What is the only physical factor that affects water balance in a pool?

Temperature is indeed the only physical factor that significantly affects water balance in a pool. It plays a crucial role because water temperature can influence the rate of evaporation and the chemical reactions occurring in the pool. As temperature increases, evaporation rates can rise, which can lead to a reduction in water volume over time. This allows for an understanding of how maintaining an appropriate temperature range can aid in managing the pool's water level and overall balance.

While pH, calcium hardness, and total alkalinity are important chemical factors that help maintain proper water quality, they do not directly affect water volume or balance in the same way temperature does. These parameters focus on the chemical stability and overall quality of the water rather than the physical state and volumes of the water present in the pool.