Evapotransplratlon and CllTnatic Data 

 Climate In the evapoti^ansplratlon process can be though 

 of as a combination of evaporative elements, such as air tempera- 

 ture, wind, dryness of the air, and solar radiation. Other fac- 

 tors of climate, such as length of daylight, may be indirectly 

 related to evaporation. 



The energy sources for the evapotransplratlon processes 

 are derived principally from direct solar radiation and advection 

 or exchangeable heat from the air. The evaporative demand of the 

 atmosphere is largely a function of those tv70 elements. However, 

 not all of the solar radiation that falls directly on the plant 

 or ground surface is used in evapotransplratlon. A portion is re 

 fleeted back into the atmosphere, a portion Is utilized in heatin 

 the air, a portion is absorbed in heating the soil, and the balan 

 is utilized in evapotransplratlon and plant growth. It is likev^i; 

 probable that the energy available from advection is not all uti- 

 lized, depending upon many factors, such as vapor pressure deficis 

 and extent of v/lnd movement. Under certain conditions, it has bei 

 demonstrated that advective cooling, as well as advecting heating 

 may occur. 



As the moisture content of the air increases through 

 evapoi'atlon and/or transpiration, the moisture gradient (vapor 

 pressure gradient) between an air mass and an evaporating surface 

 becomes less steep and retards further moisture transfer. Under 

 field conditions, the air mass near the ground is far from stable 

 Air movements act to mix moisture- saturated air near the evaporatn 



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