ABSTRACT
Isotope-hydrological investigations in small catchments in the crystalline Styrian border mountains (Eastern Alps, Austria) during storm events showed that a two-component runoff model (direct runoff, base flow) cannot sufficiently explain the course of the runoff isotope data during the events. At the very least, a “third” runoff component must exist, which cannot be attributed to either the direct runoff of precipitation water, or to the base flow before the event. Isotope data indicate that this third component consists of precipitation water, which is stored for a few weeks or months in the unsaturated zone (weathered material), whereas the base flow before a storm event consists mainly of water from the joint aquifer, with a mean residence time of about ten years. It may be concluded that the increase of the discharge during storms is primarily due to the pressing out of this third component from the previously unsaturated zone by infiltrating precipitation water. Direct runoff plays a minor role; its influence could not be detected during most of the precipitation events investigated.
