Here is a report that studied water trends from the past 50 years (pdf) and found both good/bad news.
A household in the 2008 billing year used 11,678 gallons less water annually than an identical household did in 1978.
To investigate the causes of this decline, a local study of statistically representative households of the LWC was conducted in Louisville. Adjusting for weather, water use per LWC customer fell from 208 to 187 gpd between 1990 and 2007, a decline of 21 gallons.
The bad: declines are leveling off.
Most homes have installed low-flow appliances and that accounted for the biggest drop. There was a drop in family size, from 3.38 to 2.59, and that’s not going any lower.
There were no recommendations for next steps. Future declines may be up to us.