Contents preface (VII) introduction 1—37

5.14. WATERLOGGING
In all surface water irrigation schemes, supplying the full water requirements of a crop, more water is added to the soil than is actually required to make up the deficit in the soil resulting from continuous evapotranspiration by crops. This excess water and the water that seeps into the ground from reservoirs, canals, and watercourses percolate deep into the ground to join the water table and, thus, raise the water table of the area. When the rising water table reaches the root zone, the pore spaces of the root-zone soil get saturated.
A land is said to be waterlogged when the pores of soil within the root zone of a plant gets saturated and the normal growth of the plant is adversely affected due to insufficient air circulation. The depth of the water table at which it starts affecting the plant would depend on plant and soil characteristics. A land would become waterlogged sooner for deep-rooted plants than for shallow-rooted plants. Impermeable soils generally have higher capillary rise and, hence, are waterlogged more easily than permeable soils. A land is generally waterlogged when the ground water table is within 1.5 to 2.0 m below the ground surface. Water table depth is good if the water table is below 2 m and rises to 1.8 m for a period not exceeding 30 days in a year (13). If the water table is at about 1.8 m and rises to about 1.2 m for a period not exceeding 30 days in a year, the condition is considered as fair. If the water table depth is between 1.2 to 1.8 m which may rise to 0.9 m for a period not exceeding 30 days in a year, the condition of water table depth is rather poor. In a poor condition of water table depth, the water level is less than 1.2 m from the surface and is generally rising.
A high water table increases the moisture content of the unsaturated surface soil and thus increases the permeability. There may be advantages of having water table close to the surface as it may result in higher crop yield due to favourable moisture supply. This may, however, be true only for few years after water table has risen from great depths. The favourable condition may be followed by serious decrease in the crop yield in areas where alkali salts are

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present. With slight increase in inflow to the ground, the high water table may become too close to the ground surface and when this happens the land gets waterlogged and becomes unsuitable for cultivation.

The problem of waterlogging is a world-wide phenomenon which occurs mainly due to the rise of the ground water table beyond permissible limits on account of the change in ground water balance brought about by the percolation of irrigation water. It has become a problem of great importance on account of the introduction of big irrigation projects. The land subjected to waterlogging results in reduction of agricultural production. The problem of waterlogging has already affected about 5 million hectares of culturable area in India (see Table 6.2 for more details).