More than 2,500 participants have registered to take part in the 21st edition of the World Water Week that kicked off on Monday in Sweden.

Right from the start, the message that came out from the event taking place at the Stockholm International Water Institute is that the relationship between water and cities has never been stronger. 

This year's theme is "Responding to global challenges: Water in an urbanising world''.  During the coming week attendees will treated to more than 100 seminars, discussions and side events covering topics from water monitoring and the challenges of waste water treatment to the human-rights approach of water provision and the effects of corruption among private and public water suppliers. 

Speaking at the opening session, Dr. Joan Clos, United Nations Under-Secretary General and Executive Director of UN-HABITAT warned that the world was not prepared for the type of urbanisation that was taking place in, for example, Africa and South East Asia which was not linked to industrialisation as past rural to urban migration had been.

"The challenge of water provision is not technical," he said. "We solved that problem hundreds of years ago.  But how can we provide water infrastructure in cities characterised by high unemployment, dependence on the informal economy and slums? That is the real challenge."

The adoption of the theme of urbanisation demonstrates the recognition by key water practitioners around the world of the challenge that urbanisation poses.

Also speaking at the opening, Gunilla Carlsson, the Swedish Minister for International Development Cooperation, warned that although many people thought of water provision as challenge predominantly faced by developing countries, it is actually something that faces countries in the developed and emerging countries of Europe and North America too.

"As a country's middle class grows", said Carlson, "so too does their consumption of water.  This puts a great demand on a country's resources." 

However, as drivers of the economic and social expansion, cities also provide opportunities for tackling the effects of climate change. "Urban sprawl and the unplanned expansion of cities is the real problem", continued Dr. Clos. "In low density cities the cost per capita of water provision is actually higher. We have to get back to planning basics if we are going to see real progress on urban water provision."