Key hydrodynamic principles for controlling algal blooms using emergency reservoir operation strategies

Reservoir operation strategies with low cost and high efficiency have been proposed to control algal blooms. However, the key hydrodynamic principle for performing reservoir operation strategies is still unknown, posing an obstacle to practical applications. To address this challenge, we proposed short-term emergency reservoir operation strategies (EROSs), established a three-dimensional (3D) eutrophication model of the Zipingpu Reservoir, and designed six 14-day reservoir operation cases to explore the mechanism of EROSs in controlling algal blooms. Large outflows with rapid water exchange should be adopted early in EROSs to control algal blooms in the reservoir. Small variations in the surface water temperature or the mixed layer depth/euphotic layer depth (Z_mix/Z_eu) ratio were found for different EROSs, indicating that these variations might not have been responsible for the differences in the algal blooms in the reservoir. The EROSs induced high surface flow velocity (V_s) and depth-averaged velocity (V_d) values in the reservoir, thereby controlling algal blooms by inhibiting algal growth and disrupting algal accumulation in the upper water layers. The flow of V_s against the direction of the water intake was detected during the execution of the EROSs, suggesting that increasing V_s might enhance water retention in the reservoir. Increasing V_d not only promoted water exchange to disrupt algal accumulation but also enhanced V_s to inhibit algal growth. Moreover, V_d demonstrated a strong linear relationship with the inhibition ratio of algal blooms. These results demonstrate that V_d is the key hydrodynamic indicator for performing EROSs and that accelerating V_d to exceed 0.039 m s^-1 in the near-dam region can control algal blooms. Overall, in this study, we develop a novel EROS and elucidate corresponding principles for the use of EROSs to control algal blooms in reservoirs.