Employing three ratio indices for ecological effect assessment of Manwan Dam construction in the Lancang River, China

The Lancang River, located in the upper reach of the Mekong River, has attracted worldwide attention for the large‐scale development of hydropower on its main stream, In the paper, taking Manwan dam as a case study, we adopt emergy analysis method for three ratio indices, i.e. the emergy yield ratio (EYR), environmental loading ratio (ELR) and ecosystem index for sustainability (EIS), to analyse the beneficial contribution of dam construction and operation (DCP) to regional social system, and environmental pressure as well as sustainability of river ecosystem (reservoir area and downstream channel). Different from traditional researches, the loss for ecosystem service of terrestrial ecosystem around reservoir (STER) as a significant cost is integrated into holistic analysis for ecological effect of Manwan dam construction (DC). The result show that the emergy yield of river ecosystem after Manwan DC is equal to 4.58 × 1021 sej/yr, of which the majority is from electricity generation (4.52 × 1021 sej/yr), i.e. 98.7% of the total emergy yield. The gross emergy cost of the ecosystem in the Manwan reservoir area is 6.75 × 1020 sej/yr, with the primary one being derived from sediment deposition (3.72 × 1020 sej/yr), which accounts for 55.11% of the total emergy cost. The loss of STER (1.71 × 1020 sej/yr) follows and accounts for 25.33% of the total emergy cost. We conclude that no matter whether sediments and the loss of STER are included or not, EYR of present system after DC is higher than that of former system without DC, indicating that hydropower exploitation plays an important role in improvement of the emergy yield of regional system. However, the sediment deposition and loss of STER would result in a much higher environmental pressure (increased by 44.53 times) and a lower sustainability (the index decreased by 14.43 times) to the present system. Especially, this is an underestimated conclusion due to the limitation of data available without considering the emergy loss of ecological heritage and hydrological process as well as river continuum.