FARM IRRIGATION 255 



banks, or "levees." These are built at right 

 angles, forming square or rectangular blocks of 

 land of from to 20 acres, depending upon the con- 

 tour, the head of water, the crop, and the neight of the 

 banks. On land that has a marked slope a com- 

 mon size is 50 to 150 feet square, but sometimes 

 checks only 2 or 3 rods square are necessary. In 

 the San Joaquin Valley, California, 30,000 acres of 

 alfalfa in one block are irrigated by flooding. 

 The banks are 12 to 20 inches high, 12 to 18 feet 

 wide at the base and are plowed, harrowed and 

 harvested like the enclosed spaces. 



When the field slopes considerably in but one 

 direction, the checks are made rectangular in- 

 stead of square with the long sides running 

 across the slope. This makes it possible to 

 include a larger area within the check. If the 

 slope is uneven the banks running across it will 

 naturally have to follow the contour. They are 

 from 10 to 20 inches high and 4 to 15 feet wide at 

 the bottom, so that mowers and harvesters may be 

 driven over them easily. Thus they become 

 permanent features of the farm. The ridges may 

 be thrown up by hand with shovels, but usually 

 by plowing in back-furrows and using a scraper the 

 work can be done more economically. Each 

 check should include as large an area as possible of 

 approximately the same level. 



Fillinq the Checks. In flooding a field it is 



Kf ^J 



customary first to turn the water into the highest 

 check and after this is saturated to open the bank 

 between it and the next lower check, and so on, all 

 the checks being flooded in succession from higher 

 to lower. Or the water may be taken down be- 

 tween the lines of checks and turned in on each 

 side, flooding the checks in pairs. Or all the upper 



