Hydrological parameters and climatic signals derived from long term tritium and stable isotope time series of the river Danube

Long term oxygen-18 (1968-1996) and tritium (1961-1996) records available from several stations along the river Danube have been used to derive information on the water dynamics of the Upper Danube catchment area, as far as the city of Vienna. It has been demonstrated that the long term (interannual) changes of oxygen-18 in precipitation are transmitted through the catchment and can be detected in the river water. Thus, oxygen-18 (or deuterium) can be used as an independent tracer to simulate transport processes in river systems. Because of a relatively small amplitude of the long term changes of δ18O (δ2H) in precipitation and in river water, this approach is useful in assessing the mean transit time of the fast component of the flow. For the Danube, the mean transit time derived from comparison of δ18O trend curves for precipitation and river water at Vienna is around 1 a. The time series of tritium in the Danube were modelled using the lumped parameter approach (one-box exponential model). The mean residence time of water (base flow component) in the Upper Danube catchment area is estimated at around 3 a. The applied model was unable, however, to reproduce adequately the long term trend of the tritium concentration in the Danube. It has been suggested that the buffering effect of the aquifers operating in the catchment area of the river, which becomes important during periods of large fluctuations of flow rate, may seriously influence the tritium balance in the system, thus making the predictions of the black box, steady state models unreliable. (author)