2.2 Modeling hyporheic respiration
We used previously published modeled estimates for aerobic respiration
in the hyporheic zone at the scale of NHDPLUS reaches within the two
study basins using the River Corridor Model (RCM) presented in (Son et
al. 2022b). The model computes at hourly timesteps, using reactions,
rate formulations and kinetic rate parameters described in (Fang et al.
2020). Key model inputs for each reach include hyporheic exchange fluxes
(HEF) and hyporheic residence times estimated by the Networks with
Exchange and Subsurface Storage (NEXSS) model (Gomez-Velez and Harvey
2014), and long-term (~10 year) average stream solute
concentrations (dissolved organic carbon, dissolved oxygen, and nitrate)
estimated from available observations. We gap-filled missing HEF and
residence time values for ~13% of reaches, using a
eXtreme Gradient Boosting (XGBoost, v1.5.0.2) machine learning model
(Chen and He 2023) ingesting topography variables (watershed/catchment
area, maximum/mean/minimum elevation), hydrology (mean annual
streamflow/runoff/velocity and mean annual precipitation), and stream
geometry (slope, width and length, sinuosity, bankfull depth, median
particle size, roughness, total stream length) with high accuracy
(R2>0.94). Detailed descriptions of the stream solute input
data and our XGBoost approach are included in Supplemental Information.