The National Key Laboratory of Water Disaster Prevention, Hohai University, Nanjing, 210098, China; Yangtze Institute for Conservation and Development, Hohai University, Nanjing, China; Key Laboratory of Hydrologic-Cycle and Hydrodynamic-System of Ministry of Water Resources, Hohai University, Nanjing, China.
Department of Ecohydrology and Biogeochemistry, Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), Berlin, Germany; Department of Geography, Humboldt University of Berlin, Berlin, Germany; Northern Rivers Institute, School of Geosciences, University of Aberdeen, Aberdeen, UK.
Water Res. 2024 Sep 15;262:122118. doi: 10.1016/j.watres.2024.122118. Epub 2024 Jul 19.
Catchment-scale nitrate dynamics involve complex coupling of hydrological transport and biogeochemical transformations, imposing challenges for source control of diffuse pollution. The Damköhler number (Da) offers a dimensionless dual-lens concept that integrates the timescales of exposure and processing, but quantifying both timescales in heterogeneous catchments remains methodologically challenging. Here, we propose a novel spatio-temporal framework for catchment-scale quantification of Da based on the ecohydrological modeling platform EcHO-iso that coupled isotope-aided water age tracking and nitrate modeling. We examined Da variability of soil denitrification in the heterogeneous Selke catchment (456 km, central Germany). Results showed that warm-season soil denitrification was of catchment-wide significance (Da >1), while its high spatial variations were co-determined by varying exposure times and removal efficiencies (e.g., channel-connected lowland areas are hotspots). Moreover, Da seasonally shifted from processing-dominance to transport-dominance during the wet-spring season (from >1 to <1), implying important linkages between summer terrestrial denitrification and subsequent winter river water quality. Under the prolonged 2018-2019 droughts, denitrification removal generally reduced, resulting in further accumulation in agricultural soils. Moreover, the space-time responses of Da variability indicated important implications for catchment water quality. The older water in lowland areas exhibited extra risks of groundwater contamination, whilst agricultural areas in the hydrologically responsive uplands became sensitive hotspots for export and river water pollution. Importantly, the lowland pixels intersecting river channels exhibited high removal efficiencies, as well as high resilience to the disturbances (wet-spring Da shifted to >1 under drought conditions). The proposed catchment-wide Da framework is implied by mechanistic modeling, which is transferable across various environmental conditions. This could shed light on understanding of catchment N processes, and thus providing site-specific implications of non-point source pollution controls.
集水区尺度上的硝酸盐动态涉及水文传输和生物地球化学转化的复杂耦合，这对漫射污染的源头控制提出了挑战。达克尔数（Da）提供了一种无量纲的双镜头概念，它将暴露和处理的时间尺度结合在一起，但在非均质地表集水区中定量这两个时间尺度在方法上仍然具有挑战性。在这里，我们提出了一种基于生态水文学模型平台 EcHO-iso 的集水区尺度 Da 定量的新的时空框架，该框架将同位素辅助水龄追踪和硝酸盐建模相结合。我们研究了不均匀的塞尔克集水区（456 平方公里，德国中部）中土壤反硝化作用的 Da 变化。结果表明，温暖季节的土壤反硝化作用具有全流域意义（Da>1），而其高空间变异性是由不同的暴露时间和去除效率决定的（例如，与河道相连的低地地区是热点）。此外，在春季湿润期，Da 从处理为主转变为传输为主（从>1 到<1），这意味着夏季陆地反硝化作用与随后的冬季河流水质之间存在重要联系。在 2018-2019 年持续的干旱期间，反硝化去除作用普遍减弱，导致农业土壤中进一步积累。此外，Da 变化的时空响应表明对集水区水质有重要影响。低地地区的老水有地下水污染的额外风险，而对水文响应敏感的高地农业区则成为污染物输出和河流水污染的敏感热点。重要的是，与河道相交的低地像素表现出高去除效率，并且对干扰具有高弹性（在干旱条件下，春季 Da 转移到>1）。该机制模型暗示了该集水区 Da 框架的适用性，该框架可在各种环境条件下转移。这可以帮助我们更好地理解集水区的氮过程，从而为非点源污染控制提供特定地点的启示。
