SSF Transform - Understanding the transformation of subsurface stormflow from hillslopes through the riparian zone (DFG FOR 5288)

FOR 5288/2 - TP D

Subproject within the DFG Research Unit "Fast and Invisible: Conquering Subsurface Stormflow through an Interdisciplinary Multi-Site Approach" (FOR 5288/2)

The project SSF Transform aims at gaining an in-depth understanding of the transformation processes in the riparian zone that subsurface stormflow (SSF) generated on the hillslopes and flowing towards the stream is subject to. During the first phase of the Research Unit Subsurface Stormflow, we have collected a unique dataset by installing SSF collection systems and a large number of riparian groundwater wells in four catchments, as well as comprehensive sampling campaigns and event-based sampling. Based on our results from Phase 1 to date, we have developed an understanding of the chemical signatures of SSF, the variability of riparian groundwater chemistry and the physical responses of SSF and riparian groundwater levels to precipitation events. Therefore, we now propose to investigate the transformation and attenuation of both the physical and chemical signals of SSF in the riparian zone in more detail. In Phase 2 of the project Transform, we will therefore address the following research questions: (1) How do long-term SSF volumes, dynamics and chemistry develop in the instrumented catchments? (2) Which flow pathways does SSF take through the riparian zone towards the stream? (3) Can we develop a monitoring system based on profile probes of temperature and EC sensors to monitor SSF directly moving through the riparian zone? (4) How can we characterize mixing and biogeochemical reaction conditions in the riparian zone? (5) Can we develop simple experiments and proxy procedures to characterize SSF signatures and the transformation potential of the riparian zone? We will tackle these research questions with an innovative multi-method approach. While continuing to sample SSF and riparian groundwater to add to our multi-year dataset, we will now focus on sampling riparian groundwater in higher temporal resolution during events to characterize short-term dynamics of riparian groundwater chemistry and SSF transformation processes. To this end, we will install newly developed samplers that allow the more frequent sampling in a cost-effective way. We will use vertical profiles of temperature and electrical conductivity and geophysical methods to identify flow pathways and characterize subsurface structures. The transformation potential of the riparian zone will be investigated by analyzing the riparian groundwater chemistry with respect to mixing vs. biogeochemical processing of SSF, exploring vertical chemical gradients and applying the Damköhler number concept to characterize the reaction conditions within the riparian zone. We will test various methods that could serve as proxies for the occurrence, magnitude and quality of SSF. Based on the synthesis of our findings, we will be able to evaluate the spatial transferability and the options and requirements for a 'pragmatic' but sound monitoring setup for the estimation of SSF contributions to streamwater quantity and quality.

(PIs: Dr. Luisa Hopp and Dr. Theresa Blume, GFZ German Research Center for Geosciences)