Spatial rainfall variability and runoff response during an extreme event in a semi-arid catchment in the South Pare Mountains, Tanzania

摘要

This paper describes an extreme flood event that occurred in the South PareMountains in northern Tanzania. A high spatial and temporal resolution dataset has been gathered in a previously ungauged catchment. This data wasanalysed using a multi-method approach, to gather information about theprocesses that generated the flood event. On 1 March 2006, extreme rainfalloccurred in the Makanya catchment, (300 km2), where up to 100 mm wererecorded in Bangalala village in only 3 h. The flood was devastating,inundating large parts of the flood plain. The spatial variability of therainfall during the event was very large, even in areas with the samealtitude. The Vudee sub-catchment (25.8 km2) was in the centre of therainfall event, receiving about 75 mm in 3 h divided over the two upstreamtributaries: the Upper-Vudee and Ndolwa. The peak flow at the weir site hasbeen determined using the slope-area method and gradually varied flowcalculations, indicating a peak discharge of 32 m3 s−1. Rise and fallof the flood was very sharp, with the peak flow occurring just one hour afterthe peak of the rainfall. The flow receded to 1% of the maximum flow within24 h. Hydrograph separation using hydrochemical parameters indicates that atthe floodpeak 50% of the flow was generated by direct surface runoff (alsoindicated by the large amount of sediments in the samples), whereas therecession originated from displaced groundwater (>90%). The subsequentbase flow in the river remained at 75 l s−1 for the rest of the season,which is substantially higher than the normal base flow observed during theprevious rainy seasons (15 l s−1) indicating significant groundwaterrecharge during this extreme event.