86 DIVISION OP WATER RESOURCES 



in the case of the small tank 2 feet in diameter and to a less extent in 

 the case of the 6-foot tank. The Weather Bureau pan, 4 feet in diameter 

 and 10 inches deep, is set entirely above the ground, and receives heat 

 from the sun on its sides as well as on the water surface, and it has also 

 a maximum exposure to air movement. The 2-foot tank, located on the 

 l)ank and sunk in the ground, could receive but very little heat energy 

 from the dry cold ground surrounding it, but probably some of the 

 heat received on its water surface was conducted down through the 

 water and away into the soil. Accordingly, the evaporation from the 

 2-foot tank on the bank was least ; and that from the 2-foot tank in the 

 swamp was greatest, the latter being, in fact, 8.2 times as much as the 

 former. The water in Tank No. 2 received enough heat energy from 

 the swamp water to keep it relatively warm, while the water surface 

 in Tank No. 1 remained relatively cold. The fact that the rate of 

 evaporation is relatively high when the temperature of the water is 

 greater than that of the air, has l)een pointed out by Rohwer.* 



A comparison of the evaporation from Tanks Nos. 2 and 3 shows the 

 loss from Tank No. 2 to be 1.65 times that from Tank No. 3. It is of 

 interest to note that studies at the Salton Sea in 1910, using exposed 

 tanks, showed a ratio of 1.48 to 1 for the loss from a 2-foot tank com- 

 pared to that from a 6-foot tank.** In 1916, Sleight found the ratio of 

 the loss from a 2-foot tank to that from a 6-foot tank to be 1.17 to 1 

 for tanks sunk in the ground. The higher ratio found at the Victor- 

 ville station is luidoubtedly due to the heating effect of the surrounding 

 water as noted above. 



As stated above, Table 29 indicates that the loss by evaporation 

 from the free water surfaces in the cienega is relatively high during 

 tlie winter. The reason for the high losses is that the rising water is 

 relatively warmer than the air during the winter months. 



In further demonstration of the extreme effect of exposure on 

 rates of evaporation and transjnration from tanks, it should be noted 

 that during December and January Tank No. 1 lost b.y evaporation less 

 than one-third as much as Tank No. 2, but, in Jul.v, 1931, conditions 

 were reversed and the consumptive use by evaporation and transpira- 

 tion from the tules in Tank No. 1 was nearlv four times as much as from 

 Tank No. 2. 



Mean monthly values for the entire period of record, for the 

 evaporation and use of water from tule Tanks Nos. 1, 2 and 3, together 

 with the climatological data, are given in Table 30. The mean annual 

 evaporation from the Weather Bureau pan, as shown in Table 30, 

 was 82.46 inches. Tlie mean annual consumptive use of water from 

 tule Tank No. 1, which liad the same ex]iosure on the bank as the 

 evaporation pan, was 272.24 acre-inches per acre, Avhile from tule 

 Tanks Nos. 2 and 3, located in the swamp and characteristic of swamp 

 conditions, there were used 84.45 acre-inches per acre and 78.45 acre- 

 inches per acre, respectively. Plate XVTTI shows the mean monthly 

 evaporation and use of water from Tanks Nos. 1, 2 and 3. 



* "Kvai^oration from Free Water Surfaces," by Carl Rohwer, United States 

 Department of ARriciiUure, Technical Bulletin, No. 271. (1931.) 



** Studies on the Phenomena of Evaporation of Water Over Lakes and Reservoir.s. 

 Summary of the Results of the Salton Sea Campaign. By F. H. Bigelow. Monthly 

 Weather Review, Vol. 38, No. 7. (1910.) 



