Monitoring and Modelling of Water Quantity
and Chemistry to Identify Runoff Processes
in a Mountainous Basin
Stefan Uhlenbrook, Chris Leibundgut
University of Freiburg, Institute of Hydrology, Fahnenbergplatz, D-79098 Freiburg, Germany
Monitoring of water quantity and chemistry is carried out in the Brugga basin since 1994. The objective
of this study was to identify the dominant runoff generation processes and there spatial and temporal
variability and to integrate the results of the experimental investigations into a catchment model. This
leaded to the development of better process-oriented runoff generation modules, which were
integrated into a conceptual rainfall runoff model (Fig. 1). The model was calibrated and validated
using additional data (e.g. tracer data, different hydrometric measurements) beside the simulated
discharge.
Raumgliederungsklassen
Bloc ksc huttlage
3
Zone mit Groundwater Ridging
H-Input im Bruggagebiet
Sättigungsfläc he
6000
Abfluß [l/s]
3H-Konzentrationen
[T.U.]
8000
1000
100
Siedlung, teilversiegelt
Periglazialer Hangsc hutt mit Dec kfolge
Periglaziale Dec ksc hic ht
4000
Moräne
Hoc hlage
2000
10
1950
1960
1970
1980
1990
2000
0
Zeit [Jahre]
6000
Abfluß [l/s]
model
4000
2000
0
6000
Abfluß [l/s]
25
3
sim. H-Konzentrationen
H-Messungen mit analyt. Fehler
20
4000
2000
0
3H
[T.U.]
3
23.08.98
25.08.98
27.08.98
29.08.98
500
0
500 1000 1500
Maßstab (m)
15
10
1992
1993
1994
1995
1996
1997
1998
1999
TAC
(tracer aided catchment model)
“ multiple-response validation”
Fig. 1: Holistic approach of the investigation. It is shown that experimental investigations (using
different tracer methods), field mappings and a GIS-based spatial delineation are the basis for
the development and an extensive validation of process-oriented catchment model TAC.
The Brugga basin is a mountainous basin (39.9 km 2, 438 – 1493 m a.s.l.) with a nival runoff regime,
located in the southern Black Forest, southwest Germany. 75 % of the area is wooded, 23 % is used
as pasture land. Urban land use is dominant in approximately 2 % 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 and drift 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. In addition, concentrations of the main anions (Cl -,
NO3-, SO42- and PO43-) and cations (Na+, K+, Ca2+, Mg2+) in stream discharges and in wells provided
further information about the runoff generation processes. Using the environmental tracers
18O, 3H
and
freon concentrations the residence times of the water in the different flow systems were evaluated. In
addition, the amounts of runoff components from specific source areas were determined for a period of
three years. Three main runoff components were identified: Direct runoff is generated on saturated
areas, sealed areas and boulder trains. During short periods of a few hours this component can
contribute up to 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 (shallow groundwater). The mean residence time of the water in these reservoirs is between
two and three years. With soil water displacement effects these reservoirs contribute to flood
generation, however they are also important for base flow. The deeper groundwater originates from
the hilly uplands and the crystalline hard rock aquifer and generates mainly base flow. The mean
residence time of the water is approximately 6 – 9 years. For a period of three years the contribution of
this component was estimated 20 %.
Based on the experimental investigations and using different spatial information (i.e. geology, drift
cover properties, topography and further maps) zones with the same dominating runoff generation
processes were delineated. In order to achieve this, a specific method was developed, which
accounted for the characteristics of the study site and the available data. The spatial delineation is
needed for the semi-distributed catchment model TAC (tracer aided catchment model). The model is a
conceptual model, which implies that complex hydrological processes are conceptualized using
relatively simple routines. The snow routine is based on the degree day method. The soil routine was
adopted from the HBV model. The runoff generation routine was created new, therefore specific
storage routines were developed for all zones with the same dominating runoff generation processes.
Concentrations of natural tracers (e.g. dissolved silica) can be attributed to the different runoff
components derived by by the TAC model. Consequently, the simulation of the tracer concentration in
the discharge is possible. The quality of the TAC results can be assessed by the agreement of the
simulated and the observed tracer concentration in the relation to the efficiency of the runoff
simulation.
The application of TAC in the Brugga basin, different subbasins and a neigbouring basin produced
reasonable results. The results of 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 in a first step using an independent period. In a next step, an extensive model validation on
internal stages and flows was performed using additional information (multiple-response validation).
Therefore, the modelling of the discharge in different scales, runoff components and silica
concentrations were compared with measurements. A good agreement of simulated and observed
variables was reached.
The following conclusions can be drawn: (1) 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 is suitable for an improved process oriented
modelling. (2) The potential of tracer methods was demonstrated. They are powerful tools for
identifying the runoff generation at a catchment scale. Based on these findings 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.