Transpiration refers to the process that plants use to cool themselves. As water evaporates from specialized areas of the leaf called stomata, it cools the leaf, similar to the way perspiration cools humans. If plants did not have this ability the leaf temperature would soar on a sunny day, eventually shutting down its biochemical processes and leading to death. If water becomes limited due to dry soils, tomatoes, like most plants simply close the stomata and limit water loss. When plants do this they not only lessen transpiration but also prevent carbon dioxide from entering the leaf.
Most people water their gardens with sprinklers.
Hopefully, none of you water tomatoes by standing in the garden holding a hose. Although it seems very satisfying, I guarantee that if you dig into the soil when finished, you will find that only the top inch is wet. Most of us do not have the patience to stand in the garden for the length of time needed to water properly. To illustrate this, try to guess how much water you would need to properly irrigate tomatoes planted in a bed 20 feet long and three feet wide. Tomatoes need a minimum of one inch of water per week, preferably 1.5 inches in the heat of the summer.