Water usage for the five most populated counties in Southern California (21 million people) in 2015 was nearly 3 billion gallons per day according to the USGS National Water Information System. Water sources, water deliveries, water withdrawals, water usage, population served etc. are categorized with the ability to sort by county. While Southern California by definition includes ten counties, daily water withdrawals in million gallons per day for the most populous counties were; Los Angeles (1,255), Orange (457), Riverside (440), San Diego (408), and San Bernardino (337).
Source: USGS National Water Information System. Water Use Data for California referencing Public Supply Water Withdrawal for the year 2015, refresh date June 2018.
Link to website: https://waterdata.usgs.gov/nwis/wu