Soil-to-stream export of dissolved organic carbon and its functional composition can be predicted using only widely available data

Abstract. Dissolved organic carbon (DOC) exports from soil to aquatic systems are significant components of the global carbon cycle and are of concern to the water industry owing to the role of DOC in the formation of disinfection byproducts. Modelling the processing of DOC as it flows from soil to the ocean requires distinction between compounds that are relatively more aromatic, coloured, ultraviolet-sorbent and photolabile, but resistant to microbial breakdown (T₁), from less photo-reactive but more microbially labile compounds (T₂). We assessed whether mean annual DOC concentration and its T₁ fraction (pT₁) can be predicted from data that are widely available, without in-situ measurements, to enable applications within Earth System Models. Using only spatially resolved data that are available at global scale, we estimated DOC concentration and pT₁ for a range of headwater catchments in the United Kingdom. The best fitted models predicted DOC concentration (Nash-Sutcliffe efficiency, NSE = 0.71) and pT₁ (NSE = 0.57) from land use (arable and wetland), soil type (peat or mineral), precipitation chemistry (ionic strength) and areal runoff. Application at national scale demonstrated expected spatial patterns in the distribution of DOC concentration and pT_1, with high concentrations in peatlands in the north of the country, and highest pT₁ in northern and western areas with high peat cover and surface runoff generation. The approach described here could be applied to other regions, transferring understanding of land-to-water DOC export from data-rich to data-poor areas.