Ecological Optimal Operation of Hydropower Stations to Maximize Total Phosphorus Export

Cascade reservoirs have interrupted the distribution and transportation of nutrients, which brings environmental and ecological problems. However, the current ecological operation methods focusing on the hydrologic regime cannot solve these problems. This paper presents an ecological optimal operation method that aims at restoring the transport pattern of biogenic substances. First, this paper establishes an optimization model that combines multiobjective reservoir operations with a dynamic mass balance calculation of total phosphorus (TP), taking the maximum TP export as the ecological objective, and a traditional model taking hydrologic regime as the ecological objective for further comparison. Then, the upper Mekong River Basin (i.e., the Lancang River Basin) is taken as a case study to investigate the competitive relationship between power generation and TP export under varying minimum outputs. The mechanism of TP export is then analyzed and the concept of optimum period for TP export is proposed. The results and analyses show that a competitive relationship between the power generation originates from the differences between the optimal operation processes for maximum TP export and maximum power generation. Lower water level, lower flow in nonflood season, and higher flow in flood season, especially in the optimum period for TP export are conducive to TP export. This method is then compared to the traditional method, which considers hydrologic regimes as the ecological objective under different conditions. The results show that reservoirs with higher sedimentation coefficient and larger capacity should consider more about the biogenic substances; otherwise, the hydrologic regime and biogenic substances can be substituted for each other to some extent.