The Sierra provide ample water and silt for the central valley that makes it one of the worlds ideal farming lands. It's been very actively farmed for more than 100 years.

Sure, the water situation has gone up and down with the drought, but normally there's plenty of water. A lot of the almond orchards had to be scaled back during the drought, but when there's water they use it. It's a lot more sustainable than many other farming lands because it's not dependent on an underground aquifer that took eons to fill up or water piped in from somewhere far away except for the urban areas of course.