Defining effective measures to reduce nitrate pollution in heterogeneous mesoscale catchments remains challenging when based on concentration measurements at the outlet only. One reason for this is our limited understanding of the subcatchment contributions to nitrate export and their importance at different time scales. While upstream subcatchments often disproportionally contribute to runoff generation and in turn to nutrient export, agricultural areas (which are typically found in downstream lowlands) are known to be a major source of nitrate pollution. To examine the interplay of different subcatchments, we analyzed seasonal long‐term trends and event dynamics of nitrate concentrations, loads, and the concentration–discharge relationship in three nested catchments within the Selke catchment (456 km²), Germany. The upstream subcatchments (40.4% of total catchment area, 34.5% of N input) had short transit times and dynamic concentration–discharge relationships with elevated nitrate concentrations during wet seasons and events. Consequently, the upstream subcatchments dominated nitrate export during high flow and disproportionally contributed to overall annual nitrate loads at the outlet (64.2%). The downstream subcatchment was characterized by higher N input, longer transit times, and relatively constant nitrate concentrations between seasons, dominating nitrate export during low‐flow periods. Neglecting the disproportional role of upstream subcatchments for temporally elevated nitrate concentrations and net annual loads can lead to an overestimation of the role of agricultural lowlands. Nonetheless, constantly high concentrations from nitrate legacies pose a long‐term threat to water quality in agricultural lowlands. This knowledge is crucial for an effective and site‐specific water quality management.

中文翻译：

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从事件尺度到长期尺度，分析流域异质性对硝酸盐出口动态的影响

仅基于出口处的浓度测量结果，确定减少异种中尺度流域硝酸盐污染的有效措施仍然具有挑战性。原因之一是我们对子汇水区对硝酸盐出口的贡献及其在不同时间范围内的重要性的了解有限。虽然上游子汇水区通常成比例地促进径流的产生，进而促进养分的出口，但众所周知，农业区（通常在下游低地发现）是硝酸盐污染的主要来源。为了检验不同子流域的相互作用，我们分析了塞尔克流域（456 km ^2）内三个嵌套流域的硝酸盐浓度，负荷以及浓度-流量关系的季节性长期趋势和事件动态。），德国。上游子汇水区（占总汇水面积的40.4％，氮输入的34.5％）具有短的渡越时间，并且在雨季和事件期间，动态的浓度-排放关系与硝酸盐浓度升高有关。因此，上游子汇水区在高流量期间主导了硝酸盐的出口，对出口处的年度硝酸盐总负荷的贡献不成比例（64.2％）。下游子汇水区的特征是较高的氮输入，较长的渡越时间和不同季节之间硝酸盐浓度相对恒定，这在低流量时期主导了硝酸盐的出口。忽略上游子汇水面积对硝酸盐浓度和年度净负荷的暂时性的不成比例的作用会导致对农业低地的作用的高估。尽管如此，来自硝酸盐遗留物的持续高浓度对农业低地的水质构成了长期威胁。这些知识对于有效且针对特定地点的水质管理至关重要。