Water is indispensable for human, animals and plant life. It is a part of all organisms, some of which contain more than 90 percent. Water is essential part of protoplasm. It is an important ingredient in photosynthesis. About 400 to 500 liters of water is necessary for production of a one kilogram of plant dry matter. Water is also required for translocation of nutrient and dissipation of heat.
Properties of water:
Water molecule contains two hydrogen ions and one oxygen ions. The space occupied by each water molecule is mainly due to oxygen ions while two hydrogen Ions do not occupy practically any space. The shape of the water molecule is sphere and the position of two hydrogen ions is at the corners of a tetrahedron that exists within a sphere.
The positive valences of hydrogen ions are partially neutralized by negative valency of oxygen ion. Thus, one, end of water molecule has positive charge and another end has negative charge. This makes water molecules a dipole.
Water molecules do not exist in individually. Hydrogen in water serves as connecting link from one molecule to the other and it is known as hydrogen bonding. Water sticks to it self with great energy and this property is called cohesion, where as water attaches itself to surface of many substances and this property is known as adhesion. By adhesion, water is held tightly at the soil water interface and water is retained in the soil by adhesion and cohesion. The water molecules hold other water molecules by cohesion forces. Because of these forces, water fills small pores in the soil and is in fairly thick film in large pores.
The earth’s outer solid crust is called Lithosphere. Most of the earth’s total water is contained in Oceans (96%), small portion (2%) as snow and ice and rest (2%) in the water bodies of the continents. Oceans, lakes, rivers and other water bodies of the Earth are called Hydrosphere.
Continuous circulation of water between hydrosphere, atmosphere (atm) and lithosphere is known as hydrological cycle. This has neither a beginning nor an end.
The physical and biological processes in the environment are sustained by the Hydrological cycle. Water from various water bodies evaporates due to energy provided by solar radiation and enters the atmosphere as water vapour. Oceans account for 85% of worldwide evaporation. Evaporation is the chief source of water vapour in atmosphere. Water vapour in the atmosphere constitutes only 0.001 percent of the total global water. Even if all water vapour present in the atmosphere at any amount could be precipitated, an average depth of only about 2.5 cm of water is added to the oceans. Though the quantity of water vapour present in the atmosphere is small, it provides vital hydrological link between oceans land.
Clouds are formed as the water vapour rises above in to the atmosphere. When condensation takes place in the atmosphere, water precipitates mainly as rain or to some extent as snow. Thus, water is constantly added to the atmosphere through evaporation and lost through precipitation. The annual average world precipitation is
1000 mm. As Oceans, occupy 2/3 of the total surface of earth, most of the precipitation that falls over oceans. Of the precipitation that falls over continents, about 65% is returned to Atmosphere through evapo-transpiration and the rest goes as a surface run off into the rivers and finally into the oceans. Thus, water occurs on earth in three forms viz solid, liquid and gaseous.
Water resources of the world:
About 97 percent of worlds in the oceans and this are not useful for irrigation. Of The total quantity of water, only 2.6 percent is fresh water, which is in the form of ice caps, icebergs and glaciers and only small fraction of water is present in the ground, rivers and atmosphere that can be harvested for irrigation of crops.
Water resources of India:
The average rainfall of India is 1194 mm. When considered over geographical area of 328 million hectares, this rainfall amounts to 392 million hectare meters (m. ha. m). This may round off to 400 (m. ha. m) by including the contribution of snowfall which is not yet fully determined. Out of 400 (m. ha. m) of rainfall, 75% is received during South-West Monsoon period (June to September) and rest in remaining months as shown below. A Major portion of water (215 m. ha. m) soaks into the soil, while 70 (m. ha. m) is lost as an evapo-transpiration.