JOURNAL OF ACRIMIIRAL RESEARCH 
Vol. IX Washington, D. C., May 28, 1917 No. 9 
COMPARISON OF THE HOURLY EVAPORATION RATE 
OF ATMOMETERS AND FREE WATER SURFACES WITH 
THE TRANSPIRATION RATE OF MEDICAGO SATIVA 
By Lyman J. Briggs, Biophysicist in Charge of Biophysical Investigations , and H. L. 
ShanTz, Physiologist, Alkali and Drought Resistant Plant Investigations , Bureau of 
Plant Industry, United States Department of Agriculture 
INTRODUCTION 
The rate of evaporation from a free water surface or from a moist 
porous surface is usually considered the best single-valued expression 
of the intensity of the weather factors influencing transpiration. Such 
a relationship is, however, subject to the uncertainty arising from the 
marked differences in the energy-absorbing and energy-dissipating 
properties of the transpiring and evaporating surfaces. It is evident 
that the transpiring and evaporating surfaces must be in agreement in 
this respect if the departure of transpiration from evaporation during the 
day is to be taken as evidence of a change in the transpiration coefficient, 
resulting from stomatal control or other reversible changes within the 
plant body. 
Fluctuations in transpiration from day to day appear to be reflected 
with approximately the same degree of fidelity by a number of widely 
different forms of evaporating surfaces, provided precautions are taken 
to maintain the uniformity of these surfaces throughout the period of 
observation. When the hourly transpiration rate is under consideration, 
however, the individuality of the evaporating surface to which the 
transpiration is referred can not be ignored. It is this phase of the 
question that forms the subject of the present paper. 
APPARATUS AND METHODS 
Atmometers. —Four types of porous-cup atmometers as designed by 
Livingston (1915), including white cylinders, brown cylinders, white 
spheres, and white Bellani plates, were employed in the measurements 
(PI. 4). 
Journal of Agricultural Research, 
Washington, D. C. 
ic 
(377) 
Vol. IX, No. 9 
May 28, 1917 
Key No. G— 112 
