they are devices which enable the evaluation of the moisture recime 

 of a confined soil mass, of known dimensions, in which a crop is 

 grown. Moisture changes of the crop- soil system are determined by 

 periodic or continuous welBhing, or by volumetric determination of 

 water displaced, added, and/or removed from the system. 



When used for the determination of field evapotranspiration. 

 it is particularly important that the tanks be installed in such a 

 manner that their presence does not modify the environment of the 

 measured crop. Although this technique appears to be an excellent 

 method for precise measurement of crop water use, certain factors, 

 such as the artificial restriction of crop rooting and possible 

 modification of soil heat transfer, have yet to be completely evalua- 

 ted. Research on these factors is presently being conducted by the 

 University of California. 



The use of evapot ran spirometers in the field was not com- 

 mon in California at the Initiation of this program, although tanks 

 had been used in the 1920's and 1930' s. Because soil moisture de- 

 pletion studies are not adapted to crops frequently irrigated or 

 having high water tables, small evapotranspirometers were installed 

 to provide a reliable measure of evapotranspiration under those 

 conditions . 



Alturas-Dorris Ranch Evapotranspirometer Measurements . In 

 1956, two small evapotranspirometers were Installed near Alturas to 

 measure evapotranspiration from high water table pasture. The plot 

 site was In an irrigated meadow pasture containing high moisture 

 favoring grasses, legumes, and broad-leafed plants found in improved 



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