How long will Cape Town's water supplies last? It depends on which level of government you ask.

The City of Cape Town says the Berg River Dam will provide enough water until 2020. The national Department of Water and Environment Affairs says the city will run into shortages by 2012.

Clive Justus, mayoral committee member for utility services, took issue with comments published in the Cape Times that the Berg River Dam's supplies would last until 2012, by which time water demand will meet supplies.

Justus says this is not correct. "It should be noted that the recently constructed Berg River Dam will extend the need for implementation of further new water projects to about 2020 and not 2012, as stated in the article," he said.

He adds the caveat that this depended on "demand management controls" and on the public agreeing not to waste water.

However, Water and Environment Affairs sticks to its story that, if Cape Town does not start using less water than it does now, it will run into shortages by 2012.

Deputy director general Helgard Muller said the latest information from the department's water resources planning section was that "if significant scaling down of the demand does not happen, Cape Town may run into shortages, that is, restrictions by 2012".

He said Cape Town would need another water augmentation project by 2016 - even with "effective water demand management practices" such as water conservation.

He pointed out that the length of time the city's water supplies would last depended also on whether it experienced more wet or dry cycles.

"If all dams are full, the supplies may be stretched beyond 2012 or 2016 without restrictions, and the water shortage may be obscured. The inhabitants of Cape Town may get the false impression that all is fine until the next drought strikes," Muller said.

Climate change scientists at UCT say their modelling shows that the western part of the province will become hotter and drier as the effects of climate change intensify.