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Open drains

Open drains have the advantage that they can receive overland flow directly, but the disadvantages often outweigh the advantages. The main disadvantages are the loss of land, interference with the irrigation system, the splitting-up of the land into small parcels, which hampers mechanized farming operations, and a maintenance burden.

Tile drainage

Tile drainage is an agriculture practice that removes excess water from subsurface soil. Whereas irrigation is the practice of adding additional water when the soil is naturally too dry, drainage brings soil moisture levels down for optimal crop growth. While surface water can be drained via pumping and/or open ditches, tile drainage is often the best recourse for subsurface water. Too much subsurface water can be counterproductive to agriculture by preventing root development, and inhibiting the growth of crops. Too much water also can limit access to the land, particularly by farm machinery.

Mole drainage

Heavy soils of low hydraulic conductivity (less than 0.01 m/day) often require very closely spaced drainage systems for satisfactory water control. With conventional pipe drains, the cost of such systems is usually uneconomic and hence alternative techniques are required. Surface drainage is one possibility; the other is mole drainage (Figure 8).

Mole drains are unlined circular soil channels which function like pipe

Figure 8 : Mole drains are unlined circular soil channels which function like pipe

drains.

Advantage of mole drainage is their low cost and hence they can be installed economically at very close spacing. Their disadvantage is their restricted life but, providing benefit/cost ratios are favourable, a short life may be acceptable. The success of a mole drainage system is dependent upon satisfactory water entry into the mole channel and the mole channel stability. Mole drains are formed with a mole plough (Figure 9), which comprises a cylindrical foot attached to a narrow leg, followed by a slightly larger diameter cylindrical expander. The mole plough is attached to the drawbar of a tractor and the mole channel is installed at depths between 0.4 and 0.7 m. Common lengths of run vary from 20 to 100 m.

A mole plough. The foot and expander form the drainage channel and the leg generates a slot with associated soil fissures which extends from the surface down into the channel

Figure 9 : A mole plough. The foot and expander form the drainage channel and the leg generates a slot with associated soil fissures which extends from the surface down into the channel.

Vertical drainage

Vertical drainage systems use pumped wells, either open dug wells or tube wells. Agricultural land is drained by pumped wells to improve the soils by controlling water table levels and soil salinity. Tubewell drainage refers to the technique of controlling the water table and salinity in agricultural areas by pumping, from a series of wells, an amount of groundwater equal to the drainage requirement. The success of tubewell drainage depends on many factors, including the hydrological conditions of the area, the physical properties of the aquifer to be pumped and those of the overlying fine-textured layers.

 
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