Carol ...
It's not lack of food. Some roses automatically reduce bloom size after the first flush in spring.
Roses are incredible survivors and as temps increase they will automatically put moisture into the foliage which produces food for the plant through photosynthesis and hold moisture in the root zone for continued survival. It takes a lot of plant energy to produce large blooms, so the plant allocates its resources to the parts of the plant that will help it survive best. You will notice that in fall as the temps get a bit cooler, the bloom size increases. That's in the genetics of the rose.
Of course, the drought also has an impact on this process. Even tho' you say the roses have plenty of water there are a lot of variables we can't see that can trigger a rose into a stronger survival mode during periods of drought than you would see in normal years.
Many of us are culling our gardens of plants that are having difficulty in this extreme drought. Some roses are actually more drought tolerant than others.
Good luck with your roses.
Smiles,
Lyn
