Freiburger Schriften zur Hydrologie
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Band/volume 10: UHLENBROOK S. (1999):
Untersuchung und Modellierung der Abflu▀bildung in einem mesoskaligen
One objective of this study was to increase the knowledge of the runoff
generation processes in the Brugga basin. Therefore source areas of runoff
were characterized and the contribution of runoff components during different
hydrological conditions were determined. Another objective was to integrate
the results of the experimental investigations of the runoff generation
into a catchment model. This lead to the development of a more process
oriented runoff generation routine. Based on the experimental and preliminary
work, the new developed model was applied to the Brugga basin. An evaluation
of the modelling results was possible by using additional data (multipleresponse
The Brugga basin is a mountainous basin (438 - 1493 m a.s.l.) with nival
runoff regime, located in the southern Black Forest, Southwest Germany.
75 % of the area is wooded, 22 % is used as pasture land. Urban land use
is dominant in 3 % of the area. The mean annual precipitation amounts
to 1750 mm, generating a mean annual discharge of 1220 mm. The crystalline
bedrock consists of gneiss and anatexits. The bedrock is covered by a
debris cover, which consists of moraines and periglacial deposits.
In the experimental part of this study hydrograph separations were performed
for different events using 18O, dissolved silica and chlorid as tracers.
The concentrations of the main anions and cations in discharge and in
wells provided further information about the runoff generation processes.
Using the environmental tracers 18O and 3H the residence time ofthe water
in the different flow systems were evaluated and the amounts of runoff
components were determined for aperiod of three years. Three main runoff
components were identified:
Direct runoff is generated on saturated areas, sealed areas and boulder
trains. It consists of event water and water which was stored near the
surface. During short periods of a few ho urs this component can contribute
as much as 50 % of total stream discharge, for longer periods (several
years) the contribution amounts to somewhat more than 10 %. The aquifers
of the slopes contribute about 70 % of total discharge (so-called flow
18O measurements showed that the mean residence time of the water in these
reservoirs is between two and three years. With mechanisms like the piston
flow effect and the groundwater ridging effect these reservoirs contribute
to flood formation, however they are also important for base flow. The
so-called jlow system-l originates from the hilly uplands and crystalline
hard rock aquifer and generates mainly base flow. The mean residence time
of the water is approximately 6 - 9 years, which was determined using
3H and freons measurements. For a periodof three years the contribution
of this component was estimated at 20 %.
Based on the experimental investigations and using different spatial information
(i.e. geology, properties ofthe debris cover, topography and further maps)
zones with the same dominating runoff generation processes were delineated
in the Brugga basin. 'In order to achieve this, a specific method was
developed, which accounted for the characteristics of Brugga basin and
the available data. The application of this method resulted in a spatial
delineation of zones, where it is assumed that the same runoff generation
processes dominate. This is the basis of the spatial discretization in
the newly developed catchment model TAC. The semi-distributed catchment
model TAC (tracer aided catchment model) was developed based on the experimental
investigations. The model is a conceptual model, which implies that complex
hydrological processes are conceptualized using relatively simple storage
routines. The snow routine is based on the degree day method. The soil
routine was adopted from the HBV model. The main objective was to develop
an improved, process oriented model routine of the runoff generation.
Therefore, specific routines were created for all zones with the same
dominating runoff generation processes. Concentrations of patural tracers
can be attributed to the different runoff components modelled by T AC.
The concentrations must be determined by tracer hydrological investigations.
Consequently, the simulation of the tracer concentration in the discharge
is possible. The quality of the TAC results can be assessed from the agreement
ofthe simulated and the observed tracer concentration in the relation
to the efficiency of the runoff simulation. An application of TAC in other
basins is possible, but a delineation of zones with the same dominating
runoff generation routines is required.
The application of TAC in the Brugga basin produced reasonable results.
The rainfall runoff modelling on a daily basis was at least as good as
the simulations using other conceptual models (i.e. TOPMODEL, HBV, PRMS).
The model was validated using an independent period and the quality of
the runoff simulation was equal to the calibration period. In a next step,
a model validation on internal stages and flows was tried using additional
information (multiple-response validation). Therefore, simulations of
the snow routine were compared with snow height measurements at the station
Feldberg (1480 m a.s.l.) of the German Weather Service. The general dynamic
of the snow cover was weIl modelled, however a detailed analysis of the
routine was not possible, because the data was insufficient. Additionally,
the modelling of the discharge and silica concentrations at the most frequent
runoff zone (zone with periglacial debris cover) was examined.
Therefore the simulations of TAC (discharge and silica concentrations)
were compared with measurements of aspring, which has a catchment that
is dominated by the periglacial debris cover. The discharge of the spring
was weIl modelled, and the general dynamic of the silica concentrations
was simulated adequately. Furthermore, the modelling results of TAC were
validated at the outlet with tracer measurements. A good agreement of
simulated and observed silica concentrations was reached for some periods.
Also, a comparison of the portions of the simulated runoff components
with the calculated portions using 18O and 3H measurements was performed.
The portions of the runoff components agreed far a period of almost three
years. Both methods showed the dominance ofthe flow system-2.
The modelling results, the simulation of the different hydrological processes
and the model validation using additional information (discharge, snow
height measurements, discharge at aspring, silica concentrations and runoff
components determined by environmental isotopes) lead to the following
The modelling approach of TAC, which is based on the spatial delineation
of zones with the same dominating runoff generation processes, and the
conceptualization of the runoff generation processes was suitable for
an improved process oriented modelling in the Brugga basin. In addition,
the potential of tracer methods was demonstrated. They are powerful tools
for identifying the runoff generation on catchment scale. On this basis,
better process oriented modelling concepts can be developed. The information
from tracers (e.g. tracer concentrations, calculated runoff components)
can be used to validate or disprove a modelling concept.
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