Multi-scale impact of climate change and cascade reservoirs on hydrothermal regime alteration in regulated rivers

Study region: The upper reach of the Yangtze River (URYR) basin, China. Study focus: There are few studies on the factors driving the change in hydrothermal regime and its ecological response. This study proposes a framework to quantify the impact of climate change and reservoirs on hydrothermal and fish reproduction. The framework identifies mutation years of water temperature by the TFPW-MK method and characterizes hydrothermal variability on multiple time scales using hydrologically meaningful indicators (Sample Entropy and Range of Variability Approach). The differences in hydrothermal driving forces are then clarified utilizing the Long Short-Term Memory model and contribution calculation method. In addition, we established water temperature scenarios to assess the ecological effects of hydrothermal alteration. New hydrological insights for the region: After mutation, URYR hydrothermal tends to be uniform. With the increase in hydrothermal complexity, the relative contribution of reservoirs is 98%. On an annual scale, climate change is the dominant factor in river warming. On the seasonal and monthly scales, reservoirs are used as heat sources in autumn and winter, and as cold sources in other seasons. Moreover, the reservoirs have a significant impact on hydrothermal in the nonflood season, especially in March and December, which are 0.72 degrees C (48%) and 1.17 degrees C (87%), respectively. The reservoirs delayed the spawning time for Coreius guichenoti and Myxocyprinus asiaticus but increased the duration of Myxocyprinus asiaticus (19 days).

Automated summary

This study focuses on the change in hydrothermal regime and its ecological response in the upper reach of the Yangtze River basin in China. A framework is proposed to quantify the impact of climate change and reservoirs on hydrothermal variability and fish reproduction. The results show that hydrothermal tends to be uniform after the mutation, with reservoirs playing a dominant role. Climate change is the main factor for river warming on an annual scale, while reservoirs act as heat sources in autumn and winter and as cold sources in other seasons. The reservoirs also significantly affect hydrothermal in the nonflood season, especially in March and December. They have delayed the spawning time for Coreius guichenoti and Myxocyprinus asiaticus but increased the duration of Myxocyprinus asiaticus.