Water is essential for human survival. Life without water is unthinkable. The total quantity of available water in the World is constant. According to a report of the United Nations Environmental Program, more than 97% of the available water is in the form of sea water while the quantity of fresh water is less than 3%.
Out of this fresh water, 77% is ice locked away in the glaciers and the polar ice caps and 22% is ground water.
This leaves less than 1% of fresh water supply to take part in the hydrological cycle, about half of which is found in rivers, lakes and swamps. Yet, on the global scale, availability of fresh water is more than enough to meet the demand in the present and the foreseeable future.
But this water is not available every where, all the time or in proper quality and form.
As a result, scarcity and pollution of water have led to a situation where at least 20% of the population living in Third World's cities and 75% of its rural population do not have access to reasonably safe supplies of water even for drinking.
Most parts of the country receive enough rainfall. A large portion of rain water flows to rivers, lakes, ponds and seas; some of it gets evaporated or seeps ' into the ground. During this course rain water gets contaminated with impurities. In most hilly regions there is abundant rainfall, but the people face acute shortage of drinking water as most of the rain water flows down to the plains. If we can tap rain water before it reaches the ground, the problem of providing drinking water in hills can be easily solved with least expenditure. The water so collected can be stored in safe water storage structures like tanks made of ferrocement, rainforced cement concrete, or hy-galvanized iron sheet.
Rain water is the purest form of raw water available on earth. Rain water harvesting is the process of collection of rain water from rain receiving surfaces like building roof tops, courtyards, playgrounds, hill slopes, places of worship, or specially made mini-catchment surfaces. The water is collected in storage tanks installed above ground, partly above and partly under ground, or fully under ground depending on the local situation. Since the water is collected before it comes in contact with any contaminated surface, it remains free from physical, chemical and fecal contamination. For removing suspended impurities, if any, the water is collected in reservoirs.
Rain water harvesting is essential because:
Surface water is inadequate to meet our demand and we have to depend on ground water.
Due to rapid urbanization, infiltration of rain water into the sub-soil has decreased drastically and recharging of ground water has diminished.
Serious consideration is required to manage water sources by using rain water and artificial recharging. Several dozen installations successfully operating in India constructed and maintained by CGWD.