All life on Earth depends on water, a vital natural resource. Natural resources—such as water, air, plants, wildlife, soil, and fossil fuels—are used by humans for the basic necessities of life, including food, drinking water, energy, and shelter. As a crucial resource for human life, access to fresh water has historically determined where civilizations began and thrived. Fresh water exists on Earth’s surface in lakes, rivers, and ice, as well as below the surface as groundwater. However, it is a limited resource; fresh water makes up only about three percent of all water on Earth.

Although fresh water is considered a renewable resource, the use of fresh water in some regions exceeds the ability of natural processes to replenish supplies. When the demand for fresh water cannot be met, it can lead to political tension and public-health problems. Distribution issues may arise when freshwater supplies, such as lakes, cross political boundaries. They can also occur when human activities upstream on a river adversely affect communities living downstream. Those activities can include dumping pollutants into the river or diverting large amounts of water away from where the water typically flows.

Ecosystems vary in the amount of water they hold. Places with hot and dry climates, like deserts, do not have as much fresh water as areas with cool, wet climates. North Africa and the Middle East, which are primarily desert environments, are two areas most affected by water shortages. Even in a water-rich ecosystem, such as one along a major river, the amount of available fresh water can be limited. Factors affecting water supplies in these areas include the amount of precipitation, as well as human activities, such as industry and farming, that can degrade water quality.