Modeling Riparian Hollow Controls on Nitrogen Cycling in Snowmelt
Dominated Catchments
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.