192 IRRIGATION FARMING. 



the Mexican borders. The land to be irrigated is 

 divided into compartments or checks, each of which is 

 entirely surrounded by embankments of earth. The 

 principal embankments follow contour lines, the ver- 

 tical distance from one contour levee to the next being 

 uniform, which is not always the case in the Mexican 

 fields. The contour interval usually sele6ted is six to 

 nine inches, rarely so great as one foot. It depends in 

 all cases upon the surface slope of the ground to be 

 prepared fpr irrigation, and should be less than six 

 inches if the ground is sufficiently flat to permit such 

 an arrangement without making the individual checks 

 too small. 



For ground on a slope that would require levees 

 more than one foot apart in elevation some other 

 method of irrigation should be adopted. The strips of 

 land between the contour levees are subdivided by 

 cross levees into compartments of convenient size, 

 which are generally called checks. Their area should 

 vary according to the volume of water, a good rule for 

 porous soils being not to exceed one- fourth of an acre 

 for each second-foot when a large head of water is 

 available and to make half an acre a second-foot the 

 limit for a small supply. As after the Mexican fashion, 

 water is supplied to the several checks in turn from 

 highest to lowest in each series between cross levees. 

 The irrigating lateral which leads from the supply 

 canal is usually carried in the direc5lion of greatest 

 slope, cutting the several contour levees nearly at right 

 angles. 



Irrigation commences by turning a full head of 

 water into the upper check upon one side of the irri- 



