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8 - Agriculture4. Agriculture and the environmentAgriculture has important environmental implications and significantly affects the quality andquantity of water and related resources in the Mekong Basin. The major mechanisms by whichagriculture affects the environment are: deforestation due to the clearing of land for shifting orsedentary cultivation; irrigation; and the use of chemical fertilisers and pesticides. Also significantare the conversion of acid-sulphate soils to agricultural use and human interference in the balanceof ecosystems. All of these environmental influences affect resource use and sustainability and putconstraints on agricultural activity.4.1 Environmental impacts of deforestationThe typical effects of deforestation are soilerosion, soil compacting and gullying,reduced water regulation capacity andincreased flooding, and increasedsediment in streams, rivers, andwetlands. 93 Deforestation is of particularconcern in vulnerable watershed areas –land on steep or very steep slopes at highelevations in the headwater areas of theriver system. Such land is particularlyprone to erosion. The problem isexacerbated in steep areas of Lao PDR andNortheast Thailand because contouragriculture is not practised, whichincreases the severity of erosion problemscaused by agricultural activity. A recent MRC study found that deforested critical watershed areascovered 3.5 percent of Cambodia, 20 percent of Lao PDR, and 7.5 percent of basin Viet Nam, withdata for Thailand being unavailable. 94 The impact of this situation in terms of erosion and runoff isdifficult to precisely estimate because essential data on runoff and retention rates under differentkinds of vegetation cover are not available. 95 While erosion and resultant sedimentation certainlyremain major problems, MRC water testing data indicate that levels of suspended solids are currentlydecreasing across the basin. 96 The reduction of sediment loads could be the result of the developmentof large numbers of small irrigation schemes that collect sediment in their reservoirs.4.2 Environmental impacts of irrigationIf not properly managed, agricultural development couldundermine land use sustainabilityAs the greatest user of water in the basin, irrigation schemes have implications for water levelsand water quality in the Mekong River and its tributaries. It is estimated that in Cambodia, irrigatedagriculture accounts for around 94 percent of total abstractions, with a figure of 82 percent forLao PDR, 91 percent for Thailand, and 86 percent for Viet Nam. 97 With demand for water rapidlyincreasing, there are strains on the supply of water for agricultural use. In many communities,water supply is insufficient for 2-5 months of the year, and reservoirs are often not full enough tomeet irrigation demands, especially in Thailand. 98 In the dry season month of April, irrigatedagriculture in the Vietnamese section of the delta now extracts 53 percent of the available flow inthat section of the river. 99 Water deficits are particularly noticeable in the Isarn region of Thailand.Low water levels also contribute to other environmental problems such as soil acidity and theincreasing intrusion of sea water in the Mekong Delta. These problems are likely to increase ifwater abstractions upstream lead to less water downstream to flush away contaminants. 100 With169