Hydrology and Earth System Sciences
www.hydrol-earth-syst-sci.net
1027-5606
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13
9
2009
10.5194/hess-13-1659-2009
http://www.hydrol-earth-syst-sci.net/13/1659/2009/
http://www.hydrol-earth-syst-sci.net/13/1659/2009/hess-13-1659-2009.html
http://www.hydrol-earth-syst-sci.net/13/1659/2009/hess-13-1659-2009.pdf
1659
1670
2009-09-18
Spatial rainfall variability and runoff response during an extreme event in a semi-arid catchment in the South Pare Mountains, Tanzania
M. L. Mul
m.mul@unesco-ihe.org
H. H. G. Savenije
S. Uhlenbrook
UNESCO-IHE Institute for Water Education, Delft, Westvest 7, P.O. Box 3015, 2601 DA Delft, The Netherlands
Water Resources Section, Faculty of Civil Engineering and Applied Geosciences, Delft University of Technology, Stevinweg 1, P.O. Box 5048, 2600 GA Delft, The Netherlands
This paper describes an extreme flood event that occurred in the South Pare
Mountains in northern Tanzania. A high spatial and temporal resolution data
set has been gathered in a previously ungauged catchment. This data was
analysed using a multi-method approach, to gather information about the
processes that generated the flood event. On 1 March 2006, extreme rainfall
occurred in the Makanya catchment, (300 km<sup>2</sup>), where up to 100 mm were
recorded in Bangalala village in only 3 h. The flood was devastating,
inundating large parts of the flood plain. The spatial variability of the
rainfall during the event was very large, even in areas with the same
altitude. The Vudee sub-catchment (25.8 km<sup>2</sup>) was in the centre of the
rainfall event, receiving about 75 mm in 3 h divided over the two upstream
tributaries: the Upper-Vudee and Ndolwa. The peak flow at the weir site has
been determined using the slope-area method and gradually varied flow
calculations, indicating a peak discharge of 32 m<sup>3</sup> s<sup>−1</sup>. Rise and fall
of the flood was very sharp, with the peak flow occurring just one hour after
the peak of the rainfall. The flow receded to 1% of the maximum flow within
24 h. Hydrograph separation using hydrochemical parameters indicates that at
the floodpeak 50% of the flow was generated by direct surface runoff (also
indicated by the large amount of sediments in the samples), whereas the
recession originated from displaced groundwater (>90%). The subsequent
base flow in the river remained at 75 l s<sup>−1</sup> for the rest of the season,
which is substantially higher than the normal base flow observed during the
previous rainy seasons (15 l s<sup>−1</sup>) indicating significant groundwater
recharge during this extreme event.
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