Water for people.pdf - WHO Thailand Digital Repository

S E C U R I N G F O O D F O R A G R O W I N G W O R L D P O P U L A T I O N / 2 1 9agriculture (see box 8.9), but water contamination with hazardouschemicals makes it unusable for food production.It is estimated that poor drainage and irrigation practices have ledto waterlogging and salinization of about 10 percent of the world’sirrigated lands, thereby reducing productivity. In particular, mobilizationof resident salts is a widely occurring phenomenon in irrigated riverbasins in arid regions. Waterlogging and salinization in large-scaleirrigation projects are often the result of unavailable drainageinfrastructure which was not included in the engineering design inorder to make projects look economically more attractive. Theseproblems are generally associated with large-scale irrigationdevelopment under arid and semi-arid conditions, as in the Indus(Pakistan), the Tigris-Euphrates (Middle East) and the Nile (easternAfrica) river basins. The solutions to these problems are known buttheir implementation is costly.Health and irrigationWater-related diseases are described earlier in this book, together withfigures on the burden of disease. The key irrigation-related vectorbornediseases are malaria, schistosomiasis and Japanese encephalitis.Irrigation has in the past sometimes been accompanied byadverse impacts on the health of local communities. The principalcauses are rooted in ecosystem changes that can create conditionsconducive to the transmission of vector-borne diseases, as well asdrinking water supply and sanitation conditions that lead togastrointestinal conditions. The attribution of the burden of each ofthese diseases to irrigation or components thereof in specificsettings is complex. Only where irrigation is introduced in an aridregion where the diseases previously did not occur, is theassociation between the resulting dramatic landscape changes andthe explosive rise of disease incidence and prevalence clear-cut. Inmost cases, there is a complex mixture of contextual determinantsof the diseases, combined with a number of confounding factors.For example, in parts of Africa south of the Sahara, the transmissionof malaria is so intense throughout the year that the additional riskfactors from irrigation development will not add to the diseaseburden. Schistosomiasis, rightly equated with irrigation in Africa, isalso determined by human behaviour and by the state of sanitation.Many vector-borne disease problems in irrigated areas can be tracedto absent or inadequate drainage. The various forms of surface irrigationall impose increased vector-borne disease hazards, while overheadirrigation and drip irrigation are virtually free of such hazards. Cropselection can be important. In that sense, flooded rice and sugarcaneare crops that carry increased vector-borne disease risk. Irrigatedagriculture often requires additional chemical inputs for crop protection,and the application of pesticides can disrupt the ecosystem balancefavouring certain disease vectors; it can also contribute to an accelerateddevelopment of resistance to insecticide in disease vector species.Box 8.9: Use of wastewater for irrigationThe cost of disposing of urban wastewater is all too oftenexternalized against the aquatic environment anddownstream users in rivers, estuaries and coastal zones andhardly, if ever, appears in the benefit and cost accounts.However, wastewater is recognized as a resource,particularly in water-scarce regions. If the polluter actuallypays, wastewater is free or has only a low cost, is reliable intime and close to urban markets. In addition to directbenefits to farmers who would otherwise have little or nowater for irrigation, wastewater improves soil fertility andreduces water contamination downstream. The total landirrigated with raw or partially diluted wastewater isestimated at 20 million hectares in fifty countries, somewhatbelow 10 percent of total irrigated land in developingcountries. For irrigation use, wastewater should be subjectto primary and secondary treatment, but in poor countriesthat is often not the case and raw sewage is applied.Disadvantages and risks related to use of insufficientlytreated wastewater concern the exposure of irrigationworkers and food consumers to bacterial, amoebic, viral andnematode parasites as well as organic, chemical and heavymetal contaminants. In a context of prevailing poverty, suchwater is used in the informal, unregulated sector, butsanitary concerns preclude the export of the products and,at least partially, the access to local food markets.Governments and the development community promoteefforts to lead wastewater reuse into sustainable channels,but countries and municipalities short of resources are slowin facing the cost of water treatment. Given water scarcityand the relatively high cost of obtaining potable freshwaterfor municipal uses, the use of treated wastewater in theurban context is projected to increase in the future, mostlyfor irrigation of trees in the urban and peri-urbanlandscape, including parks and golf courses.