The soils whose surface layers are saturated with water are termed waterlogged soils and the phenomenon of rise of water table to soil surface is termed water logging.
Causes of Water Logging
The followings are the causes of waterlogging in Pakistan:
Seepage of Water from Canal System
The main source of waterlogging in Pakistan is seepage of water from network of canal system. It is estimated that 45-50% of the water is lost from the main canals, branches, distributaries, main watercourses and farmer branches. Part of this water leaks into the ground. During past 60 to 100 years this leakage has gradually raised the ground water table and has caused water logging over large areas. In some cases, the water table has risen to the crop root zone or even to the soil surface.
Poor Surface Runoff and Slow Drainage
The soil surface of plains of Pakistan is flat, therefore the surface runoff is poor and drainage is slow. This results in accumulation of water on the soil surface and moves to the water table slowly under the influence of gravity. The areas of Pakistan where mean rainfall is high, the soil is clayey or loamy and possesses high water holding capacity. Therefore, much water is retained inside the soil resulting in formation of hanging water table. These waters cause rise in the water table causing water logging.
Interruption of Surface Runoff
Construction of irrigation networks and housing colonies in the path of natural drains has interrupted surface runoff at many places, resulting in the accumulation of water during monsoon season, a part of which contributes to water logging through seepage.
After heavy rainfall, rain water percolates down the pores between the soil particles under the action of gravity and finally reaches the aquifer which causes the water table to rise towards the soil surface. During rainy season, the upper layers of soil become highly saturated with water.
After heavy rainfall, the flood water spreads on the surface of the soil in the plains. Due to inappropriate drainage system, this water percolates in soil and raise the water table Which ultimately results in water logging.
Poor Water Management
Sometimes farmers use their land unscientifically. They have been depending on excessive irrigation in cultivating certain crops. Excessive irrigation and lack of adequate drainage system cause increase in water logging.
By Breaking Hardpan at a Canal Bed
During cleaning season, the digging of canals breaks down the hardpan of the canal bed. It also enhances the seepage of water to adjacent soil and results in water logging.
The construction of railway lines and roads and other development schemes in the flood plains has interrupted the passage of runoff water at many places. The water remains collected in ponds especially in rainy season. This water percolates into the soil and raises water table.
Effects of Water Logging on Plant Growth
Excess water in the root zone restrict root growth and therefore adversely affects plant growth. Some of these are:
- The air in soil pores is replaced because of excess water in the soil leading to oxygen deficit and reduced plant growth.
- The rate of diffusion of oxygen is 1000 times slower than that through air. Thus, in water logged soils the transport of gases is seriously decreased.
- Limited exchange of gases results in increase in amount of CO2 concentration that affect directly physiological processes such as transpiration and ion and water absorption.
- In the absence of oxygen, anaerobic microbes produce toxic products such methane and organic acids which accumulate in the soil and affect growth of the plants.
- In water logged soils major form of nitrogen is ammonia which is toxic to many crop plants.
- Under waterlogged conditions the concentration of phosphorus, iron, manganese and silicon increase and that of zinc decreases. The increase in concentration of ferrous iron in water logged soils is toxic to crop plants.
Reclamation of Water Logging Soils
Various control measures have been suggested for reclamation of water logged. The important ones are:
Seepage Interceptor Drains
Seepage interceptor drains may be constructed parallel to collect seepage water. These are constructed parallel to the source of seepage water. Such drains were constructed ‘both side of upper Chenab canal. However, these drains proved Ineffective in controlling the ground water level.
Surface drains help to carry seepage water and runoff waters. These have proved relatively effective as compared to seepage interceptor drains, However, due to inadequate drainage most of the drains got silted and choked with weeds.
Lining of Canals
The canals bed may be may made imperious to water to prevent seepage. These may be lined with two layers of tiles placed above one another and joined together with cement-sand plaster. Heveli canal bed was lined first at the time of its construction. The lining reduced seepage by 75 percent. Later Thal canal, Balloki Sulemanki Link and portions of BRB Link were also lined.
Pumping of Ground Water
Pumping out ground water has always been effective in lowering a shallow ground water table. It also provides additional water for irrigation where quality is suitable. The pumping by tube well has proved quite effective against water logging.