Hydrologic extremes such as floods and droughts are often spatially related, which increases management challenges and potential impacts. However, these spatial relationships in high and low flows are often overlooked in risk assessments and we know little about their differences and origins. Here, we ask how spatial relationships of both types of hydrologic extremes and their potential hydro-meteorological drivers differ and vary by season. We propose lagged upper- and lower-tail correlation as a measure of extremal dependence for temporally ordered events to build complex networks of high and low flows. We compare complex networks of overall, low and high flows, determine hydro-meteorological drivers of these networks, and map past changes in spatial relationships using a large-sample data set in Central Europe. Our network comparison shows that low flows are correlated more strongly and over longer distances than high flows and high- and low-flow networks are strongest in spring and weakest in summer. Our driver analysis shows that high-flow dependence is most strongly governed by precipitation in winter and evapotranspiration in summer while low-flow dependence is most strongly governed by snowmelt in winter and evapotranspiration in fall. Finally, our change analysis shows that changes in connectedness (i.e., the number of catchments a catchments shows strong flow correlations with) vary spatially and are mostly positive for high flows. We conclude that spatial flow correlations are considerable for both high and particularly low flows as a result of a combination of spatially related hydro-meteorological drivers whose importance varies by extreme type and season.

中文翻译：

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复杂的高流量和低流量网络在空间相关特性、驱动因素和变化方面存在差异

洪水和干旱等极端水文事件通常在空间上相关，这增加了管理挑战和潜在影响。然而，高流量和低流量的这些空间关系在风险评估中经常被忽视，我们对它们的差异和起源知之甚少。在这里，我们询问两种类型的水文极端事件及其潜在水文气象驱动因素的空间关系如何因季节而异。我们提出滞后上尾相关和下尾相关作为时间顺序事件的极值依赖性的度量，以构建复杂的高流量和低流量网络。我们比较了整体、低流量和高流量的复杂网络，确定这些网络的水文气象驱动因素，并使用中欧的大样本数据集绘制过去空间关系的变化图。我们的网络比较表明，与高流量相比，低流量相关性更强，距离更长，高流量和低流量网络在春季最强，夏季最弱。我们的驱动分析表明，高流量依赖性最受冬季降水和夏季蒸散的支配，而低流量依赖性最强烈地受冬季融雪和秋季蒸散支配。最后，我们的变化分析表明连通性的变化（即流域的数量与流域显示出很强的流量相关性）在空间上变化，并且对于高流量大多是正的。