Long-term (2002-2017) Impact of Danjiangkou Dam on Thermal Regimes of Downstream Han River (China) using Landsat Thermal Infrared Imagery

Abstract This work quantified the long-term (2002-2017) impacts of the Danjiangkou dam (China) on the thermal regimes of downstream Han River using Landsat thermal infrared imageries, with an emphasis on the effect of dam heightening. The accuracy assessment of satellite-derived temperatures indicated a root-mean-square error of 0.95 °C, suggesting an accurate retrieval of water temperature. The long-term analysis detected a seasonal cycle of thermal impacts, with strong cooling in summer, moderate cooling in spring and autumn whereas no significant changes in winter. The cycle was likely attributed to the seasonal thermal stratification in the dammed reservoir. The analysis also revealed a pronounced year-to-year variability in the thermal impacts. This was especially the case after the heightening of dam in 2013, which led to a stronger cooling effect and a longer cooling duration. The inter-annual variability can be largely explained by the varying reservoir water levels associated with dam operation and dam heightening, as well as tributary inflows and water diversion projects. The longitudinal thermal profile analysis suggested at least a length of 20 km downstream river was thermally affected. The cooling effects may be significant to the aquatic ecosystems of downstream river, especially with respect to the delay of Famous Four species of Chinese Carp spawning or the reduction and downstream shift of spawning sites. The findings provide new scientific understandings about the long-term seasonal thermal regimes of dammed rivers and the techniques can be extended to the global scale to investigate thermal effects of large dams.