16 Q CALIFORNIA FRUITS: HOW TO GROW THEM 



annual rainfall also has little relation to the amount of irrigation, 

 because neither fine shallow, nor deep coarse soils, can retain the 

 volume of water which falls upon them during the rainy season. 

 Then the varying rate of evaporation, the character of the tilth, etc., 

 enter as factors and it becomes clear that he is fortunate who knows 

 how much water to use on his own place. 



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

 tion Investigations of the U. S. Department of Agriculture to ascer- 

 tain 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 vine- 

 yards, according to local conditions 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 U. S. Department of Agri- 

 culture.* 



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. Ab- 

 sence 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 

 may not be 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 practised in 

 some localities where the rainfall 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 un- 

 der 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 foliage that, when rainfall 

 for a season is less than that amount, irrigation is at once resorted 

 to to supply the shortage. 



u , ci } ation is no* made because these publications are continually appearing 



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

 ot _ the . Irrigation InvesttRations" of the Department of Agriculture is Samuel Fortier 

 whose office is in the Post Office Building. Berkeley, Calif. 



