Jens Didszun

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.

Runoff Generation Processes on Hillslopes and Their Susceptibility to Global Change

Global change will influence hillslope hydrological processes for a variety of reasons. On the one hand, climate change might alter the hydrological input, i.e. precipitation and snow melt, which might cause an increase or decrease in the intensity of specific hillslope processes. For instance, overland flow might be amplified by increased rain intensities (Horton 1933) or by reduced infiltration due to surface crusts (Yair 1990) or increased hydrophobicity (Doerr et al. 2002), triggered by longer and more pronounced drought periods. However, overland flow could also be significantly influenced by antecedent moisture conditions of the substrate that were either altered due to wetter climate and reduced evapotranspiration at a site or due to different snow and snow melt regimes, changing the hydrological input for a specific precipitation event. On the other hand, global change in the form of land use changes will play a key role in defining the dominant runoff generation processes on hillslopes (cf. summary given in DVWK 1999).