I think you're probably right that it's lack of water. Unglazed ceramic and clay absorb quite a lot of water. I don't think the in-ground sprinklers would be sufficient.
I have to water even my plastic containers with the garden hose, in addition to the water the get from my irrigation system.
Perhaps you could check into plants that are recommended for xeriscaping in hot, dry areas. Some produce very pretty flowers.
"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams