FOREST TYPES IX CENTRAL ROCKY MOUNTAINS. 



77 



Table 21. — Maximum probable evaporation rates as indicated by computed values, 

 March, 1910, to December, 1916; actual evaporation, January, 1917, to April, 

 1921. 



Decade. 



Total evaporation per decade. 

 [Cubic centimeters per 100 square centimeters horizontal surface.] 



Year. 



Grow- 

 ing 



Jan. Feb. 



Mar. 



Apr. May. June. July. 



Aug. 



Sept. Oct. Nov. Dec. 



season. 



1 



70. 9 74. 9 

 108. 6 54. 4 

 61.7) 101.2 



110.0 

 131.1 

 97.8 



120.3, 203.0 

 95.6 209.0 

 88. 3 291. 3 



346.4 

 291.0 

 316.3 



284 5 



274.9 

 2S0.2 



199.8 

 147.6 

 215.0 



237.7 144.8' 82.6 i 

 129.5; 133.5 61.3, 54.8 



121.8 153.0 66.2! 68. 3 







2 







3 















Month. 



199. 6j 223.5 



315.7 



298. 7j 564.3 



802.8 



826.9 



433.7 



448.8 



,-. ... 



Table 22. — Variations in annual and growing-season evaporation and their relation 



to precipitation. 

 [Amounts for an area of 100 square centimeters. Evaporation computed by formula prior to 1917.] 



Year. 



1910. 

 ,1911. 

 1912. 

 1913. 

 1914. 

 1915. 

 1916. 

 1917. 

 1918. 

 1919. 

 1920. 



Average. 



Annual evap- 

 oration. 



Amount. 



C.c. 



14,063 

 3,837 

 3,446 

 2,806 



2 2,934 



3 2, S01 

 3,657 

 3, 2.^4 



* 3, 106 



s 2, 752 



2,527 



3,198 



Variation 



from 

 average. 



Per cent. 

 +27 

 +20 



+ 8 

 -12 



-8 

 -12 

 + 14 



+ 2 



-3 

 -14 

 -21 



Precipitation for 

 corresponding year. 



Amount. 



C. c. 

 4,704 

 5,002 

 4,990 

 6,740 

 6,322 

 5,718 

 4,990 

 4,374 

 4,995 

 3,771 

 5,406 



Ratio 

 to evap- 

 oration. 



1.16 

 1.30 

 1.45 

 2.40 

 2.15 

 2.04 

 1.36 

 1.34 

 1.61 

 1.37 

 2.14 



±13 



5,183 | 



1.665 



Growing-?eason 

 evaporation. 



Amount. 



C. c. 



2,232 

 1,S04 

 1,528 

 1,433 

 1,174 

 1,284 

 1,694 

 1,744 

 1,121 

 1,291 

 957 



1,478 



Variation 



from 

 Average. 



Per cent. 

 +51 

 +22 

 +3 

 -3 

 -21 

 -13 

 +15 

 +18 

 -24 

 -13 

 -35 



Growing-season 

 precipitation. 



1 Ratio 

 Amount, toevap- 



C.c. 



2,530 



2,315 



2,763 



2, 828 



3,291 



2,729 



2,492 



1,703 



2,902 



1,734 



2,9S2 



±20 



1.13 

 1.2S 

 1.81 

 1.97 

 2.80 

 2.12 

 1.47 

 0.98 

 2. 59 

 1.34 

 3.12 



" 1. S74 



1 Year from March, 1910, to February, 1911. 



' Assuming missing periods, Januarv to March, to have been "normal." 



8 Year from March, 1915, to February, 1916. 



« Year from September 21, 1917, to September 20, 1918. 



5 Year from May, 1919, to April, 1920. 



« ± 0. 100. 



» ± 0.157. 



In point of actual evaporation and relative shortage of precipita- 

 tion, the } T ear 1910 has probably been the most severe our records 

 have known, followed by 1911, both years having unusually high 

 temperatures and much sunshine. The whole year 1917, with 

 moderate evaporation and lowest precipitation (excepting 1919), 

 was only moderately severe. The year 1913, which was especially 

 deficient in sunshine, seems to have presented the most abundant 

 surplus of moisture. 



One may turn with more interest to the growing-season data, 

 and also with more confidence, because here both extremes are found 

 within the period of actual evaporation measurements, and also 

 because no shifting of the period limits is necessary. Here it is seen 

 that the lowest evaporation, that for 1920, is less than 45 per cent of 

 that for 1910, and that the most severe growing season (1917). as 

 shown by the ratio of precipitation to evaporation, is not one in 

 which the evaporation was itself excessive. Although these years 

 are only partly covered by soil-moisture records, it should be noted 

 that the exhaustion of soil moisture was in 1917 more complete than 

 in any other year of which there is knowledge. 



On the other hand, it should not be construed that the low evap- 

 oration and relatively great precipitation of 1920 constituted partic- 

 ularly favorable growing conditions. The size of rings formed has 



