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2. Where and in what forms is water available on Earth?

  • 2.1 How does water move from the atmosphere to the ground and back?
  • 2.2 How much freshwater is found at the Earth’s surface?
  • 2.3 How much freshwater can be found underground?

The world’s water exists naturally in different forms and locations: in the air, on the surface, below the ground and in the oceans.

Just 2.5% of the Earth’s water is freshwater, and most is frozen in glaciers and ice sheets. About 96% of all liquid freshwater can be found underground. The remaining small fraction is on the surface or in the air.

Knowing how water cycles through the environment can help in determining how much water is available in different parts of the world. The Earth’s water cycle is the global mechanism by which water moves from the air to the Earth (precipitation) and eventually back to the atmosphere (evaporation).

The principal natural components of this cycle are precipitation, infiltration into the soil, runoff on the surface, groundwater discharge to surface waters and the oceans, and evapotranspiration from water bodies, the soil, and plants.

“Blue water”— the water in rivers, lakes, and aquifers— can be distinguished from “green water” — which feeds plants and crops, and which is subsequently released into the air. This distinction may help managers focus on those areas which green water feeds and passes through, such as farms, forests, and wetlands. More...

2.1 How does water move from the atmosphere to the ground and back?

2.1.1 About 10% of the Earth’s freshwater that is neither frozen nor underground is found in the atmosphere. Precipitation, in the form of rain or snow, for instance, is an important form of available freshwater. About 40% of precipitation has previously evaporated from the oceans; the rest from land. The amount of precipitation varies greatly around the world, from less than 100 mm a year in desert climates to over 3 400 mm a year in tropical settings.

In temperate climates, about a third of precipitation returns to the atmosphere through evaporation, a third filters into the ground and replenishes groundwater and the remainder flows into water bodies. The drier the climate, the higher the proportion of precipitation that returns to the atmosphere and the lower the proportion that replenishes groundwater (see table 4.1). More...

Table 4.1 Precipitation distribution into surface water and groundwater components

2.1.2 A large part of the freshwater that returns to the atmosphere passes through soil and plants. Reliable figures are available only for some regions. Soil moisture is important for plant growth. Finding out how much moisture soil contains is important for such activities as farming and “river-flow forecasting”, and for understanding climate and natural and water systems. Satellite data are increasingly complementing measurements of soil moisture taken on the ground to provide a broader and more up-to-date picture to decision-makers. More...

2.2 How much freshwater is found at the Earth’s surface?

2.2.1 About three-quarters of the world’s freshwater is frozen in ice sheets and glaciers. Most remains inaccessible, located in the Arctic, Antarctica or Greenland. Land-based glaciers and permanent snow and ice, however, supply water in many countries, releasing water in amounts that vary seasonally and over longer time periods. Because of climate change, glaciers are now being more closely monitored. More...

2.2.2 Surface waters, including lakes, ponds, reservoirs, rivers, streams and wetlands hold only a small volume of the Earth’s total fresh water (0.3%). Still they represent about 80% of the renewable surface water and groundwater that is available in a given year. These water bodies perform many functions in the environment, and provide people with the prime source of drinking water, energy and recreation, as well as a means of irrigation and transport.

Lakes and other reservoirs counteract fluctuations in river flow from one season to the next because they store large amounts of water. Lakes contain by far the largest amount of fresh surface water. But the hydrology of only about 60% of the largest lakes has been studied in detail, leaving much to be learned. More...

2.2.3 River basins are a useful “natural unit” for the management of water resources, though they often extend across national borders. International river basins have drainage areas covering about 45% of the Earth’s land surface (excluding the polar regions). Some of the largest basins are the Amazon, which carries 15% of all water returning to the oceans, and the Congo-Zaire Basin, which carries one-third of all river water in Africa.

River flows can vary greatly from one season to the next and from one climatic region to another. In tropical regions, large flows are witnessed year round, whereas in drylands, rivers are often ephemeral and only flow periodically after a storm. Drylands make up about 40% of the world’s land area and have only 2% of all water runoff.

Past data records for river flow and water levels help to predict yearly or seasonal variations, though it is difficult to make accurate longer-term forecasts. Some records in industrialised countries go back up 150 to 200 years. By contrast, many developing countries started keeping records only recently and data quality is often poor. More...

2.2.4 Wetlands, including swamps, bogs, marshes, and lagoons, cover 6% of the world’s land surface and play a critical role in the conservation of water resources. Many wetlands were destroyed or converted to other uses during the last century. Those that remain can play an important role in supporting ecosystems, preventing floods, and increasing river flows. More...

2.3 How much freshwater can be found underground?

Ninety-six percent of liquid fresh water can be found underground. Groundwater feeds springs and streams, supports wetlands, helps keep land surfaces stable, and is a critical water resource.

About 60% of the water that is taken from the ground is used for farming in arid and semi-arid climates, and between 25% and 40% of the world’s drinking water comes from underground. Hundreds of cities around the world, including half of the very largest, make significant use of groundwater. This water can be especially useful during shortages of surface water.

Groundwater aquifers vary in terms of how much water they hold, their depth, and how quickly they replenish themselves. The variations also depend on specific geological features.

Much of the water underground is replenished either very slowly or not at all, and is thus termed “non-renewable”. The largest aquifers of non-renewable water are found in North Africa, the Middle East, Australia, and Siberia. There is some debate about how and when to use this water. Many aquifers that contain non‑renewable groundwater resources are shared by more than one country and need to be managed in common for the benefit of all administrative entities concerned.

If the infiltration of precipitation recharges the aquifer, the groundwater is considered “renewable” and can be used for irrigation, domestic and other purposes. While most renewable groundwater is of a high quality and does not require treatment, it should be analysed before it is used to avoid possible health impacts. However, few countries measure the quality of underground water or the rate at which it is being withdrawn. Monitoring is being improved in Europe and India, but remains minimal in many developing countries, and is deteriorating in many industrialised ones. This makes it hard to manage underground water resources sustainably. More...