RICHÁRD KISS-GÁBOR MEZŐSI1
Introduction
Studies on erosion is a traditional subject in geomorphology. In the last 20 years the geomorphology has moved away from the investigation of the denudation chronology towards the study of processes (EVANS, I. 1998). The traditional problem of geomorphology is the relationship between forms and processes. To explain the land- forms and to calculate the characteristics of the surface need a quantitative description of the relief. In the 1970’s the morphometry provided a solution for the problem of quantification of the topography (STRAHLER, A.N. 1968; ZEVENBERGEN, L.W. and THORNE, C.R. 1987; MOORE, I.D. et al., 1991). Since the middle of the 1980's GIS have proven to be a very powerful and useful tool offering advantages for the establish
ment of spatial distribution of geomorphological processes (MONTGOMERY, D. and DIETRICH W.). The surface itself reflects the potential response to the impact of past exogeneous processes.
This publication was supported by the National Scientific Research Fund (OTKA), Project No. T 029246.
Background and concept
In this project an attempt was made to find out the constraints of GIS in morphometrical analysis. The aim is not only to produce and analyse a map of vertical relief dissection, but to point out the regional differences in erosion and the translocation of the latter in a geological scale. According to our idea the map of vertical relief dissection (i.e. the negative relict surface) can be constructed for each stream order by subtracting the real surface from the summit planes fitting on the watersheds (CHURCH, M. 1992). Such maps can give information about long-term translocations of erosion and about the changes in its rate. The mapping of vertical relief dissection was based on morphometrical analyses during the 1970's, but owing to the lack of suitable methods it had been carried out only in limited conditions. The surface modelling offered by GIS have opened new prospects in this direction.
1 University of Szeged, Department of Physical Geography H-6722 Szeged, Egyetem u. 2. Hungary.
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Fig. /. The orography and drainage system of the watershed of Parádi-Tarna Stream
Method
The analyses were carried out within a catchment of approximately 100 km2 area (Fig. 7). It is situated on the north-eastern edge of the 15 million year old Mátra volcano (North Hungary), mostly built up of andesite and rhyolite tufas. The test area being more or less homogeneous geologically, it was supposed that the direction of the
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Watershed boundary 1 st order 2nd order 3rd order 4th order 5 th order 6th order
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Fig. 2. Strahler's order of the streams
streams had not changed considerably. Besides, there are uniform precipitation condi
tions, so no significant change in the rate of erosion was expected.
It was supposed that a currently six-ordered stream (according to STRAHLER) had been five-ordered previously, four-ordered even earlier, and so on. This is true only as a model, because despite of a general homogeneity, geological, climatological and orographical disturbances might occur. The suitability of the order system for statisti- cal/GIS investigation was also supposed. The aim of the research is to highlight these disturbances, which can be investigated in the drainage network. First of all, the stream
network of the area was categorised following STRAHLER's system (COSTA- CABRAL, M.C. et a i, 1994; FREEMAN, T.G. 1991; Fig. 2). More than 1000 first- ordered streams were counted. The large number of streams confirmed the statistical results of the drainage analysis. Secondly, the real surface heights were subtracted from the summit planes fitting on the watersheds of first, second, etc. order. In this way verti
cal relief dissection maps were obtained for each order using Arclnfo 7.0.3. surface modeller. As a preparation a digital terrain model of the test area was compiled.
Results relative relief grew considerably, so here the erosional intensity became more accentu
ated than on its southern counterpart (Fig. 5).
Another important change in fluvial erosion could happen during the Pliocene, when the central part of the Mátra Mountains uplifted by about 150-200 m. Therefore, the rate of erosion increased in the southern part of the catchment being closer to the centre than to the northern part.
It is well known that there is an exponential relationship between the stream or
ders and the total length of certain stream orders. Fig. 7 shows that the number of the five-ordered streams (as well as the six-ordered ones) is less than it might have been expected. Their mean length and the volumes (Fig. 8) vertical relief dissection are de
creasing radically compared to the statistical expectations. It can be explained partly by lithological reasons (the five-ordered rivers situated in the middle of the catchment, in an erosion-resistant environment) and partly by the decreasing relative relief.
The above mentioned geological and orographical intluences were studied along the Ilona valley. The Fig. 9a shows the vertical relief dissection by different or
dered streams of that valley, which runs from south to north and than turns to east. The disturbance of the curve of the vertical relief dissection, for example that belonging to the five-ordered stream - at 13,350 m far from its source - , can be explained by geo
logical reasons: here the valleys reached the lower lying lava layers, therefore the drain
age pattern became sparser. The regular rhythm of the curves can be plotted as semi
circles (Fig. 9b). The integration of the changes in rates of erosion into the exact geo
logical time-scale has not been solved yet.
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Fig. 3. 6m order relief-dissection of the test area
Watershed boundary Erosion lines (50 meter) 375 m
Vertical dissection
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Watershed boundary Erosion lines (50 meter) 345 m
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Fig. 4. 5m order relief-dissection of the test area
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Watershed boundary Erosion lines (50 meter) 345 m
Vertical dissection
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Fig. 5. 4” order relief dissection of the test area
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Fig. 6. Geological map of the test area
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Fig. 8. Volume of the relief dissection
D i s t a n c e fro m t h e e d g e o f w a t e r s h e d (m )
Fig. 9. (a) Relief-dissection along the Ilona Valley, (b) Rhythm of the relief-dissection in an ideal network
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Fig. 7. Frequency of the stream segments
R E F E R E N C E S
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Blackwell, Oxford pp. 126-143.
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FREEMAN, T. G. 1991. Calculating catchment area with divergent flow based on a regular grid. Com
puters and Geosciences vol. 17 no. 3 pp. 413-422.
MONTGOMERY, D.-DIETRICH, W. The role of GIS in watershed analysis. - In: LANE. S .- RICHARDS, K. Landform monitoring, modelling and analysis. John Wiley, Chichester pp. 241-262.
MOORE. I. D.-GRAYSON, R. B.-LADSON. A. R. 1991. Digital Terrain Modelling: A Review of Hydrological. Geomorphological. and Biological Applications. Hydrological Processes Vol. 5, pp. 3-30.
STRAHLER, A. N. 1968. Quantitative geomorphology. - In: FAIRBRIDGE, A. Encyclopedy of Geo
morphology, New Y ork
ZEVENBERGEN. L. W -THORNE, C. R. 1987. Quantitative Analysis of Land Surface Topography.
Earth Surface Processes and Landforms Vol. 12, pp. 47-56.
A. KERTÉSZ óiul F. SCHWEITZER (eds): Physico-geographical Research in Hungary Studies in Geography in Hungary 32, Geographical Research Inst. HAS, Budapest 2000, pp. 9.C l 09.