RAINFALL AND SOIL MOISTURE 11 



to June) is the driest portion of the year, with low precipitation, 

 high evaporation, and increasing temperature; the humid mid- 

 summer (July to September) is characterised by frequent thunder 

 showers, which may be light or heavy but are usually of short 

 duration ; the arid after-summer (October and November) is fre- 

 quently as dry as the fore-summer, but is moderated by lower 

 temperatures and cool nights. The rainfalls of each of the four 

 seasons form the following percentages of the annual total: win- 

 ter, 30.5 %; arid fore-summer, 5.5 %; humid mid-summer, 53.7 

 %; arid after-summer, 10.3 %. The total fall of the two rainy 

 seasons forms 84.2 % of the annual total. For the 34 year 

 record of Tucson the percentages of the seasonal rainfalls are as 

 follows: 31.7 %, 5.9 %, 50.7 %, and 11.7 %, which is in close 

 agreement with the percentages for the short record of the Desert 

 Laboratory, 



The average dates of the first and last occurrence of rains of 

 0.10 in. (0.3 cm.) or more in the summer rainy seasons of 1907 to 

 1912, are July 9 and September 10. In other words, slightly 

 more than half of the annual rainfall takes place within 63 days, 

 under normal conditions. The winter rainy season is not so reg- 

 ular in the time of its occurrence, nor is it so sharply confined to 

 a short period (see fig. 1). 



The average number of rainy days per annum is 61.5, but the 

 nuinber of days with slight rainfall is very high. The character 

 of the individual falls of rain is of the first importance to vegeta- 

 tion, and figures have been deduced from the records to show the 

 relative abundance of rainy days with falls of different intensities, 

 and also to show^ the total rainfall yield of the rains of different 

 intensities (table 1). There is an average of 17.5 days per annum 

 with a trace of rainfall, 12.6 days from with 0.01 to 0.05 in., 

 and the total number of days with less than 0.25 in. (including 

 the above) is 45.2, or slightly less than 75% of the total number 

 of rainy days. 



The addition of the total rainfall yield of the several classes 

 of rainy daj^s shows that the days with 0.25 in. (0.65 cm.) or less 

 furnish slightly less than 20% of the annual total. Although the 

 heavier falls of rain are less frequent, the totals which they jdeld 



