Multi-timescale changes of water temperature due to the Three Gorges Reservoir and climate change in the Yangtze River, China

The water temperature in the Yangtze River has been found altering over the last decades. Although these al-terations are expected to occur over multiple timescales and with multiple factors, including both reservoir operation and climate change, few efforts have been made to quantify the alterations and conduct attribution analysis on multiple timescales. It hinders the deep understanding of the thermal regime in the Yangtze River and how it responses to the driving forces. Here, a multiresolution analysis based on the maximal overlap discrete wavelet analysis is applied to quantify the multi-timescale alterations of the water temperature, and to distin-guish the impacts caused by reservoirs and climate change. The results indicate that the water temperature displays a significant upward trend in past three decades, which is mostly driven by the climate change. Reservoir operation has reduced the intra-annual variability of the water temperature, especially for the short-term and annual scales, resulting in a smoother and flatter water temperature process. The correlation between the water and air temperature is also investigated on multiple timescales based on the wavelet cross-correlation coefficients. It is found that the correlation between the air and water temperature in the natural state shows an overall increasing trend with the increasing timescale and peaks on annual scale. Due to the reservoir operation, the correlation on the moderate scales (1-4 months) has been notably weakened. While for the longer scales, the correlation between air and water temperature remains high, although their phases difference on some scales may be extended by months due to the reservoir operation.

Automated summary

The water temperature in the Yangtze River has been found to be changing over the past few decades, primarily due to climate change. The study also shows that reservoir operation has reduced the variability of water temperature, resulting in a smoother process. Additionally, the correlation between water and air temperature varies on different timescales.