Per capita water use is calculated by dividing the total amount of water withdrawn from all water suppliers by the population. Water used for irrigation and agriculture is not included. See this excellent summary report on per capita use in Florida.
The Water Resource division of the U.S. Geological Survey (USGS) publishes the Estimated Use of Water in the United States every five years. This report has detailed per capita water usage on a state by state and county by county level.
In California, Kern county has the highest per capita use of water at 274 Gallons per Day, & Mariposa has the lowest at 67 GPD. Santa Clara is 171 GPD and San Diego county is 156 GPD. Sacramento & Fresno don’t have a lot of meters and consequently they use more water at 265 GPD and 246 GPD, respectively.