Subproject within the DFG Research Unit "Fast and Invisible: Conquering Subsurface Stormflow through an Interdisciplinary Multi-Site Approach" (FOR 5288/1)
Subsurface stormflow (SSF) is known to be highly variable in space, but it is most often assessed and measured either at the local (hillslope) scale or at the catchment scale based on tracers that are thought to represent SSF. However, this means that we lack information at two scales: the spatial variability of SSF within a research catchment, as well as the variability in SSF across the many catchments where streamflow is measured but data on SSF tracers are missing.
We will tackle these two challenges with a two-pronged approach: distributed experimental investigations in the riparian zone and within the stream along entire stream reaches to measure SSF fluxes to the stream and the development of pragmatic proxies and indices of SSF that can be used for catchment intercomparison studies. Subproject E targets specifically the landscape scale/reach scale patterns and controls on hillslope-stream connectivity. Our work will enable us to answer the following questions:
(1) What are the spatio-temporal patterns of SSF entering the stream at the stream reach scale? What are major controls on these patterns and is there a threshold that causes these patterns to change?
(2) Are groundwater inflow points also SSF inflow points during events?
(3) How strongly is the SSF signal modified within the stream towards the catchment outlet?
(4) Can we use proxies, such as near-stream wells, hydrograph-based indices combined with catchment characteristics and hydro-meteorological drivers, to investigate the spatio-temporal variation in the occurrence of SSF? What are their uncertainties?
We suggest an innovative experimental approach focusing on near- and in-stream indicators of SSF. We will combine automated sequential salt-dilution gauging, radon measurements and distributed temperature sensing with a monitoring network of near-stream wells (providing data on the shallow groundwater table dynamics) and in-stream water level sensors. This will be complemented by an investigation of along-stream attenuation of the SSF signal and the consequences for hydrograph separation. Building upon a framework of checks to identify the probability of SSF occurrence in time and space, we will develop a classification scheme to assess the minimal data needs for an adequate signal observation and correct attribution which could be used in catchments where SSF cannot be studied in such detail. With the aim to simplify the determination of SSF contributions even more and facilitate catchment intercomparisons, we will assess the value of SSF indicators in streamflow hydrographs based on recession analysis. If successful, these indices will then be applied to long-term discharge time series for the test catchments to provide frequency distributions of SSF occurrence for the past decade. Due to the differences between the catchments, these distributions can be compared across a range of scales, topographies, geologies, and land uses.
(PIs: Dr. Luisa Hopp and Dr. Theresa Blume, GFZ German Research Center for Geosciences)
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