Mapping land degradation: a review from case studies in Tunisia
ISBN 978-85-88783-11-9
Authors
1Boussema, M.R.
1UNIVERSITY OF TUNIS EL MANAR Email: mrboussema@gnet.tn
Abstract
Mapping land degradation is a major challenge in most countries. Due to the spatial distribution of the factors causing land degradation, various kinds of maps are essential and unavoidable information media without which conservation actions would be risky and ineffective. Some of these maps provide qualitative information based on land degradation perceptions. Others are quantitative and allow to measure indirectly land degradation features. They are often available in different resolutions and scales. In order to be linked to a geographical information system (GIS), they should be digitized before use and need to be changed to a common resolution and scale. Globally, investigating in land degradation needs to follow a structured approach, based on reliable and updated maps. In this context, our research team has carried out during the past two decades, several research studies aiming at establishing and implementing a comprehensive methodology for the assessment and mapping of land degradation in arid and semi-arid areas in Tunisia. The results were expected to provide a better understanding of the degradation phenomena and indications for appropriate responses for conservation at all levels of scale. Available maps such as topographic base maps, geological maps, agricultural maps, etc., while combined with airborne and spatial remote sensing, GPS and GIS, made it possible to generate maps and charts that were very helpful in water erosion modeling, monitoring and quantification in watersheds of the central and northern Tunisia. They targeted in particular the study of the parameters controlling the development of the hydrographic network, the quantification of gully erosion, the identification of the most susceptible or risky areas to erosion, the analysis of either the degradation extent trends or the correlation between lithology and erosion. Available maps contain a wealth of information related to land degradation. But there was no way to improve their quality and reliability. Often, they were not at the appropriate scale. It happened that the data from input maps were updated with aerial photos and Google Earth processed images. Therefore, we were always concerned about the quality of the involved maps related to their scale or source. Similarly, the output map quality was dependent either on the spatial resolution of satellite images or the scale of aerial photographs used. It was also dependent on the accuracy of the digitization of available maps as well as aerial photographs. The maps’ scale and accuracy vary according to the operation mode and depending on whether we seek a synthetic view of the study area or a quantification at the ravine level. For example, spatial and temporal variations of the gullies’ head-cut, banks and depth have been mapped and monitored. Very often, individual maps have been generated without ambiguity to illustrate the various characteristics of land degradation. However, some were not in a very understandable form. In this paper, we present a review of the maps used and generated during our research dealing with the land degradation issue in Tunisia. We discuss their basic characteristics and their modus operandi. We propose an operating diagram detailing the required ones including optimal characteristics by type of research: modeling, monitoring or quantification of degraded arid or semi-arid land. Finally, we discuss how the national authorities dealing with land degradation assessment and conservation could endorse the operating diagram and the generated maps.
Keywords
Mapping Land Degradation; Erosion Modelling; Erosion Quantification