What would happen if stomata were located on the top of the leaf?
The plant would lose too much water.
Most stomata are located on the bottom of leaves because direct sunlight does not hit the bottom surface. If stomata were on the upper epidermis (the top side of the leaf), the direct sunlight hitting the leaf would cause more water to be lost through the stomata because of evaporation. Having the stomata on the bottom prevents this from happening.