644 SOILS: PROPERTIES AND MANAGEMENT 



the soil, and thereby intercept its flow. The need of 

 drainage may arise from several conditions. It is always 

 indicated by the occurrence of a stratum of rather com- 

 pact soil which intercepts the natural flow of water and 

 brings it within the root zone. Sometimes this obstruc- 

 tion is near the surface, sometimes it is several feet below 

 the surface. The water may be brought to the surface 

 in a single spring or in a series of springs, in the latter 

 case forming a seepage line. The retaining layer may 

 have an uneven surface and form basins and hollows 

 disguised by a covering of porous soil. For all these 

 reasons, the drainage conditions of the soil and the lines 

 of movement of water through it should be studied as 

 fully as possible before the drainage system is planned. 

 The main lines should first be located. Where the land 

 is in need of drainage in parts, a few lines of tile will 

 accomplish this. Springy holes should generally be tapped 

 by the most direct route. Often, short wing drains may 

 be necessary at the upper end, to collect the underground 

 flow. (See Fig. 65.) 



Where there is a line of seepage at nearly a uniform 

 level, a drain placed across the slope at the upper edge 

 of the wet area, and if possible cutting to the underlying 

 hard stratum, will intercept the flow and meet the needs of 

 the lower land. This is an intercepting system of drains. 



Where the land is more nearly uniform in its need of 

 drainage, a regular system is required and will usually 

 result in a saving of tile. This arrangement should 

 approximate a rectangular system, in order to avoid 

 double drainage where lateral tile join the main line. 

 This may of course be modified according to conditions. 

 The line of tile should be as long as is practicable for 

 convenience in construction. To this end, if the field 



