How do vertical and topographic riparian soil moisture patterns shape headwater dissolved organic carbon dynamics?

Dissolved organic carbon (DOC) plays a fundamental role for the aquatic ecosystem and the global carbon cycle. It also interferes with drinking water treatment processes. Its removal is costly and depends on its quantity and quality, i.e. its concentration and molecular composition. Riverine DOC concentrations have increased in Europe and North America in recent decades, primarily driven by reductions in acid deposition. Currently, changing climatic conditions such as increasing temperatures, heavy rainfall events and droughts are gaining importance in determining DOC concentrations. However, the specific mechanisms by which climate variability drives riverine DOC concentrations and its chemical composition at different time scales are not sufficiently understood. Therefore, reliable forecast about future developments are challenging. In forested headwater catchments, where riparian soils are major sources of DOC export, riparian soil moisture might be paramount to determine DOC quantity and quality. Soil moisture is driven by climate variability and controls subordinate and interdependent processes that can shape DOC quantity and quality. However, limited data of soil moisture from forested headwaters and specifically from their riparian zones are available. In this context, we will study the upper Rappbode catchment in the Harz mountains, which drains into Germany’s largest drinking water reservoir. We will relate high-frequency soil moisture observations at multiple depths (vertical dimension) at different riparian profiles with differing wetness characteristics (topographic dimension) to the corresponding DOC quantity and quality over temporal scales, including short-term, seasonal/annual, and long-term by modeling. We hypothesize that currently and in future patterns of soil moisture in the vertical and topographic dimension play a pivotal role as drivers of the temporal dynamics of DOC quantity and quality in riparian soils and subsequently in the corresponding surface waters. Initial results from our sampling campaigns highlight differences in riparian soil water chemistry between the locations of different wetness characteristics, but also distinct vertical heterogeneities. We will present further findings and results that improve the understanding of how soil moisture drives riverine DOC quantity and quality, with special consideration of vertical heterogeneities in the riparian profiles.  With a refined understanding of DOC dynamics, more reliable forecasts can be made to derive targeted adaptation strategies for safe drinking water supplies and to better assess future impacts on aquatic ecosystems and the global carbon cycle.