Study on impact of temperature flattening effect of cascade hydropower stations in Lancang River

The cumulative effect of cascade hydropower stations on river water temperature flattening cannot be ignored compared with that of a single station. Taking 11 cascade hydropower stations along the Yunnan section of the Lancang River as the study object, this study uses a replacement count method and Densimetric Froude Number Method to reveal the cumulative impacts of cascade stations on water temperature based on long-term measured water temperature data. The results show that among the existing cascade reservoirs on the Lancang River, Xiaowan and Nuozhadu are classified as thermally stratified reservoirs, Huangdeng is a transitional reservoir, and the rest are mixed-type reservoirs. These two reservoirs (Xiaowan and Nuozhadu) with strong regulatory capacities significantly affect the water temperature regime. The variation in discharged water temperature at Xiaowan Station is 5.0 ℃, compared with 10.7 ℃ for the natural water temperature at its dam site. Similarly, the variation in discharged water temperature at Nuozhadu Station is 3.2 ℃, versus 9.1 ℃ for the natural water temperature at its dam site. Cascade development in the basin has led to a prominent water temperature flattening effect, which generally intensifies along the cascade as the number of stations increases and the overall regulation capacity strengthens. The variation range of discharged water temperature from the upstream cascade station Huangdeng is 11.8 ℃, only 0.5 ℃ less than the natural water temperature range at its dam site. In contrast, the discharged water temperature range from the downstream cascade station Jinghong is 3.8 ℃, 5.3 ℃ less than the natural water temperature range at its dam site. This is mainly due to the increase in the minimum discharged water temperature induced by each reservoir. The degree of water temperature flattening caused by cascade hydropower stations varies across different basins. Under similar numbers of stations or regulatory capacities, a smaller annual variation in meteorological factors is the primary driver leading to extreme water temperature flattening.