Well, let us think about what the fuction of the stomata are. They are openings in the leaves of the plant that allow the transfer of gases and water vapor. If a plant is having to make a go of it in dry conditions, such as you are describing, would it not make sense to conserve the water supply within the plants internal system? That is exactly what this question is describing. During the hottest part of the day, the stomata remain closed to keep water vapor from escaping. At night, during the coolest part of the day, they open to allow minimal exchange of water vapor, so as not to excessively lose too much water. The adapaptation of plant leaves with fewer stomata would also make sense in dry environments, so as to have fewer exchange portals, which would cut back on the loss of water as well.