What is the only physical factor that directly affects water balance?

Temperature is the only physical factor that directly affects water balance in a pool setting. It influences the rate of evaporation and can also affect chemical reactions and stability. Warmer water tends to evaporate more quickly, leading to a rise in the need for additional water to maintain the correct levels. This impact on water loss is particularly significant in outdoor pools where temperatures can fluctuate considerably.

Other factors such as pH, flow rate, and alkalinity are important for maintaining water quality and chemical balance but do not directly affect the physical balance of water through evaporation or physical loss. pH adjusts how chemicals react, flow rate pertains to circulation and sanitation, and alkalinity stabilizes pH, but none of these contribute directly to the water balance itself in terms of volume loss or gain. Thus, temperature stands out as the critical factor affecting the physical water balance of a pool.