DOC mobilisation from forest soils governed by intermittent hydrological connectivity of subsurface water pools

Climate change in combination with forest management practices is leading to increased DOC release from a forested reservoir catchment in the western Ore Mountains in Germany. The most significant sources of DOC loadings are likely several disturbed patches of former peatland in the catchment. We suspect that the soil water availability and subsequent variable hydrological connectivity of water pools in the shallow subsurface could be major factors for DOC mobilisation impeding the drinking water production in the region.

We present data from almost one year of intensive monitoring in two small catchments (1 ha) located on i) a shallow histosol with a highly compacted mineral subsoil and ii) a regolithic cambisol. Catchment drainage was constantly observed for water levels and in situ spectroscopy to infer discharge rates and DOC concentrations. Soil moisture and temperature, surface temperature and irradiation were continuously measured along the slope gradient. During monthly campaigns in the vegetation period, pore water samples were taken from high and low points of 15 m grid cells spanning the catchments.

In our poster, we would like to discuss our findings that pore water availability is non-uniformly distributed suggesting discrete subsurface flow paths in the catchments. In low moisture conditions, subsurface water pools can be isolated from the pore water network. We argue that disconnected pools accumulate DOC during low moisture conditions, which is then released when the pools are reconnected during strong precipitation events.