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 0 3cereal base of the national diet. Thus, using a hydrologic perspective,trade in food was called trade in virtual water, that is the waterconsumed to produce an agricultural commodity. For example, a cropsuch as wheat consumes about 1 to 5 cubic metres (m 3 ) of water toproduce 1 kg of cereal. For poultry with a feed/meat conversionfactor of 4:1, the virtual water content would be 6 m 3 /kg of poultrymeat. For cattle, with a conversion factor of 10:1, the virtual watercontent of 1 kg of beef would be 15 m 3 . The amount of virtualwater imported by a country is a measure of the degree to whichthe country depends on the international market for its food supply.Manipulation of the virtual water concept is subject to somecaveats, one of which is that the water actually used by a crop mayhave stemmed partially or totally from rain, which is free of cost,whereas piped water has a cost. In the case of meat, one has tokeep in mind that free-roaming animals are efficient collectors ofvirtual water: in arid areas, the rainfed pasture they consume usuallywould have no other use.The Use of Water in AgricultureWater for food productionFor vegetative growth and development, plants require, within reach oftheir roots, water of adequate quality, in appropriate quantity and atthe right time. Most of the water a plant absorbs performs thefunction of raising dissolved nutrients from the soil to the aerialorgans; from there it is released to the atmosphere by transpiration:agricultural water use is intrinsically consumptive. Crops have specificwater requirements and these vary depending on local climaticconditions. Whereas an indicative figure for producing 1 kg of wheat isabout 1 m 3 of water which in turn is returned to the atmosphere,paddy rice may require twice this amount. The production of meatrequires between six and twenty times more water than for cereals,depending on the feed/meat conversion factor. Specific values for thewater equivalent of a selection of food products are given in table 8.3.Water required for human food intake can be derived from thesespecific values in a grossly approximate way, depending on the sizeand composition of the meals (see box 8.2).Food production: the dominant role of rainfed agricultureNon-irrigated (rainfed) agriculture depends entirely on rainfall stored inthe soil profile. This form of agriculture is possible only in regionswhere rainfall distribution ensures continuing availability of soilmoisture during the critical growing periods for crops. Non-irrigatedagriculture accounts for some 60 percent of production in thedeveloping countries. In rainfed agriculture, land management canhave a significant impact on crop yields: proper land preparationBox 8.2: Assessing freshwater needs forglobal food productionThe amount of water involved in food production issignificant, and most of it is provided directly by rainfall.A rough calculation of global water needs for foodproduction can be based on the specific water requirementsto produce food for one person. Depending on thecomposition of meals and allowing for post-harvest losses,the present average food ingest of 2,800 kcal/person/daymay require roughly 1,000 m 3 per year to be produced.Thus, with a world population of 6 billion, water needed toproduce the necessary food is 6,000 km 3 (excluding anyconveyance losses associated with irrigation systems). Mostwater used by agriculture stems from rainfall stored in thesoil profile and only about 15 percent of water for crops isprovided through irrigation. Irrigation therefore needs900 km 3 of water per year for food crops (to which somewater must be added for non-food crops). On average,about 40 percent of water withdrawn from rivers, lakes andaquifers for agriculture effectively contribute to cropproduction, the remainder being lost to evaporation, deepinfiltration or the growth of weeds. Consequently, thecurrent global water withdrawals for irrigation are estimatedto be about 2,000 to 2,500 km 3 per year.Table 8.3: Water requirement equivalent of main food productsProduct Unit Equivalent water inm 3 per unitCattle head 4,000Sheep and goats head 500Fresh beef kg 15Fresh lamb kg 10Fresh poultry kg 6Cereals kg 1.5Citrus fruits kg 1Palm oil kg 2Pulses, roots and tubers kg 1This table gives examples of water required per unit of major food products, includinglivestock, which consume the most water per unit. Cereals, oil crops, and pulses, roots andtubers consume far less water.Source: FAO, 1997a.