Forecasting the spatial extent of the annual flood in the Okavango delta, Botswana T. Gumbrichta, P. Wolskib, P. Frostc, T.S. McCarthya,* aDepartment of Geology, University of the Witwatersrand, P.O. Box 3, Wits 2050, Johannesburg 4-27125, South Africa b Harry Openheimer Okavango Research Centre, University of Botswana, Private Bag 285, Maun, Botswana cInstitute for Soil, Climate and Water, Private Bag X79, Pretoria 0001, South Africa Received 9 October 2002 revised 31 October 2003 accepted 7 November 2003 Abstract The pristine Okavango Delta wetland of northern Botswana is potentially under threat due to water abstraction from its tributaries. We have developed a statistical model which makes it possible to predict the extent of wetland loss which will arise from water abstraction. The model also permits prediction of the maximum area of flooding, and its spatial distribution, three months in advance of the flood maximum. The model was calibrated using maximum areas of seasonal inundation extracted from satellite imagery covering the period 1985���2000, which were correlated with rainfall and total flood discharge. A technique was developed to translate the modelled flood area into a flood map. The methodology can predict maximum area of flooding and its distribution with better than 90% accuracy. An important, although relatively minor, source of error in the spatial distribution of the flood arises from a secular change in flood distribution in the distal Delta which has taken place over the last 15 years. Reconstruction of flooding history back to 1934 suggests that the Delta may be subject to a quasi 80 year climatic oscillation. If this oscillation continues, the extent of flooding will increase in the coming decades. q 2004 Elsevier B.V. All rights reserved. Keywords: Okavango delta Seasonal flood Flood forecast Flood area 1. Introduction The Okavango Delta of northern Botswana (Fig. 1) is perhaps the most pristine of Africa���s large wetlands, and a major, international tourist destination. Because of its remoteness, it largely escaped colonial development, and its primary sources of water, the Quito and Cubango Rivers, which arise in central Angola and combine to form the Okavango River, also remain undeveloped. However, in recent years there has been growing interest in the water resources of the region, prompted initially by severe drought conditions in northern Botswana (Scudder, 1993). More recently, a drought in Namibia led to a proposal to pump water from the Okavango River at Rundu as an emergency measure to supply the national water distribution network of that country (Ashton and Manley, 1999). Improved rainfall, however, resulted in temporary suspension of the scheme, but the long term water development plan for Journal of Hydrology 290 (2004) 178���191 www.elsevier.com/locate/jhydrol 0022-1694/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2003.11.010 * Corresponding author. Fax: ��27-11-717-6579. E-mail address: mccarthyt@geosciences.wits.ac.za (T.S. McCarthy).

Namibia involves abstraction of water from the Okavango River (Pallett, 1997). In addition, increasing commercial agriculture along the Oka- vango River flood plain in Namibia is utilizing progressively more water from the river. Further- more, the recent end to the long civil war in Angola will result in development of the interior of that country, which will inevitably involve increased water abstraction from the tributaries of the Okavango system. Water abstraction from the Okavango River and its tributaries poses a threat to the downstream wetland habitats of the Okavango Delta, which depend to a large extent on inflow from the Okavango River, raising the spectre of conflict between development needs and conservation. Attempts have been made in the past to model the effects of water abstraction on the wetlands, but these have examined only the effect on outflow from the wetland (Ashton and Manley, 1999 UNDP, 1976 Dincer et al., 1987 Gieske, 1997). Such outflow is important, because it supplies many rural villages downstream of the Delta, and it is therefore necessary to understand the impacts water abstraction might have. However, equally important, from a conservation perspective, is the effect abstraction will have on the wetland itself, and in particular on the area of seasonal inundation. Previous models shed minimal light on this subject, because little quantitative data on the area of inundation was available. In this paper we make the first attempt to address quantitatively this extremely important issue. Local rainfall and inflow to the Delta via the Okavango River have been regularly monitored since the early 1930 s, while outflow has been recorded since the early 1970 s by government departments in Botswana. We have augmented these data with measurements of the aerial extent of annual flooding extracted from archival satellite imagery extending back to the early 1970 s. Using this data base, we have derived a statistical model which can estimate the likely maximum area of the seasonal flood. Images of previous floods of different magnitudes are used to convert the predicted area of inundation into a flood map. The model can be used to investigate the impacts Fig. 1. The Okavango Delta and its catchment. T. Gumbricht et al. / Journal of Hydrology 290 (2004) 178���191 179