StorAge Selection (SAS) functions describe how catchments selectively remove water of different ages in storage via discharge, thus controlling the transit time distribution (TTD) and solute composition of discharge. SAS‐based models have been emerging as promising tools for quantifying catchment‐scale solute export, providing a coherent framework for describing both velocity‐driven and celerity‐driven transport. Due to their application in headwaters only, the spatial heterogeneity of catchment physiographic characteristics, land use management practices, and large‐scale validation have not been adequately addressed with SAS‐based models. Here, we integrated SAS functions into the grid‐based mHM‐Nitrate model (mesoscale Hydrological Model) at both grid scale (distributed model) and catchment scale (lumped model). The proposed model provides a spatially distributed representation of nitrogen dynamics within the soil zone and a unified approach for representing both velocity‐driven and celerity‐driven subsurface transport below the soil zone. The model was tested in a heterogeneous mesoscale catchment. Simulated results show a strong spatial heterogeneity in nitrogen dynamics within the soil zone, highlighting the necessity of a spatially explicit approach for describing near‐surface nitrogen processing. The lumped model could well capture instream nitrate concentration dynamics and the concentration–discharge relationship at the catchment outlet. In addition, the model could provide insights into the relations between subsurface storage, mixing scheme, solute export, and the TTDs of discharge. The distributed model shows results that are comparable to the lumped model. Overall, the results reveal the potential for large‐scale applications of SAS‐based transport models, contributing to the understanding of water quality‐related issues in agricultural landscapes.

中文翻译：

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使用StorAge选择功能对中尺度集水区的硝酸盐出口建模

StorAge Selection（SAS）功能描述了集水区如何通过排放有选择地去除存储中不同年龄的水，从而控制排放时间分布（TTD）和排放的溶质组成。基于SAS的模型已经成为量化流域规模的溶质出口的有前途的工具，它提供了描述速度驱动和速度驱动的运输的连贯框架。由于仅在上游源头应用，基于SAS的模型尚未充分解决集水区生理特征，土地利用管理实践和大规模验证的空间异质性。在这里，我们将SAS功能集成到了网格规模（分布式模型）和流域规模（集总模型）的基于网格的mHM-硝酸盐模型（中尺度水文模型）中。所提出的模型提供了土壤区域内氮动力学的空间分布表示，以及用于表示土壤区域以下速度驱动和速度驱动的地下运输的统一方法。该模型在非均质中尺度流域中进行了测试。模拟结果表明，土壤带内氮动力学的空间异质性很强，突显了采用空间显式方法描述近地表氮加工的必要性。集总模型可以很好地捕获流域硝酸盐浓度的动态变化以及集水口出口处的浓度-流量关系。此外，该模型还可以提供有关地下存储，混合方案，溶质输出和排放TTD之间关系的见解。分布式模型显示的结果与集总模型相当。总体而言，结果揭示了大规模应用基于SAS的运输模型的潜力，有助于理解农业景观中与水质相关的问题。