Investigation of runoff generation and Radon-222 activities in a forested catchment using HydroGeoSphere

Radon (²²²Rn) is widely used as a natural tracer to investigate surface/groundwater interactions for hydrological systems. Because ²²²Rn activities in groundwater are higher compared to surface water, it can be used to quantify groundwater inflow rates into rivers and streams. Here we present a process-based model to simulate ²²²Rn emanation and transport in groundwater to investigate surface/groundwater interactions for the  Große Ohe catchment, located in the Bavarian Forest National Park (Germany). For representing surface and groundwater flow in the catchment as well as transport, decay, and emanation of ²²²Rn, the processed based hydrological model HydroGeosphere (HGS) is used. HGS is an integrated surface sub-surface hydrological model (ISSHM) which can simulate reactive transport in surface and sub-surface flow. The model was calibrated using measured in-stream ²²²Rn activities and continuous discharge observations. Main objective of this study is to investigate runoff generation in the catchment and how hydrological processes are affecting the age and residence time composition of groundwater.