Modelling deforestation and land degradation in the Guinea highlands of West Africa using remote sensing and geographic information systems.

Abstract

A dynamic spatial model of deforestation and land-use change was developed from remotely sensed data for the Fouta Djallon mountain range in the Republic of Guinea, West Africa. The objective was to simulate patterns of land clearing for shifting cultivation in terms of farmers' selection behavior for new fields based on topography and proximity to villages. Data describing the current and historic condition of the vegetation cover, land use, and erosion for a watershed in Guinea were derived from aerial photography and ground sampling. Maps of these conditions were prepared and entered in a geographic information system (GIS) together with topographic data. From these data, maps of secondary variables (slope, village proximity, site productivity, and labor) were derived using the spatial operators contained in the GIS. These variables were ranked for agricultural preference and combined following a pair-wise hierarchy to generate a composite agricultural site-preference surface. This ranking was done in iterations, using a two-year time increment, which corresponds to the typical duration of cultivation for any one field. Different variable combinations and underlying assumptions of model logic were tested to determine influence on simulation results. To validate the model, the projected landscape was compared with land-use data collected in 1989. Although the model did not simulate the farmers' selection behavior for topography and village proximity successfully, test results with individual variables suggest that site productivity as determined by the length of fallow is a critical variable in the site selection process. The addition of site quality data should improve model results. The watershed in which this study was performed is the focus of a development initiative supported by the U.S. Agency for International Development (USAID), in which viable options are being sought for regional application. Thus, aside from documenting the dynamics of shifting cultivation, this model allows planners to evaluate alternative strategies of land-use conversion with a graphic display of zones of potential hazards. Finally, the data contained in the GIS serve as a structure for monitoring long-term change in the region.