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 0.25 cm deep over the whole area. In other words, 

 if transpiration continued at this rate for 10 days, a rainfall of at least 

 2.5 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 6 12 18 6 12 



Thursday Friday Saturday 



Fig. 4.11. 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 



