WHAT IS WATERLOGGING?
The agricultural land becomes waterlogged when the soil pores within the root zone of the crops get saturated and the normal circulation of air is cutoff.
Waterlogging affects the productivity of the land and leads to a reduction in the crop yield. Waterlogging generally occurs because of over-irrigation. high water table and the poor water management.
The depth of the water table below the land surface at which the soil tends to become waterlogged and affects the crop yield depends upon the height of the capillary fringe above the water table and also on the type of crop.
For most of the agricultural soils, the height of capillary fringe usually varies between 0-9 and 1-5 m.
It is more in fine-textured soils as compared to that in coarse-textured soils. The yield of crop is usually affected when the depth of water table below the land surface is equal to or less than the values given below for different crops.
CAUSES OF WATERLOGGING
The causes of waterlogging may be summarised as follows:
1) Over irrigation:
The main cause of waterlogging is over-irrigation of the land. The excess water applied to the land percolates deep into the ground and joins the water table.
As the ground water storage augmented, the water waterlogging occurs. able rises. As soon as the water table comes close to the land surface.
2) Inadequate surface drainage: Waterlogging usually occurs when there is inadequate surface drainage of the irrigated land.
Heavy precipitation combined with inadequate surface drainage causes flooding of the land.
The prolonged flooding (or inundation) results in heavy percolation of water into the ground, which causes a rise of the water table and hence waterlogging.
3) Obstruction of natural surface drainage:
If a natural drainage (stream) near the irrigated land is obstructed by constructing an embankment for a road, a canal, a railway., etc., the flooding of the area may occur leading to waterlogging.
4) Obliteration of a natural drainage:
If an existing natural drainage is obliterated (or destroyed). it results in stoppage of natural flow and consequent flooding and waterlogging. Sometimes cultivators, while ploughing, obliterate the drainage.
5) Obstruction of natural subsurface drainage:
If there is an impermeable stratum below the land surface at a relatively low depth, it prevents the natural downwards movement of water into the subsoil.
It may result in the formation of a high perched water table which may be the cause of waterlogging. Sometimes the foundations of structures, such as causeways, obstruct the movement of water into the subsoil and may cause waterlogging.
6) Impervious top layer:
If the top layer of the land is impervious such as black-cotton soil, it obstructs the flow of water in the downward direction. Such land is prone to waterlogging due to over irrigaion and flooding.
7) Seepage from canals:
Water seeps from the bed and sides of an unlined canal. It adds to the ground water reservoir and there is a general rise in the water table, which may lead to waterlogging.
8) Construction of a reservoir:
If a large reservoir is constructed in the region, there is an increase in the water level on the upstream of the dam.
Consequently, there is an increase in the inflow to the ground water storage and a decrease in the outflow from the ground water as base flow of the river. The adjoining area may get waterlogged.
9) Defective methods of cultivation:
If the defective methods of cultivation are used, there may be ponding up of water on the land surface which may cause waterlogging.
The defective methods of cultivation include construction of high levees (bunds) which obstruct the natural drainage, inadequate preparation of land, failure to smoothen the field after tillage, improper disposal of spoil earth, improper selection of crops and growing crops which require excessive watering.
10) Defective irrigation practice:
Waterlogging may also occur due to defective irrigaion practice, such as adoping high intensity of irrigation, applying high depth of water and using defective method of application of water like wild flooding.
ILL EFFECTS OF WATERLOGGING
Waterlogging of land causes a number of ill effects. Some of the main ill effects are given below:
1) Reduction in growth of plants:
Because of waterlogging, there is absence of aeration in the roots of the plants due to which the plant growth is decreased.
2) Difficulty in cultivation:
As the land becomes waterlogged, the soil becomes slushy and puddled, and the cultivation becomes difficult.
3) Increase in salinity:
Waterlogging generally causes an increase in salinity of the soil. Salts of the soil move upward along with the water from the water table.
As the water gets evaporated from the land surface, there is an accumulation of salts and the fertility of the soil is decreased.
4) Growth of weeds:
Due to the availability of excessive water at the land surface, there is growth of water weeds. This results in a decreases of the crop yield.
5) Increase in natural plants and flora:
Due to the availability of excess water at the land surface, there is an increase in natural plants and flora. The plants such as cat tail, reeds, bull rush, grass, etc. grow in the marshy, waterlogged land and there is a reduction in the crop yield.
6) Increase in plant diseases:
Because of waterlogging, various diseases occur in the plants, which decrease their growth.
7) Fall in soil temperature:
There is a fall in soil temperature when the soil becomes waterlogged. Due to lower temperature, there may be a decrease in action of soil bacteria and the growth of plants may decrease.
8) Increase in incidence of malaria:
The waterlogged land becomes a breeding place for mosquitoes which may cause malaria. Moreover, the climate becomes damp which may affect the health of community.
MEASURES FOR PREVENTION OF WATERLOGGING
The following measures are usually adopted for prevention of waterlogging or relieving the area, which are waterlogged.
1) Controlling the intensity of irrigation:
In regions where there is a possibility of waterlogging. the annual intensity of irrigation should be kept low. In general, the average annual intensity of irrigation should not be more than 40 to 60%, i.e. the total irrigated area in a year should not be more than 40 to 60% of the culturable commanded area (CCA).
2) Providing a drainage system:
Waterlogging can be prevented by providing a properly designed drainage system, as discussed in the next section.
3) Lining of canals:
The seepage of water from the canals can be considerably reduced by lining of canals Consequently, the water table does not rise and the waterlogging is prevented.
4) Lowering of the FSL of the canals:
The seepage of water from an unlined canal can be reduced to some extent by lowering the F.S.L. of the canal.
The canal should be designed such that its F.S.L. is as low as possible, consistent with the requirements of flow irrigation for the commanded area.
A low F.S.L. results in a small difference of water levels in the canal and in the field. Consequently, the percolation losses are decreased.
Moreover, the head at the outlet is decreased which reduces the outlet discharge and prevents wasteful use of water.
5) Improving the natural drainage of the area:
The natural drainages such as streams and rivers should be improved. It involves removing obstructions to the flow such as weeds, bushes and other vegetations from the stream section.
Straightening of the streams and canalising them into shallow wide reaches improves the natural drainage. Increasing the bed slopes of the streams also improves the drainage.
The chances of waterlogging are considerably reduced if the natural drainage of the area is good.
6) Providing intercepting drains:
The water seeping from the unlined canal can be intercepted by providing intercepting drains parallel to the canal. This is especially useful when the canal has high embankments and the water table is already high.