48 



IRRIGATION PRACTICE 



water, as it evaporates from the soil, saturates the air 

 immediately above the soil surface; and thus evapora- 

 tion is diminished. Winds remove this layer of saturated 

 air, and the rate of evaporation is increased. Summer 

 showers, likewise, by establishing capillary connection 

 with the lower soil layers,' hasten evaporation. 



Finally, the wetter the soil is at the surface, the more 

 rapidly is water evaporated from it. This vitally impor- 



tant principle was observed in the Utah work, and has 

 been confirmed by Whitney and Cameron and by For- 

 tier. This law of the initial percentage declares that the 

 evaporation of water from a soil surface varies as the 

 initial percentage of soil moisture, that is, the mois- 

 ture at the beginning of the test. 



The three most important factors in determining the 

 evaporation of water from a soil are undoubtedly the 

 average temperature, the relative humidity of the air, 

 the wind, and the percentage of water held by the soil, 



