Lisa Ochola, a typical Nairobi teenager, who was a youth representative to the second World Water Forum at The Hague in the Netherlands in 2000, told the world that her family does not get running water, sometimes for months. During these periods, she said, she makes do with a glass of water for her personal hygiene. To drive home the point about the degree of water scarcity in Nairobi, she lamented that bottled water sold in supermarkets costs more than petrol. 

This is the reality that John Njoroge, who lives a middle-class life in Nairobi's Lavington suburb, also has to face as he spends Ksh10,000 a month, or $128, on the water needs of his family of five. 

In another part of the city, Kibera, one of Africa's largest slums has to deal with the phenomenon of "flying toilets": faeces discharged into cellophane bags and flung into the air to land where it will.

Nairobi's water and sanitation crisis worsened in 2000, when a scorching drought forced rationing of water and power. As in most African cities, the problem is not so much that bulk-treated water is in short supply, but that about 50 per cent of the water is wasted or unaccounted for.

The 21st century, which is the beginning of the urban millennium, is also being labelled the "century of water." There is a growing collective concern for water security in this century. In the coming decades, most of the world’s people will live in cities and megacities, where much of the world’s water will be consumed and most of its pollution generated. Sustainability in this new millennium will be largely defined in the cities, the centres of political power, public opinion and the engines of economic growth and technological innovation.

UN-Habitat, the United Nations Human Settlements Programme, has started the Water for African Cities programme in seven demonstration cities: Abidjan (Cote d'Ivoire), Accra (Ghana), Addis Ababa (Ethiopia), Dakar (Senegal), Johannesburg (South Africa), Lusaka (Zambia) and Nairobi (Kenya). UN-Habitat has also recently been mandated by the third World Water Forum to play a leading role in raising international awareness on water and cities.

According to Kalyan Ray, co-ordinator of Water for African Cities, the explosive growth of urban centres in the past 25 years or so, which continues unabated, is rapidly depleting previously bountiful fresh water resources. The urban water challenge is taking on an ominous dimension in vast areas of Africa, the Middle East and Central Asia. Rapid urbanisation, growing populations and development are overwhelming traditional water management practices.

As cities draw upon the surrounding environment for resources, their ecological footprints are expanding rapidly into their hinterlands. Over-abstraction of ground water reserves has reached critical proportions in many large cities. In many regions, intense competition is developing between cities for shared water resources. With a growing number of countries in Africa, the Middle East and Central Asia expected to face water scarcity in coming years, water could become a catalyst for regional conflicts as oil did in the 1970s.

Overall, Africa is urbanising at a rate of about 5 per cent, the fastest in the world. Africa's urban population could jump from 138 million in 1990 to 500 million in 2020, when African cities with populations of more than one million will accommodate almost 200 million people. Yet a 1990 survey of 29 sub-Saharan African countries revealed that eight were suffering from water stress or water scarcity. By 2025, this number could increase to 20 out of 29.

Lagos, for example, the commercial centre of Africa's most populous country, is a city with a current population of almost 14 million, about half that of Kenya and larger than most African countries. It is the most populous city in Africa and Europe, the sixth in the world, and could jump to number three in the world in another 20 years, with several million more inhabitants needing water access and other basic services and infrastructure. As in many other African countries, Lagos is a city on the verge of a water crisis.

Largely prompted by the Cape Town meeting of African ministers in 1997, the Water for African Cities Programme is focusing on water demand management, control of water pollution and awareness creation activities. It aims at improving water resources management in African cities, through establishment of early warning systems and catchment management to protect fresh water from pollution.

Changes are already being noticed. Addis Ababa policy makers now favour water demand management. Lusaka has experienced a reduction in water loss and inefficient use of water from 80 to 45 per cent since 2000. Dakar has improved water management and introduced early warning mechanisms to control water pollution in the Lac de Guiers. In the Accra Tema Metropolitan Area, a community-based integrated planning approach is being employed to tackle water pollution in the Densu River.

Anna Kajumulo Tibaijuka, executive director of UN-Habitat, says: "Water is essential for virtually every human activity. Yet as the world's urban population reaches the three-billion mark, it is distressing to note that one billion urban poor lack adequate access to water."

• Yima Sen is a public awareness and information specialist at UN-Habitat. A UN-Habitat Feature