32 



BULLETIN 304, U. S. DEPARTMENT OF AGRICULTURE. 



In each of Tables 8, 9, and 10 the maximum for each month is 

 shown by heavy-faced figures, as is also the yearly maximum. It 

 will be noted that the greatest mean monthly rainfall is in June in 

 two cases and in July in the other, thus showing that the maximum 

 rainfall is to be expected during the months when farming operations 

 are at their height. The mean annual precipitation for the three 

 stations is 37.39 inches. A study of the tables has shown that a 

 monthly rainfall of 6 inches occurs a little oftener than once each 

 year. A monthly rainfall of 8 inches occurs once in 3.5 years, and 

 one of 10 inches once in 11 years. A rainfall of 15 inches during a 

 period of two successive months occurs once in 3.6 years. It would 

 appear from these figures that a pumping plant should be able to 

 handle a monthly rainfall of at least 8 inches. 



Table 1 1 shows the average number of storms per year, of the given 

 intensities, for short periods of from 1 to 10 days, that may be 

 expected in the section under consideration. This table was obtained 

 from a study of the daily precipitation records of Muscatine, Peoria, 

 and St. Louis. 



A study of the rainfall records of Muscatine, Peoria, and St. Louis 

 shows that about two-thirds of the storms of the various intensities 

 given in Table 11 occurred from April to September, inclusive, the 

 months during which crops are most injured by occasional flooding. 

 During the six months from October to March, inclusive, occasional 

 flooding of the land would do little if any damage in the section under 

 consideration. Therefore the storms occurring during the winter 

 months may be disregarded in the determination of the proper 

 capacity of pumping plant. This can be done by reducing the 

 number of storms of each given intensity by one-third. For instance, 

 Table 1 1 shows that on the average a storm of 2 inches in two days 

 occurs three times a year. Only two of these storms occur at a time 

 of the year when damage to crops would be caused by flooding. 



Table 11. — Average yearly number oj storm -periods ojfrom 1 to 10 days' duration. 



[Based upon daily precipitation records of the United States Weather Bureau for Muscatine, Iowa, 1854- 

 1914; Peoria, 111., 1856-1914; and St. Louis, Mo., 1843-1914.] 





Average number of storms per year. 



Total rainfall. 



1-day 

 period. 



2-day 

 period. 



3-day 

 period. 



4-day 

 period. 



5-day 

 period. 



6-day 

 period. 



7-day 

 period. 



8-day 

 period. 



9-day 

 period. 



10-day 

 period. 





2.018 

 .479 

 .172 

 :041 

 .012 



3.059 

 .900 

 .320 

 .083 

 .024 

 .006 



3.615 

 1.172 

 .438 

 .142 

 .041 

 .012 



4.071 

 1.456 

 .545 

 .160 

 .047 

 .018 

















1.580 

 .651 

 .195 

 .071 

 .030 

 .006 



1.722 

 .775 

 .290 

 .077 

 .047 

 .006 

 .006 













0.847 

 .355 

 .100 

 .053 

 .012 

 .012 



0.912 

 .444 

 .118 

 .059 

 .024 

 .018 

 .006 









0.479 

 .165 

 .071 

 .024 

 .018 

 .006 



0.520 





.207 





.077 







.030 













.018 















.006 





















