Abstract The role of hydrological connectivity in driving the dynamics of faecal indicator organisms (FIOs) in streams is poorly characterised. Here, we demonstrate how a tracer-aided hydrological model can be used within a coupled modelling approach to explore the role of connectivity in governing stream faecal coliform (FC) dynamics. To do so, we tested a hypothesis that in northern upland catchments, the dynamics of hydrological connectivity between major landscape units (hillslopes and riparian zone) and the stream exert a dominant control on stream FC loads by facilitating generation of runoff-driven FC fluxes. This hypothesis was conceptualised within a simple FC model that was coupled to a tracer-aided hydrological model developed for a small (3.2 km2) data-rich catchment in NE Scotland. The model was dual-calibrated to daily discharge and stable isotope data for the period August 2008 to September 2009; stream FC loads were also simulated but not used as a calibration target. Behavioural models successfully captured the general dynamics of the discharge and isotope data (average Kling-Gupta efficiencies of 0.72 and 0.53, respectively), providing confidence in the realism of simulated hydrological processes. The models simulated a seasonally-varying role of connectivity in driving stream FC loads. In summer, connectivity of the catchment hillslope was crucial in providing a source of FC to the riparian zone for transfer to the stream; this countered the decline in fresh FC input to the riparian zone in summer which reflected the seasonal movement of red deer (the principal source of FC) onto higher ground. In winter when this seasonal movement caused FC to be predominantly stored in the riparian zone, simulated hillslope connectivity primarily provided water to the riparian zone that permitted increased runoff generation and associated mobilisation of FC. Comparison of observed and simulated stream FC loads revealed model performance to be variable (R2 range: 0–0.34). The better performance of the model in summer was consistent with hydrological connectivity being a dominant control on stream FC loads at this time. However, failure of the model to capture low FC loads in winter indicated that additional processes not considered in the model may also govern stream FC dynamics during this period. Incorporating the impact of freeze-thaw cycles on FC mortality, or a dilution effect of hillslope connectivity in winter, could be potential next steps in refining the hypothesis conceptualised in the FC model presented here. The novel coupled modelling approach used in this study successfully allowed a hypothesised role of connectivity in driving stream FC dynamics to be tested, contextualised by the accuracy of discharge and isotope-tracer simulations as indicators of hydrological process realism. Therefore, coupling FIO and tracer-aided hydrological models has clear promise for furthering understanding of FIO dynamics, which is a vital precursor to the successful management of microbial water quality. Based on the experiences in this study, a “roadmap” for the future development and application of coupled approaches is also presented.

中文翻译：

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水文连通性在多大程度上控制河流中粪便指示生物的动态？使用示踪辅助模型进行初始假设检验

摘要 水文连通性在驱动溪流中粪便指示生物 (FIO) 动态方面的作用尚不明确。在这里，我们展示了如何在耦合建模方法中使用示踪辅助水文模型来探索连通性在控制河流粪便大肠菌 (FC) 动力学中的作用。为此，我们测试了一个假设，即在北部高地集水区，主要景观单元（山坡和河岸带）与河流之间的水文连通性动态通过促进径流驱动的 FC 通量的产生，对河流 FC 负载施加了主导控制。该假设是在一个简单的 FC 模型中概念化的，该模型与为苏格兰东北部一个小型（3.2 平方公里）数据丰富的集水区开发的示踪辅助水文模型相结合。该模型对 2008 年 8 月至 2009 年 9 月期间的每日排放和稳定同位素数据进行了双重校准；还模拟了流 FC 负载，但未用作校准目标。行为模型成功捕获了排放和同位素数据的一般动态（平均 Kling-Gupta 效率分别为 0.72 和 0.53），为模拟水文过程的真实性提供了信心。这些模型模拟了连通性在驱动流 FC 负载中的季节性变化作用。在夏季，集水山坡的连通性对于向河岸带转移到溪流提供 FC 来源至关重要；这抵消了夏季流入河岸地区的新鲜 FC 输入的下降，这反映了马鹿（FC 的主要来源）季节性移动到更高的地方。在冬季，当这种季节性运动导致 FC 主要储存在河岸带时，模拟的山坡连通性主要向河岸带提供水，允许增加径流产生和相关的 FC 动员。观察到的和模拟的流 FC 负载的比较显示模型性能是可变的（R2 范围：0-0.34）。该模型在夏季的更好性能与此时水文连通性是对河流 FC 负载的主要控制一致。然而，该模型未能在冬季捕获低 FC 负载表明模型中未考虑的其他过程也可能在此期间控制河流 FC 动态。结合冻融循环对 FC 死亡率的影响，或冬季山坡连通性的稀释效应，可能是改进此处介绍的 FC 模型中概念化的假设的潜在下一步。本研究中使用的新型耦合建模方法成功地允许测试连接性在驱动流 FC 动力学中的假设作用，并通过排放和同位素示踪剂模拟的准确性作为水文过程真实性的指标进行背景化。因此，将 FIO 和示踪剂辅助水文模型相结合，对于进一步了解 FIO 动力学具有明确的希望，这是成功管理微生物水质的重要先导。根据本研究的经验，还提出了耦合方法未来发展和应用的“路线图”。本研究中使用的新型耦合建模方法成功地允许测试连接性在驱动流 FC 动力学中的假设作用，并通过排放和同位素示踪剂模拟的准确性作为水文过程真实性的指标进行背景化。因此，将 FIO 和示踪剂辅助水文模型相结合，对于进一步了解 FIO 动力学具有明确的希望，这是成功管理微生物水质的重要先导。根据本研究的经验，还提出了耦合方法未来发展和应用的“路线图”。本研究中使用的新型耦合建模方法成功地允许测试连接性在驱动流 FC 动力学中的假设作用，并通过排放和同位素示踪剂模拟的准确性作为水文过程真实性的指标进行背景化。因此，将 FIO 和示踪剂辅助水文模型相结合，对于进一步了解 FIO 动力学具有明确的希望，这是成功管理微生物水质的重要先导。根据本研究的经验，还提出了耦合方法未来发展和应用的“路线图”。耦合 FIO 和示踪剂辅助水文模型对于进一步了解 FIO 动力学具有明确的希望，这是成功管理微生物水质的重要先导。根据本研究的经验，还提出了耦合方法未来发展和应用的“路线图”。耦合 FIO 和示踪剂辅助水文模型对于进一步了解 FIO 动力学具有明确的希望，这是成功管理微生物水质的重要先导。根据本研究的经验，还提出了耦合方法未来发展和应用的“路线图”。