The world's next major crisis will be a lack of water for home use, including drinking water, many scientists predict. Humans can survive around 40 days without food, but much less than that without water to drink.
The scarcity of water for domestic use is becoming a critical problem, especially in rural parts of developing countries. Surface water in rivers, streams or lakes, and groundwater, are increasingly becoming contaminated with pollutants from factories, households, farms and mines. Wells dug deeper to extract groundwater are drying up. 
To meet the water demands of an exploding population, it is time to look up to the sky for the solution: harvesting rainwater as it falls.
As well as for drinking, rainwater serves various needs. It can be used domestically, for example to wash clothes, flush toilets and to water plants, and in the community, for instance in firefighting or to clean public places such as markets, and for agriculture.
If properly done, "rainwater harvesting appears to be one of the most promising alternatives for supplying freshwater in the face of increasing water scarcity and escalating demand", according to the UN Environment Programme.  Water catchments, whether it is just small ponds or large dams, can also be used for flood control.
Updating an ancient practice
Harvesting rain for domestic use has age-old roots. Ancient Romans used their villa courtyards to collect rainwater that was then stored in large underground cisterns.
Rainwater harvesting in Asia can be traced back to about the ninth century, when the small-scale collection of rain from roofs and simple dams began in rural parts of South and South-East Asia.
Today, rainwater harvesting is commonly practised in parts of East Africa, central Australia and Central America, as well as in Japan, Mexico, Singapore and Thailand, among others.
Countries in South-East Asia and the Pacific enjoy abundant rainfall spread fairly evenly throughout the year, albeit with peaks during the monsoon season that normally occurs between July and December. Annual rainfall in the region typically ranges between 1,500 and 2,500 millimetres, although mountain areas have in excess of 4,000 millimetres.  Such massive downpours often cause flooding in lowland areas.
The monsoon season is obviously the peak time for water harvesting. It makes sense for the region to consider widespread, systematic harvesting of rainwater for domestic, agricultural and industrial uses.
Modern rainwater harvesting practices in the region vary from country to country.
In Singapore, which has limited land and where most people live in high-rise buildings, rooftop rainwater harvesting is widely practised. Collected water is kept in separate roof cisterns for non-potable uses. The country's Changi Airport has a large rainwater harvesting system that collects rain from the runways and the surrounding green areas in two reservoirs. The water is used mainly for firefighting drills and toilet flushing. 
For Thailand, which has the lowest per capita volume of freshwater in Asia, rainwater harvesting provides a major alternative supply. 'Rain jars' — vessels of up to 3,000 litres that catch water from roofs — have always been part of its culture. In rural northeastern Thailand, "a home was not a home unless it had one huge rainwater jar", according to Thai writer Cezar Tigno. 
The Philippine Congress passed a law in 1989 that required each of the country's 42,000 villages to build rainwater collectors or ponds mainly for aquaculture use as well as to minimise the risk of flooding, to provide water for areas on the banks with vegetation and small parks, and to recharge badly depleted groundwater. 
However, more than two decades later, because of the lack of implementation by local governments, only a handful of these collectors have been constructed.
Directly harnessing rainwater
There appear to be four ways to bring about rainwater harvesting: introduce legislation requiring every new home to include a harvesting system before a building permit is approved; create laws requiring villages to build communal ponds; draw up legislation requiring every industrial plant or complex to build a harvesting system to meet its water needs; or to build proper drainage, water recycling or underground reservoir systems for cities. Most engineers think that this centralised system is more viable than the three other fragmented approaches.
So governments must lead the way. The mystery is why governments in South-East Asia and the Pacific have not gone all out in tapping this abundant natural water supply. To harness rainwater, what is needed is consistent public policy and political will.
This article has been produced by SciDev.Net's South-East Asia & Pacific desk.
References Luong, T.V. Harvesting the rain: A construction manual for cement rainwater jars and tanks (UNICEF East Asia and Pacific Regional Office, 2002)
 UN Environment Programme Sourcebook of Alternative Technologies for Freshwater Augmentation in Some Countries in Asia (UNEP, 1998)
 Sehgal, J.D. Roof-Top Harvesting of Rainwater: A Sustainable Water Resource in S.E. Asia (4th International Conference on Sustainable Water Environment: Innovative Technologies and Energy Efficient Solutions, 2008)
 UN Environment Programme Examples of Rainwater Harvesting and UtilisationAround the World (UNEP, retrieved 24 May 2013)
 Tigno, C. Thailand: Promoting Rainwater Harvesting,Preserving Rain Water Jar Culture (Scribd, 2007)
 Oposa T., Jr. Implement 1989 rainwater collection law (Philippine Daily Inquirer, 2009)