in regard to the general pattern of plant growth and the relation- 

 ships of the coefficient with the various factors affecting water 

 use. The soil moisture observations are believed to be of reliable 

 quality, particularly for the i960 data, where a dry soil zone v;as 

 maintained at depth below the root zone, assuring that no deep per- 

 colation of Irrigation water occurred. 



Coefficients for individual period of measurements show 

 a pattern of progressive increase from the lov; early season value 

 to a peak in July, and 'then a progressive decrease to the year's 

 end. The differences in coefficients during the early season 

 emphasized the direct relationship between the evapotranspiration 

 and percent of ground cover. The decreasing pattern of coefficients 

 after July reflects the integrated effect of decreasing ground 

 cover, physiological aging of the plants, and availability of 

 soil moisture. 



It is of interest that, although ground cover on these 

 plots reached 80 and 95 percent, the maximum coefficients were 

 reached at a ground cover of about 60 percent. This corresponds 

 in time to the boll setting. It is believed that physiological 

 factors may have had an influence on the transpiration rate at 

 this stage of plant development. Physiological factors are be- 

 lieved to have caused similar effects in other crops. With small 

 grains, for example, peak water use rates are reported to occur 

 at the heading stage. 



Late-season use of water by cotton is dependent to some 

 extent upon the amount of moisture available prior to natural or 



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