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Abstract: The annual flood cycle in the Okavango Delta is the basis of subsistence of the local population, providing water, arable land and wetland resources. Additionally, it forms the basis of a large tourism industry. Flooding in the Okavango Delta is, however, very variable both at short and long term time scales. Past research suggested the influence of factors such as tectonic activity, sedimentation, vegetation change or human influence on the spatial distribution of floodwaters. Change in the flooding regime at any given site is thus often perceived as an effect of permanent change in the system, and this perception results in unnecessary demands for engineering interventions in the system. However, it is possible that some of the perceived changes in flood regime are apparent transient effects of non-linearity of the system and variation in hydrological inputs. Hydrometric data and satellite-derived flood maps are analysed here using statistical methods (covariance analysis and double mass analysis) to distinguish between temporal and permanent changes in various parts of the Okavango Delta. The analyses reveal that system non-linearity causes the hydrological responses of the Okavango Delta to be non-proportional to the inputs. Periods of one to several consecutive years are present when flood extents and outflows are either considerably higher or lower (flow or flood regimes) than what would be expected considering the magnitude of inflow. Such effects are visible throughout the entire system, or only in some distributaries, and do not, as previously thought, represent permanent change in flood distribution. However, flood regime change that appears to result from a physical change in the system has been detected. This change has a nature of shift in flood distribution between the Thaoge and the Xudum, with the latter receiving more water after 1997 at the expense of the former and causing 2004 re-flooding of Lake Ngami. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]