[PaulDavisThe1st]

The drop comes from two critical changes:

1. improvements in domestic water usage arising from changes to in-home fixtures and appliances. Per capita use in cities across the US southwest has dropped by 30% or more over the last few decades.

2. switching over to grey/recycled water usage for a huge chunk of municipal water irrigation (eg. city parks, highway medians etc.). This uses a significant amount of water, and switching to water that has already been used once has a dramatic impact.