. Elements of ecology. Ecology. Moisture in the Air 115 layers. Water is lost from the deeper layers chiefly by absorption by roots and attendant transpiration. The magnitude of water loss by transpiration is often not appre- ciated. Perhaps some idea can be obtained from the fact that a hectare (2^4 acres) of mature oak trees will transpire as much as 25,000 liters of water per day. This volume would be equivalent to a layer of water cm deep over the whole area. In other words, if transpiration continued at this rate for 10 days, a rainfall of at least cm would be required to restore
. Elements of ecology. Ecology. Moisture in the Air 115 layers. Water is lost from the deeper layers chiefly by absorption by roots and attendant transpiration. The magnitude of water loss by transpiration is often not appre- ciated. Perhaps some idea can be obtained from the fact that a hectare (2^4 acres) of mature oak trees will transpire as much as 25,000 liters of water per day. This volume would be equivalent to a layer of water cm deep over the whole area. In other words, if transpiration continued at this rate for 10 days, a rainfall of at least cm would be required to restore the water. One might think that the maximum rate of water loss would occur from saturated soil or from a pond. Actually the rate of water loss per unit area from the ground due to the transpiration of the vegetation may be nearly twice as great as that from a free water surface due to evaporation. 40°C. 18 6 12 18 0 6 12 18 0 6 12 Thursday Friday Saturday Fig. Hygrothermograph record of temperature {broken line) and relative humidity {solid line) in the short-grass plains of central United States during 3 days in early July. (By permission from Plant Ecology by Weaver and Clements, 1938, McGraw-Hill Book Co.) Three atmospheric conditions that greatly modify the rates of evap- oration and transpiration are the saturation deficit, the temperature, and the wind velocity. The saturation deficit is the difference be- tween the actual vapor pressure and the maximum possible vapor pressure at the existing temperature. The saturation deficit thus gives more information of ecological significance than the relative humidity. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Clarke, George L. (George Leonard), 1905-. New York, Wiley
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