Hydrological processes in the subsurface investigated by water isotopes and silica

The hillslope rainfall-outflow interactions, groundwater fluxes, and hydrological balance were examined in the small mountainous headwater catchment Uhlířská, the Jizera Mountains, the Czech Republic. The hillslope soil profile is formed by paleozoic crystalline bedrock overlaid by shallow highly permeable shallow Cambisol, and by thick saturated glacial deposits in the valley, overlaid by Histosol. A quick communication of the vadose zone with the granitic bedrock via preferential subsurface flowpaths is hypothesised, in agreement with the observation of storm-caused instant water transformation to outflow through the permeable Cambisol. A quick response of a high magnitude outflow occurs regularly, although the surface runoff is very rare. Standard climatic and hydrological monitoring in the Uhlířská catchment is supplemented by the measurements of the soil moisture, soil pore water suction, subsurface hillslope stormflow in the vadose zone, and water table fluctuation in the saturated subsurface, and is accompanied by water sampling for the analyses of the contents of the isotope 18O and 2H and geochemical tracer silica in the form of SiO2. The episode based isotopic data serve for the separation of the outflow hydrograph to determine the contributions of the event and pre-event water in the hypodermic hillslope outflow and in the catchment outflow. The variation of silica content in the water cycle components was examined to assess the contributions from the soil profile and the aquifer. Up to 75% of the event catchment runoff was assigned to pre-event water, of which about 50% had been stored in the shallow soil subsurface on the hillslopes. The hypothesis was confirmed that the hillslope soil layers control the distribution of the flow into the groundwater recharge and/or the shallow subsurface flow during the rainfallrunoff episode.