44
Where drainage is used to reclaim salinized and waterlogged lands, it is an
environmentally beneficial practice, because the land is returned to its full productive
potential. The adaptation of subsurface drainage systems to serve as sub-irrigation or
controlled drainage systems leads to other benefits, i.e., the reduction of nitrate
pollution. Drainage (both surface and subsurface) is not simply the conversion of
wetlands, but the improvement of naturally inadequately drained cropland. It is
complementary to irrigation and is viewed as an essential component of irrigated
agriculture. The objective is to increase production efficiency, crop yields and
profitability on naturally poorly drained agricultural lands (FAO, 1997).
In the wetter regions, flooding and waterlogging still limit crop production in
many parts of the world. In the inland valleys of Sub-Saharan Africa with shallow
groundwater tables, controlled drainage may help to increase crop production and
improve the health of rural population. In certain lands of the humid tropics, drainage is
also needed in order to increase rice production and promote crop diversification. As the
global population and the demand for food increase, additional new drainage systems
will be installed in a broader range of climate, soil and hydrological conditions, and
existing systems will be renovated (FAO, 2007).
In many irrigation projects, crop yields are reduced due to waterlogging and
salinization of the land. In some cases, there is total loss of production and therefore the
land is abandoned. Waterlogging may also cause human health problems, particularly
malaria, because of ponded water. Of the estimated 235 million ha of irrigated land in
the world, 10 to 15% has been affected by waterlogging and salinization. Two important
causes of waterlogging and salinization are: (a) excessive application of irrigation
water; and (b) lack of adequate drainage. Thus provision of adequate drainage is a
solution to the waterlogging and salinization problems of irrigated lands. However, it
must be pointed out that improving drainage should not be a substitute for reducing
excessive application and that improved drainage should not be implemented without
first assessing whether waterlogging may be reduced by optimizing application.
Countries do not have sufficient information on their drainage problems. Existing
information is incomplete, inadequate and not reliable. According to available data,
Egypt has to work more to drain the country (FAO, 1996).