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10. DRAINAGE, SOIL SALINITY
Excess water in the crop root zone soil is harmful to plant growth. Crop yields
are drastically reduced on poorly drained soils, and, in cases of prolonged waterlogging,
plants eventually die due to a lack of oxygen in the root zone. Sources of excess soil
water that result in high water tables includes: high precipitation in humid regions;
surplus irrigation water and canal seepage in the irrigated lands; and artesian pressure.
Waterlogging in irrigated regions may result in excess soil salinity, i.e., the
accumulation of salts in the plant root zone. Artificial drainage is essential on poorly
drained agricultural fields to provide optimum air and salt environments in the root
zone. Drainage is regarded as an important water management practice, and as a
component of efficient crop production systems. World food supply and the
productivity of existing agricultural lands can only be maintained and enhanced if
drainage improvements are undertaken on cropland currently affected by the excess
water and high water tables (FAO, 1997).
The primary benefits of drainage go beyond the control of excess soil water and
accumulation of excess salts in the crop root zone. The coincident environmental and
socio-economic benefits associated with disease vector control and public health must
be fully recognized. One of the major environmental benefits of drainage is its positive
impact on improving the health of people, plants and farm animals. Drainage of wet,
swampy areas has led to a reduction in mosquito breeding sites in all parts of the world.
The effect has been a drop in the incidence and prevalence of important water related
and mosquito transmitted diseases. Furthermore, drainage of stagnant water can
eliminate foot-rot in large animals and, to a certain extent, the breeding environment of
aquatic and semi-aquatic snails, which are the intermediate host of schistosomiasis.
Drainage also reduces or eliminates mildew infections and various root rots of plants.
The overall impact of improved drainage has been an improvement in hygienic
conditions, in the health sector and in the productivity of human beings. By growing
high value food crops in well-drained soils, the health, nutrition and economic status of
rural populations can be improved. There are also increased opportunities for
employment, as new industries may develop in prosperous areas (FAO, 1997).