Uneven distribution is also a factor. For example, Africa's Niger Delta receives significant quantities of rain, while the nearby Sahara Desert gets very little. According to GEO-2000, there are 50 African rivers that provide water to two or more countries. The potential exists for armed conflict when the demand for water from these rivers outgrows the supply. Even when there are adequate water supplies, the water is not necessarily safe to drink.
Sewage is a serious threat to water sources.
In developing areas, sewage systems are often primitive, and sewage flows into rivers. In developed areas, deteriorating sewage systems can lead to contamination of water supplies. Water quality is also affected by use of pesticides and fertilizers, and by industrial waste. A large unknown in the future of water resources is the impact of human activity on the climate, and how these climatic changes will affect water supplies. Rising temperatures affect evaporation-precipitation cycles.