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5. Subsurface drainage

Surface drainage may be insufficient in poorly drained soils where the water table is naturally near the ground surface. In these cases, subsurface (tile) drainage is installed to remove the excess water and lower the water table (Figure 6). A drainage system should be able to lower the water table from the soil surface to 1-ft depth in less than 48 hours following a heavy rainfall (Ghane, 2023b).

Figure 4- Diagram of a soil profile with subsurfce drainage.
Figure 6- Diagram of a soil profile with subsurface drainage.



In subsurface drainage systems, there are three general layouts: conventional pattern (parallel), contour, and targeted layouts (Figure 7). The contour layout is a subset of the pattern layout, which is used when laterals are following the contours too allow for controlled drainage.

A targeted layout is common in rolling landscapes where surface drainage provides enough drainage for field operations on most of the field except in isolated depressional areas where removal of excess water is needed for uniform field operations (Huffman et al., 2013). If the source of water is a naturally shallow water table, a targeted subsurface layout is suitable.

Blind inlets can drain excess water from depressional areas. They are suitable in places where the source of the excess water is mainly surface runoff. To learn about blind inlets, see Ghane (2022).
If an accurate layout map is unavailable, there are a few options to locate tile drain pipes: Ground penetrating radar, tile locator, robotic pipe crawler, Google Earth images, and drone aerial imagery.

Conventional Pattern layout


Contour layout


Targeted layout
Figure 7- Subsurface drainage system layouts.