METHOD OF DRAINAGE 113 



2. Subsurface Drainage. When the damaging water is in the 

 soil, or when the water-table 3 is too high, " subsurface" or "under- 

 drainage" is necessary. This may be done through the use of 

 open ditches and underground or covered drains. 



3. Vertical Drainage. Occasionally water standing in upland 

 depressions and on some flat upland fields may be given a chance 

 to escape by making openings through the tight sqil, or subsoil, 

 so that the water may move downward into an open and dry soil 

 below, if such a substratum is to be found. 



4. Combined Methods Necessary .It is generally necessary to 

 combine different methods of drainage. On nearly all large areas 

 the best combination for thorough drainage is large open ditches 

 for outlets, and underground drains. Under certain other con- 

 ditions a surface-run combined with underdrainage gives best 



FIG. 47. Dead furrows as drains. On tight soils, especially on fields having little 

 slope, surface drainage may be had by plowing in narrow " lands " up and down the gentle 

 slopes. The surface water drains from the back furrows to the dead furrows, thence down 

 the dead furrows and off the field. B, back furrow; D, dead furrow. (See Fig. 68.) 



results. In many large areas effective drainage is accomplished 

 only through the combined use of open ditches, underdrains and 

 surface-runs (Figs. 48 and 50) . 



Method of Drainage Depends Upon Why Land is Wet. The 

 only reasonable and safe way to begin any efforts towards drainage 

 is to ascertain why the land is wet, and this knowledge should 

 determine the manner and system of drainage to be employed. 

 In most cases it is easy to determine when surface drainage is 

 needed; but underdrainage is too often overlooked. On many a 

 field the farmer never suspects a lack of sufficient drainage to be 

 the cause of low crop yields. 



To eliminate any guessing in ascertaining the need of sub- 

 surface drainage, dig a post hole about four or five feet deep, 

 preferably during seeding time or when the ground seems too wet 

 and cold to work; and if, when the hole is left open, the water 

 comes within three feet of the surface, the land is in need of 

 underdrainage. Many grasses make good growth on land satu- 



3 The surface of the free water, or ground water, in a soil, is called the 

 " ground water-table." When the free water comes near the surface, the ground 

 water-table is said to be high. 

 8 



