2 18 Journal of Agricultural Research voi. x, no. s 



EVAPORATION FROM CIRCULAR LAND TANKS OF DIFFKRENT DIAMETERS. 



This Study, the first one begun at the laboratory, was started in 

 November, 191 5, and the results of one year's work are given. Tanks 

 1,2,4 (PI. 34, C), 5, 6, and 7 (PI. 34; C, 35 B), described previously, are of 

 a type used by the Office of Public Roads and Rural Engineering and 

 at several State Experiment Stations throughout the country. This 

 general style of tank most nearly approximates the reservoir, and the 

 results obtained can be more safely extended to the reservoir of some 

 size. While of metal, but a small rim of this metal is above the ground 

 surface; hence, but little heat derived from the sun's rays is conducted 

 to the water. Further, since the tank is largely below the ground surface, 

 radiation from the tank takes place from the water surface only. The 

 same conditions are true of the reservoir. The more "unnatural" 

 evaporation pan, that setting wholly above the ground and usually made 

 of metal, is subjected in the greatest possible extent to the concentration 

 and radiation effects of the metal. It has long been known that temper- 

 ature has some effect upon the rate of evaporation. It seems quite 

 evident that results obtained from tanks of the type where the water 

 is below the ground surface, especially those of large size, can be more 

 safely extended to the reservoir than results from the tanks with all 

 sides exposed to the air. 



The detailed figures of the year's results from this set of tanks are 

 shown in Table 11. Figure 4 shows the relation graphically. In the 

 table (also the curve) the evaporation from the largest tank, No. 7, 

 which is 12 feet in diameter, is taken at 100 per cent as a basis for further 

 computation. The actual depths are given in inches as well as percent- 

 ages. The period from March 6 to November 13, 191 6, was such that 

 very little ice interfered with measurements, and the figures shown are 

 evaporation from a free and an open water surface. The percentages 

 for this period run very near to those for the entire year. Over the 

 range of areas 0.785 square foot to 11 3.1 square feet, or diameters i foot 

 to 12 feet, the range in evaporation for the year is 76.18 to 49.16 inches, 

 and in percentage 154.9 to 100 per cent. 



At all times when a water surface is free from ice capillarity is pulling 

 the water up on the metal of the tank at all points of the circumference, 

 and wave action also wets the side of the tank, thus adding to the area 

 from which evaporation takes place. These forces may or may not 

 supply the water as fast as it can be evaporated from this wet surface. 

 For the smallest tank this wet strip of metal is 3.14 feet long, or the 

 ratio of this length to the exposed area of the tank is 3.14 to 0.785 =4.0. 

 This same ratio for the tank 12 feet in diameter is 37.70 to 113. i =0.333. 

 Thus, if the wet strips are of the same width, the effect is 1 2 times as 

 great for the small tank as for the large one. However, wave action will 

 tend to wet a wider strip on the large tank and tend to equalize the 

 effect on the two tanks. 



