30 IRRIGATION PRACTICE 



soil. It is not far from the optimum water content for 

 plant-growth. 



25. Water distribution in furrow irrigation. Wher- 

 ever the water supply is plentiful, irrigation by some 

 form of the flooding method is largely employed. Where 

 water is scarce and smaller quantities must cover equal 

 areas of land, the furrow method of irrigation is almost 

 invariably practised. With certain crops, and on certain 

 lands, even if the water supply is large, the furrow 

 method of irrigation is preferred. 



Water applied in a furrow moves not only vertically 

 downward, but in every direction from the wetted fur- 

 row. The movement downward, aided by the full force 

 of gravity, is the most rapid; it diminishes as it becomes 

 more horizontal. That is, the lateral is smaller than the 

 downward movement. It is a common experience that 

 the lateral capillary movement of water near the surface 

 of deep soils, is slight. In an average loam soil it is sel- 

 dom more than 6 feet from the wetted center; in clay 

 soils larger; in sand soils smaller. The law of distribution 

 is of the same nature as for the downward movement. 



If neighboring furrows are not too far apart, the 

 moisture films moving in all directions from them finally 

 meet, until, at certain depths, depending on the nature 

 of the soil the size and distance apart of the furrows, and 

 the quantity of water used, the percentage of water is 

 practically the same, whether under or between the fur- 

 rows. Loughridge, in a study of California orchards, 

 when the furrows were from 6 to 8 feet apart, showed this 

 to be true for a variety of soils. In the Utah work, on a 

 loam soil, at depths of 5 and 6 feet, there was little dif- 

 ference in the moisture content under furrow or row, 

 when the furrows were about 3 feet apart. The longer 



