156 CALIFORNIA FRUITS: HOW TO GROW THEM 



It is interesting to note that results of close inquiry by the Irrigation 

 Investigations of the U. S. Department of Agriculture to ascertain the 

 amounts of water used by measurement of water running in main 

 ditches and by estimate of the acreage to which the water is applied, 

 do not agree closely with the growers' estimates of the amounts of 

 water which they actually use. There are, of course, always issues 

 between water-purveyors and water-buyers which can not be entered 

 upon in this connection. A rough conclusion from data secured from 

 the ditch flow, etc., is that from 12 to 30 acre-inches of water are used 

 annually in irrigated orchards and vineyards, according to local condi- 

 tions involved. It is quite clear that the amounts chiefly used would 

 not be the average but would tend toward the lower figure. The 

 details of these inquiries are found in the publications on irrigation of 

 the Office of Experiment Station, U. S. Department of Agriculture.* 



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, irrigation 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 regularly 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 satisfac- 

 tory 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 re- 

 ceiving 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 



*Definite citation is not made because these publications are continually appearing with 

 additional data on the effective use of water. The whole series should be examined. 



