Limited Headwater CO ₂ Emissions Relative to Downstream C Fluxes: Insights From a Tracer‐Enabled Reactive Transport Model

Abstract Some fraction of the total carbon (C) transported by rivers can enter the atmosphere as CO 2 via gas evasion as water transits from source to sink. Quantifying this evaded portion can be challenging due to the need to constrain chemical and physical parameters along an entire stream network using a limited number of point measurements. To address this challenge, we employed an tracer‐enabled (C and 222 Rn) reactive transport model to simulate CO 2 evasion along an entire stream network in the Little Deschutes River in the Eastern Cascades, Oregon, USA. We sampled the river network in distinct lanscape regimes and measured potential C sources including soil gas, groundwater springs, and wetland waters. Using these data, we first evaluated the reactive transport model using empirical gas transfer scaling relationships and measured groundwater chemistry. We then employed a Monte‐Carlo optimization using riverine observations of , C and 222 Rn, which yielded more accurate estimates of CO 2 evasion by improving estimates of spatially‐averaged groundwater pCO 2 and generating a site‐specific gas transfer scaling relationship. Our results demonstrate that riparian wetlands contribute to 19% of the computed CO 2 evasion flux. Lastly, we find that CO 2 evasion only accounts for 12% of the total riverine C flux, with the remaining fraction contributed by advective flux of DIC (50%) and DOC (38%) through the watershed outlet.