Water for people.pdf - WHO Thailand Digital Repository

4 2 0 / P I L O T C A S E S T U D I E S : A F O C U S O N R E A L - W O R L D E X A M P L E SRuhuna Basins, Sri LankaTable 18.2: Comparison of basic hydrological parameters of sub-basins over a thirty-year time spanBasin Catchment area Annual Annual surface Annual discharge(km 2 ) precipitation (Mm 3 ) runoff (Mm 3 ) to the sea (Mm 3 )Walawe Ganga 2,471 4,596 1,524 525Kirindi Oya 1,165 1,713 469 74Menik Ganga 1,287 2,009 352 326Malala Oya 1 402 441 74 NAOther 1 253 252 42 2 NATotal 5,578 9,011 2,4611 Ungauged rivers and streams 2 Assumed valueSource: Jayatillake, 2002b.quality is poor in the coastal lagoons and river estuaries and thequality of water appears to deteriorate further especially during thedry season (Handawela, 2002).There are two national agencies responsible for recommendingwater quality standards for drinking, bathing, irrigation and otheruses: the Sri Lanka Standards Institute and the CentralEnvironmental Agency (CEA).Human impacts on water resourcesWater resources in the basins are highly regulated to supporthydropower generation and irrigation. There are twenty largereservoirs (three of which have a capacity exceeding 100 Mm 3 ) andabout 280 smaller reservoirs, giving a total storage capacity of about900 Mm 3 . There are numerous river diversion systems, mainly forirrigation supplies, including eleven large and about 610 small crossriverstructures. Storage volume in the three main basins ranges from57 percent of annual (surface) water resources for the Kirindi Oyabasin, to 40 percent in the Walawe basin and almost zero in theMenik Ganga basin.Changes in land cover will affect water resources, of which about2,720 km 2 (about 50 percent) is under forest and scrubland. In theWalawe and Kirindi Oya basins, substantial development works havebeen carried out and the forest and scrub cover have decreased by30 percent and 23 percent respectively over the past forty years,which is a higher rate of land use change than the national average.Data and information on water resourcesThe hydrometric network includes sixteen rainfall stations, sevenagro-meteorological stations and six water level stations. The waterlevel observation network is clearly inadequate to provide soundinformation on water resources in the basins.The lack of sufficient regular flow observations have led to a largenumber of sporadic attempts by agencies involved with waterresources to collect data, mostly in response to the internalrequirements for development projects. The frequency of datacollection intensifies during project studies but generally the frequencyand quality of data observation diminish once the project is completed.Integrated Water Resources Management (IWRM) in the basinsclearly suffers from this lack of a consistent, continuous andaccurate hydrometric network. Moreover, data access and datasharing between the different agencies remain limited, restrictingthe benefits obtained from even the existing data network.Challenges to Life and Well-BeingWater for foodThe major food crop in the basins is paddy, cultivated overapproximately 52,000 hectares of which 90 percent is irrigated.Normally two crops can be grown during the year, during the twomonsoon seasons. The water used by major irrigation systems,defined as irrigation duty at the reservoir outlet, is about 1,500 mmin rainy seasons and 1,800 mm in dry. However, a proportion ofthese releases is used indirectly by small irrigation systems and fordomestic use, so actual irrigation applications are lower. Withdrawalsfor irrigation account for more than 95 percent of the total.In general large irrigation schemes achieve cropping intensitiesin excess of 150 percent, while most minor schemes are close to100 percent. Average paddy yields in major irrigation, minorirrigation and rainfed systems are 4,600, 3,600 and 2,900 kilograms(kg) per hectare, respectively.In terms of water productivity, the amount of water used toproduce 1 kg of crop has been shown in a study in the Kirindi Oyabasin to be 0.29, 0.16 and 0.14 kg/m 3 of evapotranspiration, netirrigation and gross irrigation requirement, respectively.The basins produce large amounts of marine and inland fish andcow and buffalo milk (Weerasinghe et al., 2002). Inland fisheries arebecoming popular, are receiving support from the government andare an important source of protein for the rural population. Nodetailed information is available on the productivity of inland fisheryat the basin level.