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Agricultural drainage water management in arid and semi-arid areas 3Chapter 1IntroductionNEED FOR DRAINAGE OF IRRIGATED LANDSLarge-scale development of irrigation has taken place in many arid and semi-arid areas sincethe late nineteenth century. Although irrigation has greatly increased the agricultural productionpotential, recharge brought about by seepage losses from the irrigation network and deeppercolation from farm irrigation has accumulated into the underlying groundwater. A rise inwater table results when irrigation-induced recharge is greater than the natural discharge. Inmany irrigated areas around the world, rising water tables have subsequently led to waterloggingand associated salinity problems. This has happened where drainage development has not keptpace with irrigation development or where maintenance of drainage facilities has largely beenneglected. As an example of the historical rise in the groundwater table after the introduction oflarge-scale irrigation, Figure 1 shows the elevations of the ground surface and the variations ofthe phreatic level in an irrigated area in Punjab, Pakistan.Salinization affects about 20–30 million ha of the world’s 260 million ha of irrigated landFAO (2000). To maintain favourable moisture conditions for optimal crop growth and to controlsoil salinity, drainage development is indispensable especially in saline groundwater zones.Smedema et al. (2000) estimate that current drainage improvement programmes cover lessFIGURE 1Rise of the groundwater table in Punjab, PakistanACHAJ DOABRECHNA DOABBARI DOABADepth in mabove m.s.l.190RaviSutlej180JhelumChenab17019601920160150pre irrigation0 20 40 60 80 100 kmIndusAJhelumCHAJ DOABRECHNA DOABRaviBARI DOABChenabSutlejDeghASource: Bhutta and Wolters, 2000.
4IntroductionTABLE 1Salinized and drained areas compared with total irrigated area, Central Asia and the Near EastCountry Irrigated area Salinized area Total drained area surface +subsurface drained% of% ofhaha irrigated area ha irrigated areaCentral AsiaKazakhstanKyrgyzstanTajikistanTurkmenistanUzbekistanNear EastBahrainEgyptIranJordanKuwaitLebanonMauritaniaPakistanSaudi ArabiaSyriaTunisiaTurkeySource: FAO, 1997a, 1997b.3 556 4001 077 100719 2001 744 1004 280 6003 1653 246 0007 264 19464 3004 77087 50049 20015 729 4481 608 0001 013 273385 0004 185 910242 00060 000115 000652 2902 140 5501 0651 210 0002 100 0002 2774 08060 0006.85.616.037.450.033.637.328.93.585.55.9433 100149 000328 6001 022 1262 840 0001 3002 931 00040 0004 000210 80012 7845 100 16544 000273 030162 0003 143 00012.113.845.758.666.341.190.30.66.2012.326.032.42.726.942.175.1%subsurfacedrainage ofirrigatedareathan 0.5 million ha per year, insufficient in their view to balance the current growth of affecteddrainage areas. They estimate that: 10–20 percent of the irrigated land is already equipped withdrainage; 20-40 percent of the irrigated area is not in need of any artificial drainage; while 40–60 percent is in need of drainage but remains without drainage facilities. Table 1 shows examplesfrom countries in Central Asia and the Near East to illustrate their observations.In Central Asia, the present drainage infrastructure is insufficient to control irrigation-inducedwaterlogging and salinity with a comparatively small percentage of subsurface drained land. Inaddition, the poor state of drainage networks (due to lack of maintenance) has exacerbatedwaterlogging and salinity (FAO, 1997a).In the Near East, which is a region subject to salinity problems due to the prevailing climateconditions, an average of about 29 percent of the irrigated areas have salinity problems. Table 1shows that for 12 countries in the Near East on average about 34 percent of the irrigated areahas been provided with drainage facilities. For most countries no figures are available on thearea under surface versus subsurface drainage (FAO, 1997b).In Pakistan, 13 percent of the irrigated area is reportedly suffering from severe salinityproblems in spite of the efforts made to provide drainage in irrigated areas. Salinity problemspersist because of deficiencies in water policies and the low priority attached to the allocation ofresources for the operation and maintenance (O&M) of drainage facilities in favour of initiatingnew projects (Martínez Beltrán and Kielen, 2000).On the other hand, under the influence of the growing world population and the increasingdemand for food, there is a trend of irrigation intensification. To supplement scarce surfacewater resources, groundwater is exploited through tubewell development, mainly in freshgroundwater zones, all over the world. In many of these areas, the water table is declining due0.46.119.118.516.338.50.60042.1
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Page 5 and 6: ivAcknowledgementsThe authors are o
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