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Fresh water

earth science Maturity 9-11 Next article →

Fresh water is any naturally occurring liquid or frozen water with low salt concentrations. It contains low levels of dissolved salts and other total dissolved solids. Scientists often define fresh water as having less than 500 parts per million (ppm) of dissolved salts. Some sources use higher limits, such as 1,000 ppm or 3,000 ppm. This water is distinct from seawater and brackish water. It is a finite but renewable natural resource. It is essential for the survival of most living organisms on Earth.

Earth water distribution ppm chart
Earth water distribution ppm chart

The water cycle is the mechanism that replenishes fresh water. First, water from seas, lakes, forests, and rivers evaporates into the atmosphere. This water then forms clouds. Eventually, it returns to the land as precipitation. Precipitation includes rain, snow, hail, sleet, and graupel. Once it hits the ground, it becomes surface runoff or soaks into the earth. This process moves water through various stages to create inland bodies of water. This cycle ensures that fresh water is constantly moving and being replaced.

Fresh water exists in several distinct systems. Scientists classify fresh water habitats into three main types. Lentic systems are still waters, such as ponds, lakes, swamps, and mires. Lotic systems are running-water systems, like rivers and streams. Groundwater flows through rocks and aquifers beneath the surface. There is also a hyporheic zone that bridges these systems. This zone underlies many large rivers and connects them to groundwater. It can contain much more water than the open river channel itself.

Most of the Earth's fresh water is not easily accessible. Fresh water makes up less than 3% of the world's total water resources. Of that small amount, only 1% is readily available for use. About 70% of the world's fresh water reserves are frozen in Antarctica. This includes ice sheets, ice caps, glaciers, and icebergs. Surface water, such as lakes and rivers, makes up less than 0.01% of all fresh water. Much of the rest is stored as groundwater in the saturated zone. This is the area below the water table where all spaces are filled with water.

Earth's water distribution
Earth's water distribution

Humanity relies heavily on these limited resources. Agriculture is the largest consumer, using roughly two-thirds of all extracted fresh water. Humans also use water for industrial, household, and recreational activities. However, much of the fresh water on Earth is not potable. Potable water is water that is safe for humans to drink. Much of our surface and groundwater requires treatment to be safe. Without treatment, it may be unsuitable for human consumption. We must manage these supplies carefully to avoid water scarcity.

Fresh water can be impacted by both natural and human processes. Erosion is a natural process that can cause pollution. Human activities can also pollute water, reducing its availability. In some areas, people use "fossil water" from deep underground aquifers. These aquifers formed hundreds of thousands or millions of years ago. Because they are not appreciably replenished today, they are essentially non-renewable. This makes them similar to resources like peat or lignite. Other technologies like desalination can create fresh water from the sea. However, desalination is a niche application due to high capital and energy costs.

Understanding water distribution is vital for global sustainability. The United Nations has set goals to protect these resources. The Sustainable Development Goals include targets for clean water and sanitation. Target 6.4 aims to increase water-use efficiency by the year 2030. Target 15.1 focuses on the conservation of inland freshwater ecosystems. These include forests, wetlands, mountains, and drylands. Protecting these systems helps ensure that water remains available for all life.

Earth water distribution ppm chart
Earth water distribution ppm chart
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