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COURSE TITLE: The enigmatic Okavango Delta: A large wetland in a dryland

INSTRUCTORS Fred Ellery

COURSE DESCRIPTION The Okavango Delta, with an area of approximately 15 000 km2 (almost 6 000 square miles), is southern Africa's largest wetland. Remarkably, it is situated in the Kalahari, which is semi-arid with a mean annual rainfall of around 500 mm (20 inches), while mean annual potential evapotranspiration is well over 2 000 mm (80 inches). As such the Okavango is a wetland in a dryland landscape as it is characterized by a strongly negative water balance. Runoff from rainfall in the highlands of central Angola drains south-eastwards into the Kalahari Basin, such that it disappears within a continental setting and never reaches the ocean. Of the water entering the Okavango via runoff and rainfall, 98 % evaporates. All of the clastic and 98 % of the dissolved sediment transported into the ecosystem is deposited within it. The accumulation of dissolved sediment in the ecosystem should lead to salinization of surface waters, but the ecosystem is characterized by fresh surface waters that are crystal clear.

Deposition of clastic sediment in upper channels leads to large-scale diversion of water from one part of the system to another over timescales of centuries. These events lead to combustion of peat deposits in peat fires that burn for decades. Dissolved sediment that is potentially toxic to the wetland ecosystem as carbonate salts of mainly calcium, magnesium and sodium, accumulates beneath islands where dense broad-leaved evergreen forests act as water pumps and draw down the water table. Trees are selective in their uptake of dissolved salts and exclude those that are detrimental to plant and animal life. These salts therefore accumulate in island soils, creating topographic relief. Sodium remains in solution throughout the evaporation process and poisons island soils over timescales of about 100 years. The focusing of potentially harmful salts on islands means that surface water in the wetland remain remarkably fresh. Channel switching leading to changing flow patterns allows renewal of poisoned island soils and the long-term maintenance of a freshwater wetland ecosystem where one might expect salinization.

The structure and functioning of this remarkable ecosystem are discussed in the light of research over a period of about 25 years by a fairly small group of researchers.

Fred Ellery has repeatedly said that there is nothing he would rather do than wander through and think about a wetland. His curiosity and enthusiasm about these ecosystems are epitomised in his research, teaching and learning over the length of his career.
As a young scientist he and his wife Karen worked in a remote part of the Okavango Delta in northern Botswana, an arid country north-of South Africa. As part of a fairly small multidisciplinary research team he learned to think across temporal and spatial scales, and to integrate research findings to develop understanding that he felt was essential if wetlands were to be wisely managed and conserved. More recently he worked in wetlands in the Greater St Lucia Wetland Park in northern KwaZulu-Natal in South Africa. He also headed a research program that addressed issues related to wetland rehabilitation in South Africa, where the state substantially funds wetland restoration nationally through an expanded public works program. Since moving to Rhodes University in the Eastern Cape Province in South Africa about a decade ago, he has worked on smaller wetlands, particularly those occurring on an ancient erosion surface where wetlands are tens of millions of years old.
Fred is well known nationally and has international links with scientists in Australia, Europe and North America. He is currently a joint Chairperson of the International Chapter of SWS, with responsibilities in promoting the activities of the Society in Africa.

Credit Points: 0.06

SYLLABUS/TOPICAL OUTLINE The Okavango Delta, with an area of approximately 15 000 km2 (almost 6 000 square miles), is southern Africa's largest wetland. Remarkably, it is situated in the Kalahari, which is semi-arid with a mean annual rainfall of around 500 mm (20 inches), while mean annual potential evapotranspiration is well over 2 000 mm (80 inches). As such the Okavango is a wetland in a dryland landscape as it is characterised by a strongly negative water balance. Runoff from rainfall in the highlands of central Angola drains south-eastwards into the Kalahari Basin, such that it disappears within a continental setting and never reaches the ocean. Of the water entering the Okavango via runoff and rainfall, 98 % evaporates. All of the clastic and 98 % of the dissolved sediment transported into the ecosystem is deposited within it. The accumulation of dissolved sediment in the ecosystem should lead to salinization of surface waters, but the ecosystem is characterized by fresh surface waters that are crystal clear. Deposition of clastic sediment in upper channels leads to large-scale diversion of water from one part of the system to another over timescales of centuries. These events lead to combustion of peat deposits in peat fires that burn for decades. Dissolved sediment that is potentially toxic to the wetland ecosystem as carbonate salts of mainly calcium, magnesium and sodium, accumulates beneath islands where dense broad-leaved evergreen forests act as water pumps and draw down the water table. Trees are selective in their uptake of dissolved salts and exclude those that are detrimental to plant and animal life. These salts therefore accumulate in island soils, creating topographic relief. Sodium remains in solution throughout the evaporation process and poisons island soils over timescales of about 100 years. The focusing of potentially harmful salts on islands means that surface water in the wetland remain remarkably fresh. Channel switching leading to changing flow patterns allows renewal of poisoned island soils and the long-term maintenance of a freshwater wetland ecosystem where one might expect salinization. The structure and functioning of this remarkable ecosystem are discussed in the light of research over a period of about 25 years by a fairly small group of researchers.

COURSE CONTACT

Louis Mantini
9225 CR 49, Live Oak, FL 32060
lfm@srwmd.org
P: 386.647.3144
F:

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