ROBERT A. THOMPSON-STORM-WATER IRRIGATION. 
75 
perennial streams, down whose channels flow such enormous volumes of 
storm water, immense dams can be built, whose size is determined only 
after having obtained a full knowledge of the “ordinary flow” and 
distribution of the “storm flow,” and the area of the land it is desired 
to irrigate. The building of these dams, whose height must not only be 
sufficient to raise the water to the “country level,” but also to store a 
supply above this level, entails great expense and shrewd, careful engi¬ 
neering. In India, where several large storage dams have been success¬ 
fully built, the engineers have followed closely the cross section pointed 
out by theory. 
Under the influence of the second division of the storage system can 
be brought the greater percentage of the semi-arid lands, and it will be 
seen that the great aim of this method of irrigating is to augment the 
natural supply in semi-arid and sub-humid regions, where the rainfall, 
though ordinarily sufficient to produce crops, is irregular. In this case 
only a comparatively small supply need be stored for a large area, as 
seldom will more than two waterings be required in succession. A small 
supply will tide over a short season of drouth wdiere otherwise the entire 
crop would be lost. Dams for these reservoirs can be built in most cases 
from materials near its location; either of rock or of clay mixed with 
sand or gravel, as may be most economical. It is cheaper and safer in 
all cases that the surplus water be discharged over a waste-weir built at 
one or both ends of the dam, rather than over the whole length of the 
crest; and it may be well to put particular stress on recommending that 
special attention be always given to the construction of the waste-weir. 
Where storm water is to be stored, economy always suggests the build¬ 
ing of waste-weirs for the escape of surplus waters, rather than a sub¬ 
merged dam. They should be made large enough to discharge the 
greatest flood of which there is any record. In Arizona and California 
a number of very expensive dams have been destroyed, either because of 
erroneous calculations on part of the engineer, or because of incorrect or 
insufficient data in regard to the maximum flood discharges of the 
streams. 
The third division, or the collecting and storing of storm water in ex¬ 
cavated reservoirs, is of greatest importance to orchards, gardens, and 
small farms, where methods of watering other than by gravity distribution 
can be made profitable. This method of storing water is now practiced 
extensively on a small scale in the semi-arid districts, as a source of 
water supply for man and beast. Especially is this done in those locali¬ 
ties where surface water is not abundant in the streams, or is obtained in 
wells only at great depths, or where obtained is unfit for use. A “ tank,” 
as it is commonly called, built in a bed of clay, will retain from 75 per 
cent to 90 per cent of the collected water. 
