Fast and Invisible: Conquering Subsurface Stormflow through an Interdisciplinary Multi-Site Approach | SSF
Where does water go when it rains? Where are floods generated, and how? What controls stream water quality during storm events?
These fundamental questions are central to fields such as engineering, flood protection, water and ecosystem management, and the prediction of global change impacts.
One of the most elusive processes influencing these phenomena is subsurface stormflow (SSF)—the rapid lateral movement of water through the subsurface triggered by precipitation. Despite its prevalence and significance, SSF is often underestimated due to a lack of systematic, cross-scale studies that could reveal general principles of its functioning. Such understanding is essential to developing reliable practices for experimental assessment and modeling.
In many natural landscapes, SSF plays a key role in runoff generation. It contributes directly to streamflow and indirectly through the creation of saturated zones that lead return flow. As a result, much of the streamflow response during storm events—the hydrograph—may be driven by SSF, whether directly or indirectly. Its contribution is likely larger than typically assumed.
SSF is especially important in headwater regions, which make up approximately 70% of the stream network and have a strong influence on downstream water quantity and quality. Yet, SSF remains poorly quantified due to the inaccessibility of the subsurface, spatial heterogeneity, varying source areas, and the threshold-dependent nature of the process—it occurs only during specific events. These challenges have limited our ability to study SSF systematically.
To address this gap, we propose a systematic investigation of SSF across different environments and spatial scales, using a carefully designed set of replicated and innovative methods. This effort will include novel measurement approaches, a thorough evaluation of potential proxies, and comprehensive model intercomparison and improvement. Only through such coordinated studies can we advance our understanding and management of this critical hydrological process.
We investigate novel approaches to quantifying hillslope subsurface stormflow (SSF) contributions by applying a consistent methodological framework across four representative headwater catchments in Germany’s mountainous regions—the Black Forest, Sauerland, and Ore Mountains—as well as in the Austrian Alps.
Our first objective is to improve understanding of SSF generation on hillslopes, including the threshold conditions, dominant controls, and how the physical and chemical signals of hillslope SSF are altered as water moves through the riparian zone. Secondly, we aim to identify hillslope SSF signatures at larger scales by using distributed measurements in both the riparian zone and the stream. This will allow us to analyze landscape- and reach-scale patterns and controls, ultimately bridging hillslope processes with catchment-scale hydrological responses.