Evaluating agricultural potential of a Cape Metropolitan Catchment : a fuzzy logic approach
Thesis (MSc (Geography and Environmental Studies))--University of Stellenbosch, 2006.
Sustainable use of the earth’s resources is seen by many authorities as critical to ensure the planet’s survival. In this regard agriculture is seen as a major role player and fundamental link in the chain of sustainability. South Africa, a country with relatively little favourable agricultural land, should therefore preserve high potential areas for agricultural purposes. The Western Cape, with 75% of all medium-potential arable land in South Africa, is a valuable asset. One region targeted for development and where uncertainty prevails regarding its agricultural potential, is the G21B catchment in the Atlantis Growth Corridor (AGC). The AGC is envisaged as long-term growth axis for the Cape Metropole, but conflicting opinions exist on its agricultural possibilities and suitability to absorb urban growth. Consequently, the aim of this project was to evaluate the suitability of this catchment for a number of agricultural landuses. Fuzzy logic, a modification of the land evaluation approach originally developed by the Food and Agricultural Organization of the United Nations (FAO), was used to address this problem. Input data consisted of a soil map and digital elevation model (DEM) of the area. Parameters identified from these sources and applicable to the study were soil texture of the first, second and third horizons, as well as coarse fragments in the top soil, wetness, weathered rock, average pH, effective root depth, and slope. These parameters were compared to the requirements of six landuses, i.e. wheat, wine grapes, potatoes, tomatoes, onions, and citrus, and a fuzzy representation for every landuse was constructed. Expert opinion aided in a weighting process whereby the relative weights of parameters were computed and incorporated into the evaluation. Results showed extensive areas with a relatively high agricultural potential for potatoes and wine grapes. In addition, areas with potential were also identified for wheat, citrus, and tomatoes, albeit on more limited scale. Large areas, however, were deemed unsuitable for the aforementioned agricultural crops and would therefore be suitable to absorb metropolitan growth or to be maintained as conservation areas. The fuzzy logic approach provided insightful results. Problematic parameters were easily identified and no information was lost in the evaluation process. It also allowed for an objective quantitative comparison between crops. This provided freedom in deciding which landuse should be practiced, especially if the focus is on a sustainable rather than the most productive crop.