By the Okavango Delta we mean the Moremi Game Reserve proper and the private reserves that surround it.
Traditionally, annual rainfall levels in the Okavango Delta average out at 450 mm, with the wet season beginning in December, peaking in January and February, and finishing by March. April marks the transition from summer to winter, with May being the beginning of the dry season. Temperatures drop during the winter months, as do humidity levels, and there is next to no rain between June and September. As winter draws to a close, temperatures begin to rise, and will occasionally reach 40°C or over in October.
The wet summer months are followed in March by the delta’s annual floods. Supplied by rains collected in the Angolan highlands, these begin in March, peak in mid-winter, and are lost to evapotranspiration by the end of the year. Averaging 11 billion litres per year, the annual floods almost double the delta’s size, from 900 to 1600 km2. However, the delta is virtually flat, falling just 60 odd metres over a distance of 250 km, meaning that the floods travel at speeds of a kilometre a day, and take 4 months to traverse the delta. The northern and central sectors of the delta, therefore, receive the lion’s share, while Maun and other equally southern areas collect just 2% of the seasonal waters.
So, given the pattern of local rainfall levels, the timing of the floods and the fact that the core of the delta holds water all year round, the wet season will usually see wildlife relatively well dispersed, with plenty of cover, while the dry season combines thinning cover with diminishing sources of water. The permanent swamps support a healthy and diverse range of resident wildlife, including elephant, lion, hippo, crocodile, spotted hyena, leopard, blue wildebeest, zebra, impala, wild dog, buffalo and red lechwe. Numbers hardly fluctuate, even during the dry season, as seasonal swamps absorb migrating animals. Types of safaris will depend on the time of year, and on the sector of the delta visited. As a rule of thumb, most lodges in the Okavango Delta offer both wet and dry safaris, with those situated in the south and outside the seasonal marshlands confined to shorter water based seasons than those in the centre and north.
However, northern Botswana’s dry cycle, which is said to last for between 10 and 15 years, appears to have given way to a wet cycle. With both 2008 and 2009 supplying the area with uncharacteristically long rains, and the flood levels being above average for both years, 2010 has been dubbed a ‘super year.’ Average rainfall levels for the Okavango Delta more than doubled, and measurements of the amount of water entering the delta in March of that year showed it to be the largest annual flood since the 1970s. These statistics were reflected across the region, with the Selinda Spillway and Savuti Channel once again in flood. In September 2010 the Boteti River, which runs south-east into the Kalahari, and which had been dry since 1991, reached the borders of Makgadikgadi, and in March 2011 it had reached Sukwane.
At the time of writing (2011), the effects of this new weather cycle are significant. Once dry areas are now supporting riverine habitats throughout the year. Traditional dry season water resources – such as the permanent Okavango Delta swamps – are less important now that migrating game has the option of watering at the Selinda Spillway, the Savuti Channel, Savuti Swamp and at any number of the larger Chobe pans. Predators need to work harder, as the game is dispersed for longer periods, and the cover still thick well into June.
From the point of view of the Okavango Delta, this is all to the good. With pressure taken off traditionally permanent water sources - and larger animals such as elephant predicted to utilise its outer sectors - the delta’s core and seasonal swamps will be given more time regenerate, while the previously less advantaged borders should enjoy a rise in the number of water specialists (lechwe, waterbuck, hippo and crocodile, for example), as well as in the numbers of insects, fish, amphibians and birds.