crops having variable ground cover percentages, such as alfalfa, 

 which is cut frequently. 



Crops having rapid growth rates and vigor tend to provld 

 greater ground cover more rapidly than a slow-grov/ing crop, even 

 of the same species. Thus, differences in growth rate may affect 

 evapotranspiration rates through the direct mechanism of percent 

 of ground cover, although other physiological factors, such as 

 stage of maturity or growth, may also affect evapotranspiration. 



Evapotranspiration and Soil Moisture 

 Research findings relative to the effect of variations 

 of available soil moisture upon evapotranspiration and plant grov/t 

 are varied. 



The amount of soil moisture available above the permaner 

 wilting point does not seem to affect the evapotranspiration rate 

 of crops, according to many research reports. Other research has 

 indicated that maximum growth rates are obtained only under condi- 

 tions of high moisture availability, and that growth rates and 

 yields are retarded as soil moisture availability decreases. Thes 

 concepts differ from other research investigations which have indi 

 cated a close relationship between evapotranspiration and plant 

 growth. These concepts are of particular Importance in conslderir 

 if evapotranspiration rates are affected by lov; soil moisture leves 

 vjhlch appear to affect growth rates, such as occur v;hen irrigatior 

 is deliberately withheld from grapes and cotton to change their 

 fruiting characteristics. 



Besides intentional withholding of irrigation, there are 

 also occasions of drought due to insufficient irrigation water 



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