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domingo, 20 de junio de 2010

Rainwater Harvesting—Ancient and Modern

Rainwater Harvesting—Ancient and Modern


FOR millenniums the same water has been recycled, following the natural pattern of evaporation from earth and sea, cloud formation, and precipitation. This economical system provides more than enough water for everyone on earth. Why, then, are there tremendous water problems plaguing mankind? What solutions are possible? In answer, let us look at the water situation in India.

With a population of over one billion, India finds that its water resources are being stretched to critical limits. Where does India’s water come from? In the far north, rivers are fed in springtime by melting snows and glaciers in the Himalaya Mountains. But most of the Indian subcontinent depends on annual monsoon rains to soak the dry earth, fill wells and lakes, and swell the mighty rivers crisscrossing the country. The monsoon in India is temperamental and is described as “one of the most maddeningly elusive phenomena,” which, “despite all the advances in modern technology, from satellites to powerful supercomputers . . . , can be surprisingly difficult to predict.”

A normal monsoon season lasts from three to four months, but rather than fall steadily over the entire period, much of the rain falls in brief spells in the form of torrential downpours. As a result, dams fill up and water has to be released. Rivers overflow their banks, flooding fields and homes. Because modern industrialization and urbanization have resulted in widespread deforestation, there are often not enough trees to hold the precious rain in their roots and let it gradually soak into the earth. So the torrents wash away topsoil, denuding the land. Silt builds up in lakes and ponds, making them shallow, so that they hold less water. For all practical purposes, a large portion of the precious rainwater is lost.

Then monsoon season is over. For the rest of the year, there is sunshine every day, bringing months of scorching heat! Quickly the land dries up, and the fields bake and crack. Raging rivers become trickles in wide, sandy riverbeds. Waterfalls disappear. Bore wells are sunk deeper and deeper to bring up water from underground aquifers, and the water table is lowered. When rainfall is poor, drought sets in, crops fail, cattle die, and villagers migrate to cities, adding to urban water problems.

But it was not always this way. In ancient times people throughout the length and breadth of India learned that it was best not to depend solely on the rivers and lakes, which would dry up when the monsoon rains were over. They developed the art of catching the rain where it fell, using it for immediate needs, and storing it for when the rain stopped falling. This was rainwater harvesting.
The Need for Rainwater Harvesting Today

One would imagine that with modern technology and the building of huge dams, barrages, and irrigation canals—all of which are abundant in India—there would be little if any interest in ancient systems of water catchment. After all, most of those methods fell into disuse when people could get water from a tap in their homes or villages. But there is reason for concern. Megaprojects in water management over the past 50 years have been unable to keep up with burgeoning population needs and the rapid change from an almost totally agricultural society to an industrialized one. Not enough water has been saved to satisfy the country’s thirst.

Now environmentalists and concerned authorities feel the need to promote personal involvement in water conservation. Rainwater harvesting is being encouraged in homes, factories, schools, and anywhere that water storage on a small scale is practical. Why, many cities and states have made it compulsory for new buildings to have rainwater harvesting facilities!

Millions of gallons of rainwater falls in areas where it is not channeled into storage but simply evaporates or runs off into the sea. In rainwater harvesting, though—which is based on the concept of catching water where it falls—water is saved by individuals. And unlike water from dams and canals, which has to be paid for and is thus an expensive burden for the poor, this water is free!
Taking the Lead

Many concerned people in India are therefore getting involved in water conservation. Some have been recognized internationally with awards, as was the case with Rajendra Singh, who received the prestigious Magsaysay Award for community development in 2001. Working through a nongovernmental organization, which he had established, Singh revived the almost extinct Aravari River in the state of Rajasthan, a boon to a state with 8 percent of the country’s population and only 1 percent of its water resources. Over a period of 15 years, Singh’s group planted trees and built 3,500 tanks—traditional johads for harvesting water—bringing prosperity to local villagers. Others work at saving water, unnoticed by most, but with the satisfaction of knowing that they are helping out.

Industrialists are seeing the benefits of rainwater harvesting to supplement city water supplies. In a factory on the outskirts of Bangalore, in south India, a simple, inexpensive system of roofwater harvesting was set up. Rainwater, which formerly poured onto the highway and was lost, is channeled into a tank with a capacity of 10,000 gallons [42,000 liters]. During monsoon, 1,300 gallons [6,000 liters] a day of harvested water is processed for cleaning the food vessels and the factory canteen. No water is drawn for these purposes from the city supplies.

‘A drop in the bucket,’ you might say. But imagine you have a bank account to which money is added once a year. For your daily needs, you have to withdraw from your account, but gradually you take out more than is put in. One day you will have an overdraft. If, however, for a few months of the year, you get work with more than sufficient pay to cover your daily needs, your account will have a chance to build up. Now apply this principle to water conservation. Multiply your small savings millions of times, and what do you have? Replenished water sources, a raising of the water table, a filling up of aquifers, and a supply of water to fall back on when the “pay” in the form of harvested rainwater ends. Remember, available water is limited; there can be no overdraft when no water is available.
Reviving Ancient Methods of Harvesting Rainwater

ROOF WATER: Simple and inexpensive. Slightly sloping roofs allow water to run into gutters, down pipes, and into specially prepared drums. Filters of wire mesh, sand, gravel, and charcoal clean the water. It is funneled into underground sumps or aboveground tanks. Tanks are sealed to keep out air, sunlight, and organic matter; treatment with alum reduces turbidity; and bleaching powder kills bacteria. This water can be used for gardening, flushing toilets, washing clothes. Further treatment can make it potable. Excess water can be collected into wells or directed underground to replenish the water table. Most popular urban method.

NAULAS: Stone walls built across a stream to dam the water. Shady trees planted at the sides cut down evaporation, and medicinal plants thrown into the small reservoirs purify the water.

PERCOLATION TANKS, RAPATS: Small tanks built on sandy or rocky soil to store rainwater. Some of the water is used, but the remainder percolates through to aquifers, which replenish wells.

BHANDARAS: Underground tanks built to intercept water from springs, channeling it to storage tanks for city use.

QANATS: Vertical shafts in hilly areas to catch rainwater. The water is collected in underground channels that carry it by gravity over long distances to storage wells.

INTEGRATED TANKS: Tanks that overflow into a series of lower tanks to catch the rainwater that is funneled through gutters.

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