INTRODUCTION

Africa, with an area of 30.5 million km2, is the Earth's second largest continent, and presents the most extreme climatic contrasts. The Sahara is the largest and most extreme desert; Mount Cameroon is one of the rainiest places in the world; and the Congo River, which accounts for 30% of the runoff of the entire continent, is the world's second-ranking river in terms of discharge. This great diversity of climates and hydrological structures has been accentuated by very sharp variations over time and space.

The water resources of Africa are very unevenly distributed. The arid and semi-arid zones, which account for 60% of the continent's land area, produce only 5% of the total runoff (Margat, 1991). Although this very unbalanced situation is attenuated by such major rivers as the Niger, Nile and Zambezi, it means that some countries are dependent on their neighbours for their water – a situation that may in future lead to disputes over the allocation of these resources.

In recent geological time there have been marked climatic changes (as witnessed by fossil drainage networks and rock paintings), with sudden fluctuations in rainfall levels. In the Sahel countries the drought that has persisted since the late 1960s has greatly reduced the discharge rates of the main tropical rivers and has led to the drying up of Lake Chad almost completely (Sircoulon, 1987). In contrast, however, exceptionally heavy rainfall can also occur – as it did in 1961–64 in the Upper Nile and Congo basins – leading to an abrupt rise in the water levels of the East African lakes (Piper et al., 1986) and increasing the inflows from the White Nile to the Main Nile.