10 FORREST SHREVE 



by underground seepage, by surface evaporation and by the 

 draught of plants upon it. It is obvious that the influence which 

 the individual falls of rain exert during the course of the year in 

 replenishing the soil moisture is not proportional to their actual 

 amounts. The light rains often fall on a warm, parched soil and 

 are totally converted into vapor within a few hours; the heavy 

 rains are often of such short duration that the run-off is very 

 great, resulting in a thorough wetting of upland soils and also 

 an addition to the soil water of flood plains and valleys. 



In tropical and temperate regions of copious and well distribu- 

 ted rainfall the moisture of the soil is maintained at such high 

 and constant percentages that it fails to be a differential factor in 

 plant distribution. The importance of the sustained soil mois- 

 ture of such regions is no less great, however, and the heavy 

 rainfall to which it is due exerts direct effects upon the vegeta- 

 tion which are unknown in semi-humid and arid regions. 



RAINFALL 



The record of precipitation at the Desert Laboratory, at Tuc- 

 son, Arizona, now covers eight calendar years without break — 

 1905 to 1912 inclusive — and the manuscript records of the daily 

 falls since January 1, 1907, have been available in the present 

 digest. The Laboratory is situated in the eastern foothills of a 

 low range of mountains, and exhibits a slightly higher rainfall 

 than the Arizona Experiment Station, situated 4 miles east of 

 the Laboratory on the mesa of the Santa Cruz Valley. The pre- 

 cipitation has been measured by means of the ordinary type of 

 metal gauge, in addition to which an electrical recording gauge 

 has been installed since January 1, 1912. 



The average annual rainfall from 1905 to 1912 inclusive was 

 14.60 in. (37.1 cm.), the lowest annual fall being 11.11 in. (28.2 

 cm.) in 1906 and the highest 23.32 in. (59.2 cm.) in 1905. The 

 seasonal distribution of the precipitation is such that there are 

 two well marked periods of rainfall, separated by intervals in 

 which the fall is usually extremeh^ scant. The winter months 

 (December to March) embrace a period in which gentle rains of 

 several hours duration are common; the arid fore-summer (April 



