Criteria for Selection of Plots 

 In the selection of plots for the measurement of evapo- 

 transplration^, certain physical conditions are recognized as es- 

 sential to collecting valid data. Experience over the years has 

 emphasized the Importance of certain necessary conditions. Un- 

 fortunately, the most ideal plot conditions are difficult to find, 

 and considerable time and effort have been expended over the years 

 selecting the most favorable sites. However, this is not to imply 

 that evapotranspiration rates would necessarily be different under 

 different conditions. The following criteria indicate the condi- 

 tions under which good measurements of evapotranspiration repre- 

 sentative of field conditions can be obtained. After good 

 measurements have been obtained under these conditions, the studie; 

 should be broadened to include some of the adverse soil and other 

 conditions which might affect evapotranspiration. 



1. Measurement sites should be located 200 feet inside 

 the edge of the irrigated field to avoid accentuated border effect; 



2. Generally, the land should be of smooth topography. 



3. Since the sites are located on private lands, it 

 is necessary to have freedom of access and cooperation of the 

 landowner or manager, 



4. The soil should be deep, well drained, productive, 

 and unaffected by salinity. The soil preferably should also be 

 medium textured, as very fine or very coarse textures have un- 

 favorable soil-moisture relationships. The soil profile should 

 not be stratified to such an extent as to impede moisture flow 

 or create sampling pi'-oblems. 



• 24- 



