Abstract
The Lower Middle Zambezi Basin is sandwiched between three hydropower dams; Kariba, Kafue (Itezhi-tezhi) and Cahora Bassa. The operation of the upstream dams impacts on the inflows into the downstream Cahora Bassa Dam which, in turn, affects the area inundated upstream of the Cahora Bassa Dam. This study applied a rainfall-runoff model (HEC-HMS) and GIS techniques to estimate both the gauged and ungauged runoff contribution to the water balance of Cahora Bassa. The rivers considered in the study are the Zambezi, Kafue, Luangwa, Chongwe, Musengezi and Manyame. Missing data were generated using the mean value infilling method. The DEM hydro-processing technique was used to determine the spatial extent of the ungauged area. A hydrological model, HEC- HMS, was used to simulate runoff from the ungauged catchments. Results from the study show that the ungauged catchment contributes about 12% of the total estimated inflows into the Cahora Bassa Dam. Averaged results over 30 years show total inflows of 71.73 x 10 9 m 3 /yr, total outflows of 52.25 x 10 9 m 3 / yr and a residual storage of 20 x 10 9 m 3 /yr. The study successfully estimated the water balance of the Middle Zambezi Basin which, in turn, may help to inform the operation of the Cahora Bassa Dam and management of artificial floods in the basin. Keywords: Cahora Bassa, DEM hydroprocessing, HEC-HMS, remote sensing, reservoir operation, runoff simulation
Highlights
The increased water demands in the Zambezi Basin have led to the construction of dams to store excess flows for release during periods of low flows and to be used for hydropower generation, irrigation and other requirements (Tumbare, 2005; Magadza, 2006)
The model was run for the ungauged catchments based on the Snyder Unit Hydrograph method with the basin lag time and the runoff coefficient as key parameters in the Hydrologic Engineering Centre Hydrologic Modelling System (HEC-HMS) (Adib et al, 2010)
The results show that the mean value infilling method is reliable to apply as a data quality checking method since all of the correlation coefficient values are above 0.85
Summary
The increased water demands in the Zambezi Basin have led to the construction of dams to store excess flows for release during periods of low flows and to be used for hydropower generation, irrigation and other requirements (Tumbare, 2005; Magadza, 2006). In addition to these uses, the dams have benefitted the inland fishing industry and hydro-tourism (Chenje et al, 1998; IFRC, 2007). The middle Zambezi River watercourse system, which stretches from the Victoria Falls up to the Cahora Bassa Dam wall, currently has three large hydroelectric dams namely, Kafue (Itezhi tezhi), Kariba and Cahora Bassa. As these reservoirs were constructed in the Zambezi Basin primarily for hydropower production purposes, they maintain a high hydraulic head in order to maximise power production
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