Climate change is likely to lead to increased average rainfall in the world's major river basins but weather patterns will be fickle and the timing of wet seasons may change, threatening farming and food stocks, experts said on Monday.
Furthermore, some river systems in Africa -- southern Africa's Limpopo, north Africa's Nile and West Africa's Volta -- are set to receive less rain than they do at the moment, hitting food production and fuelling international tensions.
The outlook for rain-fed agriculture was particularly bleak in the Limpopo basin, which covers parts of Botswana, South Africa, Mozambique and Zimbabwe and is home to 14 million people.
"In some parts of the Limpopo even widespread adoption of innovations like drip irrigation may not be enough to overcome the negative effects of climate change on water availability," said Simon Cook of the International Center for Tropical Agriculture.
The concerns for the Upper Blue Nile, which runs through Ethiopia to Sudan and then Egypt, centred mainly on the increased evaporation that will result from a predicted 2-5 degree Celsius increase in global temperatures.
The evaporation could "reduce the water balance of the Upper Blue Nile Basin", scientists from the Challenge Programme on Water and Food (CPWF), a global agricultural research body, said, potentially putting Cairo and Addis Ababa at loggerheads again over the river that is Egypt's economic lifeblood.
The research, which was released two weeks ahead of a major climate change conference in Durban, also looked at the future of Africa's Niger river, the Indo-Ganges, southeast Asia's Mekong, China's Yellow River, the Karekh in Iran and the Andes and Sao Francisco rivers in South America.
Overall, it found that while evaporation rates would go up, most of that loss would be offset by increases in annual rainfall as the "energised climate system turbo-charges the amount of water in the atmosphere".
However, it added climate change could lead to "flip-flops" in weather patterns that have hitherto been stable, as well as minor changes in the timing of rainy and dry seasons that have been set in stone for centuries.
"Such changes will create a management nightmare and require a much greater focus on adaptive approaches and long-term climate projections than historically have been necessary," CPWF director Alain Vidal said.
"Flood mitigation and management strategies will be crucial in areas with increasingly erratic climate and flash floods, such as the Limpopo and the Volta."
While dams remain one of the most productive large-scale ways of storing water, others are as simple as encouraging small-scale farmers to catch rain from their roofs, or pock-mark their fields with special rain-collecting 'zai pits'.