Catchment dissolved organic carbon transport: a modeling approach combining water travel times and reactivity continuum

Quantifying the transfer of organic carbon from the terrestrial to the riverine ecosystems is  of crucial importance to fully appreciate the carbon cycle at the catchment, regional and global scales. Specifically, the entity of dissolved organic carbon (DOC) fluctuations in streamflow is of particular interest also for their impact on the nutrient cycles and on water quality, with implications also for drinking water treatment. In this study, we propose a framework for modelling the flux of DOC from hillslopes to stream and river networks which couples a transport model based on water travel time distributions with the reactivity continuum theory to model DOC degradation. We test the model by applying it to the Plynlimon catchments (UK) exploiting both weekly and high-frequency (7-hour interval) time-series. Besides DOC concentration data, we use information about chloride to get an independent estimate of water travel times using the framework of StorAge Selection functions. The composition and the degradation of DOC along the flowpaths is described assuming a continuous spectrum of quality which initially follows a gamma distribution. Results show that, chiefly for high-frequency measurements, the model is able to reproduce reasonably well both chloride and DOC streamflow concentrations and to capture the complex hysteretic relation between DOC concentration and discharge. The distribution of the age of the water comprised in the streamflow proves thus a key variable to predict the quantity but also the quality of the DOC exported from soils, and the effect of hydrologic variability on this process. Starting from the proposed framework and the results obtained, we discuss how future developments could help in shedding light on the complex relations among carbon, water cycle and the metabolic balance of riverine ecosystems.