Jochen Wenninger

Identification of runoff generation processes using combined hydrometric, tracer and geophysical methods in a headwater catchment in South Africa / Identification des processus de formation du débit en combinat la méthodes hydrométrique, traceur et géophysiques dans un bassin versant sud-africain

Abstract Classical hydrometric measurements and detailed 2-D electrical resistivity imaging (ERI) surveys were combined with tracer sampling to identify the hydrological processes in a semi-arid headwater catchment in the Eastern Cape Province of South Africa. The analysis of precipitation and runoff events emphasized the strong link between precipitation and runoff formation characteristics. Soil water tension and groundwater level observations demonstrated the development of a perched water table within the soil layer. These results are supported by tracer-based runoff component separations and illustrate the important role of the shallow subsurface component. The ERI investigation permitted further insight into the structure of the subsurface. Finally, the ERI survey, in combination with time domain reflectometry (TDR) measurements, allowed the extrapolation of selective soil water content measurements. To summarize, the application and combination of different field methods led to the development of a conceptual model of the hydrological functioning of this catchment. The dominant role of the subsurface mechanisms was evaluated.

Source areas and mixing of runoff components at the hillslope scale : a multi-technical approach

Abstract Hillslope processes (i.e. water flow pathways, source areas and residence times) are essential for predicting water quantities and water quality. A multi-technical approach using classical hydrometry, natural and artificial tracers and electrical resistivity tomography (ERT) was applied to two adjacent steep hillslopes in the Black Forest Mountains, Germany. The differences in the hydrological and hydrochemical responses during three floods were larger than expected based on previously available information of topography, land use and geology. At one site a very dynamic shallow groundwater system dominated the flood generation, which could not be observed at the other site. The reasons for the heterogeneity of hillslope processes are the different soils and structures of the periglacial drift (first-order control); this is augmented by the different land use (pasture vs forest) and its effects on the near-surface processes (second-order control). The multi-technical approach proved very useful: the tracer methods enabled the detection and quantification of runoff components; geophysical methods provided further insights into the subsurface structure and, consequently, the origin of runoff components.