I don't know what's wrong with your trees exactly, but from the looks of the photo, you're going to lose them. Browned out from bottom to top. This is a dying plant.
I'm guessing that the drought is playing its part. Redwoods and other indigenous evergreens in California actually have their growth season
in the winter when it's cool and rainy, and are dormant in the summer.
Four winters without sufficient moisture could well have killed them off. I can't see a redwood putting up with that kind of deprivation.
souce: I lived on the CA north coast for over a decade, part of it in a redwood forest.