The effect of the interplay between unsteady flow and bedform in a flood event on nitrogen cycling in the hyporheic zone (HZ) remains poorly understood. In this study, a reactive transport groundwater model with different flood hydrographs was proposed to investigate the effect of modified hyporheic flow on nitrate dynamics in the HZ, including nitrate source‐sink function, response to the single‐peak flood event and removal efficiency. The results demonstrate that there exists an optimal range of river channel gradients that could enhance the biogeochemical reactions (respiration, nitrification and denitrification) in a flood event. The HZ acts as a nitrate sink especially after the flood event, and its source‐sink function is independent of the unsteady discharge/stage conditions. The nitrate in the HZ has a hysteretic response to peak stage/discharge, and its removal efficiency is decreased by up to 70% compared to steady flow conditions. These findings not only provide a better understanding of nitrogen dynamics under the effect of unsteady channel flow, but also can be applied for river restoration to efficiently remove nitrate in the HZ by modifying river channel gradients.

中文翻译：

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单峰洪水事件中河床的硝酸盐去除过程

在洪水事件中，非恒定流和床形之间的相互作用对流变区（HZ）中氮循环的影响仍然知之甚少。在这项研究中，提出了一种具有不同洪水水位图的反应性运输地下水模型，以研究改良的低渗流对HZ中硝酸盐动力学的影响，包括硝酸盐源汇功能，对单峰洪水事件的响应和清除效率。结果表明，在洪水事件中，河道坡度存在一个最佳范围，可以增强生物地球化学反应（呼吸，硝化和反硝化）。HZ尤其是在洪灾之后充当硝酸盐汇，其源汇功能与不稳定的排放/水位条件无关。HZ中的硝酸盐对峰值阶段/流量具有滞后响应，与稳定的流动条件相比，其去除效率最多降低70％。这些发现不仅可以使人们更好地了解不稳定通道水流下的氮动态，还可以用于河流修复，通过修改河道坡度来有效去除HZ中的硝酸盐。