50 Bulletin 78 



General Effects of Aridity and Rainfall 



Of all the crops discussed in this bulletin, not over half a dozen 

 grow better (other conditions being equally favorable) in a climate 

 having a very low relative humidity. All the others thrive best in 

 an atmosphere having a somewhat higher relative humidity than 

 prevails in southern Arizona, providing all other conditions are 

 favorable, and the growth of many is seriously retarded by the 

 aridity of the region. Several crops, though the temperature be 

 favorable, and though supplied with plenty of water, do not grow 

 well during those portions of the year when the relative humidity 

 is very low. Very rarely indeed is the atmosphere of the region too 

 damp for the proper development of any crop. 



As a water supply the direct effect of the local rainfall is not 

 great, comparatively little benefit or injury to crops resulting 

 from the small amount of water that falls. Indirectly, however, the 

 local rains benefit many crops. The higher relative humidity that 

 accompanies them is a benefit to most crops at any time of the year, 

 and the lower tem.peratures that accompany the summer showers 

 are a relief to most crops during that Reason. Local rains are 

 ordinarily heartily welcomed, however, chiefly because as a rule 

 rain falls at the same time in the region furnishing the supply of 

 water for irrigation. Only occasionally is the local rainfall heavy 

 enough to directly benefit the crops, and then only shallow-rooted 

 ones are much benefited, since the soil is rarely wet to as great a 

 depth as one foot during any one storm. When it is considered 

 that the total annual rainfall of the region is only five to eight 

 inches, much of which falls in such small amounts as to fail to reach 

 the roots of plants, and that amounts ranging from 20 to 50 inches 

 in depth are needed for the proper development of various crops, 

 it will be evident that the effect of the local rainfall as a water 

 supply is not great. 



The combined effects of the factors discussed above — temper- 

 ature, direct sunshine, relative humidity, and rainfall — together 

 with that of the wind, are taken into consideration in discussing the 

 crops whose relation to the weather is given in the pages that follow. 

 Upon some crops the effect of one of these factors is greatest, upon 

 others the effect of another is greatest. 



