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.
1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. There may be a hard pan that affects seepage of water to lower strata.
7. 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.