174 CALIFORNIA FRUITS: HOW TO GROW THEM 



RELATION OF RAINFALL TO IRRIGATION 



The amount of rain and the time it falls are clearly the most 

 important factors in determining the necessity for irrigation. 

 Absence of rainfall makes a desert of the richest soils at all 

 elevations and at all exposures. Its only remedy is irrigation. 

 But there are degrees of poverty in rainfall, and thorough tillage 

 will often lessen the ill effects of a scanty supply, so that an oasis 

 may be made to appear without water beyond that supplied from 

 the clouds. This is the triumph of tillage in the arid region which 

 is to be considered in another connection. 



The line between adequate and insufficient rainfall can not be 

 closely drawn. In the growth of common orchard fruits, irriga- 

 tion is not resorted to at a number of points where the local 

 rainfall sometimes is as low as 15 or 16 inches, but with less 

 than that amount, unless the soil receive additional moisture by 

 underflow, it is essential. On the other hand, irrigation is regu- 

 larly practiced in some localities where the rainfall sometimes 

 rises to 45 inches. Under average conditions of soil depth and 

 retentiveness, the amount of rainfall which may be considered 

 adequate for deciduous orchard trees under good cultivation is 

 about 20 inches. So definitely is this amount fixed in the minds 

 of some California growers as meeting the needs of the tree for 

 satisfactory growth and fruitage that, when rainfall for a season 

 is less than that amount, irrigation is at once resorted to to supply 

 the shortage. 



But owing to local conditions of soil and climate, the rainfall, 

 no matter how large, may not be relied upon to carry the trees 

 through the dry season. The fact is that the soil is not capable 

 either of receiving the heavy rainfall or of long retaining such 

 portions as actually enter it. There is, then, a considerable part 

 of the rainfall which is worse than worthless, because it does 

 injury by soil washing and soil leaching, and places where 

 extremely heavy rainfall occurs may be actually worse off than 

 other places with less rainfall. Some localities of large rainfall 

 lead in amounts of water supplied by irrigation. The converse is 

 also true, for some localities of light rainfall report success with 

 deciduous fruit trees with a minimum amount of irrigation water. 



Deciduous Fruits. Without making too much of individual 

 reports there appear instances enough to warrant the conclusion 

 that the deciduous fruit tree can winter successfully with a small 

 moisture supply and is, in fact, in less clanger from lack of moisture 

 than from oversupply at this time of the year. If there be enough 

 moisture to prevent injury from evaporation, the tree will start 

 good growth as the season advances and continue it if irrigation 



