Premises and realities of agricultural benefits from large dams

摘要

Feasibility studies of large dams attempt to optimise the sum of benefits accrued from the 'best management options' for reservoirs. The 'best management' is a result of a comprehensive analysis of economic, social and environmental benefits from power generation, agricultural production, flood and erosion control, water supply, river navigation, nature conservation, and seawater intrusion control. Agricultural benefits are estimated on the basis of increased availability of irrigation water in the dry season, reduced flooding of farmland, and diminished soil and riverbank erosion. The assumptions made during feasibility assessments of agricultural economic and social benefits may, however, never be realised. Downstream effects of large dams, suggest that farmers may not receive the benefits cited in the feasibility studies. On the contrary, increased hardship for many farmers is often the reality. In some cases, the river flow in the dry season has been reduced, and erosion and flooding of agricultural land have become more severe in both the wet and dry seasons. The disparity between plans and reality can be attributed to how dams operate. Studies on the causes of environmental damage and effects of different operation schemes are needed to prepare more realistic project plans and environmental impact assessments. The development goals for large dams can only be reached if the 'best management option' outlined in the feasibility studies are observed.