Rapid population growth, combined with industrialisation, urbanisation, and agricultural intensification and water intensive lifestyles is resulting in a global water crisis. In 2000, at least 1.1 billion of the world’s people – about one in five – did not have access to safe water. Asia contains 65 per cent of the population without safe water and Africa 28 per cent. During the 1990s, there were some positive developments: about 438 million people in developing countries gained access to safe water but due to rapid population growth, the number of urban dwellers lacking access to safe water increased by nearly 62 million.

Falling water tables are widespread and cause serious problems, both because they lead to water shortages and, in coastal areas, to salt intrusion. Both contamination of drinking water and nitrate and heavy metal pollution of rivers, lakes and reservoirs are common problems throughout the world. The world supply of freshwater cannot be increased. More and more people are becoming dependent on limited supplies of freshwater that are becoming more polluted. Water security, like food security, is becoming a major national and regional priority in many areas of the world.

Water is our most vital natural resource, supporting life and life support processes. While there is as much as 1,400 million km3 of water on earth, only one-hundredth of 1% of this amount is easily available for human use. The amount of water available for each person will continue to decrease as the world’s population expands. Unfortunately our present and future water supplies in many parts of the world are being degraded by pollution from domestic wastewater, solid waste, industrial effluent and agricultural drainage to name a few. As our natural waters become more polluted, less water is available for our needs and the needs of the natural environment. Every year, approximately 25 million people die, by either drinking polluted water or because they do not have enough water to meet their daily needs.

A single person needs at least half a liter (0.11 gallons) per day to meet basic survival needs and two liters (0.44 gallons) per day to avoid thirst. Some 27 to 200 liters (6 to 44 gallons) are needed per person per day for drinking, sanitation, bathing and cooking. Household water needs vary depending on the type of dwelling, number of residents and type of plumbing fixtures. (1 gallon=3.785ltrs) More important is for each one of us to learn as how to save even a drop of water.

For all the above problems, one of the best remedy & simple, cost effective solution is Rain water harvesting system, today, scarcity of good quality water has become a major cause of concern. However Rain water which is pure and of good quality is lost as runoff.

Rainwater is a free source of nearly pure water and rainwater harvesting refers to collection and storage of rainwater and other activities aimed at harvesting surface and groundwater. It also includes prevention of losses through evaporation and seepage and all other hydrological and engineering interventions, aimed at conservation and efficient utilisation of the limited water endowment of physiographic unit such as a watershed. In general, water harvesting is the activity of direct collection of rainwater. The rainwater collected can be stored for direct use or can be recharged into the ground water. Rain is the first form of water that we know in the hydrological cycle, hence is a primary source of water for us.

Rivers, lakes and groundwater are all secondary sources of water. In present times, we depend entirely on such Secondary sources of water. In the process, generally, it is forgotten that rain is the ultimate source that feeds all these secondary sources. Water harvesting means making optimum use of rainwater at the place where it falls so as to attain self-sufficiency in water supply, without being dependent on remote water sources.

Cities get lot of rain, yet cities have water shortage. Why? Because people living there have not reflected enough on the value of the raindrop. The annual rainfall over India is computed to be 1,170 mm (46 inches). This is higher compared to the global average of 800 mm (32 inches). However, this rainfall occurs during short spells of high intensity. Because of such intensities and short duration of heavy rain, most of the rain falling on the surface tends to flow away rapidly, leaving very little for the recharge of groundwater. This makes most parts of India experience lack of water even for domestic uses. Ironically,

Even Cherrapunji, India, which receives about 11,000 mm of rainfall annually, suffers from acute shortage of drinking water. This is because the rainwater is not conserved and is allowed to drain away. Thus it does not matter as to how much rain falls at a place, if it is not captured or harvested there for use. This highlights the need to implement measures to ensure that the rain falling over a region is tapped as fully as possible through water harvesting, either by recharging it into the groundwater aquifers or storing it for direct use.

Many urban centers in Asia and other regions are facing an ironical situation today. On the one hand there is an acute water scarcity and on the other, streets are generally flooded during rains. This has led to serious problems with quality and quantity of groundwater. One of the solutions to the urban water crisis is rainwater harvesting – Capturing the runoff. The advantage of Rainwater Harvesting is more where surface water is inadequate to meet our demand and exploitation of groundwater resource has resulted in decline in water levels in most part of the Country.

Bangalore (South India) Bangalore receives 970 mm rainfall annually and the number of rainy days is 60.highest amount of rainfall is received during April to November, while the rest of the months receive scanty rainfall. Peak runoff is 50 millimeters per hour. Due to the availability of rainwater throughout the year, water is basically stored in the rainwater harvesting systems and used for non-potable purposes. Water from the rooftops is led into storage structures. Providing an extra length of pipe to collect the polluted 2.5 mm of rainfall normally does the first flushing. Filters are made of sponge and a mixture of sand, gravel and charcoal. After first flushing and filtration water is led into underground sumps (which are very common in Bangalore) or to a new storage tank. The overflow from this tank is taken to an open well to recharge the aquifer.

The geological formations are predominantly granite and granitic gneiss, with joints and fractures in abundance due to intense chemical weathering of rocks. The depth of weathering varies from 0.2 m to 20 m. This geological set-up offers an immense scope for recharging of ground aquifers. The undulating terrain with gentle slopes draining into lakes offers an ideal situation for water harvesting. In the urban area of Bangalore water bodies cover about 5 per cent of land.

A study made by the Centre for Ecological Studies and Indian Institute of Sciences revealed that out of 262 lakes in 1960 only 82 exist now, of which less than 10 have water. Forty per cent of the city population is dependent on groundwater. The demand supply gap is met by groundwater exploitation. Even the surface water is pumped from Cauvery River flowing at a distance of 95 kilometers and about 500 meters below the city necessitating huge pumping costs and energy usage.

In Kolkata, India, about half the population that lives in the slum or squatter settlements collect water from stand posts. The rest of the slum population do not have access to the municipal water supply and have to make their own arrangements – for instance relying on hand pumps/drawing from tube wells. In Bangalore, India a city of some 6 million inhabitants, it is estimated that more than half depends on public fountains. Almost a third of the population has partial or no access to piped water.

To further illustrate, India’s population as per 2001 census is 1027.02 million. Over 60 per cent of households in India meet their drinking water requirements from underground water sources such as hand pumps, tube wells and wells. In urban areas while 68.7 per cent households use tap water, 29 per cent of the households directly use those underground water resources. Intense use of underground water has resulted in depletion of subterrene water resources in many parts of India.

The World population has more than doubled since 1950 and reached 6.15 billion in 2001. The most recent population forecasts from the United Nations indicate that, under a medium-fertility scenario, global population is likely to peak at about 8.9 billion in 2050.

In parallel with these demographic changes, there have been profound demographic shifts as people continue to migrate from rural to urban areas in search of work and new opportunities. The number of people living in urban areas has jumped from 750 million in 1950 to nearly 2.93 billion in 2001. Currently, some 61 million people are added to cities each year through rural to urban migration, natural increase within cities, and the transformation of villages into urban areas. By 2025, the total urban population is projected to increase to more than five billion, and 90 per cent of this increase is expected to occur in developing countries. Sixty per cent of the global population is living in Asia. Urban population growth in Asia at 2.7 per cent per annum is 27 per cent higher than the global average. Asia’s population living in urban areas is projected at 43.0 per cent for 2010 and will represent 50.8 per cent of world’s total urban population. Asia is expected to double its urban population by the year 2020. By 2025, the majority of this region’s population will live in cities. By 2015, there will be 153 cities of one million inhabitants, 22 cities with 8 or more million people and 15 with 10 to 20 million people.

Types of Rainwater Harvesting Systems

Typically, a rainwater harvesting system consists of three basic elements: the collection system, the conveyance system, and the storage system.Collection systems can vary from simple types within a household to bigger systems where a large catchment area contributes to an impounding reservoir from which water is either gravitated or pumped to water treatment plants. The categorization of rainwater harvesting systems depends on factors like the size and nature of the catchment areas and whether the systems are in urban or rural settings. Some of the systems are described below.