Last October, when Dubliners thought they were being charged for water on the basis of how much they used it appears to have given them pause for thought. Water usage dropped to its lowest for the year.

The previous month, which was to be the last month of “free” water, consumption was at its highest.

Consumption dropped from an average of 543 million litres a day in September to less than 524 million litres in October, a decline in usage of nearly 20 million litres in a month.

There was perhaps a “last hurrah” attitude at play in September, with gardens across the capital getting their final drink of the season.

Just over a month later, the Government had changed its mind and decided not to start charging for water until January and to keep everyone on a flat rate until 2018.

The effect was certainly noteworthy, according to Pat Cronin, an executive engineer with the council.

“The drop in demand around October 1st, 2014, was quite noticeable, and, leaving out normal seasonal and demand variability, is quite noteworthy and indicates the impact of customer conservation measures on demand and water use,” said Mr Cronin .

State’s population

That 20 million litres of water a day equates to a reduction in usage of 3.5 per cent across the Dublin region, which takes in parts of Wicklow and Kildare as well as Dublin city and county and accounts for a third of the State’s population.

A 3.5 per cent drop may not sound like much to get excited about, but Dublin City Council has stressed for decades that water supply in the region is “on a knife edge”, with no more than 1 per cent spare capacity.

Small increases in demand have in recent years led to night-time water cuts.

In December 2010, for example, restrictions were in place when demand was outstripping supply from the Dublin reservoirs by about two million litres per day.

The cost is also significant; not having to produce 20 million litres of water saves about €25,000 a day.

Which begs the question why was Irish Water not highlighting this apparent conservation effect last November amid all the controversy over the introduction of water charges?

Statistical proof

Unfortunately, the drop in consumption did not last long enough, says Ray O’Dwyer, head of operations with Irish Water.

“We noticed it happened, but we didn’t come out in relation to it because it didn’t last for a sufficiently long period.”

Statistically, there was not enough proof to draw any conclusions from the figures.

However, he says that does not change the fact that charging people for the water they use results in them using less of it.

“We know for a fact, from international experience, when everybody is metered it does reduce the water usage. It can go down by 10 per cent.”

With flat charges now in place until 2018, assessing the conservation effect will have to wait.