154 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



The main object of this discussion is to make clear the need for 

 evaporation records day by day, and for a finer analysis of the sea- 

 sonal records than has been the custom in recent ecological studies. 



Instrumental Methods. 



The instrumental methods of measuring evaporation may be of 

 two kinds, employing respectively free-water bodies, as in the open 

 tank, and evaporating surfaces which are kept continually moist 

 but in which the water is retained and to some extent withheld from 

 evaporation by capillary action. The latter, for brevity's sake, will 

 be spoken of as nonfree-water methods and instruments. The litera- 

 ture of instrumental evaporation studies is very extensive and has 

 been fully annoted by G. Livingston (162). This discussion must 

 be confined to a few of the more recent efforts. 



FREE- WATER SURFACE. 



The free-water surface receptacle is used almost exclusively in 

 broad climatological and irrigation studies and only to a limited 

 extent by ecologists. The Weather Bureau has now adopted a stand- 

 ard free-water surface evaporating pan, 10 inches deep. 48 inches in 

 diameter, and constructed of 22-gauge galvanized iron, which is de- 

 scribed by Kadel (155). Largely through the work of Bigelow (152), 

 it was determined to be essential, that the receptacle have a surface 

 of known area, that the water exposed in the pan or tank have a 

 known volume, that the surface of the water have a known and as 

 nearly constant distance below the margin of the vessel as possible, 

 and that the material and shape of the receptacle be the same in all 

 cases. All of these conditions are principally effective on the tem- 

 perature attained by the water, and variation in any one of them is 

 apt to affect the evaporation. The size of the vessel and the water 

 it contains, for example, have much to do with the hourly rate of 

 evaporation. A small vessel of a given aerial surface and depth will 

 give a higher evaporation than will a vessel five times as large, since 

 the water in the smaller vessel will more readily adjust itself to con- 

 ditions of the surrounding air than the water in the larger pan. The 

 temperature of the evaporating medium, of course, is of great im- 

 portance in determining the rate at which vapor rises from a pure 

 liquid. It is now the standard procedure to select a very open site 

 for evaporation measurements, avoiding objects which might shade 

 the pan or give it reflected light. The pan is placed on a low plat- 

 form or crib, only a few inches above the ground, vet allowing air 

 circulation all around it. 



Measurements. 



The amount of water evaporated is determined by the use of a 

 hook gauge, the loss in any 24-hour period hem- corrected for pr< 



