208 



Bulletin 37. 



tains in the form of rain flows away within a few days or weeks. 

 The remainder slowly percolates through the soil and rocks to the 

 stream beds, thus maintaining a flow which, even though contin- 

 uous, is not sufficient to supply with water the lands needing it. 

 The water that falls as snow comes down to the valleys below 

 more gradually, furnishing an increased supply for irrigation for 

 one or two months. 



While precipitation may occur at any time of the year, in 

 most of southern Arizona, there are two seasons during which the 

 fall is heavier than during the remainder of the year, and the con- 

 sequent supply of irrigating water much greater. The greatest 

 precipitation occurs from July to September, inclusive, the other 

 rainy season occurring from December to February, inclusive. 

 The rain and snow falling in the mountains during the latter 

 period usually furnish an increased supply of irrigating water un- 

 til the end of March. From the latter month until July the rainfall 

 is light and the supply of water usually gradually diminishes, be- 

 coming very low during June. The summer rains swell the 

 streams and increase the supply of irrigating water temporarily. 

 Then follow about three months during which the supply is again 

 usually less than the demand, in many valleys of the territory. 

 The following monthly averages of the rainfall recorded at the 

 thirty-eight stations situated in the valleys of southern Arizona 

 and in the watersheds furnishing them water for irrigation, and 

 having a record of five or more years, will indicate how the rains 

 are distributed throughout the year : 



Temperature and Relative Humidity. 



The coolest months in southern Arizona are December and 

 January, during which heavy frosts are frequent in most of the 

 valleys. During February the weather becomes warm enough to 

 start the buds of some deciduous trees, and by the end of March 

 all are partially or wholly leaved out. The principal part of the 



