A new report has found there is no alternative to taking water from the Shannon to service the greater Dublin region.
The study, due to be presented next week, has found that options put forward by opposition groups in the Shannon area are not sustainable.
Dublin City Council said water supplies in the area always operate to a very fine margin.
It said it is still repairing leaks from winter's cold spell and June's good weather has reduced stocks.
It said there is only so much water that can be treated each day.
Today they issued an appeal for residents to conserve their supply.
In the long term the Dublin area needs a new supply as extraction from the Liffey source is reaching its maximum.
The proposal to take 3% of the Shannon met with opposition.
It will be up to An Bord Pleanála to decide.
