SBR- Content.pmd - INBO

State of the Basin Report - 2003the riparian countries all endeavouring toincrease rice production, and henceextending irrigation systems, there is likelyto be growing water scarcity and water-useconflicts between the agricultural, industrial,and domestic sectors. Diversification ofproduction into crops that use less water thanrice could be one solution to the problem.The establishment of irrigation reservoirs cancause changes to the river’s flow regime,which have important environmentalimplications. Reservoirs can reduce severeflooding and shorten the duration of flooding,which generally assist wet season cultivation in the flood plain. They also can delay the onset offloods, which is especially beneficial in the lower parts of the river. The associated reduction in theflood plain area does, however, adversely affect fish production and thus fish catches. This, in turn,has serious consequences for the millions dependent on the fishery for subsistence and livelihoods.Also, the ability of reservoirs to reduce theflood peak under extreme conditions isuncertain. Emergency releases from reservoirscan quickly cause flood flows equal to orgreater than natural river flows and causemajor damage to cropping (and to life andproperty) in areas where reduced floodingduring normal years creates a sense ofcomplacency. Irrigation reservoirs aremanaged primarily to meet the demands ofirrigation, and are normally kept as full aspossible to guarantee dry season requirements.Greater understanding of inflows couldimprove their potential for flood control.Much of the LMB suffers from both chronic floodingand droughtIn the Viet Nam Delta, inundation of seawaterthreatens agricultural productionExcessive irrigation in the dry season and irrigation with poor drainage can, in combinationwith certain soil and water conditions, cause salinity problems. Salt can accumulate in the subsoiland at the surface, and cause immediate damage to agriculture and fish production andproductivity, as well as long-term soil deterioration. Salinity problems in the Korat Plateau inNortheast Thailand stem from the irrigationof saline soils and rock-salt exploitation. 101Sodium chloride concentrations in this arearange from 2,000-10,000 mg/L. 102 Similarproblems occur in the Vientiane Plain.In the Viet Nam Delta, salinity is caused bythe intrusion of seawater. This is a concern tofarmers near the mouth of the river, whereabout 750,000 ha is affected by saline waterduring the dry season. 103 Efforts to controlsalinity intrusion by blocking seawater leadto other problems. Water stagnates and mayresult in fish-killing algae blooms, and waterDykes and other infrastructure protect delta farmland from salt water intrusion170