46 IRRIGATION PRACTICE 



Fortier reports an average weekly evaporation from a 

 wet soil of 4.75 inches and from a water surface placed 

 under like conditions only 1.88 inches two and one-half 

 times as much. The explanation of this must of course 

 be sought in the higher temperature of the soil due to a 

 lower specific heat and a higher absorptive capacity 

 for heat. 



This strong tendency of water to return to the air 

 by way of evaporation makes it fundamentally impor- 

 tant to devise and put into operation methods that will 

 prevent, to the largest possible degree, this form of the dis- 

 sipation of water. (Fig. 10.) 



35. Conditions determining evaporation. Many fac- 

 tors are concerned in the evaporation of water from the 

 surface of water or any moist substance such as an irri- 

 gated soil. These may be classified as follows: 



1. Nature of soil. 



(a) Physical. 

 (6) Chemical, 

 (c) Depth. 



2. Meteorological conditions. 



(a) Temperature. 

 (6) Sunshine. 



(c) Relative humidity/ 



(d) Winds. 



(e) Showers. 



3. Initial percentage of water. 



4. Condition of top soil. 



(a) Plowing. 

 (6) Cultivation. 



(c) Rolling. 



(d) Packing 



