Regulating flow to maximize riverine nitrogen removal by riparian zones downstream of reservoirs

Abstract

Flood waves generated by reservoir discharge play a crucial role in the potential for riparian denitrification downstream. The efficiency of nitrogen (N) removal from river water by riparian zones is typically enhanced with greater wave amplitude (A) and duration (T). However, a knowledge gap exists with respect to the impact of flood waves on N removal when considering a fixed-value integral over time of flood waves (IOFW). This study explored N transport and transformation in the riparian zone downstream of the dammed Inbuk River, Korea under various wave conditions with a fixed IOFW. The results revealed a logarithmic enhancement in solute infiltration into the riverbank with an increasing wave amplitude/duration ratio (A/T ratio). The solute residence time within the riparian zone displays an initial increase followed by a decrease with the increment in the A/T ratio. We identified a threshold for the wave A/T ratio that maximizes riverine N removal by the riparian zone, a phenomenon observed across various dammed rivers. Our findings indicated that riparian denitrification is reaction time-limited when the wave A/T ratio exceeds the threshold; contrarily, it becomes transport capacity-limited. Additionally, we observed a significant relationship between the net nitrate removal and the return/infiltration time ratio of water (r² = 0.93, p < 0.05). The research results contribute to a better understanding of the impact of water level fluctuations on the N cycle in riparian zones, and provide valuable insights for developing sustainable reservoir operation strategies.