Scientists have warned Asian countries that they face chronic food shortages and likely social unrest if they do not improve water management. The water experts are meeting at a UN-sponsored conference in Sweden.
They say countries in south and east Asia must spend billions of dollars to improve antiquated crop irrigation to cope with rapid population increases.
That estimate does not yet take into account the possible impact of global warming on water supplies, they said.
Asia's population is forecast to increase by 1.5bn people over the next 40 years.
Going hungry
The findings are published in a new joint report by the UN Food and Agricultural Organisation and the International Water Management Institute (IWMI).
They suggest that Asian countries will need to import more than a quarter of their rice and other staples to feed their populations.
"Asia's food and feed demand is expected to double by 2050," said IWMI director general Colin Chartres.
"Relying on trade to meet a large part of this demand will impose a huge and politically untenable burden on the economies of many developing countries.
"The best bet for Asia lies in revitalising its vast irrigation systems, which account for 70% of the world's total irrigated land," he said.
With new agricultural land in short supply, the solution, he said, is to intensify irrigation methods, modernising old systems built in the 1970s and 1980s.
But that, he says will require billions of dollars of investment.
'Scary scenarios'
At the same time as needing to import more food, the prices of those cereals are likely to continue to rise due to increasingly volatile international markets.
The report says millions of farmers have taken the responsibility for irrigation into their own hands, mainly using out-of-date and inefficient pump technology.
This means they can extract as much water as they like from their land, draining a precious natural resource.
"Governments' inability to regulate this practice is giving rise to scary scenarios of groundwater over-exploitation, which could lead to regional food crises and widespread social unrest," said the IWMI's Tushaar Shah, a co-author of the report.
Asian governments must join with the private sector to invest in modern, and more efficient methods of using water, the study concluded.
"Without water productivity gains, south Asia would need 57% more water for irrigated agriculture and east Asia 70% more," the study found.
"Given the scarcity of land and water, and growing water needs for cities, such a scenario is untenable," it said.
The scenarios forecast do not factor in the impact of global warming, which will likely make rainfall more erratic and less plentiful in some agricultural regions over the coming decades.
