The body has no water storage. Fluid is continuously lost through urine, skin, breathing and digestion, and significantly more through sweat during training. These losses are continuous — the intake is not. This is why the balance fluctuates throughout the day.
In healthy people, thirst regulates this reliably. It is not an emergency signal that only activates when a deficiency already exists, but rather part of a finely tuned control system. A deficit only becomes relevant from about two percent body mass loss: from that point on, strength and endurance performance measurably decline.