It is the process of removal of excess water as free or gravitational water from the surface and the subsurface of farm lands with a view to avoid water logging and creates favourable soil conditions for optimum plant growth.
It is generally assumed that in arid region drainage is not necessary and water logging is not a problem. Even in arid region, due to over irrigation and seepage from reservoirs, canals etc., and drainage becomes necessary. Irrigation and drainage are complementary practices in arid region to have optimum soil water balance. In humid region, drainage is of greater necessity mainly due to heavy precipitation.
Drainage is required under the following conditions.
• High water table.
• Water ponding on the surface for longer periods.
• Excessive soil moisture content above FC not draining easily as in clay soil.
• Areas of salinity and alkalinity where annual evaporation exceeds rainfall and capillary rise of ground water occurs.
• Humid region with continuous or intermittent heavy rainfall.
• Flat land with fine textured soil.
• Low lying flat areas surrounded by hills.
(ii) Characteristics of good drainage system
• It should be permanent.
• It must have adequate capacity to drain the area completely.
• There should be minimum interference with cultural operations.
• There should be minimum loss of cultivable area.
• It should intercept or collect water and remove it quickly within shorter period.
(iii) Methods of drainage
There are two methods;
1. surface method, and
2. sub surface method
1. Surface drainage – This is designed primarily to remove excess water from the surface of soil profile. This can be done by developing slope in the land so that excess water drains by gravity.
It is suitable for:
• slowly permeable clay and shallow soil
• region of high intensity rainfall
• to fields where adequate outlets are not available
• the land with less than 1.5% slope. It can be made by land smoothening and making field ditches.
The surface drainage can be further classified as:
• Lift drainage
• Gravity drainage
• Field surface drainage
• Ditch drainage
(a) Lift drainage – To drain from low lying areas or areas having water due to embankment, lift drainage is used. Water to be drained is lifted normally by open devices, unscoops or by pumping or by mechanical means. This method is costly, cumbersome and time consuming.
(b) Gravity drainage – Water is allowed to drain from the areas under higher elevation to lower reaches through the regulated gravity flow through the outlet of various types. This system is practiced in wet land rice with gentle to moderate slopes. This method is less costly, easy and effective. However, the area to be drained should be leveled smooth and slightly elevated from the drainage source.
(c) Field surface drainage – The excess water received from the rain or irrigation is drained through this method. The irrigated basins or furrows are connected with the drainage under lower elevation, which is connected to the main outlet and to the farm pond used for water harvesting. If the slope of the land is sufficient to drain excess water from the individual plot, this drain water may be collected and stored locally in reservoir for recycling for life saving irrigation. This drainage method is cheap and effective but there is possibility of soil erosion and distribution of weed seeds along the flow of drainage water.
(d) Ditch drainage – Ditches of different dimensions are constructed at distances to drain the excess water accumulated on the surface and inside the soil up to the depth of ditch. Such ditches may be interceptors or relief drains. This method is adopted in nurseries, seed beds and rainfed crops.
This is an effective and efficient method but requires smoothening of surface and construction of ditches. This involves cost and wastage of crop lands. Shifting of soil, restriction for the movement of farm machineries, reconstruction and renovation of ditches during the crop duration and harvesting of crops are the problems in this method. In flat land, bed or parallel field ditches may be constructed. The collector ditches should be across the field ditches.
2. Sub surface drainage system –
Sub surface drains are underground artificial channels through which excess water may flow to a suitable outlet. The purpose is to lower the ground water level below the root zone of the crop. The movement of water into subsurface drains is influenced by the hydraulic conductivity of soil, depth of drain below ground surface and the horizontal distance between individual drains. Underground drainage is mostly needed to the medium textured soil, high value crops and high soil productivity.
There are four types of sub surface drainage.
• Tile drainage
• Mole drainage
• Vertical drainage
• Well Drainage/or Drainage wells.
• There is no loss of cultivable land
• No interference for field operation
• Maintenance cost is less
• Effectively drains sub soil and creates better soil environments.
• Initial cost is high
• It requires constant attention
• It is effective for soils having low permeability.
1. Tile drainage – This consists of continuous line of tiles laid at a specific depth and grade so that the excess water enters through the tiles and flow out by gravity. Laterals collect water from soil and drain into sub main and then to main and finally to the out let. The drains are made with clay and concrete. Tiles should be strong enough to withstand the pressure and also resistant to erosive action of chemicals in soil water.
2. Mole drainage – Mole drains are unlined circular earthen channels formed within the soil by a mole plough. The mole plough has a long blade like shank to which a cylindrical bullet nosed plug is attached, known as mole. As the plough is drawn through the soil, the mole forms the cavity to a set depth. Mole drainage is not effective in the loose soil since the channels produced by the mole will collapse. This is also not suitable for heavy plastic soil where mole seals the soil to the movement of water.
3. Vertical drainage – Vertical drainage is the disposal of drainage water through well into porous layers of earth. Such a layer must be capable of taking large volume of water rapidly. Such layers are found in river-beds.
4. Drainage wells – The wells are used for the drainage of agricultural lands especially in irrigated areas.
D. Systems of Drainage
There are four systems of drainage:
1. Random – This is used where the wet areas are scattered and isolated from each other. The lines are laid more or less at random to drain these wet areas. The main is located in the largest natural depression while the sub mains and laterals extend to the individual wet areas.
2. Herringbone – In this system, the mains are in a narrow depression and the laterals enter the main from both sides at an angle of 45° like the bones of a fish.
3. Gridiron – The gridiron is similar to herringbone but the laterals enter the main only from one side at right angels. It is adopted in flat regularly shaped fields. This is an efficient drainage system.
4. Interceptor – Ditches of different dimensions are constructed at distances to drain the excess water accumulated on the surface and inside the soil up to the depth of ditch. Such ditches may be interceptors or relief drains. This method is adopted in nurseries, seedbeds and rainfed crops. This is an effective and efficient method but requires smoothening of surface and construction of soil.
Restriction for the movement of farm machineries, reconstruction and renovation of ditches during the crop duration and harvesting of crops are the problems in this method. In flat land, bed or parallel field ditches may be constructed. The collector ditches should be across the field ditches.
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