Spatial and temporal characterisation of stable isotopes in river water as indicators of groundwater contribution and confirmation of modelling results; a study of the Weser river, Germany

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 parts per thousand and 1 to 2 parts 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.