In India, environmentalists have launched a campaign to revive an ancient water conservation method to meet growing scarcities of this crucial resource.

It's called rainwater harvesting, or as conservationists term it "catching the raindrops."

For centuries, it was a popular system in South Asia where the annual rainfall comes in intense spells of just 100 hours every year. To save this precious resource, villages, forts and palaces built networks of tanks and wells that stored virtually every drop.

The system is simple, slope the land towards a well or a pit, pave the surrounding area, and capture rain that would otherwise have been lost.

But water harvesting was mostly forgotten as the government began to harness rivers to build large dams, provided piped water to cities, and irrigation facilities to villages.

However, the country's steadily-growing population made it impossible to keep pace with demand. Rivers and lakes began to shrink. In the hot summer season, water supplies to cities and villages became a trickle. In rural areas, women started walking miles for a pot of water. In urban areas, people began to tap underground water.

Some years ago, environmentalists from a public policy institute, The Center for Science and Environment, undertook a survey to study the impact of growing water shortages and drought that had hit parts of the country. Eklavya Prasad from the center says experts came up with an unusual answer, tap into the wisdom of ancient civilizations to mitigate critical water shortages.

"We came across villages which were still water-sufficient after the third year of drought," said Mr. Prasad. "That's when things started ticking for us, and we thought why is it so, why are these villages still water-sufficient, and if you just come out of that village and go a kilometer off north, you will see a totally dry village there. And then we realized these are the villages which have been harvesting rainwater on a regular basis."

After intensive surveys and studies, engineers developed simple and inexpensive systems to gather rainwater in urban homes and villages. Experts call it a low technology and environmentally friendly solution. They say rainwater has the potential to meet most of the water shortages in the country.

In cities, pipelines carry the rainwater from sloped terraces or roofs to a storage facility. On unpaved surfaces, a system of channels diverts the water into the ground to replenish water tables that are dropping rapidly due to widespread use of bore wells. The stored water is used for gardens and toilets.

Savita Gokhale is one of several volunteers at the Center for Science and Environment promoting the system. She said water harvesting is vital for cities where paved surfaces prevent rainwater from percolating into the ground.

"It helps you to replenish the groundwater. And groundwater storing is the safest and the most economical way of storing it. Once you are storing it in the ground, it is not lost due to evaporation, it is not contaminated," Ms. Gokhale said.

The campaign to harvest rainwater is picking up gradually in several Indian cities. Non-government organizations are spreading awareness in villages. The federal government is installing the system on new government buildings, including the stately presidential palace in New Delhi. Several state governments are offering city residents incentives to adopt the system.

In New Delhi, a handful of schools and colleges have begun water harvesting, not only to beat water shortages, but also to raise awareness among young people about the need to conserve a resource that is fast running out.

One of them is Janaki Devi College. A college teacher, Aruna Ludra, explains how rainwater that once went to waste is now being stored. "On the right hand side there is a dead well. Earlier whenever the rain used to come the whole water from the hockey field, here, everywhere, the ridge, it used to come down and go past into the road. Now the water is going straight to the dead well," she said.

At Mira Model School, rainwater harvesting has provided sufficient water to grow grass on dry and dusty playfields. A teacher, Atul Bhalla, said a short spell of rain in May has helped them through water shortages, even though the monsoon season has been mostly dry.

"We were ready when it rained in the end of May. All my tanks were full at that time," Mr. Bhalla said.

Environmentalists say the need for traditional water storage systems will be highlighted this year when monsoon rains have been deficient in large parts of the country, and millions of people across India are battling acute water scarcities.