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Water occurs in three phases - solid, liquid (ice/snow) and gaseous (water vapour).
In liquid phase it is vital for the existence of life on this planet. In gaseous phase it is important for respiration in organisms and cycling, although the main reservoir pool of water is not the gaseous phase.Therefore, water cycle is considered separately as "Hydrological Cycle".
Importance of Water
i) Required for solubilization of chemicals and several biochemical reactions like hydrolytic digestion of polymeric nutrients, photosynthesis, etc.ii) Important for the working of macromolecules, as a good ionizer, transport of materials, etc.
iii) Acts as a habitat for hydropyhtic and aquatic animals and as an agent of geological change.iv) Acts as an agent of energy transfer and use. Water is a tremendous factor in neutralising heat radiations of sunlight, so also acts as a 'temperature buffer'.
Types of Water Cycle
There are two overlapping water cycles operating in nature.1) Global water cycle
2) Biological water cycle.Global water cycle does not involve life. Water evaporates from water bodies like seas and oceans (more than 75% of total evaporation), lakes and rivers. Condensation of vapour leads to precipitation in the form of rain, snow or hailstorm, which fall on the soil.
Most of the rainwater percolates into the soil, becomes a part of water table and seep into the ocean. Surface rain water and water formed by melting of ice and snow, flow into the rivers, streams and finally joins the sea.A large amount of water remains underground as well as in the form of perennial snow in the polar regions and mountain peaks above snow line.
Global water cycle is in 'steady state' as total precipitation is balanced by total evaporation.Biological water cycle involves the entry of water into living beings and then return to the physical environment. Underground water acts as a starting point.
Water Utilisation
a) Aquatic organisms take water from their surrounding.
b) Terrestrial plants absorb water from the soil through their roots.c) Terrestrial animals obtain water from the plants they consume or take in water directly from different water bodies.
Water Production
1) Animals release water into the atmosphere by respiration in the form of vapours or to soil as fluid excretion.
2) Water returns to the environment after the death and decay of organisms (plants and animals).3) A large part of water is given back to nature by plants, as they lose most of the absorbed water, by the process of transpiration.
Water vapour thus formed results in the formation of clouds, followed by rain and so enters the global water cycle. Thus, there is always the exchange of water between the global water cycle and biological water cycle.
