SOILS. 



pensable; while in the Santa Clara valley of central California, 

 with an average rainfall of 15 inches falling through the 

 winter and spring, the growth of all ordinary field crops has, 

 for fifty years not failed oftener than is commonly the case in 

 the humid region of the North Central states. This is be- 

 cause in California the winter and spring are the growing sea- 

 sons, while the rainless summers do not stand in the way, 

 for crops are already harvested; and the deep rooting of trees 

 and vines provides these with the needful moisture from the 

 depths of the substrata (see chap. 10, pp. 163 to 173). 



It would thus seem that twenty inches of irrigation water 

 properly applied ought to be sufficient for all purposes, when 

 added to the natural rainfall, which is rarely entirely absent. 

 Yet in actual practice less than 24 acre-inches is rarely used, 

 and much more is the rule ; 72 to 96 ins. being sometimes used 

 in Arizona. Evidently enormous losses occur in practice, and 

 it is of the utmost importance to discover the causes of these. 



Causes of Loss. Since irrigation water is commonly mea- 

 sured at the distributing weirs, loss from seepage and evapora- 

 tion on the way to the fields is an obvious source of an over- 

 estimate of the water actually supplied to the farmer. In 

 sandy districts the loss thus incurred is reliably estimated at 

 nearly 50% in many cases. The apparent duty of the water 

 is thus at once reduced to half its effect, and four instead of 

 two feet of water are supposed to have been used, and are 

 charged for. 



Evaporation resulting from surface flooding or use in shal- 

 low furrows may, again, cause the loss of from 30 to 50% 

 of the water that actually reaches the land ; so that in the lat- 

 ter case, between seepage and evaporation the irrigator may 

 lose the effect of three-fourths of the water he pays for. 



Loss b\ Percolation. Finally, the water may be wasted on 

 the land itself in leachy soils by over-use, /. e. , it may percolate 

 to a large extent beyond the reach of the roots when the flow 

 is continued too long; as will always be the case when the 

 head (supply) ditches are laid too far apart, so that the water 

 may be wasting into the country drainage just below the upper 

 ditch long before the water in the furrow reaches the lower 

 one; as illustrated in the upper one of the subjoined diagrams. 



