While I live in Michigan, I can tell you that roses are party girls - they like to eat and drink. They need about 5 gallons of water a week. If you have been feeding them chemical fertilizers they may be in decline from excessive salts. If you "wash" the soil with a product called Clearex and begin using organic fertilizers, such as Rose Tone by Espoma, alfalfa, and cotton seed meal. Lots of rose growers also use a handful of Epsom salts in spring and again in early summer to provide magnesium. Lack of iron with cause yellowing too. California is rose country so I would chat with folks at a good local independent garden center that sells lots of roses. A soil test is also a good idea before you begin playing chemist. Too much of a good thing can do great harm. Happy Yardening, Nancy.