104 BULLETIN 1233, L\ S. DEPARTMENT OE AGRICULTURE. 



Table 30. — Probable total evaporation during the period in which the mean soil 

 temperature, for decades, at a depth of 1 foot, is below S2° F. — Continued. 





Type. 



Decade. 



Cubic centimeters, per 100 square centimeters. 



Total 

 for 



No. 1 



Jan. 



Feb. 



Mar. 



Apr. 



May- 

 Sept. 



Oct. 



Nov. 



Dec. 



frozen 

 period. 





Canyon spruce 



do 



High spruce, burn . 



Pikes Peak, tim- 

 ber line. 



! l 



iMonth. 



f 1 

 2 



1 3 



[Month. 



f 1 



2 



1 ^ 



[Month. 



f 1 



2 



1 * 



[Month. 



F. 

 F. 



F. 



F. 

 F. 

 F. 



F. 



F. 

 F. 











F. 

 F. 

 F. 















F-3 







F. 













9.8 



5. 5 



30.1 







7.2 



7.4 



00. 





F. 

 F. 

 F. 



F. 

 F. 



F. 



F. 

 F. 

 F. 



F. 









F. 

 F. 

 F. 

















F-5 



















36. 4 



41.4 



50. 2 



14.4 







25.0 



175.4 





F. 

 F. 

 F. 



F. 



F. 

 F. 



F. 

 F. 

 F. 



F. 

 F. 









F. 



F. 

 F. 















W-D 







F. 

















64. 4 



102. 



104.2 



49.5 ! 





23. 5 



60 4 



404.0 





F. 

 F. 

 F. 



F. 

 F. 

 F. 



F. 



F. 

 F. 



F. 

 F. 

 F. 







F. 

 F. 

 F. 



F. 

 F. 

 F. 















F-16.... 





F. 







23. S 



13.8 



40.1 



62.6 



90.6 



141.,') 



44.2 



416.6 



These data are based on (1) the period, by decades, in which the 

 mean soil temperature is below 32°, the recorded temperature being 

 corrected by one-half the amount indicated in Table 24, and each 

 such decade being shown in Table 30 by a letter F; (2) the relative 

 evaporation by months, for the particular station, as shown by Table 

 23; (3) the average evaporation for any decade or month at the 

 control station, as shown by Table 22. The result, of course, is the 

 merest approximation to the probable evaporation stresses of the 

 average season, and does not indicate at all the extreme conditions 

 that might be met. In a consideration of these data the fact should 

 be borne in mind that at most of the local stations evaporation has 

 been recorded 7 to 12 inches above the ground surface, but at others 

 the height of instruments has been 5 to -20 feet. (See Table 23.) 



Table 30 is based on so many assumptions that it can not be taken 

 as conclusive, yet in a broad way its indications are felt to be import- 

 ant and valuable. The general effect of using the mean soil tempera- 

 tures for several years is probably to make the period of soil freezing 

 appear longer than it actually is in the average individual year. 

 This is shown by using the actual data for the season of 1919-20, 

 when a good many evaporimeters were in operation. At a well 

 insolated station like the control the soil temperatures in a single 

 season are found to consist of several depressions well below the 

 freezing point, with intervals in which the soil moisture is very evi- 

 dently available. The maximum continuous stress, therefore, is 

 here only one-third as great as indicated by Table 30, and probably in 

 the most severe of winters it would not be more than one-half as 

 great. On the other hand, even a small amount of shade seems to 

 be sufficient to prevent the thawing of the soil on warm days. The 

 use of the mean soil temperatures, rather than maxima, of course 

 introduces another chance of prolonging the freezing period beyond 

 its actual limits. 



Both the possibility of occasional thawing on well-insolated sites 

 'that i<. maxima above 32 ) to a depth of 1 foot, and the probability 

 that, on such sites moisture nearer to the surface will he frequently 



