscosity effect does not disappear during the growing season but is just
smaller than the evapotranspiration effect, the DOC concentration pattern is not
changing between the dormant and growing seasons. The different controls of diurnal
fluctuations of stream-flow and water quality concentrations need to be carefully
considered in order to better understand the different patterns in catchment hydrology. |
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