Groundwater-Runoff relationships.

Stable environmental isotope (2HH, 8O)O) techniques were applied to an Alpine watershed of 18.7 km2 and 670-1801 m a.s.l. From this example some insights into the runoff generation in a natural hydrologic system are developed. Isotopic hydrograph separations into direct and indirect runoff fractions, based on these techniques, show that the indirect component contributes dominantly to the total runoff. Contributions of 70-80% are frequent even during peak discharge from rainstorms or snowmelt events. The subsurface reservoir spontaneously responds to an input impetus. This effect has been ascribed to rapid dislocations of the subsurface pressure head. The indirect component describes distinct hydrographs substantially similar to total discharge hydrographs. Further differentiations of the indirect component are restricted to special cases, and they involve increased experimental efforts. The isotope-discharge relationships occasionally give more detailed informations. The most reliable approximations to reality by two-component separations are limited to snowmelt periods with almost homogeneous isotope inputs from the melting snow covers. The long-term isotope studies, that yielded sinusoidal input and distinctly damped output functions, enable us to make in combination with hydrological basic data and specific modeling procedures assessments of the turnover dynamics in the subsurface system. The average contribution of the indirect component is 2/3 of the runoff during the observation period of 3 years. This then corresponds to an annual infiltration rate of 1000 mm. The traditional concept of runoff generation dominated by the surface respectively overland flow should in any case be fundamentally revised