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The NLC has developed a standard land-cover classification scheme comprising 31 classes of land-cover (Table 1). These have been designed to conform to internationally accepted standards and conventions in order to ensure cross-border compatibility and integration with existing national and international land-cover classifications systems and datasets. An 'Illustrated Field Guide' has also been produced which contains the land-cover classes together with example photographs.
To provide a broad overview of the nature and extent of land-cover in a particular catchment these classes were aggregated into four generalised land-cover categories: agriculture, degraded, natural and urban (Table 2). Most of the aggregated land-cover classes are self-explanatory, however, in certain cases, some compromise was required.
The class 'improved grassland', for example, includes recreational man-managed grasslands such as golf courses, but also includes agriculture-related land-cover such as grazing, and hay or turf production (Thompson, 1996). For this analysis, 'improved grassland' was placed under the category 'Agriculture'. Fairbanks et al. (2000) also placed 'improved grassland' under the category of 'Cultivated Lands' in their aggregation of the NLC land-cover classes.
Degraded vegetation classes (NLC classes 13-17) are typically associated with rural subsistence level farming and livestock grazing, however, they are predominantly related to severe soil erosion problems associated with overgrazing and/or excessive wood-resource removal (Thompson, 1996). Although Fairbanks et al. (2000) placed these classes in the 'Cultivated Lands' category, in this analysis these classes, together with 'dongas and sheet erosion scars', were placed in a separate category - Degraded. The NLC land-cover classification also distinguishes between degraded lands and subsistence cultivation (NLC class 23).
In the classes 'waterbodies' and 'wetlands', no distinction is made between natural and man-made areas. Both classes were included under the category Natural in this study. It should be noted, however, that artificial impoundments can and often do, have a major impact on the functioning of rivers and estuaries.
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