After reading this article you will learn about the drainage of irrigation lands.
Irrigation and drainage are complementary to each other. In humid area, drainage attains much greater importance than in arid regions. Irrigated lands require adequate drainage to remain capable of producing crops. Adequate drainage of fertile lands require lowering of shallow water table and this forms the first and basic step in reclamation of waterlogged, saline and alkali soils.
The drainage of irrigated lands:
(i) Improves soil structure and increases the soil productivity,
(ii) Facilitates early ploughing and planting,
(iii) Increases the depth of root zone thereby increasing the available soil moisture and plant food,
(iv) Increases soil ventilation,
(v) Increases water infiltration into the ground thereby decreasing soil erosion on the surface,
(vi) Creates favourable conditions for growth of soil bacteria,
(vii) Leaches excess salts from soil,
(viii) Maintains favourable soil temperature, and
(ix) Improves sanitary and health conditions for the residents of the area.
Water table can be lowered by eliminating or controlling sources of excess water. Improvement in natural drainage system and provision of artificial drainage system will be of considerable help in lowering the water table. Natural drainage systems can be properly maintained at low costs and are feasible means to protect irrigated lands from excessive percolation.
Artificial drainage also aims at lowering of water table and is accomplished by any of the following methods:
(i) Open drains
(ii) Closed drains
(iii) Pumping of ground water
Open drains are used to convey excess water to distant outlets. These accelerate removal of storm water and thus reduce the detention time thereby decreasing the percolation of water into the ground. Open drains can be either shallow surface drains or deep open drains. Shallow surface drains do not affect subsurface drainage. Deep open drains act as outlet drains for a closed drain system and collect surface drainage too.
The alignment of open drains follows the paths of natural drainage and low contours. The drains are not aligned across a pond or marshy land. Every drain has an outlet the elevation of which decides the bed and water surface elevations of the drain at maximum flow. The longitudinal slope of drain should be as large as possible and is decided on the basis of non-scouring velocities.
The bed slope ranges from 0.0005 to 0.0015. The depths of about 1.5 to 3.5 m are generally adopted for open drains. The side slopes depend largely on the type of embankment soil and may vary from 1/2 H: IV (in very stiff and compact clays) to 3H: IV (in loose, sandy formations).
The open drains should be designed to carry part of storm runoff also. The cross-section of an open drain is decided using the general principles of channel design. The channel will be in cutting and the height of banks will be small.
If the drain has to receive both seepage and storm water, it may be desirable to have a small drain in the bed of larger open drain. This will keep the bed of the drain dry for most part of the year and maintenance problems will be considerably less. Only the central deeper section would require maintenance.
Flow of clear water at low velocities permit considerable weed growth on the channel surface. The open drains have, therefore, to be cleaned frequently. Besides manual cleaning, chemical weed killers are also used. But, at times the drain water is being used for catties and the weed poison may be harmful to the catties. Aquatic life is also adversely affected by the chemical weed killers.
Closed drains are usually made of porous earthenware. These are laid below the ground surface and are covered by the earth. These do not occupy land surface and, hence, are useful when land value is high. If such drains are to be placed in impervious soil, the drains should be surrounded by filter of a coarser material to increase the permeability and also prevent migration of soil particles and blocking of drains.
Vertical relief wells tapping water from confined stratum and connected to the closed drain system form a very effective drainage system. The drains are placed at about 1.8 to 2.0 m below the ground level so that the water table can be lowered to about 1.5 m below the ground surface.
The major drawback of the gravity drainage system is that it is not capable of lowering the water table to large depths. Pumping ground water is more effective drainage method of lowering water table to relatively large depths. Pumping methods consist of withdrawing water from wells drilled or already existing in the waterlogged area.