6 



BULLETIN 33, U. S. DEPARTMENT OF AGRICULTURE. 



Table II. — Monthly precipitation and evaporation (from a free water surface) 

 at the Dickinson substation for the crop season (April to August) of the 

 years 1907 to 1913, inclusive. 



[Data (in inches) from the records of the Biophysical Laboratory of the Bureau of Plant Industry.] 





April. 



May. 



June. 



July. 



August. 



Total. 































Ratio 



Year. 



© 



£ 

 o 



a 



03 

 U 

 



ft 

 03 

 > 



a 

 _o 



-^ 

 .& 



'a 

 oj 

 u 

 to 



a 



O 



u 



o 

 ft 



03 



> 



R 



_o 



"o3 

 ft 



'3 



S 



_o 

 +3 

 03 



O 

 ft 



o3 



> 



a 



o 



1 

 S 



0J 

 Fh 



ft 



fl 

 ,o 

 +j 



03 



o 



ft 



03 



> 



03 

 'ft 



'o 

 <c 



d 

 o 



"S 



u 

 o 

 ft 



53 

 > 



d 



+3 

 | 

 .ft 

 'o 



O) 



ft 



d 



O 

 ft 

 03 



t> 



of 

 precipi- 

 tation 



to 

 evapo- 

 ration. 



1907 



0.30 



3.200 



1.38 



4.765 



2.68 



6.019 



4.82 



5.886 



1.93 



6.755 



11.11 



26.625 



1:2.40 



190S 



1.27 



3.3C0 



3.79 



4.450 



4.06 



5.898 



1.34 



7.506 



1.36 



6.844 



11.82 



27. 998 



1:2.37 



1909 



.60 



3.170 



6.04 



4.922 



3.02 



4.924 



1.87 



5.360 



5.54 



6.813 



17.07 



25. 189 



1:1.48 



1910 



1.71 



5.420 



1.26 



5.187 



3.03 



7.097 



2.35 



8.678 



1.48 



5.665 



9.83 



32. 047 



1:3.25 



1911 



.4S 



5.290 



1.63 



5.794 



2.61 



7.028 



1.27 



9.259 



1.69 



5.478 



7.68 



32. 849 



1:4.28 



1912 



2.51 



3.650 



3.99 



4.671 



2.06 



5.528 



3.90 



6.020 



2.71 



5.922 



15.17 



25. 791 



1:1.70 



1913 



.59 



4.150 



1.63 



3.978 



1.83 



6.951 



1.26 



7.211 



2.79 



6.882 



8.10 



29. 172 



1:3.60 



Average. 



1.07 



4.030 



2.82 



4.825 



2.76 



6.206 



2.40 



7.131 



2.50 



6.337 



11.54 



29. 524 



1:2.56 



The average precipitation during the growing season for the seven 

 years from 1907 to 1913, inclusive, as shown in Table II, is 11.54 

 inches, or seven-tenths inch greater than the 22-year normal (Table 

 I) for the same months. Table II also shows the seasonal precipi- 

 tation for the years 1907 and 1908 to be very close to normal; that 

 of 1909 about one-third greater; of 1910 and 1911 considerably less; 

 of 1912 greater ; and of 1913 less than normal. 



EVAPORATION. 



Next to the seasonal precipitation, the seasonal evaporation is 

 probably the most important factor influencing the growth of crops at 

 Dickinson. The daily evaporation has been recorded at the Dickinson 

 substation, and the total amount (inches) per month and for the 

 growing season is shown in Table II. The evaporation is determined 

 from a free water surface, the method being that employed at all of 

 the stations where the Biophysical Laboratory of the Bureau of 

 Plant Industry has been cooperating. 1 The average evaporation for 

 the five months from April to August, inclusive, for the seven years 

 from 1907 to 1913 was 29.524 inches. The lowest total evaporation — 

 25.189 inches — was recorded in 1909, the year of the greatest rainfall. 

 The largest total evaporation — 32.849 inches — was recorded in 1911. 

 the year of the lowest seasonal rainfall. Thus the amount of evapo- 

 ration usually varies inversely with the amount of precipitation. 



The ratio of precipitation to evaporation, also given in Table II, 

 shows the evaporation for the seven years to be 2.50 times the precipi- 

 tation. In 1909 the ratio was the narrowest, the evaporation for 

 that year being only 1.48 times the precipitation. In 1911 the ratio 

 was the widest, when the evaporation was 4.28 times the precipita- 



1 Briggs, L. J., and Belz, J. O. Dry farming in relation to rainfall and evaporation. 

 V. S. Dept. Agr., Bur. Plant Indus. Bui. 188, p. 16-20, 1910. 



