There is enough fresh water in the world to double food production in the next decades -- the problem is its inefficient use, according to a study released Monday at the World Water Congress meeting in Brazil.

"There is clearly sufficient water to sustain food, energy, industrial and environmental needs during the 21st century," read the report, published in a special edition of the journal Water International.

The five-year study involving experts in 30 countries "is the most comprehensive effort to date to assess how ... human societies are coping with the growing need for water."

The world's most pressing problem is doubling food production to feed a growing population "expected to expand from seven to 9.5 billion people by 2050," the report read.

Water scarcity "is not affecting our ability to grow enough food today," said Alain Vidal with the Challenge Program on Water and Food (CPWF), the group that produced the report.

"Yes, there is scarcity in certain areas, but our findings show that the problem overall is a failure to make efficient and fair use of the water available in these river basins.

"This is ultimately a political challenge, not a resource concern," he said.

According to Vidal, "huge volumes of rainwater are lost or never used, particularly in the rain-fed regions of sub-Saharan Africa. With modest improvements, we can generate two to three times more food than we are producing today."

Simon Cook, one of the study coordinators, told AFP that the capacity to increase food production "is there, but only if we use the water in a balanced way."

Water management must be seen in their totality for a balanced use, Cook said.

The report notes "the increasingly political role" of managing the competing demands for water. These include water for crops and pastures, to generate electricity, for industrial use, and for city residents.

The experts studied 10 river basins around the world, including the Indus-Ganges, Mekong, and Yellow in Asia; the Limpopo, Nile and Volta basins in Africa; and the Sao Francisco and the Andes river basin in South America.

The XIV World Water Congress meets through Thursday at the resort town of Porto de Galhinhas in north-eastern Brazil.