Kiser explained how his arena drag can move sand and simultaneously apply water, for an exact depth and hydration. I can completely control the moisture. I don't water every time I drag the arena, but I monitor moisture all the time. A spray boom gives him precise control of the water added. Even indoors, footing is affected by temperature and humidity.
Intense lighting can dry out the ground.
Kiser observes the horses' performances, in warm-up sessions and in the event. I see how the horse can get up underneath himself. Some horses will go deeper than others. I watch for pulled shoes. If many pull when they stop, chances are the ground is too sandy, too slick.