A sub –surface drainage system is required for water table control and for maintaining a favourable salt balance in crop root zone.
It may comprise of buried horizontal pipelines, deep opne channels or a system of drainage wells.
Sub-surface drainage may be obtained mainly by horizontal sub surface drainage systems called tile drainage and vertical sub-surface drainage system comprising of a system of drainage wells.
Horizontal sub-surface drainage systems comprises of a network of buried pipes as lateral drains, connected to buried collector drains leading to open ditches serving as outlet drains. The laterals may be made of burnt clay tiles or cement concerted or corrugated performed PVC pipes , 10-15 cm in diameter. The depth of lateral drains usually varies from 1 to 1.5 m and are spaced 30 to 100 m. apart. The collector drains are laid deeper than the deepest lateral drain in a tile network. The collector drains are of larger diameter in order to collect the flow from many lateral drains. Collector drains are of non –perforated type.
Cylindrical or oval shaped underground earthen channel formed within highly cohesive soils or fibrous organic soils by a mole plough. They have only limited application.
Advantages of Sub-surface Drainage:
1. Removes the gravitational on free water, which is not directly available to plants and thus provides aeration and optimum soil temperature.
2. Increase the volume of the soil from which the roots can obtain food.
3. Permits increase of the bacterial activity in the soil, which improves the soil structures, and makes the plant food more readily available.
4. Increase the moisture available to the more plants because the root zone is deepened.
5. Reduce soil erosion, since the well –drained soil has more capacity to hold rainfall resulting in less runoff.
6. Improves soil moisture condition in relation to the operations of tillage, plantings and harvesting machines.
7. Removes toxic sub-stances such as alkali from the root zone.
Comparison of Surface or Sub-surface Drains:
Sub –surface drains unlike open drains occupy no land surface. They do not harbour weeds or interfere with farming operations. On most fields, how ever, a combination of subsurface and surface drains provide the required drainage to the area.
The file drainage system consists of a drainage outlet, collector drains and laterals. The laterals remove the free water from the soil and the collector drains carry the tile water to the drainage outlet.