juiyao. I9I7 Evaporation J yom IV ater a^dd River-Bed Materials 225 



In the case of the 9 square-foot area the ratio, perimeter divided by 

 area, is 0.15 greater in the case of the square tank than for the circular 

 one. This has apparently caused an increase in evaporation of 2.7 per 

 cent. For the tanks of 3.14 square feet area there is a corresponding 

 increase of 0.26 in the ratio and an increase of 3.5 per cent in evapora- 

 tion. 



Although these figures do not show the exact relation, the comparison 

 between the results from round and square tanks is sufficient to show 

 that the great difference in the ratio, perimeter to area, for tanks ranging 

 from I foot to 1 2 feet in diameter has an important part in the difference 

 in evaporation depths from these tanks. No difference in mean water 

 temperature could be measured between tanks 2 and 15, and 3 and 4. 



VARIATION OF EVAPORATION WITH DEPTH OF TANK SET IN THE GROUND 



Tanks 18 to 22 and tank 3 are 2 feet in diameter. They range in 

 depth of water from 0.25 foot to 5.75 feet. Tanks 16 and 17 are 6 feet 

 in diameter and, taken with No. 5, the set of three range in depth of 

 water from 0.75 foot to 2.75 feet. These two sets were installed quite 

 late in the season (May 25), but the results are representative. Table V 

 shows the weekly evaporation depths for the nine tanks and the ratio 

 of the evaporation from the tanks of lesser depth to that from the deepest 

 of each set, expressed as a percentage. It is quite evident that the dif- 

 ference in evaporation over the range of depths is due to temperature. 

 During the months when the cooling effects of the night were not so 

 great, the shallow tanks show the greater evaporation; but later, when 

 the day temperatures and the heat storage of the shallow tanks are more 

 than offset by the low night temperatures, the shallow tanks indicate a 

 lesser evaporation. 



This difference in evaporation is not great, but for general use a tank 

 not less than 2 feet deep is recommended, since its contents will not 

 become heated or cool as quickly as those of the shallower tank. The 

 difference between the results from the 6-foot tank and the 3-foot one 

 is so slight that under all ordinary conditions there is no necessity for 

 using a tank deeper than 3 feet. The evaporation as measured from 

 tanks from 2 to 3 feet in depth may be more safely extended to the 

 reservoir than that from shallower tanks, since the deeper tanks operate 

 more in accordance with the reservoir under natural conditions. 

 98976°— 17 2 



