One type of vehicle is always within sight on Yemen’s roads: the water truck.

The brightly coloured, dilapidated tankers, often driven by Kalashnikov-wielding tribesmen, travel winding mountain roads and cross deserts to bring Yemenis a commodity more precious than petrol. It is one that increasingly only the rich can afford, with supply through the water mains regularly cut off. Others must rely on scarce rain, charity or crime to stave off thirst.

Yemen is set to be the first country in the world to run out of water, providing a taste of the conflict and mass movement of populations that may spread across the world if population growth outstrips natural resources.

Government and experts agree that the capital, Sanaa, has about ten years at current rates before its wells run dry but the city of two million continues to grow as people are forced to leave other areas because of water shortages.

In Yemen, which is fighting three insurgencies, the battle lines of tribal wars have traditionally followed the lines of the wadis, desert valleys that become rivers when the rare rains fall. Amid one of the world’s highest rates of population growth — 3.46 per cent last year — the water shortage has become critical and is driving civil unrest.

Hannan, an 18-year-old mother of one from Lahej, near Aden, said that only the comparatively well-off could plan for cuts in supply. “In a good week we’ll have a water supply all week but then the following week there will be water only for a day or two,” she said.

She and her husband, a factory worker, pay 3,000 riyals (£9) for a week’s supply of water from a touring water truck when the taps run dry. With an income of only 20,000 riyals (£60) per month, this means the family often spend half their income on water.

“There are a lot of people who can’t afford it and they have to rely on their neighbours to help,” she said.

Her neighbour, Anisa, 40, said: “When the water goes, it’s a sign of trouble in the community.”

Water available across Yemen amounts to 100 to 200 cubic metres per person per year, far below the international water poverty line of 1,000 cubic metres.

Groundwater reserves are being used faster than they can replenish themselves, especially in the Sanaa basin, where water once found 20 metres below the surface is now 200 metres deep, and despite the rainwater tanks on the roofs of most houses.

In desperation some citizens have dug unlicensed wells, compounding the problem. In Taiz, in the south, tapwater is available only once every 45 days. In the mountainous Malhan district in the north, women and children climb a 1,500m mountain to collect water from a spring, often in the small hours to avoid long queues.

Hosny Khordagui, director of the water governance programme in Arab states at the United Nations Development Programme, said: “If they do not find a solution we will see people encroaching on big cities, the formation of slums, a rise in crime, venereal disease, violence, even fanaticism. Fanatics will find very fertile ground to recruit and develop their infrastructure.”

Yemeni citizens have lived on scarce water supplies for thousands of years but the problem has been exacerbated by widespread production of the local drug of choice, qat, which consumes up to 40 per cent of water. About 70 per cent of Yemeni men chew the leaves each day, and bushy qat trees are often the only spots of green in the dry landscape.

The Deputy Planning Minister, Hisham Sharaf, admitted: “We have a water shortage which reflects itself in fighting between the people . . . If we continue spending this much water on qat Sanaa has ten to fifteen years.”

The Government is considering a desalination plant for seawater, but this is an expensive solution and may come too late. The only other option is to cut down on the agriculture industry, importing even more food.

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