the rates of evaporation at the two places, Salton Sea, Cal., and San Carlos, Ariz., are to each other as the excesses above freezing of the temperatures of their respective water surfaces. "But the average air temperature at Salton Sea is about 74° F. and at San Carlos 64° F., and hence the average of the water surface temperatures in the neighborhood of 68° F. and 60° F., respectively, above freezing. "Hence we get the proportion: The yearly evaporation at Salton Sea, 72 inches, is to the yearly evaporation at San Carlos as 36 is to 28; that is, the yearly evaporation at San Carlos on the above assumption is in the neighborhood of 56 inches.' Very respectfully, WILLIS L. MOORE, Chief, United States Weather Bureau. 4. The rainfall at San Carlos averages 13 inches. If the evaporating influences are the same at San Carlos and at Salton Sea, then the evaporation from the San Carlos reservoir would be 69 inches (or 72) less 13 inches, equal 56 inches (or 59). Thirteen inches of rainfall is deducted from the total evaporation notwithstanding that the rain falling upon a portion of the reservoir is already figured in the water supply itself, since between the dam and the points where run-off determinations were made there is a drainage area tributary to the reservoir which can be counted on to supply enough water to offset the rainfall on the portion of the reservoir referred to. 5. As regards the difference in elevation between the Salton Sea and the San Carlos reservoir, we may quote Prof. Bigelow (see also Prof. Humphreys in the letter quoted above) to the effect that "there is no evidence that the water in pans of the same size evaporates faster on the plateau levels, 4,000 feet, than at sea level." 6. In report of Advisory Board of Army Engineers on Water Supply for San Francisco, page 80, it is stated that Records of the United States Geological Survey show that at Kingsburg, on Kings River, during a four-year period (1881-1885) it (evaporation) varied from 2.69 feet to 4.68 feet, the mean being 3.85 feet. At Sweetwater Reservoir, during a period of 10 years (1889-1899), it varied from 3.75 to 5.17 feet. From a tank at Reno, Nev., in 1894, it is given as 3.35 feet. The mean for Clear Lake, Lake County, is given as 32 inches. Mr. Schussler, of Spring Valley Water Co., states that several years' observation at Crystal Springs Reservoir showed evaporation at the rate of one-fourth inch per day for 200 days in the year, or 50 inches per year. For a region with fogs so prevalent as is the (San Francisco) peninsula, this rate of evaporation, as compared with those from the other places mentioned above, seems abnormally high. However, it has been given weight in accepting 4 feet as the average annual depth of evaporation from surfaces of Sierra and Coast Range reservoirs. 7. As to pan observations, Prof. Bigelow states: The purpose of studying evaporation from pans has been to develop formulas and tables applicable to water surfaces without using any pan, because of the insuperable difficulties of making accurate transfers from pan to lake, on account of the very complex nature of the pan action, which has now been fully explained and illustrated. The temperature of the water, the vapor pressure, and the wind are all very different at the lake from what they are at the pan, and there is no reliable way known to go from one to the other. There can be no more erroneous procedure than to assume that the evaporation of a pan at some distance from a large water surface is equal to that at the lake. In dry climates the large water surface evaporates from one-third to two-thirds the rate of a 1 square meter pan. (Bulletin of the Argentine Meteorological Office, Walter G. Davis, director, March, 1911.) He also states that "water evaporates much faster from small pans than from large pans," and that "the evaporation from a 4-foot standard pan, when corrected for temperature and wind, and multiplied by the factor 66 per cent, is about what observation suggests" for near-by water surfaces. (Monthly Weather Review, 1910.) From the Salton Sea experiments it would appear that this factor, 0.66, is correct only when applied to evaporation measured in 4-foot pans placed immediately above the water surface of the lake or reservoir, and is too large for evaporation measured in pans on the land. 8. The great differences in evaporation measured at the same place under different circumstances are well illustrated by the following figures taken from the Monthly Weather Review for 1910: (a) Evaporation from a pan 2 feet in diameter on the ground, 1,500 feet inland from the shore of the Salton Sea.... (b) Same, except pan 40 feet above ground (on tower).. Inches. 164. 50 193.44 (c) Evaporation from a 4-foot pan, 2 feet above water surface, on tower 500 feet off-shore in Salton Sea.. 108. 65 (d) Same, except pan 45 feet above water surface (on tower).. 137. 71 (e) Tower, 7,500 feet off shore in Salton Sea; pan 4 feet in diameter, 2 feet above water surface.... 106. 45 Same, pan 45 feet above water surface.. 140. 02 (g) Evaporation from a 2-foot pan on ground at Granite Reef Dam, Arizona.. (h) Same, 3-foot pan, elevated 10 feet above ground.. 115. 18 Same, 4-foot pan, floating...... 136. 84 97.74 Many additional observations could be quoted to show the effect of elevating the pan 10 feet or more. While this throws no light on the probable evaporation from a reservoir at San Carlos, it indicates that pan evaporation, when the circumstances surrounding the observations are not stated, may be misleading. For example, the figures given in (a) and (b) above, viz., 164.5 and 193.44 inches, considered alone would lead one to believe that the evaporation from the Salton Sea must be greater than the measured value, 69 (or 72) inches. There is close accord between (c) and (e), and (d) and (f), respectively. The factor 0.66 applied to (c) or (e) gives about 70.1, which is very close to the determined evaporation. If we assume that the evaporation from the Salt River at the Granite Reef Dam is the same as the evaporation from the Roosevelt Reservoir, then it appears from (i) that evaporation from a floating pan is considerably greater than from the water in which the pan is floating. The pan evaporation would depend, among other things, on the depth of water in the pan, but it is likely that on account of difference in depth of the river and reservoir, motion of the river, etc., the evaporation from the river at Granite Reef Dam is greater than from the reservoir. These considerations may account in some measure for the discrepancy between the figure given in (i), viz, 97.74, and the accepted depth of evaporation from the reservoir, viz, 53.03. It may be noted that 97.74 × 0.66 63.9. = 9. In the following letter it is stated that (pan) evaporation near El Paso amounts to 84.84 inches: To Mr. M. O. Leighton, UNITED STATES GEOLOGICAL SURVEY, Chief Hydrographer, United States Geological Survey, Washington, D. C. In connection with the discussions concerning the San Carlos Reservoir, there has been some difference of opinion regarding evaporation, and you may be interested in the following figures compiled by H. J. Gault, engineer, showing the results of experiments on evaporation carried on by the United States Reclamation Service in connection with Prof. Bigelow, of the Weather Bureau. These observations were carried on on the Rio Grande where the altitude is higher and the climate not near so warm or so arid. The evaporation in the Rio Grande Valley should be less than that at San Carlos. A. P. DAVIS, This presumably is the total evaporation; i. e., including rainfall. If we apply the factor 0.66 and subtract the rainfall at San Carlos, we have for evaporation from the San Carlos reservoir, 84.84 x 0.661343 inches. 10. The results given below were obtained by Prof. R. H. Forbes from investigations carried out under his direction at the University of Arizona, Tucson: Prof. Forbes states that this figure, 85.51, includes rainfall. If we assume the evaporation at San Carlos to be the same as at Tucson, apply the factor 0.66 and subtract the San Carlos rainfall we have for evaporation from the San Carlos reservoir, 86.51 × 0.66-13 = 44 inches. 11. The consulting engineers, J. D. Schuyler and H. Hawgood, in their report of September 27, 1910, to Chas. R. Sligh, say in regard to the adoption by Mr. Leighton in his report of February 8, 1910, of Lippincott's figure (91 inches): Further, the provision for evaporation of 91 inches per annum, founded on experiments made some 20 years ago, is excessive, and not supported by the more recent and searching investigations made by the United States Weather Bureau at Reno and at Salton Sea, and if the Department of Agriculture on the slopes of Mount Whitney, Cal., which clearly indicate that the probable evaporation from a reservoir surface of the elevation and location of San Carlos would be in the neighborhood of 66 and not 91 inches. 12. Evaporation at the Roosevelt Reservoir is estimated by Prof. R. H. Forbes to be as in the following table: The measured evaporation at the Station Farm includes rainfall. The circumstances surrounding these observations are thought to be such as to make the measured evaporation approximate that from an open water surface. 13. Lippincott, in Water Supply Paper No. 33, assumed the depth of evaporation from the proposed San Carlos Reservoir to be as in the following table: Applying the factor 0.66, we have 60.1 inches. 91 14. Pan evaporation at Independence, Cal., under desert conditions, is reported to be 5.467 feet, as result of partial observations taken in 1908 and 1911, and observations extending through the years 1909-10. (Water Supply Paper 294, p. 118.) 15. In 1912 observations of variations in the level of Owens Lake, Cal., measurement of inflow, including rainfall, showed that the depth of evaporation was 4.61 feet. The water of this lake is heavily alkaline. 16. Collecting the above figures, we have: Salton Sea. San Carlos Reservoir (from Salton Sea, Chief of Weather Bureau) 1 69.00 56.00 2 56.00 Rio Grande, pan evaporation (Davis). 84. 84 San Carlos Reservoir, from Rio Grande, applying factor 0.66 and difference in rainfall 43.00 Tucson, pan evaporation (Forbes). 86.51 San Carlos Reservoir, from Tucson, applying factor 0.66 and difference in rainfall..... 44.00 17. From the above, it seems justifiable to assume net evaporation from the San Carlos Reservoir at 60 inches, or a gross evaporation of 73 inches. 18. The following table shows the monthly percentages of the total annual evaporation as deduced in the Tucson and Rio Grande experiments and as used by Lippincott in Water Supply Paper No. 33, respectively: 19. The Tucson percentages will be assumed for the San Carlos Reservoir. Net monthly evaporation from San Carlos Reservoir. Determined from Weather Bureau rainfall records at San Carlos extending over 31 years. C. H. MCKINSTRY. |