SSF FORCING - Temporal dynamics in subsurface storm flow

Stefan Achleitner, Universität Innsbruck, Arbeitsbereich für Wasserbau Bernhard Kohl, Bundesforschungszentrum für Wald - BFW, Institut für Naturgefahren How do floods develop and how does the runoff enter the channel? In many natural landscapes, a rapid subsurface runoff component (subsurface stormflow - SSF) plays an important role in addition to surface runoff. This contribution to the total runoff is difficult to quantify and is probably larger than assumed so far. Measurement on this is very difficult for various reasons: the inaccessibility of the subsurface, the large spatial variability and heterogeneity, the variable sources, and the fact that it is a threshold-driven process that only occurs during certain events. The FWF-funded project SSF-FORCING addresses this problem as part of the DFG/FWF research group "Fast and Invisible: Conquering Subsurface Stormflow through an Interdisciplinary Multi-Site Approach". This research group is funded for 4 years by the German Research Foundation DFG (FOR 5288/1) and the Austrian Science Fund FWF (I 5960-N) and consists of 7 individual projects with 12 participating scientists from Germany, Austria and Switzerland. The research group is dedicated to its systematic cross-scale investigation in different landscapes where interflow SSF is a dominant runoff process. In order to explore the functional principles of SSF, a strong emphasis is put on experimental work in four different test areas, complemented by modeling at different scales. The Austrian subproject A - SSF-FORCING, located at the Department of Hydraulic Engineering of the University of Innsbruck and at the Federal Forest Research Center (BFW) Innsbruck, focuses on the experimental and numerical investigation and assessment of driving factors of surface and interflow. The experimental area (5x10m and 10x40m) is seen as the lowest common denominator to assess the SSF on a larger scale (slope and catchment level). Different initial and boundary conditions as well as soil and land use characteristics that influence the formation and temporal dynamics of SSF are investigated. The goal is to derive threshold values and thus possible parameterizations to describe the process dynamics. This goal is pursued by new, innovative irrigation experiments. The data obtained will also be supplemented with existing irrigation simulation data and analyzed in combination with these. The planned investigations thus comprise quantitative measurements in combination with qualitative evaluations of flows and conditions.