5. The lrrln:ated field should be located in typical 

 lrri,e^ated areas, not on the frlnee of irrigated areas. 



6. There should be an adequate supply of irrigation 

 v/ater. It is highly desirable that there be possibilities for 

 controllinr; and measurin,"; the amount of vvater applied to the test 

 plot. 



7. Except for measurements which are made by evapo- 

 transplrometers, no water table should exist within or near the 

 root zone of the crop, 



Evapotranspiration Measurement Techniques and 

 Discussion of Development and Current Methods 



The tools and techniques used in this study to measure 



evapotranspiration fall into two general categories. One is field 



plot sampling, and the other is evapotranspirometer measurements. 



Field Plot Sampling - Gravimetric Method 



Periodic measurement of soil moisture provides a means 

 of determining total change of water content within a selected 

 portion of the soil profile. Evapotranspiration may be determined 

 from data on soil moisture change and precipitation. Soil moisture 

 must be sampled or measured each time at or near the same location 

 in each plot, with several locations being situated in each plot. 

 Moreover, the moisture determinations must be made at least twice 

 following wetting of the soil by any heavy irrigation or heavy 

 precipitation. To obtain satisfactory results, it is necessary 

 that sufficient time lapse be permitted following thorough wetting 

 of the soil (usually several days) before making the first moisture 



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