River runoff is cyclical in nature, with alternating cycles of wet and dry years. Significant deviations from average values differ in duration and magnitude. For example, 1940-44, 1965-68 and 1977-79 are clearly low periods in terms of total runoff from the world's rivers. During these periods, the runoff was estimated at 1 600-2 900 km3 below the average value. By contrast, 1926-27, 1949-52 and 1973-75 saw much greater levels of river runoff (Shiklomanov, 1999). The last two decades have witnessed increasing runoff in South America and decreasing runoff in Africa.
From collection: Vital Water Graphics
Philippe Rekacewicz, Delphine Digout, UNEP/GRID-Arendal