The amount of water currently on the Earth is essentially constant, but it is always moving around in a complex cycle that takes it on multiple routes between the atmosphere, underground, glaciers, and surface waters such as lakes, oceans and rivers.
For example, the Sun heats the oceans causing liquid water to evaporate and form clouds, which may deposit their water over land as rain. The rainwater takes multiple routes: some enters rivers and runs back into the oceans, another portion will evaporate back into the atmosphere, while yet another amount will seep into the ground and enter aquifers - water bearing rock formations below the Earth's surface.
Most of the Earth's fresh water is in the form of ice – glaciers and ice sheets.
Image: Artwork illustrating the water cycle (credit: Gary Hincks/SPL)
Bizarre and ancient life forms in the freezing depths of the world's deepest lake.
The cold here could kill someone in under a minute. The cameraman wore a dry suit and poured boiling water over his regulator before diving to stop it freezing. The seals are extremely shy, so the cameraman had to position remote cameras along a predicted route from their ice lairs. These are the first images of the Baikal seals under the ice.
Iain Stewart shows how the ancient Garamantes flourished in the Sahara desert.
Professor Iain Stewart shows how an ancient group of people called the Garamantes exploited water below the ground, known as groundwater, to flourish in the Sahara desert. The water they were using came from a time when the Saharan climate was wet.
Iain Stewart paraglides over a waterfall on a journey through the water cycle.
Professor Iain Stewart paraglides over a waterfall on a journey through the water cycle, which constantly circulates water around our planet.
The water cycle, also known as the hydrological cycle, describes the continuous movement of water on, above and below the surface of the Earth. The mass of water on Earth remains fairly constant over time but the partitioning of the water into the major reservoirs of ice, fresh water, saline water and atmospheric water is variable depending on a wide range of climatic variables. The water moves from one reservoir to another, such as from river to ocean, or from the ocean to the atmosphere, by the physical processes of evaporation, condensation, precipitation, infiltration, surface runoff, and subsurface flow. In doing so, the water goes through different phases: liquid, solid (ice) and vapor.
The water cycle involves the exchange of energy, which leads to temperature changes. For instance, when water evaporates, it takes up energy from its surroundings and cools the environment. When it condenses, it releases energy and warms the environment. These heat exchanges influence climate.
The evaporative phase of the cycle purifies water which then replenishes the land with freshwater. The flow of liquid water and ice transports minerals across the globe. It is also involved in reshaping the geological features of the Earth, through processes including erosion and sedimentation. The water cycle is also essential for the maintenance of most life and ecosystems on the planet.