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| Titel |
What causes cooling water temperature gradients in a forested stream reach? |
| VerfasserIn |
G. Garner, I. A. Malcolm, J. P. Sadler, D. M. Hannah |
| 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 (2014-12-20), S.5361-5376 |
| Datensatznummer |
250120575
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| Publikation (Nr.) |
 copernicus.org/hess-18-5361-2014.pdf |
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| Zusammenfassung |
| Previous studies have suggested that shading by riparian vegetation may
reduce maximum water temperatures and provide refugia for temperature-sensitive aquatic organisms. Longitudinal cooling gradients have been
observed during the daytime for stream reaches shaded by coniferous trees
downstream of clear cuts or deciduous woodland downstream of open moorland.
However, little is known about the energy exchange processes that drive such
gradients, especially in semi-natural woodland contexts without confounding
cool groundwater inflows. To address this gap, this study quantified and
modelled variability in stream temperature and heat fluxes along an upland
reach of the Girnock Burn (a tributary of the Aberdeenshire Dee, Scotland)
where riparian land use transitions from open moorland to semi-natural,
predominantly deciduous woodland. Observations were made along a 1050 m
reach using a spatially distributed network of 10 water temperature
data loggers, 3 automatic weather stations and 211 hemispherical
photographs that were used to estimate incoming solar radiation. These data
parameterised a high-resolution energy flux model incorporating
flow routing, which predicted spatio-temporal variability in stream
temperature. Variability in stream temperature was controlled largely by
energy fluxes at the water-column–atmosphere interface. Net energy gains
occurred along the reach, predominantly during daylight hours, and heat
exchange across the bed–water-column interface accounted for <1%
of the net energy budget. For periods when daytime net radiation gains were
high (under clear skies), differences between water temperature observations
increased in the streamwise direction; a maximum instantaneous difference of
2.5 °C was observed between the upstream reach boundary and 1050 m
downstream. Furthermore, daily maximum water temperature at 1050 m
downstream was ≤1 °C cooler than at the upstream reach
boundary and lagged by >1 h. Temperature gradients were not
generated by cooling of stream water but rather by a combination of reduced
rates of heating in the woodland reach and advection of cooler (overnight
and early morning) water from the upstream moorland catchment. Longitudinal
thermal gradients were indistinct at night and on days when net radiation
gains were low (under overcast skies), thus when changes in net energy
gains or losses did not vary significantly in space and time, and heat
advected into the reach was reasonably consistent. The findings of the study
and the modelling approach employed are useful tools for assessing optimal
planting strategies for mitigating against ecologically damaging stream
temperature maxima. |
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