Salt movement by rising water tables in surface irrigation, and by pumping brackish water from groundwater systems, can lead to land salinization and reduced crop productivity. The primary investments in waterlogging and salinity control involve irrigation and drainage infrastructure and improved irrigation water management to control seepage losses from canals and reservoirs. This is vital where natural leaching of excessive salts from the soil is not adequate and where geological conditions lead to salinity in irrigated areas. Proper management and provision for subsurface drainage in irrigation systems can address salinity problems and enhance productivity and sustainability of irrigation systems. Waterlogging and salinity problems often require some form of drainage to allow sustainable agriculture production. This must be an integral part of irrigation system investments. However, poor irrigation and agronomic practices have led salinity, sodicity, and waterlogging to affect 40-50 percent of the world’s 270 million hectares of land currently under irrigation. Drainage investments are needed to control waterlogging and salinity on 60-85 million hectares of currently irrigated lands. This investment can yield significant economic benefits (box 9.13). Box 9.13 Egypt: economic impact of drainage Egypt’s National Drainage Program (1992-00) Implementation Completion Report rated drainage components highly satisfactory and estimated an economic rate of return of 19 percent. Drainage is one of the most important investments for raising agricultural productivity in a country with limited water resources and high population growth. Drainage is estimated to have increased production in 1998-99 as follows: Seed cotton........95,000 tons Rice............... ...240,000 tons Wheat............... 380,000 tons Maize ................1,050,000 tons
The annual contribution of drainage to the Gross Domestic Product was estimated at US$0.9 billion or 8 percent of agricultural value added. Source: Ali, van Leeuwen, and Koopmans 2001 |
Waterlogging Waterlogging occurs when soil pores stay filled with water, resulting in oxygen deficiency that impairs root growth and a plant’s ability to absorb nutrients. Rice plants are an exception, as they transfer air to roots and are able to grow well even when roots are submerged for long periods. Irrigation systems are vulnerable to waterlogging at critical locations such as irrigation supply canals and seepage from reservoirs. Low areas within a command area are vulnerable if farmers on higher ground do not practice good water management. High water tables result from deep percolation and lateral underground water flows. Some soils develop high water tables due to low drainage porosity, whereas others maintain high water tables because of low water-transmitting properties. Slowly falling water tables may affect plant growth even when levels eventually fall below root zones.     
|
RSS