Population growth and water stress are driving Earth to a food and environmental crunch that only better farming techniques and smarter use of the ecosystem will avert, a UN report issued on Monday said.

The number of humans is expected to rise from seven billion in 2011 to at least nine billion by 2050, boosting demands for water that are already extreme in many countries and set to worsen through global warming.

"Currently, 1.6 billion people live in areas of physical water scarcity and this could easily grow to two billion soon if we stay on the present course," according to the report.

"With the same (farming) practices, increased urbanisation and dietary patterns, the amount of water required for agriculture in terms of evapotranspiration would increase from 7,130 cubic kilometres (1,711 cubic miles) today to 70-90 percent more to feed nine billion people by 2050."

The 35-page assessment was compiled by the UN Environment Programme (UNEP) and the International Water Management Institute (IWMI), drawing mainly on estimates in peer-reviewed journals.

It was released at the start of World Water Week in Stockholm, a forum on water issues.

The report said that in many high-intensity food-producing regions, water limits are already being "reached or breached."

They include the plains of northern China, India's Punjab and the western United States.

Climate change will accentuate scarcity as it will altering patterns and intensity of rainfall. In Africa alone, agricultural output could be reduced by 15-30 percent by century's end.

Using today's farm techniques, focussing on always higher yields and ever-wider use of land, would be disastrous, said the report.

"If the same agriculture practices continue to be used, it would result in the inevitable degration or complete destruction of the terrestrial freshwater and coastal ecosystems that are vital to life itself," it warned.

The report, An Ecosystem Services Approach to Water and Food Security, called for innovation to improve yields and end hunger but also be less damaging to the environment.

Ideas include better training for farmers, including incentives for environmentally-sound practices.

Crops should be selected that are more suited to scarce or erratic rainfall, better irrigation techniques would improve the efficiency of water use and catchment ponds in hot countries could be invaluable mini-reservoirs, helping small farmers to survive in times of absent rain, said the report.

Planting trees and shrubs on the perimeter of fields discourages water runoff and retains soil moisture, thus helping crops. It also enables habitat links for species living in fragmented patches of forest.

The report stressed better governance, in which ecosystems are managed holistically - in other words, governments, farmers, urban dwellers and specialists come together to look at how to balance the needs of all water users with those of the environment.

By putting a dollar figure on the value of natural resources, farmers and consumers would get a better idea of the need to conserve, it said.

It cited a rough estimate of 70 billion dollars for the global economic value of wetlands, of which 5.25 billion is generated in Africa and 37.1 billion in Asia.