Stefan
Uhlenbrook
The spatial and temporal (event and seasonal time scale)
variability of major runoff components
in the mountainous Brugga basin (Black Forest, Germany) were
examined. The meso-scale (40 km2)
study basin represented an extraordinary challenge as comparable
studies have been undertaken
mainly in smaller headwater basins. Discharge data, tracer
concentrations of 18O, 3H, CFCs,
dissolved silica and major anions and cations were analyzed
during single events and over a period
of three years. Three main runoff components were defined:
event water with a residence time of
several hours to a few days contributed up to 50% during flood
peaks, quantified by a classical
hydrograph separation technique using 18O.
However, this component is of minor importance for longer
periods, comprising about 11.1% of
total runoff as estimated for the period August 1995-April
1998. The other two flow components
originated from shallow and deep ground water. Source areas
for these are the upper drift and
debris cover for the shallow ground water and the deeper drift,
weathering zone and hard rock
aquifer for the deep ground water. Mean residence times ranged
from 28 to 36 months based on
18O data for the shallow ground water, and from 6 to 9 years
based on 3H and CFC data for the
deep ground water.
The importance of the upper drift and debris cover of the
slopes for runoff generation at the test
site was clearly demonstrated at the seasonal time scale,
showing a contribution of 69.4% based
on a mixing model with a monthly time step. The deep ground
water contribution was 19.5%.
With this information , a conceptual model of runoff generation
for the study site was constructed. |