The Full Picture

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change in rangeland condition over a ten-year period

(1989–1999), as judged by local experts.

Besides land degradation, other forms of land cover and land

use change (e.g. expanding subsistence agriculture or informal

settlements) as well as natural processes can also cause a reduc-

tion in production per unit of rainfall. The present method should

thus be used as a regional indicator to identify potential problem

areas that can then be investigated in greater detail, using high-

resolution remote sensing data and field data. It is unlikely that

any method that depends exclusively on remote sensing from a

single sensor will be able to unequivocally map the complex

ecological process of degradation. Maps depicting the trends in

vegetation production per unit of rainfall, as shown here, require

field verification before management and policy decisions can be

based on them. The Department of Agriculture is therefore

actively involved in the verification process. In contrast to previ-

ous maps of land degradation in SA that were mainly based on

expert opinions, the remote sensing approach is objective and

repeatable, and all indications are that it will furnish a valuable

regional monitoring tool.

The long-term, high temporal resolution, satellite data that are

being promoted by GEOSS are indispensable, and significant efforts

are in progress to improve the quality of the data. The AVHRR instru-

ments were never intended for the uses to which they are now put

and the quality of the data, while appropriate for the original appli-

cation in weather forecasting, are not optimal for remote sensing of

vegetation. Newer sensors such as MODIS are now in service, but

these are experimental systems, not intended for long-term opera-

tional observations. Thus the role of GEOSS in encouraging

long-term data collection is very important in the context of mapping

and monitoring desertification.

Photo: K Wessels

Degradation continues to threaten the local resource base upon

which rural communal livelihoods depend

Trends in vegetation production per unit of rainfall

Left: Temporal trends in vegetation production per unit of rainfall in the summer rainfall region of South Africa

(red – negative, green – positive) derived with NOAA AVHRR data from 1985 to 2003. Right: The rate of rangeland

degradation as judged by experts (after Hoffman et al. 1999). The two maps agree in many parts of the region.

Note the high spatial resolution of the satellite observations

Source: K Wessels

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