Water cycle is another important material cycle. Water is one of the important substances necessary for life. On average water constitutes 70% of the body weight of an organism. It is an important ecological factor that determines the structure and function of the ecosystem. Cycling of all other elements is also dependent upon water as it provides the transportation to different compartments and is also a solvent medium for their uptake by organisms. It is needed along with carbon dioxide in photosynthesis and has moderating effect on the temperature of the surrounding areas by virtue of its heat absorbing ability.
Protoplasm, the physical basis life, is made up of 85-90% water. Human blood, too, contains 90% water. However, significant amounts of water are incorporated by the ecosystem in protoplasm synthesis and a substantial return to the atmosphere occurs by way of transpiration from living plants and evaporation from animals.
Water covers about 75% of the earth’s surface, occurring in lakes, rivers, seas and oceans. The oceans alone contain 97% of all the water on earth. Much of the remainder is frozen in the polar ice and glaciers. Less than 1% water is present in the form of ice-free fresh water I rivers, lakes, and aquifers. Yet this relatively negligible portion of the planet’s water is crucially important to all forms of terrestrial and aquatic life. There is also underground supply of water. Soils near the surface also serve as reservoir for enormous quantities of water.
The earth’s supply of water is stable and water is used over and over again. About one third of all solar energy is dissipated in driving the water cycle. Sunshine evaporates water from the oceans, lakes and streams, from the moist soil surfaces and from bodies of living organisms. Water vapor gathers in the form of clouds, which move with the winds over the earth’s surface. After cooling and condensation, water falls in the form of rains or snow. This constant movement of water from the earth into the atmosphere and back is known as the water cycle. Some of the water, which falls on the land percolates through the soil until it, reaches a zone of saturation. Below the zone of saturation is a solid rock through which water cannot percolate. The upper surface of this zone of saturation is known as the water table. The extra water runs off in the form of streams which coverage and joins to form rivers. Finally, water is returned to the ocean.