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Modeling Riparian Hollow Controls on Nitrogen Cycling in Snowmelt Dominated Catchments
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  • D. Brian Rogers,
  • Michelle Newcomer,
  • Jonathan Raberg,
  • Carl Steefel,
  • Nicholas Bouskill,
  • Peter Nico,
  • Patricia Fox,
  • Mark Conrad,
  • Markus Bill,
  • Eoin Brodie,
  • Nicola Falco,
  • Susan Hubbard
D. Brian Rogers
Lawrence Berkeley National Laboratory

Corresponding Author:dbrtw2@mst.edu

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Michelle Newcomer
Lawrence Berkeley National Laboratory
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Jonathan Raberg
University of Colorado at Boulder
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Carl Steefel
Lawrence Berkeley National Laboratory
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Nicholas Bouskill
Lawrence Berkeley National Laboratory
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Peter Nico
Lawrence Berkeley National Laboratory
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Patricia Fox
Lawrence Berkeley National Laboratory
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Mark Conrad
Lawrence Berkeley National Laboratory
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Markus Bill
Lawrence Berkeley National Laboratory
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Eoin Brodie
Lawrence Berkeley National Laboratory
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Nicola Falco
Lawrence Berkeley National Laboratory
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Susan Hubbard
Lawrence Berkeley National Laboratory
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Abstract

A multi-scale understanding of processes controlling the nitrogen budget is essential for predicting how nitrogen loads will be affected by climate-induced disturbances. Recent studies in snowmelt-dominated catchments have documented changes in nitrogen retention over time, such as declines in watershed exports of nitrogen, though there is a limited understanding of the controlling processes driving these trends. Working in the mountainous headwater East River Colorado watershed, our study aims to refine this process-based understanding by exploring the effects of riparian hollows as nitrogen cycling hotspots. The objectives of this study are to (1) quantify the influence of riparian hollows on nitrogen retention in snowmelt-dominated catchments, (2) understand how disturbances (i.e. early snowmelt, long summer droughts) and heterogeneities affect the nitrogen-retention capacity of riparian hollows, and (3) quantify the relative contribution of riparian hollows to the watershed nitrogen budget using high-resolution LIDAR watershed data. We used a multi-component flow and reactive transport model, MIN3P, to simulate the biogeochemical kinetics of riparian hollows, using data from the East River watershed to parameterize, constrain, and validate the model. Several hydrological, biogeochemical, and geological perturbations were then imposed across simulations to assess the effects of abrupt and gradual perturbations on riparian hollow hydrobiogeochemical dynamics. Topographic position and wetness indices were used to scale the net yearly storage and flux terms from riparian hollows, and reveal the significant impacts hollows can have on aggregated watershed biogeochemistry. Initial model results suggest that riparian hollows serve as significant nitrogen sinks, and that earlier snowmelt and extended dry season considerably limit denitrifying processes. Our work linking remote sensing and empirical scaling techniques to numerical biogeochemical simulations is an important first-step in assessing nitrogen-retaining features relative to the watershed nitrogen budget.