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The Geology and Petrography of Crater Lake National Park, 1902

 PART I.

CRATER LAKE.

EVAPORATION FROM CRATER LAKE.

The observations upon the oscillations of the lake surface appeared to show a greater loss of water than would be accounted for by evaporation alone, but to get more definite data concerning the matter arrangements were made to determine approximately the rate of evaporation on the lake surface. For this purpose a tin evaporating pan 13 inches square and 6 inches deep was used. To avoid the bright reflections its inside was painted dirt brown with pitch and soil. It was supported on a raft (Pl. XIII, B) so that the lower part of the pan was in the water of the lake. The raft was anchored in an open bay where the water was 7 feet deep and freely agitated by the winds, but the waves did not move quite so high as on the lake. For this reason the evaporation from the pan was possibly less than that of the lake.

The following table gives the observations made at the lake in 1901:

Observations of evaporation from Crater Lake.

During the day the air and the water in the pan became warmer than the water of the lake, and during the night they became colder. Twice a day the temperature of lake, air, and water in pan agreed, but generally the temperature of the water in the pan was nearer that of the air than was that of the lake. If the amount of evaporation is in large measure proportional to the difference of temperature between the air and water, the rate of evaporation from the lake must have been greater than from the pan. However, it is believed that the higher temperatures of the pan water under the dry summer atmosphere of Crater Lake more than counterbalanced the effect of greater temperature difference, and that the rate of evaporation from the pan was as great as that from the lake.

The average evaporation for the 10 days’ record used is 0.0125 feet per day. At this rate per annum the evaporation would be 55 inches. During cloudy winter weather evaporation is much reduced, so that 55 inches must be greater than the annual rate.

Professor Russell, in his mapa, makes the evaporation per annum for the Crater Lake region about 46 inches, which appears to be a close approximation.

^aFourteenth Ann. Rept. U. S. weather Bureau, Pl. VI.

Observations on evaporation were made also in camp on the rim of Crater Lake. Two pans 13 inches square and 5 inches deep were used. One pan (1) was bright and the other (2) browned like that on the raft by pitch and soil, and both were placed in the sun. Observations were made twice daily and the temperature of the air and water noted, as in the accompanying table. The rate varied considerably. The average loss per day in the bright pan was 0.0186 of a foot, while that in the browned pan by its side for the same time was 0.0256 of a foot. The dirt-colored pan unexpectedly gave the highest rate of evaporation, and the rate from both pans exceeded that on the lake. The temperature of the water in this case was nearer that of the air than that in the case of the raft pan, and yet the evaporation was so much greater on account of the higher temperature. These observations tend to indicate that the evaporation from the lake can not be greater than that measured by the pan on the raft, and this gives me more confidence in the conclusion concerning the amount of water that escapes by filtration.

Observations to determine evaporation at Crater Lake.

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