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 heck out your garden in the next prolonged dry spell. After a few days of regular watering the soil crust will probably take on a grey-white tinge. It is a sign that chemicals--mostly salt--even in the water from a domestic tap are accumulating on the surface.
Consider then what this means for irrigated agriculture in the world's driest and arid areas where water application beyond the actual needs of the crop is a must to compensate for evapotranspiration. Every time irrigation water goes on a field it adds salt to the root zone but this salt concentration has to be kept within acceptable levels or yield and quality may suffer.
It is no mean task for farmers in the dry areas who are pleased to apply any irrigation water at all. If the salinity of good quality irrigation water is 500 mg/liter TDS (total dissolved solids), every 10,000 cu m of water contains 5,000 kg of salts. But that's not all. Additional salt comes from naturally-occurring salt in the soil structure, and from consumptive use of capillary ground water.
In an experiment in Syria conducted by ICARDA, in collaboration with Aleppo University and funded by the Canadian International Development Research Center (IDRC), preliminary results show that bread wheat irrigated by saline groundwater of about 5 dS/m (1dS/m = 650 mg/l or 650 parts per million total dissolved solids) produced 1 kg of grain per cubic meter of water. The same crop irrigated with water of double the salinity produced just 0.3 kg grain/cu m of water.
In Egypt, where about a third of the 2.7 million hectares irrigated land in the early 1970s was salt-affected, reported results show salinity reducing yields from conventional agriculture by 30%. An intensive program was started to improve sub-surface drainage on at least two million hectares by 2000.
More than half of the 2.32 million hectares of irrigated land in Uzbekistan is salt-affected, and the build-up of salinity is seriously threatening productivity. ICARDA, with national scientists, has established a research site near the Arys-Turkestan Canal in Kazakstan.
The research there aims to improve irrigation efficiency with attention to leaching; to improving drainage in order to reduce soil salinity; to irrigation regime; to crop rotation, and other experimental treatments. An important component of the project is building awareness among farmers of the importance of drainage and that better management can lead to higher yield with less water used and better soil quality. Early results from 1999 look very promising, having produced significantly more cotton yield with about 40% less applied irrigation water.
Where natural drainage is insufficient to leach accumulated salts below or out of the root zone, artificial drains must be installed.
To avoid upward movement of saline water from the shallow groundwater table, drains need to be dug deeper in irrigated, arid land than is generally necessary in humid climates. These drains are needed to remove excess water and salt from the soil at a rate that will permit normal plant growth.
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Adequate drainage is an important agricultural practice, particularly in arid and semi-arid regions to prevent soil salinization and waterlogging-related problems. Traditionally, collected drainage water is disposed of to sea, in a river, lake, or drainage reservoir. When drainage water contains naturally-occurring and potentially toxic trace elements such as selenium and boron, or pesticide residues which impact negatively on the environment at high concentrations, new drainage disposal strategies have to be developed.
The logical thing to do in areas such as West, Central, and North Africa, among other dry areas of the world where water is scarce, is to develop strategies to use such water and other wastewaters in agricultural production. It all depends on the salinity and other chemical concentration levels in the drainage. This water can be used for conventional agriculture under good and modified water management practices or it can be used for non-conventional agriculture, such as the production of halophytic forage or industrial plants.
Yields of up to double the usual forage output in 1999 have given ICARDA's Central Asia Soil and Water Project a very promising start to the use of treated wastewater for irrigation. In the Sorbulak area, north of Almaty, Kazakstan, the project is developing a sustainable agronomic system which safely utilizes treated wastewater to produce various forage and industrial crops. At least 40,000,000 cubic meters of treated wastewater from the Almaty area is potentially available each year for agricultural use, enough for 10,000 hectares of land designated to produce forage crops under irrigation.
The city wastewater travels a 100-km-long canal to Sorbulak, passing through a series of ponds and mechanical and biological treatments on the way, which are designed to remove solids and restore the oxygen content. Instead of then being discharged into a river as in the past, it is now of sufficient quality to be applied to crops. ICARDA is assessing the benefit to the crops of the nutrients in the wastewater which has a high nitrogen content.
Egypt is also trying to benefit from drainage to improve not only the quality of its natural resource and its sustainability but to expand agriculture production horizontally, based partly on drainage water as a source for irrigation. The officially-reused drainage water increased from 2.6 billion cu m per year in the 1980s to about 4.2 billion cu m per year in the early 1990s. Two projects, the El Ummum drain and the Salam Canal, coming on stream will bring the total reused drainage water in the Nile Delta to about 7.2 billion cubic meters per year.
Irrigation water guidelines indicate that waters of salinity below 0.7 dS/m can be used for agriculture without restriction. If salinity is between 0.7 to 3.0 dS/m, slight restriction on use is warranted, and if the salinity of waters is greater than 3 dS/m then some salinity problems and perhaps a reduction in crop yield should be expected.
In the Jordan Valley, 98% of the total irrigated area of 24,300 ha is being irrigated with irrigation water of salinity between 0.7 and 3 dS/m. This is possible because of the active drainage systems permitting sustainability of this land for conventional agriculture production.
A similar approach could be adopted elsewhere in the dry areas to help solve drainage-induced environmental concerns or potential problems. By treating the drainage as a 'resource' rather than as a waste, it is possible to help the growers, as well as to contribute to the sustainability of agricultural systems.
Dr Fawzi Karajeh is Marginal-Water Management Specialist at ICARDA.
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