Introducing the project “Non-invasive identification and characterization of the subsurface structures and their control on subsurface stormflow processes”

Subsurface stormflow (SSF) is a direct subsurface response to a precipitation event, contributing to streamflow generation. SSF is thus all subsurface flow reaching the stream during an event, including near-stream saturation-excess overland flow triggered by SSF and return flow. Generally, SSF develops in vertically structured soils where the bedrock or a less permeable soil layer is overlaid by a permeable soil layer and vertically percolating water is, at least partially, deflected in a lateral downslope direction. SSF can also occur if groundwater levels rise into more permeable layers and water flows laterally to the stream. SSF is an elusive yet prevalent component of the runoff processes, often underestimated because a general understanding based on systematic studies across scales and sites is still lacking. However, only a standardized methodical procedure can allow us to advance our understanding by reveal general principles of SSF functioning and to provide protocols and best practices for its assessment, both experimentally and with respect to modeling. As part of this, identifying and characterizing the soil heterogeneity and the subsurface setting of the hydrologically relevant structures are among the major challenges on the way towards understanding SSF processes. These involve a high variability, presumably in combination with strongly organized patterns.

With this contribution, we introduce the research project “Non-invasive identification and characterization of the subsurface structures and their control on SSF processes”, part of the Research Unit “Fast and invisible: conquering subsurface stormflow through an interdisciplinary multi-site approach”, recently funded by the German Research Foundation (DFG).

This 4-years project addresses the current limitations in linking the experimental identification of subsurface structures to their numerical parameterization as required by numerical models working at larger scales. It builds on the integration of classical pedology for soil mapping and non-invasive geophysical imaging of the subsurface, and it will develop a workflow capable of accounting for such multi-source information, supported by emerging theoretical concepts for up-scaling the physical parameters to the larger domain. The systematic experimental setup provided by the Research Unit will give us the opportunity to test our approach at selected hillslopes in four highly instrumented catchments hence to evaluate the experimental results in a wider context. The subsurface characterization and derived parameterization will support various numerical models.

The major goal of the project is to develop an operational framework for identifying and characterizing the soil heterogeneity and the subsurface structures locally and to extrapolate the physical parameters of the subsurface to the catchment scale, and ultimately gain insights into the subsurface controls on SSF generation processes and their threshold behavior.