RAINFALL AND SOIL MOISTURE 25 



has been made by the writer for six altitudes in the Santa Cat- 

 alina Mountains, near Tucson, from 3000 ft. to 8000 ft. for the 

 arid fore-summer, the season in which there is the maximum dif- 

 ferentiation of the conditions at the base and the summit of this 

 desert mountain range. ^ The ratio of soil moisture to evapora- 

 tion at 3000 ft. is to the ratio for 8000 ft. as 9.7 is to 1. In other 

 words the ratios for the base and summit of the mountain range 

 are to each other in the same proportion as the maximum and 

 minimum ratios for the year at the Desert Laboratory. The 

 annual range of moisture conditions at the Desert Laboratory is 

 as great, then, as it is through 5000 ft. of altitude in the adjacent 

 mountains in the most arid season of the year. 



SUMMARY 



The chief importance of rainfall to vegetation lies in its restor- 

 ation of the supplies of soil moisture. In desert regions the 

 rainfall consists largely of light, insignificant showers and heavy 

 rains with high run-ofT. In a study of desert rainfall in relation 

 to the march of soil moisture it is necessary to determine the 

 lower limit of amount of significant rainfalls. It is also neces- 

 sary, in relation to vegetation, to ascertain the lengths of periods 

 without significant rainfall. 



The average annual rainfall at the Desert Laboratory is 14.60 

 in. There are two rainy seasons, that of the summer averages 

 63 days in length, but yields 54% of the annual rainfall. The 

 average number of rainy days is 61.5 per annum, on 46.2 of 

 which the rainfall is less than 0.25 in. In six years there were 

 32 days with more than 0.75 in, of rain, and they yielded 46 % 

 of the total rainfall of the six years. There have been periods 

 of 140 days without rainfall of sufficient amount (0.15 in.) to 

 affect the soil moisture. 



The march of soil moisture during the year is closeh' related 

 to the amounts of the significant falls of rain, and the changes of 

 moisture content are conservative at the lower depths (15 cm. 

 and 30 cm.) in the heavy clay soil investigated. The moisture 



^ See Carnegie Institution Year Book Xo. 11, p. ;j9. 1912. 



