| dc.description.abstract | The Lower Middle Zambezi catchment is sandwiched between three major dams; Kariba,
Kafue (Itezhi-tezhi) and Cahora Bassa. The upstream dams have an impact on the inflows in
the downstream Cahora Bassa Dam and, also, on the area inundated upstream of the Cahora
Bassa. This study aimed at estimating the water balance of the Cahora Bassa using available
meteorological and hydrological data sets such as rainfall, temperature and runoff for the
period 1980 to 2010. The flow data analysed includes the discharge from the Kariba, Kafue
and Cahora Bassa dams and river flows for Luangwa, Chongwe, Musengezi and Manyame.
Missing flow data was generated using regression and mean value infilling methods. GIS
was used to calculate the isohyets for the average annual rainfall in the lake drainage basin
using the nearest neighbour interpolation technique. GIS was further used to process the
Digital Elevation Model (DEM) of the study area through the DEM hydroprocessing
algorithm in ILWIS thus establishing the ungauged catchment area of the lake.
Two gauging stations were installed at Manyame Mapomha and Angwa Bridge on the
Manyame and Angwa rivers, respectively, as part of the project’s effort of moving from
ungauged to gauged flow. The results from these stations were however too short to give
any meaningful indications as to the discharge from the associated catchments. As a result, a
hydrological model, the HEC-HMS, was then used to simulate runoff from the ungauged
catchments which showed that these areas contribute about 12% of the total estimated
inflows into the Cahora Bassa Dam. The average flow for one hydrological year revealed a
total inflow of 71.73 x 109 m3/year, total outflows averaged 52.25 x 109 m3/year and a
residual storage of 20 x 109 m3/year. While the high flood level for the Cahora Bassa Dam
lies at the 329 masl contour line, which translates to an inundated area of 3,246 km2, the
actual maximum water level for the period considered (1980 - 2010) reached only 328.18 m
contour thus giving an inundated area of 2,920 km2.
Analysis of the river flow hydrographs and the associated lake water level indicates that
there is a close link between reservoir operations and flood occurrence in Luangwa,
Kanyemba and Muzarabani areas. The flooding effect of the reservoirs is only felt when the
water level in the Cahora Bassa Dam is above 324.
The study concludes that the ungauged lower middle Zambezi contributes a significant
amount which averages 8.5 x 109 m3/year, which is about 12 %of the total inflows into
Cahora Bassa while the gauged flows from the Kariba, Kafue, Luangwa, Chongwe,
Manyame and Musengezi contribute about 85 % of the total flows into the Cahora Bassa.
The remaining 2.5 % comes from direct rainfall on the lake surface. Furthermore, the study
showed that flooding is most likely when high water levels are accompanied by high flows
in the associated rivers such as the Luangwa, the Mwanza Mtanda and the Musengezi. | en_ZW |
