surface with drier air from above. Wind speeds and surface rough- 

 ness Influence the relative turbulence of the air, moving the 

 moisture away from the evaporating surface and bringing In drier 

 air to further the evaporation process. Thus, it is apparent that 

 the evaporative demand of the atmosphere is determined by the inter- 

 action of several climatic elements. 



Progress is being made in determining the relationships 

 between the aforementioned climatic factors to arrive at a quan- 

 titative approach to estimating evapotranspiration. 



Evapotransplratlon and Plant Conditions 

 The term "evapotransplratlon" implies the sum of evapo- 

 ration plus transpiration. In the case of plants that are actively 

 growing and well supplied with moisture, transpiration is related 

 and responsive to climatic conditions. Evaporation from soils, hov/- 

 ever, is related more closely to, and limited by, the moisture con- 

 tent of the exposed soil surface than to climatic conditions. In 

 most irrigated areas in California, rain is sparse during the growing 

 season and, except for areas of high water tables, soil svirfaces 

 soon dry through evaporation following irrigation. As a result, 

 under California irrigation conditions, transpiration is usually 

 the larger of the two components comprising evapotransplratlon. 



The primary plant parameter affecting evapotransplratlon 

 rate appears to be the percent of ground cover. This is an im- 

 portant consideration when determining evapotranspiration for an- 

 nual field crops, such as sugar beets and cotton, and for other 



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