Unit 12 ( Irrigation Engineering)
Water Logging and Drainage and Ground Water Re-charge
Definition of water logging:-
The natural process of making saturation of soil and unproductive due to excessive in its pore is known as the water logging.
That area where it occurred is called water logging area.
Equation:-
I = O+S
Where,
I= inflow of water
O = outflow of water
S= storage of water
Cause of water logging:-
Topographic condition: - when the surface of soil is not uniform, then the rain water collects and make small pound. So due to permeability of soil, this water increases the water table, hence water logging occurred.
Inadequate drainage:-
If the drainage facility is not adequate then, it will increase the chance of water logging by increasing the water table.
Canal seepage:-
Seepage of water from the earthen canal also raises the water table and hence water logging occurred.
Over irrigation:-
Irrigation water if used in excess over the field’s .then it will raises the water table and hence water logging occurred.
Rainfall and flood:-
Excess rainfall or flood will also cause the increasing in water table. By which water logging occurred.
Effects of water logging:-
Anchorage problems of plants and trees:-
If the soil becomes saturated then, the plant roots become very shallow and are easily uprooted when wind blows.
Growth of water loving wild plants:-
When the soil is water- logged water loving plant grow up which are harmful to the crop growth.
Increase in harmful salts:-
When the water table rise, then the upward moment of water brings harmful salts in the crop root zone. After evaporation, the water leaves behind salt, which reduce the production of crops.
Lowering of soil temperature:-
If moisture is present in the soil pores, the temperature of soil lowers down. Then bacteria activity retarded, which affects growth of crops badly.
Reduction of time for maturity:-
When the underwater is logged, then the crop period shortens which reduces the crop’s maturity time.
Detection of water logged area:-
1. The water logged area can be easily detected by knowing the intensity of rainfall and the amount of runoff. So to calculate the quantity of rain water which infiltrate into the sub soil and help in raising the level of water table.
2. If the above information is no available then, a hole is made to be drilled to determine the level of underwater below the surface. If the water is available nearer to the root zone of the crops, then the area is called waterlogged area.
Preventive measures:-
1. Controlling seepage from the canals: -
By following measures should be adopted to reduce seepage from the canals.
a) By lowering the F.S.L of the canal:-
When the full supply level of canal is lowered then the loss of water’s seepage loss is reduced, and hence water logging is also reduced.
b) By lining of canal :-
The bed and sides of the canal should be lined by protective materials so that seepage loss is reduced and hence water logging is also reduced.
Disposal of rain water:-
Rain water as soon as, it falls on the earth’s surface should be disposed of as soon as possible otherwise it will increase water logging.
Reducing the intensity of irrigation:-
The intensity should be reduced in this area where water logging occurred. Irrigation should be done rotation wise in different seasons.
REMEDIAL MEASURES:-
The followings methods are adopted to reclaim the water logged areas.
Installation of lift irrigation system:-
When a tube well systems are introduce, then the level of underground water goes down and hence water logging is reduced.
Implementation of drainage scheme:-
Area is reclaimed by introducing overland and subsurface drainage schemes. Surface drainage may be of:-
1) Providing seepage drain
2) By providing storm water or surface drain.
3) By providing lining of canal.
4) Implementation of tube well in fields or water logging area.
Surface drain: -
Surface drains are that construction may be natural of artificial which remove surplus water from any area and placed over the surface of the soil. While aligning surface drains followings points which are given below, should be considered.
1) Drain should follow lowest contour in a natural drainage line.
2) The total alignment of drain should be straight, so that length of the drain is reduced and all this reduces the cost of construction of that drain.
3) The drain should not pass through any ponds and it also should not cross irrigation canals
Sub surface drains:-
When the depth of the surface drain increases, then the drain scheme becomes uneconomical. In this situation subsurface drain is used. Subsurface drain is pipe drain laid in permeable stratum below a ground water table. These drains are circular pipes made of vitrified clay. The trench is excavated in the ground up to the required depth and tile line is laid on 15 cm sand bed.
Ground water recharge:-
It is a process where water moves downward from surface water to ground water. This process usually below plant root. The water table recharge occurs naturally of artificially.
Natural Ground water is recharge by rain water, by melting of snow, by river and lake through permeability of soil
Artificial ground water is recharge by making a pond, reservoirs and by storing rain watered.
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