194 IRRIGATION PRACTICE 



ble, for by that method only a part of the surface is cov- 

 ered with water, and that part may be covered with loose 

 earth by cultivation soon after irrigation. Other soils, 

 after having been wetted, as they dry, fall apart, form- 

 ing natural mulches. On these soils, flooding is quite 

 safe. 



On relatively level land, either flooding or furrowing 

 may be adopted. Flooding is best done when the slope of 

 the land is not great, especially if the soil tends to "wash" 

 easily. On steeper lands, furrowing must be employed. 

 The heavier the soil, the steeper may be the inclination; 

 the lighter the soil, the gentler must be the inclination. 

 On the relatively steep slopes, frequently used for orchards, 

 furrowing, alone, is employed, and the sharp descents are 

 overcome by carrying the furrows back and forth around 

 the 'slopes with any desired fall. While no definite rule 

 can be laid down as to the permissible inclination of 

 lands under irrigation, yet a farmer soon learns by experi- 

 ence the practice best suited to his land. Farm irriga- 

 tion systems should be laid out with reference to the con- 

 tour of the land and, therefore, the irrigation farmer 

 should first secure contour maps of the land which he in- 

 tends to bring under irrigation. 



By the "head" is understood the volume of water 

 supplied to the unit of time. Under some systems of canal 

 management, farmers are given large streams of water 

 for short times; under other systems, small streams are 

 available for longer periods. The total quantity of water, 

 at the end of the period, may in either case be practically 

 the same. A high head of water pushes rapidly over the 

 land. Loose, sandy soils that absorb water rapidly must 

 be irrigated with a high head of water, especially under 

 the flooding method, or the water may all be drawn into 



