Quantitative Assessment of Anthropogenic Activities on Precipitation–Flood Relations in the Middle-Lower Reaches of the Yangtze River Since the 21st Century

Abstract

Quantitatively distinguishing between climate change and anthropogenic activities is crucial for flood management in the middle-lower reaches of the Yangtze River (MLRYR). However, the quantitative contribution of climate change and anthropogenic activities to flood occurrences in the region over long time scales (spanning more than one climatic state) and across extensive spatial areas remains unclear. This study aims to address such issue by analysing runoff data from 12 hydrological stations in the MLRYR from 1961 to 2020. By combining the Peak Over Threshold (POT) method with the detection of flood event initiation and termination days, we analysed changes in mean annual flood duration days (MAFD) across two distinct periods: 1961–1987 (Phase I) and 2001–2020 (Phase II). Our results demonstrate a significant positive correlation between precipitation and flood in the MLRYR throughout the study period, though this correlation is weaker in Phase II compared to Phase I. By applying the double mass curve method, we quantified the contributions of climate change and anthropogenic activities to changes in MAFD in the MLRYR, comparing the period of rapid urbanisation since the 21st century to Phase I, when urbanisation was at its initial stage. The results indicate that anthropogenic activities have significantly suppressed average annual flood occurrences in Phase II, with contribution rates ranging from 118% to 199%. Conversely, changes in precipitation have positively influenced flood occurrences in Phase II, with contribution rates ranging from 18% to 99%. The extensive construction of hydraulic engineering has played a crucial role in regulating the runoff of the main and tributary rivers in the MLRYR, thereby weakening the precipitation–flood relations since the 21st century. This study suggests that the continued construction of hydraulic engineering in the MLRYR may remain a crucial measure for mitigating the increased flood risks associated with global warming and accelerated urbanisation in the future.