Roses are a very thirsty bush. For the best blooms and growth they should drink at least several gallons of water a week. The hotter your climate, the more water a rose needs. Check out this terrific article from the Santa Clarita Valley Rose Society: Water and Roses
Another problem can crop up if watering is done improperly: black spot disease. The best way to prevent black spot disease is to keep the foliage dry. The question then becomes: “How best to water?”
I have found the answer to be watering with a slow-release container. You can make one of these easily with an empty gallon milk carton or a kitty litter container. Drill or punch a hole in one corner. Fill the container as quickly as possible with water from a hose. Put your finger over the hole on the bottom and place the carton beside your rose.( It helps to loosen the dirt around the rose with a trowel.) The water slowly trickles out of the carton, and sinks into the dirt without running out too quickly and away from the rose bush.
This might not look pretty, but it works amazingly well. Try it out for yourself.