The temperature will be a lot higher , because the heat of the sun will make the water move a lot faster. The heat can also make the water evaporate , making the water we have into water vapor. Because the heat is making the water move a lot faster then normal, then that’s what causes the water to start to evaporate and become water vapor. Basically meaning that the gathering of water is slowly decreasing, while the water vapor is increasing little by little , because of the heat. If the gathering of water and water vapor reaches equilibrium, then they both have to be equal.