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 



* fj 



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 ta 

 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 $0% 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 l 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' t . 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 

 \vere 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. y\ Office Expt 

 Stations. 



