Sub - Irrigation 39 7 



is that of applying sufficient water to thoroughly wet 

 the surface, and yet those who have not tried the 

 plan feel confident that there will be a great saving 

 in this direction ; but the rate of capillary movement 

 of water in soil is relatively so slow, and percolation 

 so rapid in most cases, that it becomes nearly imper- 

 ative that water shall be placed upon the surface, 

 where it is most needed and is of greatest service. 



It has been shown under furrow irrigation, where 

 the water is applied at the surface, that the streams 

 must usually be led as close as every four feet, to wet 

 the whole ground, and from this it follows that lines 

 of tile laid even closer than this would be required 

 in sub -irrigation. In Fig. 123 is shown the wetting 

 of the surface which occurred by distributing the 

 water through 3 -inch tile placed 18 inches below the 

 surface, in which hydrostatic pressure was maintained 

 sufficient to cause the water to rise one or two inches 

 above the top of the ground. In this experiment 

 the tile were arranged as represented at D, Fig. 

 124, 10 feet apart, and it will be seen that only 

 about 3 feet in width above each line of tile has been 

 wet, and yet water enough has been applied to cover 

 the area more than 6 inches deep. Even at C, Fig. 

 124, where the tile are only 5 feet apart, it was 

 necessary to apply 19.68 inches of water in depth to 

 completely wet the surface, but in this case the sub- 

 soil was more open than it was at D. It is plain, 

 therefore, that in order to thoroughly wet the sur- 

 face of the ground by sub -irrigation, much more 

 water will be required than by furrow irrigation, 



