PRESENT AND FUTURE HYDROLOGICAL CONDITIONS IN THE OKAVANGO DELTA, BOTSWANA, AND THEIR IMPLICATIONS FOR PEAT FORMATION AND DEGRADATION

Okavango Delta is a 12000 km² flood-pulsed wetland developed on a large alluvial fan, located in semi-arid Botswana. Flooding in this system varies on a seasonal, annual and multi-annual basis, in response to short and long term climatic variability. Additionally, flood distribution is affected by geomorphological processes such as channel aggradation and possibly tectonic activity. This paper reviews the current understanding of the relationship between hydrological conditions and peat formation and degradation in the Okavango Delta. In general, the seasonally inundated parts of the wetland have a dominantly mineral substratum, while peat accumulation occurs within the permanently inundated zone. Change in inundation conditions resulting from long term climate and geomorphological dynamics lead to change in peat distribution. Desiccation of previously permanently inundated areas result in the loss of accumulated peat and organic matter to fire and slow oxidation, while permanent flooding of previously seasonally inundated areas lead to peat accumulation. Furthermore, the paper presents scenarios of future (20-50 years) hydro-ecological conditions in the Okavango Delta and assesses their implications for the status of the permanent swamp. Conditions considered are: a) future climatic conditions simulated by GCMs, b) future climatic conditions simulated using stochastic time series modelling, c) future hydrological conditions possibly resulting from the observed geomorphological change, and d) hydrological conditions resulting from possible human development in the catchment. Change in climatic conditions as simulated by various GCMs differs in the magnitude and direction of change. As a consequence, the area of permanent swamps is simulated to either increase from the current 4500 km² to 6500 km² or decrease to 2000 km², depending on the GCM used. The simulations using stochastic time series modelling indicate the increase in permanent swamp acreage to 6500 km². Simulations of geomorphological change suggest virtual desiccation of the eastern part of the system, and development of permanent inundation conditions favouring peat accumulation within 4500 km² of the western distributaries. Water abstractions for human use may also have a dramatic effect on the acreage of permanent swamps.