U, S, Weather Bureau pans. Table 2 indicates the mean monthly 

 difference of evaporation between Livingston spherical black 

 and white atmometers. 



At the Initiation of the proo'ram in 195^j little was 

 known of the effects of the immediate ground cover environment 

 on evaporation from atmometers and pans. Furthermore, little 

 consideration had ever been given to the effects on evaporation 

 rates of surrounding land areas or cleanliness of pans at sta- 

 tions having apparently similar immediate environmental conditions. 

 In analyzing the data it became apparent that certain of these 

 factors are extremely Important. 



In the initial tabulations of evaporation data, great 

 differences were noted between adjacent stations having dissimilar 

 environmental conditions. A tabulation on the basis of station 

 environment shows this to be especially true for evaporation pans, 

 as may be noted in Table 1, For example. Table 1 indicates that 

 the May through September total of the mean monthly evaporation 

 from pans located on dry-farmed rangelands was more than 25 per- 

 cent greater than evaporation from pans situated on irrigated 

 pasture. This difference became increasingly greater during the 

 summer months. The higher and increasingly greater evaporation 

 on dry-farmed rangelands resulted from the greater availability 

 of energy in surrounding dryland areas, and the increase of ad- 

 vectlve heating that results as the drylands exhaust moisture 

 carried over from wintertime precipitation during the summer. 



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