How Much Does Stream-Groundwater Exchange Influence Whole-Stream
Metabolism in a Small Mountain Stream?
Abstract
To estimate whole-stream metabolism, the open-channel oxygen method has
traditionally provided underlying assumptions for modeled estimates of
gross primary productivity (GPP) and ecosystem respiration (ER). The
open-channel oxygen method employs the diel dissolved oxygen (DO) curve,
which attributes stream metabolism to four processes: photosynthesis by
primary producers, oxidative respiration, reaeration, and groundwater
flux. Of these processes, groundwater flux is often assumed to be
negligible when modeling whole-stream metabolism, which may introduce
bias in estimates of GPP and ER. For example, if net groundwater flux is
into the main channel, we may expect an overestimation of modeled ER due
to dissolved oxygen dilution effects from influent groundwater. Although
this error is recognized, there is a lack of continuous and spatial data
that quantifies the extent of bias that is introduced by not including
groundwater flux in model parameters. To investigate this bias, we
measured whole-stream metabolism and groundwater flux in Como Creek, a
headwater catchment 26 km west of Boulder, CO. DO sensors were deployed
in the stream and groundwater wells in June 2018 at 3 sites along 500 m
of the reach. BASE (Bayesian Single-station Estimation), a package
available through R, was used for modeling whole-stream metabolism
between peak streamflow and baseflow. BASE also optimizes the reaeration
coefficient, which was estimated both including and neglecting
groundwater discharge and DO concentration. Preliminary results indicate
that Como Creek has a net groundwater flux out of the stream, resulting
in higher rates of GPP in the groundwater-corrected model output, and
indicating the potential for bias in uncorrected models.