Water budget and subsurface runoff determination for small Alpine catchments using WaSIM

Assessing the water balance including subsurface runoff in high Alpine catchments is still a major challenge due to environmental and meteorological complexity, and mostly data-lacking hydrology. The aim of this study is the determination of the water balance components and water budget with focus on approximation of interflow, subsurface runoff and groundwater interactions, depending on sediment and bedrock properties.

In this process we investigate a small, high data providing Alpine catchment in the Wipp Valley (Tyrol, AT) to evaluate the best modelling approach in order to apply it on catchments along the Austrian Brenner axis. Thus, a direct model comparison of the main study catchment, with its (moderate data providing) neighbouring valley is carried out. The main study catchment (Padaster Valley) covers 11.2 km² and is located east of Steinach am Brenner in the Wipp Valley. Due to its partially usage as a deposital site, respectively a landfill for the tunnel excavation material of the Brenner Base Tunnel, this valley represents a highly interesting site in a hydrological aspect. Thus, the Padaster Valley is highly monitored and hence predestined for hydrological investigations. Hydrological data such as discharge is measured high frequently on four gauges, meteorological data on two gauges. An additional study catchment (Navis Valley) covers 63 km² and is located northerly next the Padaster Valley. Seven gauges provide meteorological data, however, continuous discharge data is just measured at the valley mouth. Further meteorological data for both areas will be contributed by the ZAMG (Zentralanstalt für Meteorologie und Geodynamik), whose INCA model provide a high spatial resolution dataset of 1km. However, in order to gain a better overall understanding of subsurface runoff and hydrogeological processes, geological data will be considered and incorporated/integrated in the modelling process. This includes geological maps, - cross sections and geophysical analysis, which help to estimate the bedrock topography, and consequently the volume as well as deeper seated hydrogeological properties of the sediment cover. In this context, continuous data from 7 groundwater observation wells provide information regarding groundwater levels and hydraulic head. To increase the model accuracy regarding subsurface flow processes, subsurface-depending runoff types after Pirkl & Sausgruber (2015) are applied. Furthermore, several maps such as land use, surface runoff coefficient and soil map including grain size distribution of the layers have been compiled by in-situ fieldwork for this study. In order to model the water budget, subsurface runoff and overall hydrological slope properties, the distributed hydrological Model WaSIM (Richards version; Schulla, 1997) is applied. The model is based on a modular system which uses physically-based algorithms.

The present study is been carried out by the Austrian Research Centre for Forests (BFW) in collaboration with the Brenner Base Tunnel (BBT-SE).