Integrated process studies and modelling simulations of hillslope hydrology and interflow dynamics using the HILLS model

Interflow is of major importance for runoff generation and groundwater recharge in mountainous and hilly catchments. However, integrated process studies and model simulation of the hydrological dynamics of hillslope drainage by interflow are still scarce. Therefore, such a study has been carried out on the Kiefer-Hang test hillslope in the catchment of the Sieg River, located in the middle mountain range of the Rheinische Schiefergebirge in Germany. The overall objective was to analyse and to simulate soil moisture, interflow and groundwater recharge with a high resolution in time and space. For the simulation study the two-dimensional HILLS numerical hillslope model was selected, parameterized and calibrated by detailed field surveys. The simulations were undertaken using measured hydro–meteorological data with 5-minute intervals. The process analyses of the study revealed the hydrological dynamics of the test slope, and the results of the simulations proved the ability of the HILLS model to simulate interflow. However, some extreme storms and dry weather periods were simulated poorly because: (i) the soil-physics is simplified in the model as compared to the complex structure of the test slope, and (ii) HILLS in its present version does not account for feedback from the level of the adjacent Breidenbach Creek.