Impact assessment of hydropower dam on erosion risk within a hilly agricultural area: example of the Ban Chat dam (Northern Vietnam)

In Vietnam, intensification and densification of sloping lands led to impass environmental programs despite the technical support policies of conservation agriculture. On the one hand, these programs were intended to improve or replace the systems supposedly unsuitable for traditional farming communities. On the other hand, many programs have promoted reforestation for soil protection and upstream watershed management. But the success of these initiatives was never expected that, both as regards the improvement of living conditions as environmental benefits. Our study dealt with the impacts of the building of 3 major hydropower dams (Son La, Huoi Quang and Ban Chat) regarding the land-use change and the environmental sustainability based on the erosion risk within the example of the Ban Chat dam in Lai Chau Provinces. In this region of small mountains, the annual crops, particularly maize and cassava, showed significant erosion risk that can exceed 30 t/ha/yr. But the surface area used for agricultural lands is often very low, below 20%. Then the regional erosion risk at the dam watershed level seems quite low, generally around 5 t/ha/yr. However this study underlines the erosion increases of 20% over the last ten years in Ban Chat watershed, with a maximum land-use change early before the dam building. An acceleration of the disappearance of the fallow zones occupied by shrubs (decrease of 17 800 hectares, i.e. -18%) is observed to be replaced by tea plantations and planted forest due to the opening access to economic market, driven by the hydropower planning. These trends suggest increasing fears of severe agricultural pressure on slopes, driving economic and environmental problems. At last, we suggest some pathways to elaborate an appropriate land-use assessment system for project monitoring and improved land planning before the dam construction, to be sure to identify project’s opportunities for legitimacy and relevancy.