EVAPORATION PROM SOILS 137 



out. This part of the soil is the hygroscopic moisture 

 spoken of in the last chapter. 



Moreover, it must be kept in mind that evapora- 

 tion does not occur as rapidly from wet soil as from 

 a water surface, unless all the soil pores are so 

 completely filled with water that the soil surface 

 is practically a water surface. The reason for this 

 reduced evaporation from a wet soil is almost self- 

 evident. There is a comparatively strong attraction 

 between soil and water, which enables the moisture 

 to cling as a thin capillary film around the soil par- 

 ticles, against the force of gravity. Ordinarily, 

 only capillary water is found in well-tilled soil, and 

 the force causing evaporation must be strong enough 

 to overcome this attraction besides changing the 

 water into vapor. 



The less water there is in a soil, the thinner the 

 water film, and the more firmly is the water held. 

 Hence, the rate of evaporation decreases with the 

 decrease in soil-moisture. This law is confirmed by 

 actual field tests. For instance, as an average of 

 274 trials made at the Utah Station, it was found that 

 three soils, othei*wise alike, that contained, respec- 

 tively, 22.63 per cent, 17.14 per cent, and 12.75 per 

 cent of water lost in two weeks, to a depth of eight 

 feet, respectively 21.0, 17.1, and 10.0 pounds of water 

 per square foot. Similar experiments conducted 

 elsewhere also furnish proof of the correctness of 

 this principle. From this point of view the dry- 



