Climatology • 507 



A third interesting example of localized precipitation is found 

 in the Sulphur Spring Valley. McNeal, having an altitude of 4150 

 feet, receives a mean annual precipitation of 15.70 inches; Cochise, 

 altitude 4219 feet, receives 11.41 inches; and Willcox, about the 

 same elevation as McNeal, receives only 10.67 inches. Table IV 

 records mean monthly precipitation, and for convenience in study 

 is grouped into four districts as follows: (1) The Little Colorado 

 Drainage Basin; (2) Other localities above 4000 feet elevation; (3) 

 Localities between 2000 and 4000 feet; (4) Localities below 2000 

 feet. 



While the mean annual precipitation is indicative, the distribu- 

 tion of rainfall is of primary importance. For illustration, indi- 

 vidual showers of one-quarter or one-third inch, occurring at inter- 

 \als of two or three days have no real value, and often necessitate 

 considerable work to preserve a mulch. An inch of precipitation 

 falling thus intermittently may be detrimental ; whereas, if the 

 showers had come very close together, or if an inch of precipitation 

 had fallen in one or two showers, the effect would have been bene- 

 ficial. While an inch of precipitation falling in one hour moistens 

 the soil to some extent, it is largely lost by surface runoff'; whereas 

 an inch of precipitation falling over a longer time more completely 

 enters the ground. 



The amount of moisture lost by surface runoff' is great, often 

 being sufficient to close all trafffc for several hours at a time even 

 on main roads. Since so much value is lost in this manner one of 

 the problems of successful farming is ])roper utilization of floods. 

 In this y\rizona Indians are masters. There are thousands of acres 

 within the State which may be irrigated occasionally by these floods. 

 Figures 4, 5, 6, and 7 graphically illustrate the amount and distribu- 

 ticjn of precipitation at representative Arizona localities. 



