Headwater gas exchange quantified from O₂ mass balances at the reach scale

Headwater streams are important in the carbon cycle and there is a need to better parametrize and quantify exchange of carbon-relevant gases. Thus, we characterized variability in the gas exchange coefficient (k ₂) and dissolved oxygen (O₂) gas transfer velocity (k) in two lowland headwaters of the River Avon (UK). The traditional one-station open-water method was complemented by in situ quantification of riverine sources and sinks of O₂ (i.e., groundwater inflow, photosynthesis, and respiration in both the water column and benthic compartment) enabling direct hourly estimates of k ₂ at the reach-scale (~ 150 m) without relying on the nighttime regression method. Obtained k ₂ values ranged from 0.001 h^-1 to 0.600 h^-1. Average daytime k ₂ were a factor two higher than values at night, likely due to diel changes in water temperature and wind. Temperature contributed up to 46% of the variability in k on an hourly scale, but clustering temperature incrementally strengthened the statistical relationship. Our analysis suggested that k variability is aligned with dominant temperature trends rather than with short-term changes. Similarly, wind correlation with k increased when clustering wind speeds in increments correspondent with dominant variations (1 m s^-1). Time scale is thus an important consideration when resolving physical drivers of gas exchange. Mean estimates of k ₆₀₀ from recent parametrizations proposed for upscaling, when applied to the settings of this study, were found to be in agreement with our independent O₂ budget assessment (within < 10%), adding further support to the validity of upscaling efforts aiming at quantifying large-scale riverine gas emissions.