Abstract The prevailing opinion on the temporal origin of water in a hillslope stormflow hydrograph is that the pre-event water represents a dominant fraction. Such conclusion is usually based on hydrograph separation techniques using stable water isotopes (or other conservative tracers) in conjunction with a mass balance approach. In this study, a two-dimensional dual-continuum model was used to study preferential flow of water and transport of Oxygen-18 (O-18) in a vertical cross-section of a hillslope located in a temperate spruce forest. The effects of hydrodynamic mixing and the spatiotemporal variability of isotopic signatures on estimated pre-event/event water fractions in the hillslope discharge were studied by means of numerical simulation experiments. Pre-event and event water contributions to hillslope stormflow were evaluated using a two-component mass balance approach combined with the 2D flow and transport simulations involving real as well as synthetic O-18 signatures. Long-term simulations of O-18 transport in the hillslope segment were compared with the observed O-18 content in soil water and in the hillslope effluent. The results of the long-term simulations indicated significant mixing of pre-event and event water occurring near the subsurface trench and in the soil above the soil–bedrock interface where the transfer of O-18 from the soil matrix to the preferential pathways takes place. Despite the dominant role of preferential flow in the generation of hillslope stormflow, the pre-event water formed 52–84% of total subsurface stormflow. The mass balance approach failed in partitioning the hillslope discharge into the pre-event/event water components for two thirds of the selected rainfall–runoff episodes due to similar natural isotopic signatures of pre-event and event water. The analysis showed that spatially and temporally variable exchange of O-18 between the soil matrix and preferential pathways exerted a primary control on the estimates of the temporal origin of water in the hillslope runoff. It was demonstrated that the degree of hydrodynamic mixing in the flow domain played an important role in the interpretation of the isotope-based hydrograph separation.

中文翻译：

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山坡水文过程线分离：大孔土壤中可变同位素特征和水动力混合的影响

摘要 关于山坡暴雨流水位线中水的时间起源的普遍观点是，事前水代表了主要部分。此类结论通常基于使用稳定水同位素（或其他保守示踪剂）并结合质量平衡方法的水文图分离技术。在这项研究中，二维双连续介质模型用于研究位于温带云杉林的山坡垂直截面中水的优先流动和氧气 18 (O-18) 的运输。通过数值模拟实验研究了流体动力混合和同位素特征的时空变化对山坡排放中估计的事件前/事件水分数的影响。使用双组分质量平衡方法结合涉及真实和合成 O-18 特征的 2D 流动和传输模拟，评估了事件前和事件水对山坡暴雨流的贡献。将山坡段中 O-18 迁移的长期模拟与观察到的土壤水和山坡流出物中的 O-18 含量进行比较。长期模拟的结果表明，在地下沟槽附近和土壤-基岩界面上方的土壤中发生了事前和事后水的显着混合，其中 O-18 从土壤基质转移到优先途径发生. 尽管优先流在山坡暴雨流的产生中起主导作用，但事件前的水形成了总地下暴雨流的 52-84%。由于事件前和事件水的自然同位素特征相似，质量平衡方法未能将山坡排放划分为三分之二选定降雨-径流事件的事件前/事件水分量。分析表明，土壤基质和优先途径之间 O-18 的空间和时间可变交换对山坡径流中水的时间起源的估计施加了主要控制。结果表明，流动域中的流体动力混合程度在基于同位素的水文图分离解释中起着重要作用。分析表明，土壤基质和优先途径之间 O-18 的空间和时间可变交换对山坡径流中水的时间起源的估计施加了主要控制。结果表明，流动域中的流体动力混合程度在基于同位素的水文图分离解释中起着重要作用。分析表明，土壤基质和优先途径之间 O-18 的空间和时间可变交换对山坡径流中水的时间起源的估计施加了主要控制。结果表明，流动域中的流体动力混合程度在基于同位素的水文图分离解释中起着重要作用。