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River water samples were analysed for stable isotopes (deuterium and oxygen-18) collected from 46 sites during spring 2008, and from monthly samples at the outlets of seven sub-basins of the River Weser (46,200 km(2) basin area in total) over a five year period from 2003 to 2007, to characterise temporal and spatial isotope patterns of river water. Results indicate a pronounced elevation effect (0.2 per thousand and 1 to 2 per thousand per 100 m for delta(18)O and delta(2)H, respectively) as well as influence of seawater mixing for a few coastal locations. A lumped parameter modelling approach was used to compare residence times and relative amounts of direct flow, fast and slow groundwater with those derived from a combined water balance and tritium balance modelling approach. Residence times of direct runoff were estimated to be between one and three and a half months. Much longer groundwater residence times are necessary to explain tritium recession in river water. The modelling fits for stable isotope data in river water, derived with residence times and base flow amounts combined from a water and tritium balance approach, emphasise that beneath a characterisation of a direct flow component, seasonal variations of stable isotope values in river water carry information on groundwater contribution.

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.

Resume Des mesures hydrometriques classiques et l'imagerie 2-D de resistivite electrique (IRE) detaillee ont ete combinees avec le prelevement de traceurs pour identifier les processus hydrologiques en jeu dans un bassin versant semi-aride de la province Cap-Oriental, en Afrique du Sud. L'etude des evenements de precipitation et d'ecoulement a souligne la forte relation qui existe entre les precipitations et les caracteristiques de la generation des ecoulements. L'observation du potentiel hydrique du sol ainsi que du niveau d'eau souterraine a mis en evidence le developpement d'une nappe perchee dans le sol. Ces resultats sont confirmes par des separations d'hydrogrammes a base de traceurs et illustrent le role important des ecoulements souterrains superficiels. L'etude par IRE a permis de mieux connaitre la structure du sous-sol. Finalement, l'etude par IRE, combinee avec des mesures obtenues par reflectometrie en domaine temporel (TDR), a permis d'extrapoler a des mesures selectives de teneur en eau. Po...