Bewertung von Erosionsprozessen und Hangabtrag im Einzugsgebiet der Alpen mit Hilfe von mathematischen Modellen und geographischen Informationsmodellen : Schutzgut Boden: Bewertung und Bewahrung

1998

S. 97-118

13 Lit. Ang.

Unselbständiges Werk

200050309

This work has been carried out in the framework of the R.I.V.E.T. project, which was aimed to the integrated research on slopes degradation in mountain areas. The R.I.V.E.T. Project was supported by the European Union within the third framework program "environment and climate". A modern approach to problems of erosion in a watershed consists in linking distributed-parameters models to Geographical Information Systems (G.I.S.). The G.I.S. output layers may also be used directly to determine locations, within a watershed, that are critical for erosion processes and sediment yield. The scope of the work included the assessment of slopes erodibility through the integrated analysis of thematic maps by crossing raster data in the G.I.S., the parallel implementation and setup of a distributed mathematical model linked with the G.I.S. and the comparison of the results obtained with the two methods. The research was conducted in the Alpone watershed, a 77.15 km¬ catchment located near Verona in the western Veneto region, that drains into the Adige river. Vineyard and forest lands are the two major land uses/land covers, accounting for 49.2 and 24.2 % of the total land area, respectively. The average surface slope is about 18 %. The main stream is 22 km long, with an average slope of 1 %. Soils are dominantly clayey (57.9 %), and therefore the watershed has a very low permeability to water. The watershed was monitored for runoff, sediment and water flow at an automatic sampling station located next to the town of Monteforte dÖAlpone. The production of a derived thematic map representing slopes stability involved the following steps: - production of basic raster thematic maps of soils and rocks erodibility, soil permeability and slope, assigning a specific index value to each aspect and level; - crossing of data of these three maps, obtaining a raster thematic map of potential soil stability index; - production of a basic raster map of land use, assigning an index value to each land use condition; - crossing of data of the potential soil stability map and land use map, obtaining an effective slopes stability map showing a five stability level classification. For the same watershed, the Agricultural Non-point Source Pollution Model (AGNPS) (Young et al., 1991), linked with WODITEM G.I.S., was used to assess runoff and soil erosion respectively for 20 and 7 observed events that occurred in 1992 and 1993. AGNPS is an event-based model that operates on a cell basis, so that the spatial variation in parameters of each cell can be accounted for the analysis throughout the whole watershed. The output from the event of October 4, 1992, with a rainfall amount of 149.4 mm in 24 hours, was examined in detail to locate cells having erosion rates exceeding soil loss tolerance levels. About 39 % of the total sediment load entering the main stream is estimated to have originated in five sub-watersheds whose area amounts to about 19 % of the entire watershed. A further 28 % of sediment load comes from sub-watersheds covering about 28 % of the total area. The comparison of results obtained with both methods, the integrated analysis of thematic maps by crossing raster data in the G.I.S. and the application of a distributed mathematical model linked with the G.I.S., showed a good accordance in terms of identification of areas with high erosion potential. It is to be stressed that, however, areas with higher sediment production are not necessarily coincident with areas showing a low value of stability index. The main difference between the two methods lies in the fact that the thematic maps analysis, considering aspects such as erodibility, soil permeability, slope and land use, needs to be set-up not only by means of data coming from monitoring stations, but also with extensive use of on-field surveys, which are characterised by a certain degree of subjectivity. The distributed AGNPS model integrated with G.I.S., has proven to be, in the opinion of Authors, a useful tool for an high-resolution analysis of runoff and erosion processes and their spatial distribution in middle-large size watersheds with prevailing vineyard use.