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Freiburger Geographische Hefte, Vol 57

Irene Marzolff (1998): Monitoring of vegetation development and geomorphodynamics in Aragón (Spain) using large-scale imagery taken from a hot-air blimp and Geographic Information Systems


Favoured by semi-arid and arid climate and by a long-term history of human settlement and land use, land degradation and desertification are today threatening large areas in the Medi terra nean. The Iberian Peninsula in particular has during the outgoing century been subject to considerable land use changes inducing large-scale landscape modifications and often bringing about the triggering or intensification of degradation processes. Causing irreversible damage to the landscape, soil erosion by water has been proven to be the most severe degradation process in the Mediterranean belt. The documentation and investigation of geo morphological processes, of their development and their relationships with vegetation, soils, relief and climate gets to be the key for understanding and counteracting land degradation.

Aerial photographs and satellite images together with image processing and GIS technology are increasingly employed for monitoring of land degradation and soil erosion at regional or even global scale. However, neither spatial nor temporal resolution of this imagery corresponds to the magnitude and frequency of geomorphological processes which are often transitory and occur in close relation to complex patterns of vegetation cover and forms of the micro-relief. The investigation of the actual geomorphodynamics requires detailed and repeatable surveys of these small patterns of forms and processes which is not easily accomplished even with labour-intensive field work.

Since 1995, the EPRODESERT research project of the Institutes of Physical Geography at Frankfurt and Freiburg University has been studying the reasons and consequences of soil erosion and land degradation on areas under extensified land use, mostly abandoned fields and set-aside land, in North East Spain. The study presented in this dissertation is one of the core issues of the EPRODESERT project; it is concerned with the development and application of a remote sensing system which is specifically aimed at monitoring the project’s 24 m x 36 m test areas and their surroundings focussing on geomorphodynamics and vegetation succession. Major demands which had to be met at the development stage were high variability of flying heights (and thus image scale and resolution corresponding to various scales of investigation), temporal flexibility (and thus image repeatability), low cost, ease of use and transportability in remote areas.

The system employed for this study is a specially designed hot-air blimp (100 m³, approx. 10 m long) running on propane gas. It serves as a stable and vibration free platform for a revolvable twin camera system which comprises two 35-mm single-lens reflex cameras loaded with normal colour and infrared slide film. Burner and camera functions are remotely controlled from the ground while tether ropes enable to precisely position the blimp at up to 400 m height over the area to be photographed. With this system, photographic monitoring of several test sites was conducted during 6 field trips between 1995 and 1998, yielding several hundred images at scales between 1:200 and 1:10 000 (areal coverage approximately 35 m² to 10 ha).

A selection of photographs was subsequently digitised onto Kodak Photo-CD. Using image processing and GIS software for rectification and image analysis, a Process-Geomorphological Information System was developed consisting of digital georeferenced test area maps with 2.5 cm resolution. Visual photo interpretation combined with on-screen digitising, digital image classification and hybrid visual/digital classification methods enabled the detailed mapping of geomorphological processes, density and patterns of vegetation cover as well as plant life forms. Texture and fourier analysis were employed to automatically delineate micromorpho logical structures caused by ploughing. After calibration of the cameras, digital elevation models could be generated from stereoscopic images by photogrammetric analysis. The DEMs with 25 cm resolution and an orientation accuracy of 3-6 cm in height were used for computing slope and curvature maps as well as for the simulation of potential flow paths over the test areas.

This Process-Geomorphological Information System served for statistical analysis (predomi nantly by cross-tabulations and correspondence analyses) and interpretation of the develop ment and relationships between vegetation cover and geomorphodynamics under the influ ence of relief and precipitation at three test sites in the semi-arid Ebro Basin, the sub-humid Prepyrenees and the humid High Pyrenees. A generally negative correlation of geomorpholo gical process activity with vegetation cover density can be observed for all study sites, while the strength of correlation varies between climatic regions and within those for different plant life forms. An influence of micro-relief on actual geomorphodynamics exists mainly for linear erosion forms. Only when vegetation cover is very sparse outweighs the influence of relief as a factor controlling erosion processes the role of vegetation density; in this case, nano-relief (tilling pattern on set-aside field) can also considerably influence pattern and intensity of geomorphological processes. In addition to and supplementing these studies on vegetation/ geo mor phodynamics-relations, several chapters of this work are dedicated to detailed ob serva tions of geomorphological forms specific to the individual study areas (rill and gully erosion on fallow land of different age in the Ebro Basin, accumulation of rock fragments at the soil surface of the abandoned field in the Prepyrenees).

As a most important result can be concluded, that on the observed test areas erosion processes are occurring in patterns of high spatial frequency at far higher percentages of vegetation cover than tends to be assumed by most investigations into land degradation. Generally, a vegetation cover of 30-40% is taken as a threshold beyond which runoff and soil erosion rates reach negligible amounts. In contrast, results of the study presented here show sheet erosion on fallow land with up to 70% overall vegetation cover. Looking at small-scale patterns, moderate sheet erosion can even be observed at up to 90% vegetation density, and process dynamics may during several observation periods intensify even with increasing vegetation cover. In the semi-arid region in particular, where vegetation succession is limited by water stress, at least 60% of small-scale increases of vegetation cover are required in order to bring prevailing sheet erosion processes to a halt.

At the spatial scales between nano- and micro-relief of test areas, the presented study could provide evidence of distinct relations between spatially recordable control factors and pro cesses of land degradation (relief form, rock fragment cover, state and change of vegetation cover and actual geomorphodynamics). Unquestionably, these degradation processes and relationships continue to affect the landscape on higher scale levels and in larger geomorpho logical systems. The study thus represents a step towards the comprehension of process structures within the complex of vegetation succession and geomorphodynamics and towards the extra polation of these processes into geomorphological systems of different sizes. Further more, this work demonstrates the as yet little exploited potential of small format aerial photo graphy integrated with Image Processing and Geographic Information Systems. For geomor phological process research and above that for other space-related questions, the remote sensing method presented here allows to close the gap between terrestrial photography and conventional aerial photographs.