102 ARID AGRICULTURE. 



advantages : The top soil, usually the best, is re- 

 moved to form the banks; even after levelling, 

 the checks usually have enough slope so that the 

 water stands considerably deeper on the lower 

 end of check than on the upper, which produces 

 unevenness in the crop, some getting too much 

 and some too little water. Considerable area is 

 withdrawn from cultivation by the levees; it is 

 hard to cross levees with heavy machinery. The 

 great advantage is in the ease of irrigating and 

 the large head of water that may be quickly ap- 

 plied and thoroughly controlled by the irrigator. 

 The "contour check" method differs from the 

 rectangular only in the way the levees are made. 

 In this system surveys are made of level, or 

 "contour" lines, spaced as far apart as it is prac- 

 tical to control the water when irrigating. This 

 system may be used on steeper slopes than the 

 preceding, in which case the checks are long and 

 narrow and usually somewhat curved in outline, 

 following the natural slope. 



NATURAL This is a method that applies only where cer- 



tain rather unusual conditions exist. These are : 

 First, an impervious hardpan or rock stratum a 

 short distance below the surface and parallel 

 with it. Second, a shallow bed of gravel or 

 coarse sand above this, through which water can 

 seep easily from a ditch at the upper end of the 

 field. Third, a rather fine soil not so deep, but 



