Keeping your footing moist is an important part of a maintenance routine and helps to improve the stability of the footing and with keeping the fibers bonded together with the sand particles.
If your arena is an indoor or covered ring then you will need to have a system to be able to keep the footing moist. Whether that be a sprinkler system, a water tank or by utilizing the magical powers of MAG to avoid the need to water altogether. The method you use does not really matter as long as you can achieve the desired result.
In an outdoor arena, the necessity to water largely depends on your climate and how much you can take advantage of mother nature. In some instances, our trainers are able to get away with barely watering their outdoor arena, if at all. If you are located in a hot/dry climate then you would need to be more proactive with watering of course.