Temperature controls production but hydrology regulates export of dissolved organic carbon at the catchment scale
Temperature controls production but hydrology regulates export of dissolved organic carbon at the catchment scale
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DOI：
10.5194/hess-24-945-2020
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发表时间：
2020-02
影响因子：
6.3
通讯作者：
H. Wen;J. Perdrial;Benjamin W. Abbott;S. Bernal;R. Dupas;S. Godsey;A. Harpold;D. Rizzo;Kristen L. Underwood;T. Adler;Gary Sterle;Li Li-Li
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作者：
H. Wen;J. Perdrial;Benjamin W. Abbott;S. Bernal;R. Dupas;S. Godsey;A. Harpold;D. Rizzo;Kristen L. Underwood;T. Adler;Gary Sterle;Li Li-Li
抽象的。河流网络的横向碳通量是全球碳收支的一个重要组成部分，但知之甚少。这项工作调查了温度和水文如何控制美国宾夕法尼亚州萨斯奎哈纳页岩山临界区天文台溶解有机碳(DOC)的生产和出口。通过对流域日径流量、蒸散量和河流DOC浓度的现场观测，我们建立了流域尺度的生物地球化学反应输送模型BioRT-Flux-PIHM(Biogogo-Chemical Transport-Flux-Penn State集成水文模型，BFP)，该模型达到了纳什-萨特克利夫效率(NSE)大于0.5的满意标准。我们使用校准后的模型来估计和比较日DOC产生率(Rp；单个网格单元中本地DOC产生率的总和)和出口率(Re；河流出口或负荷的浓度和排放的乘积)。结果表明，日Rp的变化不到一个数量级，主要取决于季节温度。相反，日Re的变化超过3个数量级，并与流量和水文连通性的变化密切相关。夏季，高温和蒸散作用使坡面干涸，坡面与溪流断开，Rp达到最大，Re达到最小。在此期间，这条河流只从河流边缘的沼泽中的贫有机地下水和富含有机的土壤水中输出DOC。DOC在山坡中积累，在湿冷期(冬春季)重新达到峰值时，随着溪流与上坡和Rp的重新连接达到最低值，DOC被冲走。该模型再现了观测到的浓度-流量(C-Q)关系，其特征是具有不寻常的冲刷-稀释模式，最大浓度出现在中间排放处，表明源水有三个端元。敏感性分析表明，这种非线性是由不同水流条件下不同源水对河流相对贡献的变化引起的。在低流量时，溪水反映了贫有机地下水的化学成分；在中等流量时，溪水主要由来自沼泽的富含有机质的土壤水主导；在高流量时，溪水反映了上坡土壤水，DOC浓度中等。无论DOC产量如何，只要深层地下水流量的贡献率保持较低(占径流的18%)，这种模式就会持续存在。当地下水流量增加18%以上时，相同数量的地下水和沼泽土壤水混合在溪流中，掩盖了唾液中较高的DOC浓度。在这种情况下，在高排放时，随着DOC浓度的增加，C-Q模式转变为仅冲洗模式。这些结果描述了一个概念性的模型，即在干热条件下，集水区作为DOC的生产者和储存库，在湿条件和冷条件下，该集水区转变为DOC输出者。这项研究还说明了对DOC生产和出口的不同控制--分别是温度和水文流动路径--如何在集水区尺度上造成时间上的不同步。未来变暖和日益加剧的水文极端可能会加剧这种不同步，DOC的产生主要发生在旱季，而DOC的横向输出在主要风暴事件中占主导地位。
Abstract. Lateral carbon flux through river networks is an important and poorly understood component of the global carbon budget. This work investigates how temperature and hydrology control the production and export of dissolved organic carbon (DOC) in the Susquehanna Shale Hills Critical Zone Observatory in Pennsylvania, USA. Using field measurements of daily stream discharge, evapotranspiration, and stream DOC concentration, we calibrated the catchment-scale biogeochemical reactive transport model BioRT-Flux-PIHM (Biogeochemical Reactive Transport–Flux–Penn State Integrated Hydrologic Model, BFP), which met the satisfactory standard of a Nash–Sutcliffe efficiency (NSE) value greater than 0.5. We used the calibrated model to estimate and compare the daily DOC production rates (Rp; the sum of the local DOC production rates in individual grid cells) and export rate (Re; the product of the concentration and discharge at the stream outlet, or load). Results showed that daily Rp varied by less than an order of magnitude, primarily depending on seasonal temperature. In contrast, daily Re varied by more than 3 orders of magnitude and was strongly associated with variation in discharge and hydrological connectivity. In summer, high temperature and evapotranspiration dried and disconnected hillslopes from the stream, driving Rp to its maximum but Re to its minimum. During this period, the stream only exported DOC from the organic-poor groundwater and from organic-rich soil water in the swales bordering the stream. The DOC produced accumulated in hillslopes and was later flushed out during the wet and cold period (winter and spring) when Re peaked as the stream reconnected with uphill and Rp reached its minimum. The model reproduced the observed concentration–discharge (C–Q) relationship characterized by an unusual flushing–dilution pattern with maximum concentrations at intermediate discharge, indicating three end-members of source waters. A sensitivity analysis indicated that this nonlinearity was caused by shifts in the relative contribution of different source waters to the stream under different flow conditions. At low discharge, stream water reflected the chemistry of organic-poor groundwater; at intermediate discharge, stream water was dominated by the organic-rich soil water from swales; at high discharge, the stream reflected uphill soil water with an intermediate DOC concentration. This pattern persisted regardless of the DOC production rate as long as the contribution of deeper groundwater flow remained low (<18 % of the streamflow). When groundwater flow increased above 18 %, comparable amounts of groundwater and swale soil water mixed in the stream and masked the high DOC concentration from swales. In that case, the C–Q patterns switched to a flushing-only pattern with increasing DOC concentration at high discharge. These results depict a conceptual model that the catchment serves as a producer and storage reservoir for DOC under hot and dry conditions and transitions into a DOC exporter under wet and cold conditions. This study also illustrates how different controls on DOC production and export – temperature and hydrological flow paths, respectively – can create temporal asynchrony at the catchment scale. Future warming and increasing hydrological extremes could accentuate this asynchrony, with DOC production occurring primarily during dry periods and lateral export of DOC dominating in major storm events.