Freiburger Schriften zur Hydrologie
|
[ << Band/volume 18 ] [
Freiburger Schriften zur Hydrologie ] [
>> Band/volume 21 ]
Band/volume 19: DIDSZUN J. (2004):
Experimentelle Untersuchungen zur Skalenabhängigkeit der Abflussbildung
The objective of this study was to investigate the scale dependence of
runoff generation proc-esses and runoff components with discharge samplings
on an event basis in catchments of low mountainous regions. The motivation
results from the increasing number of applications of process-oriented
catchment models to mesoscale basins, the process knowledge, however,
implemented in these models was achieved by experimental research in microscale
catch-ments. This research was part of the cooperative project "runoff
generation and catchment modelling", funded by the German Research
Foundation (Deutsche Forschungsgemeinschaft, DFG). This offered the possibility
of research activities not only in the Dreisam basin (258 km², Black
Forest Mountains), but also in the Rotherdbach catchment (0,09 km²,
Ore Mountains) and the Brachtpe catchment (2,6 km², Sauerland, Central
West Germany). Be-sides some microscale experiments the research focused
on numerous event samplings, i.e. simultaneous runoff sampling during
precipitation events in several nested catchments. The samples were analysed
for the major ions, dissolved silica and stable isotopes (Oxygen-18, Deuterium).
The experimental investigations in the microscale Rotherdbach catchment
supported the pre-vious perceptions of runoff generation in this basin.
Dye-tracer experiments provided evi-dence of fast lateral flow paths in
the vicinity of the channel, delivering either pre-event or event water
to the stream during precipitation events. The dominant runoff components
of two sub-catchments were quantified by the analysis of the natural tracers
and the application of hydrograph separation techniques. It became evident
that one sub-catchment was dominated by fast interflow and surface saturation
excess flow originating in the riparian zone. The inter-flow in the other
sub-catchment was significantly slower, peaking one to three days after
the precipitation maximum, the saturated areas in this sub-catchment being
variable in size and of different vegetation structure.
In the Brugga basin, a sub-basin of the Dreisam, spring water of the elevated
regions, hill-slope water and stream runoff could clearly be distinguished
by their hydrochemical composi-tion. However, it was not possible to verify
spatially and hydrochemically uniform hillslope water components. Neither
the position on the hillslope nor the depth of the sampling location had
systematic influences on the composition of the natural tracers. This
can be explained by the significant heterogeneity of the subsurface flow
paths in the unsorted and permeable mate-rial of the periglacial drift
covers. The event sampling of a small forested catchment (1.5 ha) drained
by a steep channel showed clear differences in comparison with the hydrochemical
event reaction of the downstream catchment (1.1 km²) and the basin
outlet (40 km²). The small hillslope channel is representing only
one hydrotope which is dominated by fast inter-flow. In contrast, the
typical sub-catchments of the Brugga basin (1?2 km²) include already
the majority of runoff generation processes and runoff components respectively
and therefore show a similar event reaction as to that of the basin outlet.
The event sampling in the Brachtpe catchment included four microscale
sub-catchments, each about 0.5 km² in size, and the catchment outlet.
A hydrological characterization of the sub-catchments and a quantification
of the dominant runoff components was achieved by the analysis of the
natural tracers and the use of the hydrograph separation technique. Differences
between the sub-basins exist e.g. in the velocity of the runoff reaction
caused by the fraction and the position of the saturated or quickly saturated
areas. Normally, these areas also pro-duce increased fractions of event-water
and surface runoff respectively. Only minor influ-ences on the runoff
generation were shown for the landuse. This can be explained by the fact
that the fast reaction is dominated by components originating in the swampy,
saturated or quickly saturated areas near the streams especially in the
headwaters. But these areas are more or less independent from the landuse.
Overall, these results support the perceptions of the re-search in the
Brugga basin: In the microscale sub-catchments it was possible to detect
differ-ences in the hydrological and hydrochemical reaction. But at the
basin outlet these differences could not be attributed to the respective
sub-catchments anymore.
In both, the Brugga and the Dreisam basin, the spatial heterogeneity of
the natural tracers was determined by synoptic sampling during low flow
periods. The small sub-catchments showed the highest heterogeneity which
decreased with increasing catchment size. On the one hand detectable thresholds
are caused by the typical sub-catchment size in the Dreisam basin. On
the other hand they are controlled by the distribution of the sampled
sub-catchment sizes. A simulation of the tracer concentrations by using
catchment properties (e.g. size, mean altitude, geology) and the landuse
fractions was not possible.
Three event samplings in up to eight sub-catchments of the Brugga basin
showed only minor differences in the systematic event reaction except
for the effects of runoff concentration. Al-though the hydrological answer
of the small sub-catchments (ca. 1.5 km²) was partly more pronounced
than that of the rest, the reactions were not systematic. This means that
there were differences in the hydrometric and hydrochemical reaction between
the small sub-catchments during a single event and the reactions also
varied between the events. The differences be-tween the bigger sub-catchments
originate basically from the heterogeneity of the events (pre-cipitation,
antecedent moisture). Regional distinctions in the runoff generation processes
could not be found because the size of the investigated sub-catchments
was big enough to comprise the majority of the runoff generation processes
and therefore the sub-basins show similar hydrologic reactions. But it
has to be considered that a prerequisite for the analogous reactions is
the similarity of the catchment properties (topography, geology, precipitation)
in these sub-catchments. In contrast, two event samplings in seven sub-catchments
of the Drei-sam basin showed significant differences in the runoff generation
of those sub-basins which differ from the Brugga basin with respect to
geology and topography or exhibit a bigger frac-tion of urban areas (settlements,
roads).
Catchments with these characteristics are mainly big-ger in size (>
40 km²), which results in a scale dependence of runoff generation
in the Drei-sam basin. But this scale dependence does not originate from
the basin size itself. It is rather caused by the scale dependent catchment
properties. Although the effect of urban areas be-came evident only in
the bigger catchments it can be treated as a hydrotope as well. The ap-plication
of the statistic method principle component analysis on the event data
from the Brugga and Dreisam basin on the one hand confirmed the systematic
similarity of the differ-ent data sets, on the other hand, however, pointed
out the variability of precipitation events. It was furthermore possible
to statistically highlight the influences of urban areas and of runoff
concentration on the hydrochemical event reaction.
In this work the effect of catchment size on the spatial heterogeneity
of natural tracers and the systematic hydrological event reaction was
presented. The work also offered the possibility of experimentally detecting
the runoff generation processes, which take effect only in the mesoscale,
and of comparing larger sub-basins in this regard. Thus, for the application
of process-oriented catchment models to mesoscale basins both, microscale
and mesoscale proc-ess knowledge is necessary. The limitations of quantifying
runoff components highlighted in this work accentuate the need for future
research in the field of new tracers and improved quantification strategies.
PDF-Download (4,0 MB)
|