Browsing by Author "Green, M. B."

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    Identifying Controls on Nitrate Sources and Flowpaths in a Forested Catchment Using a Hydropedological Framework
    Pardo, Linda H.; Green, M. B.; Bailey, S. W.; McGuire, Kevin J.; McDowell, W. H. (American Geophysical Union, 2022-02-01)
    Catchment-scale assessments of nitrogen retention and loss rarely account for soil and landscape heterogeneity and are, thus, unable to account for the suite of nitrogen cycling processes that ultimately affect the export of nitrate via stream water. Long-term study at the Hubbard Brook Experimental Forest, NH has generated a unique data set that facilitates spatially explicit examination of interactions among hydrology, soil development, and nitrogen cycling processes. Using high-frequency streamwater chemistry data with intensive subsurface hydrology and solute monitoring, we tracked areas of the catchment that are hydrologically active under different flow conditions to identify the source area of streamwater nitrate. We hypothesize that as the drainage network expands, increasing hydrologic connection to bedrock outcrop-associated soils, streamwater nitrate concentration, and flux at the catchment outlet increase. Most nitrate export (>80%) occurred during high flows when high nitrate, bedrock-controlled areas of the catchment were most connected hydrologically to the drainage network (∼15% of the time). End-member mixing analysis demonstrated that the bedrock-controlled upper part of the catchment influences nitrate concentration at the outlet and contributes most to catchment nitrate export compared to the near-stream soil units and seeps. Most of the time, nitrate at the catchment outlet comes from seeps and the near-stream zone; under high flow conditions, bedrock-controlled hotspots for nitrate production contribute more to export of nitrate. This analysis demonstrates how the source area of streamwater nitrate varies under different flow conditions, suggesting that long-term nitrate dynamics may be driven primarily by a relatively small part of the catchment.
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    Linking water age and solute dynamics in streamflow at the Hubbard Brook Experimental Forest, NH, USA
    Benettin, Paolo; Bailey, Scott W.; Campbell, John L.; Green, M. B.; Rinaldo, Andrea; Likens, Gene E.; McGuire, Kevin J.; Botter, Gianluca (American Geophysical Union, 2015-11-01)
    We combine experimental and modeling results from a headwater catchment at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, USA, to explore the link between stream solute dynamics and water age. A theoretical framework based on water age dynamics, which represents a general basis for characterizing solute transport at the catchment scale, is here applied to conservative and weatheringderived solutes. Based on the available information about the hydrology of the site, an integrated transport model was developed and used to compute hydrochemical fluxes. The model was designed to reproduce the deuterium content of streamflow and allowed for the estimate of catchment water storage and dynamic travel time distributions (TTDs). The innovative contribution of this paper is the simulation of dissolved silicon and sodium concentration in streamflow, achieved by implementing first-order chemical kinetics based explicitly on dynamic TTD, thus upscaling local geochemical processes to catchment scale. Our results highlight the key role of water stored within the subsoil glacial material in both the short-term and long-term solute circulation. The travel time analysis provided an estimate of streamflow age distributions and their evolution in time related to catchment wetness conditions. The use of age information to reproduce a 14 year data set of silicon and sodium stream concentration shows that, at catchment scales, the dynamics of such geogenic solutes are mostly controlled by hydrologic drivers, which determine the contact times between the water and mineral interfaces. Justifications and limitations toward a general theory of reactive solute circulation at catchment scales are discussed.