MEASUREMENT OF TRANSPIRATION 



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measurement are not all apparent at first consideration. It would appear 

 that the rate of evaporation could be measured by the rate at which water 

 disappeared from an open pan exposed to the atmosphere. While such 

 methods have been used they are subject to numerous errors and limitations. 

 The rate of evaporation for such a pan is con- 

 trolled not only by environmental conditions, but 

 also by the size, shape, and color of the pan, and 

 the depth of water below the rim. Other errors 

 arise from the accumulation of rainwater and 

 the splashing of water due to wind or other 

 causes. 



The many difficulties encountered in the use 

 of open pans of water for measuring evaporation 

 rates led to the devising of more suitable instru- 

 ments for this purpose. Many measurements of 

 evaporation rates have been made with instru- 

 ments called atmometers (Fig. 38). These con- 

 sist essentially of surfaces of porous clay moulded 

 in the form of hollow cylinders or spheres. 

 Water evaporates from such surfaces in essen- 

 tially the same way that it evaporates from a free 

 surface of water. Atmometers are attached to 

 a reservoir of water as shown in the figure, and 

 are usually provided with mercury valves which 

 prevent absorption of rain. The loss of water 

 from such instruments can be determined either 

 by measuring the decrease in volume of water, 

 or the loss in weight of the instrument. Al- 

 though atmometers are not subject to many of 

 the errors inherent in the use of open-pan 

 evaporimeters numerous precautions must be 

 observed in employing them. Details of atmo- 

 metric technique are discussed by Livingston 



(1935). 



Measurement of Transpiration. — Four general methods are in use for 



measuring transpiration, but only the first two of these as listed can be used 



for quantitative determinations of the rate of loss of water-vapor from plants 



which are rooted in the soil. 



I. Method of Weighing Potted Plants. — This method can be employed 



only with plants which are rooted in pots or other suitable containers. For 



Fig. 38. Arrangement 

 for the measurement of 

 evaporation rates by means 

 of an atmometer. {A) por- 

 ous clay sphere, {B) mercury 

 valve, (C) reservoir. 



