Long-term variations of annual flows of the Okavango and Zambezi Rivers

Annual inflows and outflows from the Okavango Delta are analysed for possible changes over time. The magnitude of inflows determines the area inundated annually over the delta, and therefore the delta ecosystem on which the thriving tourism depends. Inhabitants of the delta area greatly depend on harvesting aquatic and terrestrial resources, and the abundance of these resources is affected by flooding. Annual flows, annual maximum flows, and annual minimum flows of the Okavango River are analysed for the presence of abrupt or gradual changes and cycles in order to establish the nature of these perceived changes. Change points have been identified in annual flows of the Okavango River and Zambezi River. Other time series except the annual minimum flows are free of trends. Significant cycles were identified on annual flows, annual maximum flows, and annual minimum flows of both the Okavango and Zambezi Rivers. Residuals obtained after removing the cyclic component from each of these time series are trend free. The apparent decline of inflows into the Okavango Delta is explained by the existence of this cyclic component, rather than gradual or abrupt decline in flows over time. The cyclic component identified on the Okavango River has a regional scale as flows on the neighbouring Zambezi River have similar features. Possible causes of the cyclic behaviour are not known. (c) 2006 Published by Elsevier Ltd.