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, 1940-1944, 1965-1968 and 1977-1979, 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. By contrast, 1926-1927, 1949-1952 and 1973-1975 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.
Tropical regions typically exhibit greater river runoff volumes. The Amazon carries 15% of all the water returning to the world’s oceans, while the Congo-Zaire basin carries 33% of the river flow in Africa. Arid and semi-arid regions, which make up an estimated 40% of the world’s land, have only 2% of its runoff (Gleick, 1993).
Water quality information is required for sustainable water resource management. Land-based activities can affect water chemistry through pollution, and play a role in transporting sediments in rivers. Sediments carry many types of pollutants from point and non-point sources, the quantity of which depends on the general land use and activity in the drainage basin of origin.