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
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
is the global mechanism by which water moves from the air to the
Earth (precipitation) and eventually back to the
The principal natural components of this cycle are
precipitation, infiltration into the soil, runoff on the
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
2.1 How does water move from the atmosphere to the ground and back?
About 10% of the Earth’s
freshwater that is neither
frozen nor underground is found in the
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).
Table 4.1 Precipitation distribution into surface water and
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.
2.2 How much freshwater is found at the Earth’s surface?
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.
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.
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
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.
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
preventing floods, and increasing river flows.
2.3 How much freshwater can be found underground?
Ninety-six percent of liquid
fresh water can be found
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.
aquifers vary in terms of
how much water they hold, their depth, and how quickly they
replenish themselves. The variations also depend on specific
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
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