SSF TRANSFORM
Project staff:
Dr. Luisa Hopp
Dr. Theresa Blume
Lars Schindler
Description:
The SSF Transform project investigates the transformation of subsurface stormflow (SSF) signals as water moves from hillslopes through the riparian zone to streams. SSF, a key hydrological process in headwater catchments, generates distinct physical and chemical signatures on hillslopes, but these signals are often altered during transport through the riparian zone—acting as a dynamic, three-dimensional reactor for mixing, storage, and biogeochemical reactions. Understanding this transformation is essential for accurately identifying SSF contributions to streamflow, especially given the high spatial and temporal variability of the process. Transform builds on Phase 1 of the DFG Research Unit 5288, where trench-based SSF collection and riparian groundwater monitoring revealed site-specific chemical signatures and marked small-scale variability in riparian zone chemistry. However, snapshot sampling limited insights into dynamic processes. In Phase 2, Transform advances this work by conducting event-based, high-frequency sampling of SSF, riparian groundwater, and stream water across three German catchments. This allows for the first time a detailed analysis of how SSF signatures evolve during rainfall-runoff events. The project integrates multi-level samplers, temperature and electrical conductivity (EC) profile probes, and geophysical methods (ERT, GPR, SP) to monitor subsurface flow pathways and residence times. A key innovation is the application of the Damkohler number concept to quantify the transformation potential of riparian zones based on flow dynamics and reaction times. The project also develops simple, pragmatic proxies—such as soil batch experiments, EC/temperature profiles, and outflow from roadside ditches—to estimate SSF contributions without trench installations. By combining field observations, advanced monitoring, and conceptual modeling, Transform aims to establish a general framework for understanding SSF transformation across scales and landscapes, enabling more accurate assessments of water quality and hydrological connectivity in complex catchments.
Research Questions
1. How do long-term SSF volumes, dynamics and chemistry develop in the instrumented catchments?
2. Can we identify a long-term chemical signature of SSF and riparian zone groundwater and/or characterize seasonal patterns?
3. Which flow pathways does SSF take through the riparian zone towards the stream? How long does SSF reside in the riparian zone? When and where does it mix with stream water (parafluvial flow)? Which characteristics of the riparian zone, such as spatial dimension and subsurface structures, influence the residence time of SSF?
4. Can we develop a monitoring system based on profile probes of temperature and electrical conductivity sensors and geophysical methods to monitor SSF directly in the subsurface, thus removing the barrier of the observation well pipe and its standing water table?
5. How can we conceptualize the hillslope-stream connection and the extent and the processes of transformation of SSF signatures in the riparian zone (physical mixing vs. biogeochemical processes)?
6. Can we develop simple experiments to characterize SSF signatures and the transformation potential of the riparian zone, with the final goal to estimate the contribution of SSF to streamflow for a particular site?
Methods/ Approach
Transform employs a multi-method, process-based approach to study SSF transformation in the riparian zone. Core methods include event-based, high-frequency sampling of SSF (trenches), riparian groundwater (multi-level and standard wells), and stream water using newly developed PAUL automated samplers, replacing older ISCO systems. These campaigns, conducted across 8 events in different seasons, enable detailed tracking of chemical signature changes. To monitor subsurface dynamics, temperature and EC profile probes are installed at 5 depths (1.5–2 m) in riparian zones to detect preferential flow and flow direction. These data are combined with geophysical methods: self-potential (SP) and point velocity probes (PVPs) to map flow paths, and ERT/GPR (with full-waveform inversion) to image subsurface structures and tracer movement. Multi-level samplers (MLS) are deployed to resolve vertical hydrochemical gradients in riparian groundwater. The project tests simple proxies—soil batch experiments, long-term water table dynamics, and outflow from roadside ditches—to estimate SSF signatures and transformation potential. A central innovation is the application of the Damkohler number concept, linking residence time in the riparian zone to biogeochemical reaction times. This is evaluated using topographic, hydraulic, and hydrological data. All methods are integrated with collaborative projects: Connect (temperature proxies), Subsurface Controls (geophysics), Novel Tracers (DOM), and Patterns and Proxies (scaling). The project also includes a shared Postdoctoral Researcher for cross-project analysis. Data are uploaded to the SSFDW, following FAIR principles. This integrated approach enables a mechanistic understanding of SSF transformation, advancing both experimental design and predictive hydrology.