Spatio-Temporal Variability in Hydroclimate over the Upper Yangtze River Basin, China

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

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.

Evidence for Intensification in Meteorological Drought Since the 1950s and Recent Dryness–Wetness Forecasting in China

Drought is one of the major environmental stressors; drought is increasingly threatening the living environment of mankind. The standardized precipitation evapotranspiration index (SPEI) with a 12-month timescale was adopted to monitor dry–wet status over China from 1951 to 2021. The modified Mann–Kendall (MMK) and Pettitt tests were used to assess the temporal trend and nonlinear behavior of annual drought variability. The analysis focuses on the spatio-temporal structure of the dry–wet transition and its general connections with climate change processes. In addition, the seasonal autoregressive integrated moving average (SARIMA) model was applied to forecast the dry–wet behavior in the next year (2022) at 160 stations, and the hotspot areas for extreme dryness–wetness in China were identified in the near term. The results indicate that the dry–wet climate in China overall exhibits interannual variability characterized by intensified drought. The climate in the Northeast China (NEC), North China (NC), Northwest China (NWC), and Southwest China (SWC) has experienced a significant (p < 0.05) drying trend; however, the dry–wet changes in the East China (EC) and South Central China (SCC) are highly spatially heterogeneous. The significant uptrend in precipitation is mainly concentrated to the west of 100° E; the rising magnitude of precipitation is higher in Eastern China near 30° N, with a changing rate of 20–40 mm/decade. Each of the sub-regions has experienced significant (p < 0.01) warming over the past 71 years. Geographically, the increase in temperature north of 30° N is noticeably higher than that south of 30° N, with trend magnitudes of 0.30–0.50 °C/decade and 0.15–0.30 °C/decade, respectively. The response of the northern part of Eastern China to the warming trend had already emerged as early as the 1980s; these responses were earlier and more intense than those south of 40° N latitude (1990s). The drying trends are statistically significant in the northern and southern regions, bounded by 30° N, with trend magnitudes of −0.30–−0.20/decade and −0.20–−0.10/decade, respectively. The northern and southwestern parts of China have experienced a significant (p < 0.05) increase in the drought level since the 1950s, which is closely related to significant warming in recent decades. This study reveals the consistency of the spatial distribution of variations in precipitation and the SPEI along 30° N latitude. A weak uptrend in the SPEI, i.e., an increase in wetness, is shown in Eastern China surrounding 30° N, with a changing rate of 0.003–0.10/decade; this is closely associated with increasing precipitation in the area. Drought forecasting indicates that recent drying areas are located in NWC, the western part of NC, the western part of SWC, and the southern part of SCC. The climate is expected to show wetting characteristics in NEC, the southeastern part of NC, and the eastern part of EC. The dry–wet conditions spanning the area between 30–40° N and 100–110° E exhibit a greater spatial variability. The region between 20–50° N and 80–105° E will continue to face intense challenges from drought in the near future. This study provides compelling evidence for the temporal variability of meteorological drought in different sub-regions of China. The findings may contribute to understanding the spatio-temporal effect of historical climate change on dry–wet variation in the region since the 1950s, particularly in the context of global warming.