supplies. Due to the foregoing reasons, crops are frequently sub- 

 jected to drought for periods of time varying from a few hours up 

 to several weeks . 



Therefore, In the studies reported here, an evaluation 

 was made of the effect of available moisture upon evapotransplratlon, 



Available moisture was determined for the principal root 

 zone for each crop from selected neutron probe soil moisture data. 

 In the case of the alfalfa, a perennial crop, a single zone from 

 0-12 feet was used for the entire study. For cotton, an annual 

 crop, the zone was increased from the 0-1-foot depth to the 0-11- 

 foot depth as the crop grew and the root system developed and ex- 

 panded. The results of the evaluations are discussed further under 

 the sections on crop coefficients. 



Other Factors Affecting Evapotransplratlon 

 Soil fertility and other physical factors of the soil, 

 such as texture, structure, salinity, and even color, affect the 

 growth rate of a crop. Soil properties, such as texture, struc- 

 ture, and salinity may also affect, to some degree, moisture move- 

 ment and utilization. These factors have an undetermined, and 

 probably much lesser, effect on evapotransplratlon than drought, 

 climate, and plant conditions. 



Determination of Coefficients 

 Results of various research projects have indicated 

 that the processes of evapotransplratlon and evaporation are both 

 responsive to the same factors. As will be discussed in ensuing 



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