juiy3o. I9I7 Evaporation from Water and River-Bed Materials 253 



Table XXIII. — Actual evaporation from South Platte graded river-bed material, i, 7vifh 

 water table at different depths below the surface. All tanks 2.5 feet in diameter 



Period ending < 



Actual depth of water evaporated 

 (inches). 



Evaporation 

 from water 

 surface of tank 

 2(2 feet in 

 diameter, 3 

 feet deep). 



Water table below the 

 surface. 



3 inches. 12 inches, 



Aug. 9. 



IS- 



17- 



29. 



Sept. 12. 



25- 



29. 



Oct. 4- . 



ro. . 



16.. 



Total 



Equivalent for tank 1.96 feet in diameter. 

 Percentage c 



.85 

 .69 



•54 



3-54 

 4.44 



2-73 

 .91 



I. 23 

 •99 



I. 00 

 .68 



•39 



.42 



1-75 

 2.52 

 I. 90 

 ■59 

 .63 

 •51 

 .63 



0.74 



•45 



.18 



.29 



I. 01 



.49 

 ■72 



• 14 

 .06 

 . 00 



• 17 



17.92 



II. 02 

 ^ II. 82 

 66.0 



4.25 

 & 4. .S8 

 24. 2 



o The period began on Aug. 4, 1916. 



b See Table XXIV for ratio for this correction. 



<^ Water surface taken as 100 per cent. 



From the list of tanks used it will be seen that No. 53 is a duplicate 

 of Nos. 33 and 34, and that No. 54 duplicates Nos. 37 and 38, with the 

 exception that 53 and 54 are 30 inches in diameter, while the others are 

 23>^ inches. From work on evaporation from water surfaces, the differ- 

 ence in evaporation depths due to difference in size of the evaporation 

 pan has been noted. Data were not at hand to show whether or not this 

 relation would hold for the sand tanks. Table XXIV gives the results 

 from the sand tanks of two diameters. The final figures indicate that 

 for the period of the sand-tank work the evaopration from the surface 

 of the sand from the smaller tank, approximatel)^ 2 feet in diameter, 

 v/as about 7X per cent greater than from the larger tank. This figure 

 does not check that found for the v^ater tanks, the corresponding differ- 

 ence there being 3^2 per cent. The data do not indicate the cause of 

 this difference, but the assumption is that it is due to a temperature 

 effect. It is probable that the moist sand acts in a different capacity 

 as a heat reservoir than does a tank of water. 



