Abstract Increasingly available high-frequency data during storm events, when hydrological dynamics most likely activate nitrate storage-flux exchanges, reveal insights into catchment nitrate dynamics. In this study, we explored impacts of seasonality and landscape gradients on nitrate concentration-discharge (C-Q) hysteresis patterns in the Selke catchment, central Germany, which has heterogeneous combinations of meteorological, hydrogeological and land use conditions. Three nested gauging stations established along the main Selke River captured flow and nitrate export dynamics from the uppermost subcatchment (mixed forest and arable land), middle subcatchment (pure steep forest) and lowermost subcatchment (arable and urban land). We collected continuous high-frequency (15-min) discharge and nitrate concentration data from 2012-2017 and analyzed the 223 events detected at all three stations. A dominant hysteresis pattern in the uppermost and middle subcatchments was counter-clockwise and combined with an accretion effect, indicating many proximal and mobilized distal nitrate sources. However, 66% of all events at the catchment outlet experienced a dilution effect, possibly due to mechanisms that vary seasonally. During wetting/wet periods (October-March), it was combined mainly with a counter-clockwise pattern due to the dominance of event runoff volume from the uppermost and middle subcatchments. During drying/dry periods (April-September), however, it was combined mainly with a clockwise pattern due to occasional quick surface flows from lowland near-stream urban areas. In addition, the clockwise hysteresis occurred mainly from May-October during mostly drying/dry periods at all three sites, indicating little distal nitrate transport in response to the low terrestrial hydrological connectivity, especially in the lowermost dry and flat subcatchment. This comprehensive analysis (i.e., clockwise vs. counter-clockwise, accretion vs. dilution) enables in-depth analysis of nitrate export mechanisms during certain periods under different landscape conditions. Specific combination of C-Q relationships could identify target locations for agricultural management actions that decrease nitrate output. Therefore, we strongly encourage long-term multisite and high-frequency monitoring strategies in heterogeneous nested catchment(s), which can help understand process mechanisms, generate data for physical-based water-quality modeling and provide guidance for water and agricultural management.

中文翻译：

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基于嵌套高频监测的季节性和景观梯度对事件尺度硝酸盐排放动态的影响分析

摘要 风暴事件期间越来越多的高频数据，当水文动力学最有可能激活硝酸盐储存通量交换时，揭示了对集水区硝酸盐动力学的见解。在这项研究中，我们探讨了季节性和景观梯度对德国中部塞尔克流域硝酸盐浓度-排放 (CQ) 滞后模式的影响，该流域具有气象、水文地质和土地利用条件的异质组合。沿主要 Selke 河建立的三个嵌套测量站捕获来自最上部子汇水面积（混交林和耕地）、中间子汇水面积（纯陡峭森林）和最下部子汇水面积（耕地和城市土地）的流量和硝酸盐输出动态。我们收集了 2012 年至 2017 年的连续高频（15 分钟）排放和硝酸盐浓度数据，并分析了在所有三个站点检测到的 223 起事件。最上部和中部子汇水面积的主要滞后模式是逆时针的，并与吸积效应相结合，表明有许多近端和移动的远端硝酸盐源。然而，流域出口 66% 的事件经历了稀释效应，这可能是由于季节性变化的机制。在湿润/湿润时期（10 月至 3 月），由于来自最上部和中部子汇水面积的事件径流量占主导地位，它主要与逆时针模式相结合。然而，在干燥/干燥时期（4 月至 9 月），由于偶尔来自低地近流城市地区的快速地表水流，它主要与顺时针模式相结合。此外，顺时针滞后主要发生在 5 月至 10 月期间，在所有三个地点的大部分干燥/干燥时期，表明响应低陆地水文连通性的远端硝酸盐传输很少，特别是在最底层干燥平坦的子汇水面积。这种综合分析（即顺时针与逆时针、吸积与稀释）可以深入分析不同景观条件下特定时期的硝酸盐输出机制。CQ 关系的特定组合可以确定减少硝酸盐产量的农业管理行动的目标位置。因此，我们强烈鼓励在异构嵌套流域中的长期多站点和高频监控策略，这有助于理解过程机制，