Water resources are sources of water that are useful or potentially useful to
humans. It is important because it is needed for life to exist. Many uses of
water include agricultural, industrial, household, recreational and
environmental activities. Virtually all of these human uses require fresh water.
Only 3% of water on the Earth is fresh water, and over two thirds of this is
frozen in glaciers and polar ice caps. Water demand already exceeds supply in
many parts of the world, and many more areas are expected to experience this
imbalance in the near future. The framework for allocating water resources to
water users (where such a framework exists) is known as water rights.
Sources of fresh water
Surface water is water in a river, lake or fresh water wetland. Surface water
is naturally replenished by precipitation and naturally lost through discharge
to the oceans, evaporation and sub-surface seepage. Although the only natural
input to any surface water system is precipitation within its watershed, the
total quantity of water in that system at any given time is also dependent on
many other factors. These factors include storage capacity in lakes, wetlands
and artificial reservoirs, the permeability of the soil beneath these storage
bodies, the runoff characteristics of the land in the watershed, the timing of
the precipitation and local evaporation rates. All of these factors also affect
the proportions of water lost through discharge to the oceans, evaporation and
Human activities can have a large impact on these factors. Humans often
increase storage capacity by constructing reservoirs and decrease it by draining
wetlands. Humans often increase runoff quantities and velocities by paving areas
and channelizing stream flow. The total quantity of water available at any given
time is an important consideration. Some human water users have an intermittent
need for water. For example, many farms require large quantities of water in the
spring, and no water at all in the winter. To supply such a farm with water, a
surface water system may require a large storage capacity to collect water
throughout the year and release it in a short period of time. Other users have a
continuous need for water, such as a power plant that requires water for
cooling. To supply such a power plant with water, a surface water system only
needs enough storage capacity to fill in when average stream flow is below the
power plant's need.
Nevertheless, over the long term the average rate of precipitation within a
watershed is the upper bound for average consumption of natural surface water
from that watershed. Natural surface water can be augmented by importing surface
water from another watershed through a canal or pipeline. It can also be
artificially augmented from any of the other sources listed here, however in
practice the quantities are negligible. Humans can also cause surface water to
be "lost" (i.e. become unusable) through pollution.
Sub-Surface water, or groundwater, is fresh water located in the pore space
of soil and rocks. It is also water that is flowing within aquifers below the
water table. Sometimes it is useful to make a distinction between sub-surface
water that is closely associated with surface water and deep sub-surface water
in an aquifer (sometimes called "fossil water"). Sub-surface water can be
thought of in the same terms as surface water: inputs, outputs and storage. The
critical difference is that for sub-surface water, storage is generally much
larger compared to inputs than it is for surface water. This difference makes it
easy for humans to use sub-surface water unsustainably for a long time without
severe consequences. Nevertheless, over the long term the average rate of
seepage above a sub-surface water source is the upper bound for average
consumption of water from that source.
The natural input to sub-surface water is seepage from surface water. The
natural outputs from sub-surface water are springs and seepage to the oceans.
If the surface water source is also subject to substantial evaporation, a
sub-surface water source may become saline. This situation can occur naturally
under endorheic bodies of water, or artificially under irrigated farmland. In
coastal areas, human use of a sub-surface water source may cause the direction
of seepage to ocean to reverse which can also cause salinization. Humans can
also cause sub-surface water to be "lost" (i.e. become unusable) through
pollution. Humans can increase the input to a sub-surface water source by
building reservoirs or detention ponds. Water in the ground are in sections
called aquifers. Rain rolls down and comes into these. Normally an aquifer is
near to the equilibrium in its water content. The water content of an aquifier
normally depends on the grain sizes. This means that the rate of extraction may
be limited by poor permiability.
Desalination is an artificial process by which saline water (generally ocean
water) is converted to fresh water. The most common desalinization processes are
distillation and reverse osmosis. Desalinization is currently very expensive
compared to most alternative sources of water, and only a very small fraction of
total human use is satisfied by desalination. It is only economically practical
for high-valued uses (such as household and industrial uses) in arid areas. The
most extensive use is in the Persian Gulf.
Several schemes have been proposed to make use of
icebergs as a
water source, however to date this has only been done for novelty purposes.
Glacier runoff is considered to be surface water.
Water Filters -