Kris-
In Southern California (I know both San Diego and Santa Barbara do this) they release a “percentage” value weekly that is based on ET data.
Their philosophy is you set your sprinkler system to water based on a water calculator for JULY (highest amount of water) and the ET percentage will adjust from there.
See here:
http://www.sbwater.org/waterindex.aspx?id=450
For example – on my system they would suggest watering for about 32 minutes per week (per station) at the maximum.
Then, on lower volume weeks, the % value would adjust the watering time (open sprinkler already has a % value built in).