a unique desert that is humid, and a worthy addition to UNESCO
The list of the world’s most extraordinary natural places, the UNESCO World Heritage List, gained its 1,000th entry this week with the addition of the Okavango Delta in northern Botswana.
To be chosen for the UNESCO list, a site must be deemed to have “outstanding universal value”. It is fitting that the delta is such a choice of landmark because it is simply unique: a wetland located in a desert, a delta that does not flow into the sea. It may seem improbable, but in reality it provides an example. of how biological, hydrological, biogeochemical and climatic processes have interacted to give us an ecosystem of exceptional importance and astonishing biological diversity.
A desert, humid even in the dry season
The Okavango Delta is unusual in many ways. In October of each year, the rains that fall in the highlands of Angola flow along the Cubango and Cuito rivers which, on the border between Angola and Namibia, join to form the Okavango River. This pulse of water, which makes up over 60% of the water entering the delta, flows slowly downstream for four to seven months, reaching the northern part of the delta between February and May.
The pulse of water enters the delta through a wide valley in northern Botswana called handshake. Here, the water is relatively deep and fast-flowing, crossing the sands of the Kalahari, before a perpendicular geological fault caused by tectonic activity causes the water to slowly spread in a fan, reaching the southern regions of the delta in August-September. In the southern hemisphere, these are the winter months, and in this particular region, it is also the dry season. Therefore, the maximum extent of flooding actually occurs at the driest time of the year.
This impulse of water causes the size of the delta to fluctuate considerably, approximately doubling its size by 6,000 km2 at 12,000 km2 every year during the dry season. Local precipitation during the summer months accounts for less than 40% of the river flow in the delta, but still plays an important role in influencing the extent of the area flooded by the delta. Together, they ensure that the local wildlife has water to thrive for much of the year, transforming the delta into a huge green oasis that provides terrestrial and aquatic habitats for an abundance of plants and animals.
A persistent freshwater oasis in a wetland
Another remarkable feature of the delta is that in the scorching heat of the desert, 98% of its water is lost through evaporation and transpiration (water absorbed by plants and evaporating from their leaves). It is another paradox of the delta, that its waters remain cool and non-saline even with a strong subtropical sun. This is due to a complex set of interactions, recently understood, which involve termites, trees and over 150,000 islands.
It is believed that most of the islands in the delta originated from young trees growing on termite mounds. As trees grow, water and solutes are sucked from the soil into the plants and released through transpiration. This lowers the water table over time, which draws water from the surrounding floodplains to replace it, and thus keeps the replenished area freshwater.
The salts concentrate below the islands creating areas of hyper-saline soils in which few organisms can survive, but as the salt concentrations below the islands increase, the salts become dense and heavy enough to sink to the bottom, ensuring effectively a layer of cool water. water above.
The dynamic and diversified delta
The Okavango Delta is one of the last virgin wetlands in Africa and, since 1997, it has been ranked as the largest in the world Wetland of international importance. It has suffered little damage from human activity, partly because of its nutrient-poor waters which translate into low fish yields, and partly because of a tourism policy that generates considerable income with a minimal impact.
However, these long periods of available water and the nutrient-rich grasslands and floodplains of the delta have given rise to a flourishing life, one of the largest concentrations of wildlife in Africa: the largest elephant population, hippos and many endangered species such as the wild dog, black and white rhinoceros, and cheetahs. Many of the 450 bird species recorded in the delta are also globally threatened, such as slate egrets and wattled cranes. This diversity is no accident; it is closely linked to the exceptional diversity of habitats induced by the unique dynamics of the pulsation of floods during the driest season.
The natural variation in precipitation and the internal hydrology of the region means that there is always variation in the size and extent of flood waters. For example, after a period of contraction in the late 1990s-2000s, the past few years have seen the delta flood to a degree not seen since the 1960s. It may be inevitable that a warmer world will affect the region. , and although the effects are not well understood, preliminary results suggest that floodwaters will decline.
But another clear and present danger, and the effects of which could be felt much sooner, is the threat posed to water and wildlife by the Botswana government. granting of 41 mining licenses for diamonds, base metals and oil in the Okavango Delta earlier this year. Even on the UNESCO list, nothing is certain.