The Okavango River drains an area of over 140,000 km2, including the uplands of central Angola. Here, ample precipitation falls in the Southern Hemisphere summer months, and then flows towards the delta and fan. The Okavango River is thus subject to large seasonal variations in flow. Water level and extent in the delta region ebbs and floods accordingly. In addition to the seasonal cycle, river flow variations on longer timescales of 60 to 80 years have also been documented.
Using statistical analyses and hydrological modelling, Piotr Wolski and colleagues find that these multidecadal swings between wet and dry phases mainly stem from variations in rainfall, with little influence from temperature-driven evaporation (J. Hydrol. http://doi.org/jrx; 2012). They attribute these rainfall variations to internal feedback mechanisms between the ocean, atmosphere and land, as opposed to external influences, for example from humans or solar variability. According to climate model projections, these multidecadal oscillations are likely to continue at similar amplitudes throughout the twenty-first century. At the same time, conditions are expected to become progressively drier in the long term, as a result of higher temperatures and thus evaporation. The multidecadal swings are therefore likely to alternatingly intensify and offset the long-term anthropogenic drying trend.
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