METHOD OF IRRIGATION 193 



119. Surface irrigation. Surface irrigation is the 

 method generally adopted hi all irrigated countries. 

 There is a great variety of methods of surface irrigation, 

 most of which are scarcely worth consideration, because 

 they either fail to recognize the natural laws underlying 

 irrigation, or then* cost of installation is beyond practi- 

 cability. 



The approved methods of surface irrigation may be 

 classified under two heads: first, the flooding methodj 

 second, the furrowing rnethod. Byjbhe flooding method, 

 all the soil is covered by the water applied; by the furrow- 

 ingjnethod, the water is guided in furrows-jor-channels 

 which traverse the whole field, but the water covers only 

 a part of the soil surface. Both flooding and furrowing 

 are used extensively hi all irrigated regions. In one 

 locality flooding may be the general method; hi anotlier, 

 furrowing. The adoption of one or the other of these 

 methods depends sometimes upon careful trials, but more 

 often upon custom following the first practices. 



The chief factors determining the choice between flood- 

 ing and furrowing, are: (1) the nature of the soil, (2) the 

 contour of the land, (3) the head of the water stream, 

 (4) the quantity of water available, and (5) the nature of 

 the crop. 



If the soil is light and "washes" a condition exist- 

 ing over large areas of the irrigated section furrowing 

 is the only really practicable method. On such soils, the 

 soil-washing due to flooding often results in large chan- 

 nels, gullies or "washes" being cut in the soil. On heavier 

 soils, flooding may be practised safely, as far -as erosion is 

 concerned. Many soils, after having been wetted, bake and 

 form a hard crust, which is injurious to the soil and to 

 the plant. On such soils the furrowing method is advisa- 



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