What factor can affect a horse's water requirements significantly?

Temperature significantly influences a horse's water requirements due to its direct impact on the horse's hydration needs. As the ambient temperature rises, horses tend to sweat more to help regulate their body temperature. This increased sweating leads to higher fluid loss, necessitating greater water intake to maintain hydration levels. Conversely, in cooler temperatures, horses generally sweat less and may require less water.

Other factors, while potentially relevant, do not have as substantial an effect as temperature. For example, a horse's color might have some influence on heat absorption and, thus, its activity level, but it is not a direct determinant of water needs. The size of the stable or time of year may influence conditions, but temperature is the most direct variable affecting water requirements through its direct correlation with hydration demands and sweating.