Using travel time distributions and reactivity continuum to model terrestrial dissolved organic carbon export and reactivity in an Alpine catchment

Quantifying the transfer of organic carbon (OC) from the terrestrial to the riverine ecosystems is of crucial importance to fully appreciate the carbon cycle at the catchment, regional and global scales. Dissolved Organic Carbon (DOC) represents one of the main forms in which terrestrial OC is leached to inland waters and the oceans. Its concentration in streams, rivers and lakes is critical for aquatic metabolism but also for the transport of metals and pollutants. In the past years, several studies in different experimental catchments observed increasing trends in DOC concentration in rivers, possibly linked to changes in land use and hydrologic regime.  Moreover, these studies unveiled how hydrologic variability imposes a strong control on DOC transport with streamflow producing events disproportionately contributing to the overall DOC export.  In this study, we explore the interaction between water and carbon cycles in the critical zone of an alpine catchment in order to quantify the flux of DOC exported from the soil to the stream via superficial or subsuperficial runoff. We couple the Water Age  theory to unravel the time water spends within hillslopes with the Reactivity Continuum model to quantify the degradation of DOC along the transport. The model is applied to the Oberer Seebach basin (Austria) for which extensive time series of streamflow DOC concentration and hydrological variables are available at high-frequency resolution (sub-daily measurements). For a subset of the DOC samples, the excitation emission matrices and the absorbance spectra are also available and allow deriving information on the quality and reactivity of DOC. We reproduce DOC concentration estimating the travel time distribution of water and assuming it dictates the time available for the continuous degradation of the DOC mixture. Results show that the model is able to well reproduce DOC streamflow concentration, capturing its complex relation with streamflow discharge over the three years of observations. In addition, the framework allows the estimation of the reactivity distribution of the exported DOC. To validate these results, we compare the estimated average reactivity with multiple fluorescence and absorbance indexes calculated from data, revealing significant correlations.