In the hydrologic cycle, two processes--evaporation and transpiration--are responsible for returning water to the atmosphere. Evapotranspiration is the sum of these processes. Evaporation is the loss of water vapor from the surface of objects containing liquid water, such as lakes and streams, soil, and organisms. Temperature is a major factor controlling evaporation. In general, the higher the temperature, the faster the rate of evaporation. Other major factors influencing evaporation rate include the amount of water vapor already held by the air (humidity) and air movement. Low humidity and high wind speed promote evaporation.

Transpiration is the loss of water vapor primarily through the stomata of leaves and green stems of plants. All above-ground parts of plants are covered by a waxy cuticle that helps prevent water loss. As a result, the internal spaces within the plant are nearly saturated with water vapor. Stomata are microscopic pores in plants' surface layers that open or close to allow gas exchange for photosynthesis while regulating water loss to prevent desiccating the plant. Cottonwoods near Dallas, Texas, can transpire up to 120 liters of water per tree per day. In the Amazon rain forest, a single large tree can transpire up to 1,180 liters of water per tree per day.

Transpiration provides the force necessary to pull water and dissolved nutrients up from the roots to the tips of the highest branches. Within the plant, water and dissolved minerals travel in specialized xylem cells that are arranged end to end in long files. Xylem cells are programmed to die once they mature; they then form hollow tubes to transport water. When filled with water, the xylem acts like a straw, and transpiration from the top draws water up from the bottom. Transpiration provides enough suction force that water could be pulled through the xylem to the top of a tree much taller than the tallest living redwood, higher than a fortystory building.

Air temperature directly affects evapotranspiration; any increase in temperature will increase evapotranspiration as long as adequate water is available. In tropical rain forests, transpiration increases beginning at sunrise, and the released water vapor rises in the heated air until it condenses in cooler air at high elevation, thus forming clouds. These clouds deliver afternoon rain showers that replenish soil moisture to repeat the process the next day.

Some studies suggest that extensive clear-cutting of the forest has decreased transpiration sufficiently to decrease local rainfall. If enough water is not available during the hot daytime, stomata of the remaining trees close and transpiration is shut down. Less water vapor is returned to the atmosphere, and the daily water cycle is broken. Less water returned to the atmosphere can exacerbate drought conditions and have a snowballing effect on climate. Some scientists predict that even the Amazon rain forest could be transformed to a savanna if too many trees are cut down and there remains too little evapotranspiration to sustain regular rainfall. Similar changes can occur in temperate grasslands and in temperate forests, making them more arid.

Decreasing rates of transpiration that are due to global warming and local drying also affect photosynthesis. Some plants, classified as carbon 4 plants, are more efficient at photosynthesizing at higher temperatures and in drier conditions. These plants, which are most common in the southern United States, will spread northward as temperatures rise. By contrast, carbon 3 plants, including most trees and vegetable crops, tend to be less drought tolerant and will not cope as well with increasing temperatures. Carbon 3 plants can be expected to migrate toward cooler latitudes if their existing locations become untenable. Plant migration is generally slow, and it is questionable if a poleward migration of biomes and plant types could keep pace with a global warming trend. Even if plants could migrate fast enough, the soils of higher latitudes are not as fertile and productive as those in current temperate areas, the "breadbaskets" of the world, because they have not supported extensive plant growth for millennia.

Water loss from transpiration often can exceed evaporative water loss from a water surface of the same area. For instance, a sugarcane field in Hawaii can transpire 120 percent as much water vapor as a similar area of open water. This is due not only to the amount of leaf surface area containing stomata but also to extensive root systems that can tap soil water in excess of the flat surface area of water. Some studies predict that such natural pumping of soil water, in response to a warming environment, will deplete soil water in the same way that pumping irrigation water from underground aquifers depletes groundwater.

Bibliography:

1)         Angelo, Claudio. "Punctuated Disequilibrium." Scientific American 292, no. 2 (February, 2005): 22-23.

2)         Huxley, Anthony. Green Inheritance: The World Wildlife Fund Book of Plants. New York: Anchor Press, 1985.

3)         Pimm, Stuart L. The World According to Pimm: A Scientist Audits the Earth. New YorK: McGraw-Hill, 2001.

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