Water for people.pdf - WHO Thailand Digital Repository

8 0 / A L O O K A T T H E W O R L D ’ S F R E S H W A T E R R E S O U R C E SThe Natural Water CycleTable 4.4: Groundwater use for agricultural irrigation inselected nationsCountry Irrigated Irrigation % ofarea (M ha) use (km 3 /year) groundwaterIndia 50.1 460 53China 48.0 408 18Pakistan 14.3 151 34Iran 7.3 64 50Mexico 5.4 61 27Bangladesh 3.8 13 69Argentina 1.6 19 25Morocco 1.1 10 31Groundwater is more widely used for irrigation than surface water in countries such as India,Bangladesh and Iran. Note that arid countries such as Saudi Arabia are not listed in the table despitetheir using almost 100 percent groundwater from irrigation.Sources: Burke and Moench, 2000; Foster et al., 2000.Aquifer replenishment – controls and uncertaintiesGroundwater is in slow motion from areas of aquifer recharge(which favour the infiltration of excess rainfall and/or surfacerunoff) to areas of aquifer discharge as springs and seepages towatercourses, wetlands and coastal zones (Zektser, 1999). The largestorage capacity of many aquifers over long time periods (see table4.3) transforms a highly variable recharge regime into a much moreconstant discharge regime.The contemporary rate of aquifer recharge (replenishmentthrough deep infiltration) is often used as an indicator ofgroundwater resource availability. However, average aquiferrecharge rate is not necessarily a constant parameter, and is alsoFigure 4.3: Typical groundwater flow regimes and residence times undersemi-arid climatic conditionsAs a result of the very large storage capacity and very low flow velocity ofgroundwater systems, aquifer residence times can often be counted in decades orcenturies, and sometimes in millennia.Source: British Geological Survey.frequently subject to considerable uncertainty (Foster et al., 2000),since it varies considerably with:■ changes in land use and vegetation cover, notably introductionof irrigated agriculture with imported surface water, but alsowith natural vegetation clearance, soil compaction, etc.;■ changes in surface water regime, especially diversion of river flow;■ lowering of the water table, by groundwater abstraction and/orland drainage, leading to increased infiltration; and■ longer-term climatic cycles, with considerable uncertaintyremaining over the impacts on groundwater systems of thecurrent global warming trend.These variations mean that groundwater recharge estimates havealways to be treated with caution.As a result of the very large storage of groundwater systems,aquifer residence times can often be counted in decades orcenturies and sometimes in millennia (figure 4.3). Evidence of this,and the major influence on aquifer recharge of climate changeduring Quaternary history, has been revealed through analyses ofenvironmental isotopes.Development and application of these techniques, promoted bythe IAEA-Isotope Hydrology Section, has demonstrated that much ofthe deeper groundwater in large geological basins and thick coastaldeposits originated as recharge infiltrating during wetter epochs,often 10,000 or more years ago. In some more arid regions this‘fossil groundwater’ may be the only resource, and thus should beused judiciously.Groundwater development: the risk of unsustainabilityThe rapid expansion in groundwater exploitation has led to majorsocial and economic benefits, but is also encountering significantproblems (see table 4.5). For example, it has been estimated thatmining of groundwater storage is occurring at a rate of about10 km 3 /year on the North China Plain within the Hai He basin, andabout 5 km 3 /year in the 100 or so recognized Mexican aquifers.This abstraction is not physically sustainable in the longer term.In both cases most of the consumptive use of the pumpedgroundwater is by irrigated agriculture, but there is also competitionwith urban water supply abstraction coupled with inadequateattention to wastewater in general and to opportunities thatintegrated planning could provide.A significant fraction of total aquifer replenishment is commonlyrequired to maintain dry-weather river flows and/or to sustain sometypes of aquatic and terrestrial ecosystems (Foster et al., 2000;Alley, 1999). Groundwater abstraction reduces (in some cases