Distribution simulation and risk assessment of total phosphorus in mega-scale reservoir under different water diversion operations and upstream inflow scenarios: A case study in China

Water quality variations and the relevant environmental effects on reservoir water bodies have been a hot topic in the field of hydro-environment in recent years. Previously developed hydrodynamic-water quality coupling models for hydro-engineering rarely comprehensively simulate and accurately reflect the influence of hydrological variation on water quality migration and transformation under synergetic variations in artificial water allocation regulation and natural upstream inflow changes. This study develops an Environmental Fluid Dynamics Code-based hydrodynamic-water quality coupling model to simulate and analyze the distribution and migration characteristics of total phosphorus concentration variations in the case of a typical mega water diversion hydro-engineering Reservoir and its tributary Hanjiang River, China. Holistic scenario combination simulation demonstrates the importance of coupling hydrodynamic patterns that can substantially influence and mediate TP migration, showing that when the upstream inflow of the Hanjiang River increases by 50 %, the TP concentration simultaneously increases by 50 % compared to the average baseline regime at the water diversion intake point. The increased-inflow event in the flood season easily causes the TP concentration at the water diversion intake to be higher than the Class-II surface water quality standard threshold (0.025 mg/L) of China, while generally maintaining a good status (maximum specific point < 0.05 mg/L). The increase in water intake site will not disturb the reservoir hydrodynamics, thus not causing the transport of high-concentration total phosphorus and associated water quality risks. This study not only quantifies the synergistic impacts of inflow fluctuations and diversion operations on reservoir water quality but also provides a mechanistic modeling framework to guide risk-informed management strategies for mega-reservoirs facing complex water resource allocation pressures.