The water cycle is the continuous distribution of water on the global Earth. Water moves dynamically along Earth's water reservoirs, such as rivers, lakes, oceans and atmosphere, by processes, including condensation, evaporation, precipitation, runoff and filtration. In this dynamic movement, water changes states from a solid to a liquid and from a liquid to a gas. The water cycle includes the processes that drive the movement of water across the hydrosphere, thus, the water cycle is an endless cycle that helps maintain life and ecosystems of the world.
Evaporation and precipitation
The principal exchange process between the surface of the Earth and the atmosphere occurs by evaporation and precipitation. Water that is evaporated from surface water reservoirs, such as rivers, lakes, and the ocean, along with plants and soil, is returned to the atmosphere in the form of water vapor. In the atmosphere,cooler temperatures turn water vapor to condense into clouds. These clouds are then moved by air currents across the Earth. The water vapor, then, falls back to the surface of the Earth as precipitation on land or the ocean or as hail or snow, accumulating in ice caps and glaciers.
Importance of the water cycle
Rainfall water may fall on mountainous valleys, forming lakes and running down through rivers. These currents of water may infiltrate to the underground, replenishing underground aquifers and forming fresh water springs. Water runoff may evaporate into the air or move from one reservoir of water into another by infiltration. Water may also be used for agricultural purposes by man. Eventually all this water reaches the ocean, where it starts a new water cycle. Due to the close relation between global energy and the carbon cycle, the water cycle is of great importance to the Earth's ecological system.
How the water cycle transforms the Earth
The patterns of global water distribution determine the features that are characteristic to the various ecosystems of the world, such as tropical rainforests, temperate forests and deserts, among others. Water helps at transporting minerals and nutrients across the globe. It purifies water by filtration and replenishes land water reservoirs with freshwater. Water also reshapes the Earth geologically by processes, including erosion and sedimentation. The water cycle links the ocean to the Earth's water reservoirs through evaporation and precipitation and influences biodiversity on Earth.
The time that water spends on the surface of the Earth depends on the type of water reservoir. Water that accumulates on the Earth's soil remains only very briefly due to evaporation or transpiration. The time that evaporated water remains in the atmosphere is approximately 9 days. Underground water can remain there for over 10,000 years before coming to the surface of the Earth. The ice sheets of Greenland and Antarctica can store ice for about 800,000 years. Scientists estimate water residence by dividing the volume of water in a reservoir by the rate at which water enters or leaves the reservoir.
The ocean and climate
The ocean contains 97% of the world' total volume of water. The rest of the water is contained within the polar ice caps, glaciers, lakes, rivers, soil, underground and the atmosphere. It has been estimated that the world's oceans contribute about 90% of the water that is included in the water cycle. The ocean is a huge storage of heat. When water evaporates from the ocean, a lot of heat is released in the process. The transfer of energy between the ocean and atmosphere creates winds and ocean currents,
Influencing the world's global climate.
It has been proved that the water variability around the world and climate change will have a profound effect in the water cycle. Studies demonstrate that some dry regions of the world will become drier and wet regions will turn wetter due to global warming. Human activities that are known to alter the water cycle include deforestation, agriculture, urbanization, industry and CO2 emissions into the atmosphere, among others. According to climate.nasa.gov, slight changes in atmospheric and ocean contents can have significant effects on the world's global climate.