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| Titel |
An analytical framework for extracting hydrological information from time series of small reservoirs in a semi-arid region |
| VerfasserIn |
Frank Annor, Nick van de Giesen, Thom Bogaard, Dirk Eilander |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250075744
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| Zusammenfassung |
| Small water reservoirs for water resources management have as important socio-economic
advantage that they bring water close to villages and households. This proximity allows for
many water uses in addition to irrigation, such as fisheries, household water, building
materials (loam, reeds), tourism and recreation, and cattle watering. These positive aspects
are offset by the relatively large evaporative losses in comparison to larger reservoirs,
although, it is not exactly known how large these losses are. For decision makers, investors
and donors, the decision to construct a small reservoir should be multifactored; and based on
economic, socio-cultural and environmental factors. For the latter, getting the water balance
and the energy budget of small reservoirs right is key for any environmental impact
analyses.
For Northern Ghana, the relation between volume of a small reservoir and its’ surface
area has been established in a robust equation as: Volume = 0.00857Area1.4367 with the
surface area explaining more than 95% of the variation in water volume of the reservoirs.
This allows the use of remote sensing observations for estimating water volume of small
reservoirs in northern Ghana. Hydrological analyses of time series of small reservoir areas
comprises estimates of evaporation fluxes and cumulative surface runoff curves. Once the
reservoirs are full, spillage will occur and volumes and surface areas remain stable at their
maximum extents. This implies that the time series of reservoir surface area contains
information concerning the on-set of downstream surface runoff. This on-set does not
coincide with the on-set of the rainy season but largely depends on the distribution of rainfall
events and storage capacity in the subsurface. The main requirement for this analysis is that
the reservoir has negligible seepage losses or water influx from the underlying
subsurface.
In our research, we carried out a time series analysis of surface area extent for about 45
small reservoirs in the Upper East Region of Ghana. Reservoirs without obvious large
seepage losses (field survey) were selected. To verify this, stable water isotopic samples are
collected from groundwater upstream and downstream from the reservoir. By looking at
possible enrichment of downstream groundwater, a good estimate of seepage can be made in
addition to estimates on evaporation. We estimated the evaporative losses and compared those
with field measurements using eddy correlation measurements. Lastly, we determined
the cumulative surface runoff curves for the small reservoirs .We will present this
analytical framework for extracting hydrological information from time series of
small reservoirs and show the first results for our study region of northern Ghana. |
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