The amount of water in the earth's environment never changes,
whether it is as a liquid (fresh water, seawater, rain, tiny
droplets in clouds), as a gas (water vapour) or in its solid state
(snow, ice or hail). There is also water inside living organisms.
Water continually circulates between the land, the oceans and the
atmosphere. This circulation is called the Water Cycle or Hydrologic
the water cycle work?
Heat from the sun causes water to become a gas, or vapour. This
is called evaporation. As the earth's surface warms, rising currents
of air carry the water vapour upwards. The water vapour becomes
cooler as it rises, and condenses into tiny drops, forming clouds.
These drops join together and fall back to the earth as rain, hail
- evaporate directly from water,
land or vegetation
- run off the land into streams
- soak a little way into the
ground, be absorbed by plant roots and then return to the water
vapour in the air by evapotranspiration from the leaves
- soak deeper into the ground
and add to the groundwater, moving slowly along the direction of
groundwater flow towards rivers, wetlands or the sea.
This cycle has existed since water was formed on earth, but human
activities change the way water moves through the landscape.
The Natural Water Cycle
The water present
on earth today is the same water that was present during the
time of the dinosaurs.
In a continuous
process called the hydrologic cycle, water evaporates into the
atmosphere from lakes, rivers, oceans and plants, forms clouds,
and then returns to earth in the form of rain, snow, sleet and
sometimes known as the universal solvent because it tends to
dissolve small amounts of almost any substance it touches.
As it falls to
earth, it may dissolve impurities it comes in contact with from
On earth, it may
dissolve surface materials, and as it seeps down through layers
of rock and soil, it may dissolve additional substances, such as
minerals like calcium and iron.
substances such as herbicides, insecticides, TCB's and TCE's may
enter the picture. Besides the many sources of industrial and
agricultural pollution, water may also pick up contaminants like
lead, sand, or rust particles in its journey from wells or
through municipal and residential distribution systems.
Over the last 100
years, the earth's population has nearly tripled. During this
time, we have discovered nuclear energy, synthetic chemicals,
and cryptosporidium, among other things.