Tritium balance in macro-scale river basins analysed through distributed hydrological modelling

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The recession of bomb tritium in river discharge of large basins indicates a contribution of slowly moving water. For an appropriate interpretation it is necessary to consider different runoff components (e.g. direct runoff and ground water components) and varying residence times of tritium in these components. The spatially distributed catchment model (tracer aided catchment model, distributed; TACD) and a tritium balance model (TRIBIL) were combined to model process-based tritium balances in a large German river basin (Weser 46 240 km2) and seven embedded sub-basins. The hydrological model (monthly time step, 2×2 km2) estimated the three major runoff components: direct runoff, fast-moving and slow-moving ground water for the period of 1950 to 1999. The model incorporated topography, land use, geomorphology, geology and hydro-meteorological data. The results for the different basins indicated a contribution of direct runoff of 30-50% and varying amounts for fast and slow ground water components. Combining these results with the TRIBIL model allowed us to estimate the residence time of the components. Mean residence times of 8 to 14 years were found for the fast ground water component, 21 to 93 years for the slow ground water component and 14 to 50 years for an overall mean residence time within these basins. Balance calculations for the Weser basin indicate an over-estimation of loss of tritium through evapotranspiration (more than 60%) and decay (10%). About 28% were carried in stream-flow where direct runoff contributed about 12% and ground water runoff 13% in relation to precipitation input over the studied 50-year period. Neighbouring basins and nuclear power plants contributed about 1% each over this time period.

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