We report the complex spatial and temporal dynamics of hyporheic exchange flows (HEFs) and nitrogen exchange in an upwelling reach of a 200 m groundwater‐fed river. We show how research combining hydrological measurement, geophysics and isotopes, together with nutrient speciation techniques provides insight on nitrogen pathways and transformations that could not have been captured otherwise, including a zone of vertical preferential discharge of nitrate from deeper groundwater, and a zone of rapid denitrification linking the floodplain with the riverbed. Nitrate attenuation in the reach is dominated by denitrification but is spatially highly variable. This variability is driven by groundwater flow pathways and landscape setting, which influences hyporheic flow, residence time and nitrate removal. We observed the spatial connectivity of the river to the riparian zone is important because zones of horizontal preferential discharge supply organic matter from the floodplain and create anoxic riverbed conditions with overlapping zones of nitrification potential and denitrification activity that peaked 10–20 cm below the riverbed. Our data also show that temporal variability in water pathways in the reach is driven by changes in stage of the order of tens of centimetres and by strength of water flux, which may influence the depth of delivery of dissolved organic carbon. The temporal variability is sensitive to changes to river flows under UK climate projections that anticipate a 14%–15% increase in regional median winter rainfall and a 14%–19% reduction in summer rainfall. Superimposed on seasonal projections is more intensive storm activity that will likely lead to a more dynamic and inherently complex (hydrologically and biogeochemically) hyporheic zone. We recorded direct evidence of suppression of upwelling groundwater (flow reversal) during rainfall events. Such flow reversal may fuel riverbed sediments whereby delivery of organic carbon to depth, and higher denitrification rates in HEFs might act in concert to make nitrate removal in the riverbed more efficient.

中文翻译：

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在英国气候变化情景下，地下水流上游的氮交换时空动态及其对降雨变化对摄动变化的潜在响应

我们报道了在200 m地下水喂养的河流的上升流中，低速交换流（HEFs）和氮交换的复杂时空动态。我们将展示结合水文测量，地球物理和同位素以及养分形成技术的研究如何提供对氮途径和转化的了解，而氮是其他途径无法捕获的，包括从较深的地下水中垂直优先排放硝酸盐的区域和快速变化的区域。将河滩与河床联系起来的反硝化作用。河道中硝酸盐的衰减主要由反硝化作用引起，但在空间上变化很大。这种可变性是由地下水流动路径和景观设置驱动的，这会影响水流，停留时间和硝酸盐的去除。我们观察到河流与河岸带的空间连通性很重要，因为水平优先排放区从洪泛区供应有机质，并创造出缺氧的河床条件，硝化电位和反硝化活性区重叠在河床以下10–20 cm处。我们的数据还表明，河段中水路的时间变化是由几十厘米左右的阶跃变化和水通量强度驱动的，这可能会影响溶解的有机碳的输送深度。在英国的气候预测下，时间变化对河流流量的变化很敏感，这些气候变化预计区域冬季降水中位数将增加14％至15％，夏季降水将减少14％至19％。在季节性预测上叠加的是更强烈的风暴活动，这很可能导致更加动态和本质上复杂的（在水文和生物地球化学方面）流变带。我们记录了降雨事件期间抑制上升流地下水（逆流）的直接证据。这种逆流可能会给河床沉积物加油，从而将有机碳输送到深处，而HEF中较高的反硝化率可能会协同作用，从而使河床中的硝酸盐去除效率更高。