Project E CONNECT - Longitudinal Patterns of SSF-Stream Connections

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Project E CONNECT aims at the identification of subsurface stormflow (SSF) contributions to the stream using distributed measurements in the riparian zone and in the stream, targeting specifically the landscape scale/reach scale patterns and controls of four different catchments.Identification of SSF occurrence at the hillslope is challenging because it is invisible as well as spatially highly variable due to a multitude of interacting controls and processes. However, actually identifying SSF contributions in the stream is an even greater challenge. Not only can the SSF signal be changed in the riparian zone, but once in the stream this signal can again be diluted or lost due to hyporheic exchange.We will tackle these challenges with a two-pronged approach: extensive experimental investigations in the riparian zone and the stream to identify SSF contributions and, based on this understanding, the development of pragmatic proxies and indices of SSF that can be used for catchment intercomparison studies. Our work will enable us to answer the following questions:1) What are the spatio-temporal patterns, controls and thresholds of SSF entering the stream?2) Do groundwater inflow patterns during baseflow differ from SSF inflow patterns during events?3) How strongly is the SSF signal modified within the stream towards the catchment outlet and what does this mean for hydrograph separation?4) Can we use proxies, such as near-stream wells, and 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 a mix of 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 and in-stream water level sensors. This will be complemented by an investigation of along-stream attenuation of the SSF signal and the resulting 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 which will help 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.