So 100% of reservoir loss is due to evaporation? In the analogy, filling the glasses before EBMUD turns the water off is like filling a reservoir during (even brief) rainy seasons.
As it stands California’s reservoirs never seem to fill completely anymore. Adding more reservoirs won’t help store extra water if it simply isn’t falling from the sky.
You should not stop all the water from reaching the ocean in the first place, bad things happen when you do that. We are reaching the point where we just have enough water to keep the rivers flowing so we don't kill all the wildlife that exists in them, we're not adding significant amounts of water to storage.