Anthropogenic intensification of the eco-hydrothermal regime transition in regulated rivers: The cumulative effect of cascade reservoirs

Abstract

Although more reservoirs are under construction, research on the cumulative effects of cascade reservoirs remains insufficient. In addition, there is a lack of detailed metrics for the extreme hydrothermal events. We develop a comprehensive framework that focuses on changes in the eco-hydrothermal regime of rivers and the cumulative effects of cascade reservoirs. The framework integrates 29 eco-hydrothermal indicators (existing and newly proposed), including Magnitude, Thermal suitability, Timing, Rate of change, and Extreme hydrothermal conditions. A coupled method of the long short-term memory (LSTM) and air2stream models is used to reconstruct multi-scenario water temperature under non-cascade reservoir regulation. Results show that the eco-hydrothermal regime in the upper Yangtze River has undergone significant changes, with the most pronounced change observed in extreme hydrothermal events, where the duration, severity, and kurtosis have increased, with an overall RVA value of 0.87. Cascade reservoir regulation is the main cause of eco-hydrothermal changes, promoting the homogenization of hydrothermal processes, reducing thermal suitability for the Four Famous Major Carp and Chinese Sturgeon during spawning, and delaying the window of thermal signals. The study found that while climate change has enhanced extreme thermal conditions in the upper Yangtze River, the aggravation of extreme high and low hydrothermal conditions is primarily controlled by cascade reservoirs. In terms of the impacts calculated, the duration corresponds to 104–117 days and 32–36 days, severity corresponds to 277.85–287.21 °C2 and −84.53∼−90.2 °C2, and kurtosis corresponds to 1.94–2.42 °C2 and −0.85∼−1.15 °C2.