Contribution of Hydrological Processes in the Occurrence of Extreme Hydrological Events in the Middle Zambezi River Basin

Abstract

The middle Zambezi has experienced hydrological extremes in alternating intervals. The impacts are mostly felt by the people who live close to the river and mostly around Lake Cahora Bassa. A study on the analysis of the contribution of the middle Zambezi in the occurrence of these hydrological extremes was carried out to determine the contribution of each middle Zambezi catchment in the occurrence of floods and droughts, in terms of magnitude and frequency. Rainfall analysis was also employed to augment the results obtained from the hydrological analysis. This was because; the middle Zambezi had many impoundments which might contribute in the occurrence of either floods or hydrological drought due to reservoir operation. Hence rainfall was incorporated to discern natural floods and hydrological droughts from those which could be human induced. A total of 33 rainfall stations were analysed to get the average rainfall received in the catchment using the Thiessen Polygon approach. Since the Manyame Catchment was poorly gauged, a Soil Conservation Service Curve Number model was used to compute the runoff from this catchment. The other tributaries of the middle Zambezi which were studied included Luangwa River (as measured from Luangwa Bridge), Kafue River (Kafue Gorge releases) as well as the Zambezi River (Victoria Falls and Kariba releases). Pearson Correlation and a double mass analysis were employed in order to determine the influence of each tributary on the Lake levels of Cahora Bassa, and also contribution of each tributary in terms of magnitude and frequency of occurrence of floods and droughts. The results showed that the levels of the Cahora Bassa were not influenced by individual tributary but were influenced by the combined flows. In terms of magnitude, the releases from Kariba contributed most, but were least frequent, while the Luangwa contributed most in terms of frequency and Manyame River was second to Kariba in both cases; magnitude and frequency. Occurrences of floods and hydrological droughts in the middle Zambezi River basin were correlated with rainfall anomalies within the catchment. This showed that hydrological extremes were a natural result as opposed to human induced. On the contrary, the Kariba dam attenuated most floods which occurred in Victoria Falls and further curtailed the impacts of hydrological droughts by increasing the low flows. In conclusion, the hydrological extreme events experienced in the middle Zambezi were mainly natural and were mainly caused by hydrological processes of the tributaries of middle Zambezi from Kariba to Cahora Bassa.