IRRIGATION IN HUMID CLIMATES 



407 



242. Dry seasons. It is an elementary fact of weather 

 science that neither the total annual rainfall nor its dis- 

 tribution is exactly the same from year to year. The 

 average of many years does not, perhaps, vary greatly, 

 but from year to year there is a considerable difference. 

 This constitutes the main reason for irrigation in humid 

 districts. For instance, Williams has compiled the follow- 

 ing table from the United States Weather Bureau, covering 

 ten years, from 1899 to 1909, showing the average annual 

 rainfall, the number of droughts or periods of fifteen days 

 with less than 1 inch of rainfall, for five points in the 

 United States, representing five great divisions of the 

 country. In the first column is shown the point at which 

 the observation was made; in the second column, the 

 average annual rainfall in inches; in the third column, the 

 number of fifteen-day periods or over with less than 1 inch 

 of ram, or periods of drought; in the fourth column, the 

 number of days when irrigation was required, meaning 

 the number of days beyond the fifteen days during which 

 less than 1 inch rainfall was received. 



It may be noted hi the above table that at Ames, 

 Iowa, with an average rainfall of over 30 inches, there were, 

 in ten years, twenty-three periods of drought, with 190 

 days when irrigation would have been beneficial. At 



