Long-term variations in hydraulic residence time of floodplain lakes and their response to water conservancy projects

Hydraulic residence time, a key indicator to evaluate the hydrodynamic processes of a lake, especially a floodplain lake, critically affects environmental processes such as water self-purification and algal transport. The hydraulic residence time of Poyang Lake in the east of China, a typical floodplain lake, exhibits significant and complicated variations. Focusing on Poyang Lake, this study adopted the hydraulic data from 1979-2018 to calculate the hydraulic residence time of the lake. Different methods, such as the Mann-Kendall method and hydrodynamic modeling, were used to reveal the long-term variations in the hydraulic residence time and explore the reasons for these variations. Subsequently, this study predicted the possible effects of the planned water conservancy project on the hydraulic residence time of the floodplain lake. The main results can be summarized as follows: (1) In the past 40 years, the annual average hydraulic residence time of Poyang Lake varied between 8.6 and 20.1 d, with a significant reduction noted. Furthermore, the monthly average hydraulic residence time decreased considerably, with the decline in July-September being the most significant. (2) The annual average outflow rate of Poyang Lake increased insignificantly, while the annual average lake volume decreased significantly, which were possibly the main reasons for variations in the annual average hydraulic residence time. (3) In the future, with the planned Poyang Lake water conservancy project (PLWCP), the low water level of the lake is expected to be compensated for by increasing the water level during the regulation period; thus, the hydraulic residence time of the lake would be extended. The findings of this study could deepen the understanding of the hydrodynamic characteristics of floodplain lakes and their responses to water conservancy projects; they could also provide technical foundations for water resource management and environmental protection of floodplain lakes.