Governments in the Middle East and North Africa need to invest now if they want to avoid severe water shortages in the future, the World Bank has warned.
The amount of water available per person in the arid region will halve by 2050, a report from the bank estimates.
It blames climate change and population growth for new pressures on supplies.
Governments in the region should tackle water waste, build more efficient networks and reduce water use, the World Bank says.
Farming challenge
The bank's report suggests agriculture is a key target area.
With 85% of water-use devoted to agriculture, the report suggests countries such as Morocco will have to cut back on irrigation and switch to crops that require less water but earn more money.
"If we plan for the future, it's a lot simpler than crisis management further down the line"
According to its figures, declining water quality has already knocked around 1% off gross domestic product in Morocco, Algeria and Egypt, and nearly 3% in Iran.
"We've simply got to reduce the amount of water used, especially in agriculture," said Julia Bucknall, natural resource management specialist at the World Bank, addressing reporters Rabat, Morocco.
She added that water firms needed to cut water lost by evaporation.
"If we plan for the future, it's a lot simpler than crisis management further down the line," said Ms Bucknall.
Governments must increase public education projects on water and develop new infrastructure projects, including desalination plants, the bank's report says.
Bank officials identified Tunisia and Jordan as being strong performers in managing water demand and making the most of available water resources.
Morocco has invested in dams to catch water but in farming regions around Agadir, the water table is seven times lower than in 1982.
The BBC's James Copnall in Rabat says North African and Middle Eastern countries have had years of experience in coping with water shortages, but the challenge, it seems, is about to get much harder.
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