All life on Earth depends on water, a vital natural resource. Natural resources—such as water, air, plants, wildlife, soil, and even 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 freshwater has historically determined where civilizations began and thrived. Freshwater exists on Earth’s surface in lakes, rivers, and ice, as well as below the surface as groundwater. However, it is a limited resource; freshwater makes up only about three percent of all water on Earth.

Although freshwater is considered a renewable resource, the use of freshwater in some regions exceeds the ability of natural processes to replenish supplies. When the demand for freshwater 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 freshwater as areas with cool, wet climates. North Africa and the Middle East, which are primarily desert environments (according to the Koppen Classification System), 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 freshwater 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.