The world's water crisis is so severe it could take almost 30 years to eradicate hunger, the United Nations says.
By Alex Kirby
BBC News Online environment correspondent
It believes the goal of halving the proportion of hungry people by 2015 may be unattainable.
Many people around the world must carry water long distances
By 2020, the average water supply per person worldwide is expected be a third smaller than now.
The UN says political inaction and a lack of awareness are worsening the crisis.
The warning comes from the World Water Assessment Programme (WWAP), which combines the efforts of 23 UN agencies. It is based in Unesco, the UN Educational, Scientific and Cultural Organisation.
The programme has prepared a report, Water For People, Water For Life, for the third World Water Forum, to be held in the Japanese city of Kyoto from 16 to 23 March.
The report's chapter on agriculture says about 25,000 people die daily from hunger, with an estimated 815 million people suffering from malnutrition.
Stretched to the limit
One of the Millennium Development Goals (MDGs) agreed internationally promises to halve the proportion of hungry people by 2015.
But the report says this may not be possible before 2030, because previous estimates of food availability failed to distinguish between rain-fed and irrigated crops.
It says another 45m hectares (111 million acres) will be under irrigation by 2030, requiring an increase of 14% in water used in this way. Irrigation already accounts for about 70% of global water use.
THE WATER STRUGGLE
6,000 people die daily of diarrhoea
Water consumption has almost doubled since 1950
There is more polluted water than in the ten largest river basins
Yet 20 of about 170 countries covered in the report are already using more than 40% of their renewable water resources for irrigation.
It describes this as "a threshold used to flag the level at which countries are forced to make difficult choices between their agricultural and urban water supply sectors".
So many countries may decide they cannot afford the water they will need to grow the food that could end hunger.
The report says another 16 countries use more than 20% of their water for irrigation, "which can indicate impending water scarcity. By 2030 South Asia will on average have reached the 40% level, and the Near East and North Africa not less than 58%.
"By contrast, sub-Saharan Africa, Latin America and East Asia are likely to remain far below the critical threshold. These regions will see the bulk of agricultural expansion in the next 30 years."
The report says almost 60% of the water used in irrigation is wasted. But average grain yields doubled between 1962 and 1996.
It says: "Towards 2050, the world could enjoy access to food for all. The fact that 815m are presently ravaged by chronic undernourishment is not due to a lack of capacity to produce the required food.
"It is because of global and national social, economic and political contexts that permit, and sometimes cause, unacceptable levels of poverty to perpetuate."
Unesco's director-general, Koichiro Matsuura, said: "Water supplies are falling while the demand is dramatically growing at an unsustainable rate.
"Over the next 20 years, the average supply of water world-wide per person is expected to drop by a third."
Building the peace
The report says: "Globally, the challenge lies in raising the political will to implement water-related commitments.
"Attitude and behaviour problems lie at the heart of the crisis, inertia at leadership level, and a world population not fully aware of the scale of the problem.
"Financing the Millennium Development Goals will probably be one of the most important challenges that the international community will have to face over the next 15 years."