For large landscaped areas, it makes sense to be as accurate as possible with turf and landscaping water needs to lower water use and water bills. Several weather stations are located in the region to provide evapotranspiration data to be used to create accurate water budgets and water schedules. For daily evapotranspiration data for the Santa Rosa area visit the California Irrigation Management Information System (CIMIS), then click on the "data reports" option on the upper left corner of the web page, then click on the sample report option and select Sonoma County.
A basic formula for developing a water budget for your site measured in CCF (100 cubic feet of water or 748 gallons) per year is (Eto × crop co-efficient) − (effective rainfall × square feet ÷ 1,200) ÷ 52 = CCF week
For more information on creating water budgets for your site, please contact the Sonoma County Water Agency's Water Conservation Program at 707-547-1906.
Another great source of irrigation information and training is the Cal Poly Irrigation Training and Research Center in San Luis Obispo. They can be reached by calling 805-756-2434.