The amount of water evaporating either from a water-surface, or from a wet or moist soil, varies greatly according to the climatic conditions, and the state of the weather; also according to the condition of the soil-surface. There are damp climates, and days or periods when, the air being nearly saturated with moisture, evaporation even from a water-surface will be almost insensible. On the other hand, with dry air and a high temperature, enormous quantities of water may be evaporated in the course of a day. The evaporation from water-surfaces interests deeply those who supply, as well as those who are supplied with, water from storage reservoirs; evaporation from the soil-surface interests deeply all farmers, and more especially irrigators whose water-supply is scanty, or is paid for by them by measurement. Light rains, as well as light surface irrigations, may at times evaporate almost wholly without any effect save a lowering of the temperature of the soil. In the case of snow, it is a well-known fact in the northern arid regions that a light snowfall may in winter evaporate entirely without imparting any liquid moisture to the soil. A loss of 50% of the water actually brought upon land by surface irrigation is of common occurrence in some portions of the irrigated region. The dependence of evaporation upon air-temperature under conditions otherwise identical, is well illustrated by the experiments made in 1904 by S. Fortier[96] on the Experiment Station grounds at Berkeley, California, at a time when under the influence of the sea breeze the average saturation of the air might be assumed at about 70%. The tests were conducted in six tanks sunk into the ground so as to place the water-surfaces on a level with it, and the water-temperatures were maintained in four of the tanks by means of ice or heating lamps. The results are shown in the following table: SUMMARY OF AVERAGE WEEKLY LOSSES BY EVAPORATION, WITH VARYING TEMPERATURES OF WATER, AT BERKELEY, CAL., IN JULY AND AUGUST, 1904. =======================+============ Temperature of water. | Weekly |evaporation. -----------------------+------------ Degrees Fahrenheit: | Inches. 55.5 | 0.42 62.0 | 0.77 69.2 | 1.54 80.1 | 3.08 89.2 | 3.92 -----------------------+------------ [96] Progress Report on Coöperative Irrigations in Calif.; Cir. No. 56, Office Exp’t Stations.