System-oriented approach for catchment characterization in terms of typical landscape subunits based on the geophysical models

Abstract

Hydrological conditions in a catchment depend on many factors such as climatic, geological, geomorphological, biological and human, which interact with each other and influence water balance in a catchment. This interaction leads to the subordination in the landscape structure, namely the weak elements subordinate to the powerful elements. Thereby, geological and geomorphological factors play an essential role in catchment development and organization, because the main lithological features keep their properties in the landscape structure during a long time. We consequently can allocate a hillslope to one class of the representative units because the important flow processes run at the hillslope. Moreover, we can subdivide a hillslope into stratigraphic subsurface units and significant hillslope areas based on the lithological change of contrasting interfaces. The knowledge of subsurface structures is necessary to understand and predicate complex hydrological processes in a catchment. Geophysical techniques provide a good opportunity to explore the subsurface. It is impossible to achieve a complete geophysical investigation of subsurface in a catchment with difficult environmental conditions because of large time effort in the field, equipment logistic, and ambiguity in the data interpretation. The research study demonstrates how we can investigate a catchment using geophysical methods in an effective manner in terms of characterization across representative units with respect to a functional role in the catchment. This research study aims to develop combined resistivity and seismic velocity hillslope subsurface models for the distinction of representative functional units. In order to identify the contrasting interfaces of the hillslope, to localize significant hillslope areas, and to address the ambiguity in the geophysical data interpretation, the research study combined resistivity surveys (vertical electrical soundings and electrical resistivity tomography) with refraction seismic method, and conducted these measurements at one single profile along the hillslope transect and perpendicular to this transect. The measurements along the hillslope transect deliver the two-dimensional hillslope section of resistivity and seismic velocity distribution with contrasting stratigraphic interfaces, whereas the measurements perpendicular to the hillslope transect obtained from vertical electrical soundings survey localize significant hillslope areas indicating existence of two-dimensional features in the subsurface. To demonstrate the suitability of the suggested approach, we carried out resistivity and refraction seismic measurements at two hillslopes with different environmental conditions (geology, geomorphology, slope inclination, land use). The first hillslope is locating in the forested Weierbach catchment in the Ardennes massif, the mid-western part of the Grand Duchy of Luxembourg. The second hillslope is placing in the grassland Schaefertal catchment in the Lower Harz, the Central Germany. Besides, both hillslopes are characterized by Pleistocene periglacial slope deposits, which plays an important role in the ecosystem functioning. The obtained resistivity and seismic hillslope models for each study site complement well one another. The hillslope models identify significant hillslope areas along the hillslope called as elementary functional units, and electrical vertical stratigraphic units and seismic vertical stratigraphic units that agree with lithological stratigraphy of these study sites. Therefore, the suggested geophysical approach is suitable to characterise a hillslope as the representative unit only at a single transect in the efficient manner in contrast to the expensive 3D-measurements. In conclusions, to amplify the suitability of this approach, we constructed the geophysical hillslope model in the three-dimensional image using an additional cross profile. This three-dimensional image proofs the correctness of our approach. Based on the obtained results, we can provide a conceptual hydrogeological model of the representative hillslope.