AMOUNT OF EVAPORATION 



131 



sTION^ 



iTROM 



possible, direct evaporation of water from the soil 

 must be prevented. 



Few farmers really realize the immense possible 

 annual evaporation in the dry-farm territory. It is 

 always much larger than the total annual rainfall. 

 In fact, an arid region may be defined as one in which 

 under natural conditions several times more water 

 evaporates annually from a free water surface than 

 falls as rain and snow. For that reason many stu- 

 dents of aridity pay little attention to temperature, 

 relative humidity, or winds, and simply measure the 

 evaporation from a free water surface in 

 the locality in question. In order to ob- 

 tain a measure of the aridity, MacDougal 

 has constructed the following table, show- 

 ing the annual precipitation and the an- 

 nual evaporation at several well-known 

 localities in the dry-farm territory. 



True, the locahties included in the fol- 

 lowing table are extreme, but they 

 illustrate the large possible evap- 

 oration, ranging from about six to 

 thirty-five times the precipitation. 

 (See Fig. 32.) At the same time 

 it must be borne in mind that 

 while such rates of evaporation 

 may occur from free water sur- 

 faces, the evaporation from agricultural soils under 

 like conditions is very much smaller. 



RAlNfALO 



WATER! 



SURTACE 



Fig. 32. Annual rain- 

 fall and evaporation 

 in arid region com- 

 pared. The high evap- 

 oration rate makes 

 necessary thorough 

 farming. 



