Long-term variations of annual flows of the Okavango and Zambezi Rivers Dominic Mazvimavi * , Piotr Wolski Harry Oppenheimer Okavango Research Centre, University of Botswana, P. Bag 285, Maun, Botswana Abstract Annual inflows and outflows from the Okavango Delta are analysed for possible changes over time. The magnitude of inflows deter- mines 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 flood- ing. 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 fea- tures. Possible causes of the cyclic behaviour are not known. Ó 2006 Published by Elsevier Ltd. Keywords: Cyclicity; Trends; Annual flows; Okavango Delta 1. Introduction The investigation of the presence of abrupt, gradual or cyclic changes of river flows is a key research issue since cli- mate change or land cover changes have the potential to cause such changes (Kundzewicz, 2004; Kundzewicz and Robson, 2004). Several studies have investigated the possi- ble existence of changes over time in runoff time series. Garcia and Mechoso (2005) identified increasing trends starting during the 1970s on major South American rivers. Winter flows were noted to have increased while summer flows had decreased on a basin located on the western part of Canada (Burn et al., 2004). A decreasing trend was noted in annual flows and annual minimum flows of the Yangtze River in China (Xiong and Guo, 2004). Swedish rivers were observed to have some periods with high or low flows (Lindstro ¨m and Bergstro ¨m, 2004). On Lake Malawi, the causes of the occurrence of periods with very low lake levels (1915–35) and others with very high lake levels (1979–84) have been of concern (Shela, 2000). Gradual or abrupt changes over time of inflows into wetlands have the potential to cause adverse effects on ter- restrial and aquatic ecosystems associated with wetlands. Large wetlands such as the Kafue Flats and Barotse Flood Plain in Zambia and Okavango Delta in Botswana, have evolved due to the existence of specific flow regimes. Local communities depend on these wetlands for subsistence e.g. flood plain agriculture, fishing, provision of construction materials. These wetlands are also important tourist desti- nation sites as they support significant aquatic and terres- trial wildlife. Significant changes in the seasonality, magnitude and inter-annual variability of inflows into these wetlands can alter their ecosystems which are important sources of livelihoods. During stakeholder consultations undertaken as part of the development of the Okavango 1474-7065/$ - see front matter Ó 2006 Published by Elsevier Ltd. doi:10.1016/j.pce.2006.08.016 * Corresponding author. Tel.: +267 686 1833; fax: +267 686 1835. E-mail address: dmazvimavi@orc.ub.bw (D. Mazvimavi). www.elsevier.com/locate/pce Physics and Chemistry of the Earth 31 (2006) 944–951