254 SOILS. 



also the depression of temperature which, as a rule, is un- 

 favorable to the best development of vegetation. It is only 

 in case of extreme stress from hot, drying wind that such 

 evaporation and the consequent depression of the temperature 

 of the surface soil can be of advantage to the farmer. 



The amount of water evaporating either from a water-sur- 

 face, or from a wet or moist soil, varies greatly according to 

 the climatic conditions, and the state of the weather; also ac- 

 cording to the condition of the soil-surface. There are damp 

 climates, and days or periods when, the air being nearly sat- 

 urated 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 ex- 

 periments made in 1004 by S. Fortier 1 on the Experiment 

 Station grounds at Berkeley, California, at a time when un- 

 der the influence of the sea breeze the average saturation of the 

 air might be assumed at about 70%. The tests were con- 

 ducted 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 heat- 

 ing lamps. The results are shown in the following table : 



1 Progress Report on Cooperative Irrigations in Calif. ; Cir. No. 56, Office Exp't 

 Stations. 



