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| Titel |
An eye on small reservoirs: remote sensing of storage volumes, their use as remotely sensed runoff gauges, and evaporation losses |
| VerfasserIn |
J. R. Liebe, N. van de Giesen, M. S. Andreini, M. T. Walter, T. S. Steenhuis |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250025907
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| Zusammenfassung |
| Small reservoirs are important sources of water supply for the scattered rural population in
semi-arid areas. Due to their small size, and their existence in large numbers, such reservoirs
have not been studied much. Little is known about their storage volumes, their impact on their
watershed’s contribution to runoff downstream, and their evaporation losses are frequently
stated to be prohibitive.
Satellite remote sensing can be used to assess and monitor small reservoirs’ storage
volumes with regional area-volume equations. Radar remote sensing of small reservoirs was
found suitable especially during the rainy season due to its capability to penetrate clouds, but
is affected by wind and lack of vegetation context during the dry season. Reservoirs
were extracted most often successfully with a quasi-manual classification approach,
as stringent classification rules often failed under less than optimal conditions.
Especially wind speeds above 2.6 m s-1 at the time of image acquisition were
detrimental (Bragg scattering) to the extraction of reservoirs. Due to lower wind
speeds, the use of night time acquisitions was more effective than the use of daytime
images.
With a time series of radar images, small reservoirs were used as remotely sensed
runoff gauges, and to calibrate hydrological rainfall-runoff models. Eight small
reservoirs in the Upper East Region of Ghana, and Togo, were monitored to calibrate
modified Thornthwaite-Mather models, in which increasing precipitation leads to
exponentially increasing contributing areas. Model results indicate that the reservoirs
captured, on the average, 34% of the quick flow, and 15% of overall runoff from their
watersheds.
Reservoir evaporation losses were measured directly with a floating evaporation pan and
were compared to evaporation rates determined from the reservoir’s energy budget, and
Penman’s equation. The direct pan measurements were generally lower than the evaporation
determined with the energy budget or Penman. Compared to land based potential evaporation,
the reservoir evaporation was not excessive. Regional wind patterns influence evaporation
dynamics from the reservoir. Northeast winds with a high saturation deficit lead to significant
evaporation losses, while the evaporation losses under moister, more prevalent southwest
winds were moderate. |
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