Event-based high-resolution water quality measurements in rural headwater catchments: DOC quality and DOC export

In this study, the DOC concentration and DOC quality as well as the discharge of a small spring catchment area in the North Hessian low mountain range were investigated over a period of 1 ½ years. The Nesselbach near Kassel is a small siliceous low mountain stream with an average flow of around 8.5 l/s. The catchment area is primarily characterized by agriculture (75% area share) and is home to a small settlement and forest areas (15% area share). The DOC concentration and the quality indices SUVA₂₅₄ and SL₂₇₀ and SL₂₉₀ were determined using a UV-Vis in-situ probe with a resolution of 5 minutes. These indices can be used to evaluate the strength of the aromatic compounds of the DOC molecules (SUVA₂₅₄) or the molecular weight (SL₂₇₅ and SL₂₉₀). Good DOC quality is generally associated with strong aromatic bonds and a high molecular weight. Spearman correlations were calculated to investigate factors such as water temperature and event characteristics on DOC quality and DOC export. In addition, hysteresis analyses were carried out to interpret the discharge-DOC load relationship with a time lag. The results of the study show an unexpectedly low DOC export from the headwater catchment compared to the relevant literature, as well as a strong correlation between DOC export and DOC quality. The observed fluctuations in DOC quality are mainly determined by the season and the changing land use.