IRRIGATION AND DRAINAGE 



325 



irrigating systems have been built by the federal government, 

 and about ten million acres of land have been converted by irri- 

 gation from worthless farms into fields rich in crops (Fig. 195). 

 Most irrigating systems use centrifugal pumps to force water 

 over long distances and to supply it in quantities sufficient for 

 vast agricultural needs. In many regions, the success of a 

 farm or a ranch depends upon the irrigation furnished in dry 

 seasons, or upon man's ability to pump water from a region of 

 abundance to a remote region of scarcity (Fig. 196). 



FIG. 196. Rice for its growth needs periodical flooding, and irrigation often 

 supplies the necessary water. 



The draining of land is also a matter of importance. Swamps 

 and marshes which were at one time considered useless have 

 been drained and thus reclaimed and converted into good farm- 

 ing land. The surplus water is removed from the soil by cen- 

 trifugal pumps. Sand and sticks which are in the water would 

 clog the valves of an ordinary pump but are passed along with- 

 out difficulty by the rotating wheel. 



