EVAPORATION AND TRANSPIRATION are the two processes by which water is removed from the Earth’s atmosphere. Because it can sometimes be difficult to ascertain which one is occurring, the term evapo-transpiration is often used to describe the sum total of both processes. Evapotranspiration is an important part of the planet’s hydrological cycle, and it is likely to change in a warming world.
Evaporation is the process of changing water from a liquid to a gaseous state. Water molecules are in motion, and if they generate enough speed and are going in the right direction, they essentially heat themselves to the equivalent of boiling, at which point, they have enough energy to escape the liquid state and pass into the atmosphere. Only a small percent of molecules will find themselves in these favorable conditions at any one point, so the overall rate of evaporation from any body of water is relatively low. Evaporation rates ebb and flow depending on local conditions. Bodies of water with a large surface area evaporate more than smaller bodies, as there are more water molecules near the surface. If the relative humidity is low, the evaporative rate is correspondingly higher. High temperatures also increase evaporation, as do high or sustained winds. In all, about 80 percent of evaporation comes from the Earth’s oceans, with the remaining 20 percent coming from inland waterways and vegetation.
Transpiration is the evaporation of water from plants and trees. Approximately 90 percent of all water taken up by a plant is lost to transpiration. Generally, the ratio of transpiration is 200 to 1,000 kg. of water evaporates for every kg. of dry matter produced. One acre of corn will release 400,000 gallons of water into the atmosphere in a growing season, and a 49 ft. (15-m.) tall maple tree could lose 58 gallons (220 liters) of water per hour on an average summer’s day.
Water evaporates through small openings in the leaf surface called stomata. These openings expand when light hits the surface of the plant, and close up at night. The stomata allow the leaf to expel oxygen and carbon dioxide. Transpiration helps a plant by pulling water up from the ground, and transporting minerals and fluids throughout the plant; the release of water also cools the plant.
Several forces act on a plant to increase transpiration. Unless the air around the plant is at 100 percent of relative humidity, the plant quickly begins to dry out and starts pulling moisture from the soil. Wind tends to enhance the effect by blowing humid air away from the plant. Sunlight warms up the leaves and opens the stomata; warm air temperatures create the same response. If the soil around the plant becomes too dry, the stomata close off to preserve as much moisture as possible, and the plant quickly begins to wilt.
The Earth’s water budget is fixed and cyclical, and evapotranspiration is one of the processes by which water is exchanged between the surface and the atmosphere. The water vapor expelled by evaporation and transpiration forms clouds, which travel high into the atmosphere, cool, and produce rain, which then falls back to the surface, where the cycle begins anew. Some of the water that reaches the surface nourishes plant life, which transpires most of that water back into the atmosphere. Some of it sinks far enough into the soil to reach the water table, and recharges freshwater sources. The amount of water vapor in the atmosphere remains relatively constant at 12,900 cu. km., but each year, approximately 490,000 cu. km. of vapor cycles through the whole system.
Increased evapotranspiration is one of likely outcomes of global warming. Higher air temperature enhances both evaporation and transpiration. This will contribute to longer and more frequent periods of drought in the lower and middle latitudes in coming years.
For example, one computer model built in the 1990s estimated that by the end of the 21st century, summertime evapotranspiration in the American southwest could increase by 27 percent. The model found similar increases in 12 regions around the world. Between the growing human population in need of drinking water and the heightened probability of drought, the world’s water deficit will probably increase in coming years.