Drainage means the process of removing water from the soil that is in excess of the needs of crop plants. Drainage is the removal of excess gravitational water from the soil by artificial means to enhance crop production.
A SOIL MAY NEED ARTIFICIAL DRAINAGE FOR ONE OR TWO REASONS.
- When there is a high water table that should be lowered or
- When excess surface water cannot move downward through the soil or ever the surface of the soil fast enough to prevent the plant roots from suffocating.
ADVANTAGES OF DRAINAGE:
- The field will net get waterlogged and crop can get sufficient water and air
- After the rains are received, the soil comes in tilth earlier and it is possible to carryout agriculture operations properly and in time.
- The structure of soil improves
- There is good aeration and warmth in the root zone which are essential for proper growth.
- Bacteria that change organic matter into plant foods get necessary air and warm temperature in the soil.
- Desirable chemical reactions take place and nutrient become available to the plants easily.
- There is proper root development and absorption of nutrients is accelerated.
- Seeds germinate faster and better stand of crop is obtained.
- Due to healthy growth of plants they can resist the attack of pest and diseases better.
- Weed growth can be checked by timely weeding and inter culturing operations.
- Roots go down deep and can draw up on moisture at greater depth and with stand periods of through better and
- Good drainage permits the removal of many toxic salts and thus, reduces damage to crops.
Drainage problem occur on lands, which we consider as an arid. The causes of drainage problems are as follows. This is also termed as causes of bad drainage or why soils become water logged or ill drained.
- EXCESSIVE USE OF WATER: Water that is plentiful and cheap often is used in excess. The result is general water logged condition. Wild flooding continuous irrigation or excessively long irrigation turns to promote water logging.
- SEEPAGE OF CANALS LATERALS OR DITCHES: The seepage enters underground strata at elevations higher than those of irrigated lands enter and often becomes a direct source of water logging of low lying areas.
- INTERNAL STRATIFICATION OR IRRIGATED SOILS: The internal natural drainage of soils is often poor. The slowly permeable soils, which when irrigation water is applied, impede the percolation of the excess water. The water cannot move down wards fast enough and accumulate on the surface forming a thin layer and obstruct aeration.
- LOW LYING AREA: The area is low lying and excess rain cannot be carried away as a surface runoff rapidly into the drain causing water logged condition.
- The water table may be high and the additional gravitational water just accumulates and checks the air spaces and saturates the surface and sub soil.
- There may be a hard pan that affects seepage of water to lower strata.
- There may be salts affecting water absorption by roots.
PRINCIPLES OF DRAINAGE:
The main purpose of artificial draining is to remove the water that is harmful for plant growth. In areas with rolling topography, the excess water is carried away as a surface run off seepage water through natural depressions into the nalas and rivers. But in flat areas and in soils having an imperious substratum, the natural drainage system is not well developed and therefore water saturates and accumulates in low lying areas until evaporated or drained out slowly. The soils that remains saturated for long time needs artificial drainage. The artificial of soil water consists of providing man made channels through which the free water is carried away to natural drains such as nalas, rivers. This can done either by digging open channels to the required depth or by laying underground tile pipelines of suitable dimensions at the proper intervals and at required depth. When such artificial openings are provided in saturated soil, the water in the underground water table is lowered until it reaches the bottom level of the drainage line. The surface line of the water table does not remain horizontal but it depress over the drains. This happens, because water over the drains has the shortest distance to travel and it has the least resistance to flow through the pore spaces of the soil. The horizontal distance over which water will flow in the drains depends upon the type of soil. If the soil porous the distance is grater. Therefore, drainage must be at short intervals and at shallower depth if the soil is sandy and porous. Thus, it can be seen that the factors, which determine the depth and spacing of the drainage system, are the soil type and the desired of the water table.
TYPE OF DRAINAGE
DRAINAGE IS OF TWO FORMS
- Surface drainage and
- Sub surface drainage or underground drainage.
- A) SURFACE DRAINAGE (NATURAL SYSTEM OF DRAINAGE):
It may consist of open ditches that are laid out by eye judgment, leading from one wet spot to another and finally into a nala or river. This is often called natural system.
- Open ditch drains: The pattern of ditches is regular. The method is adopted to land that has uniform slope.
- Field ditches: Field ditches for surface drains may be either narrow with nearly vertical sides or V shaped with flat side slopes. V shaped ditches have the advantages of being easier to cross with large machinery.
- Narrow ditches: Narrow ditches are most common where large farm machinery is not used. In level areas, a collecting ditch may need to be installed at one side of the field and shallow shaped ditches are constructed to discharge into the collecting ditch. The field ditches should be laid out parallel and spaced 15 to 45 meters or more apart as required by the soil surface conditions and crop to be grown. They should be 30 to 60 cm deep depending upon the depth of the collecting ditch. Farming operations should be parallel to the field ditches. The care that a ditch will drain satisfactorily depends up on how quickly water runs into the ditch how much rain falls on the land, slope, and the condition of the soil and plant cover.
- B) SUB SURFACE OR UNDER GROUND DRAINAGE:
A sub surface or underground drainage will remove excess soil water. It percolates in to themselves, just like open drains. These underground drains afford the great advantages that the surface of the field is not cut off, no wastage of lad and do not interfere with farm operations. On the other hand, they are costly to lie and are not effective in slowly permeable clay soils.
UNDERGROUND DRAINS MAY BE CLASSIFIED AS:
- Tile or pipe drain
- Box drains
- Rubble (coarse stones or gravels filled) drains
- Mole drains and
- Use of pumps for drainage.
- Tile drain:
It consists of digging a narrow trench, placing short section of tiles at the bottom and covering the tiles with earth. The loose joints between two section of the tiles serve as a place where drainage water may enter into the drainage system. Water moves by gravity into the joins between tiles and through tile walls. Porous tile gives no better drainage than tiles that water does not percolate and porous tile can easily broken or crushed. the drains are two types of tiles in use. Tile should be always placed at least 75 cm deep to prevent breakage by heavy machinery.
- Box drains:
Instead of pipes, underground drains may be made in V shaped cut or trench, sides of which are reverted with soil, restoring the surface of the field. Depth may be 90 cm below ground.
- Rubble drains:
A somewhat equally substitute for tile drains is made by cutting narrow V shaped drains or rectangular in section, as for box drains, filling them up with rough stones large and small and then covering the whole up with soil level with surface field soil. Depth may be 90 cm.
- Mole drains:
They are often used in clay, clay loam soils. A moling machine is one that draws a bullet nosed cylinder; usually 10-15 cm in diameter is therefore formed. A mole drain should be at least 75 cm below the surface to prevent closing of the holes by compaction from farming operations. Mole drains are extremely used in Europe.
- Use of pumps for drainage:
The pumps are used in U.S.A. and many other countries for drainage. River bottoms, lakes and costal plains, peat lands and irrigated lands are the main types of lands reclaimed by pump drainage. The subsequent must be sufficiently permeable for the ground water to move to the pipes enough for effective pumping.