School of Geography and the Environment

Land Degradation in the Karoo, South Africa

The idea of 'desertification' has been widely applied to recent landscape change in South Africa. Alarmist fears of creeping desertification have been expressed. Most researchers have concentrated on vegetation, identifying a change from mixed grass and shrub to predominantly shrub in the semi-arid Karoo. For farmers, the implication is a loss of palatable grazing.

The project is gaining new insights into land degradation by studying changes in specific landscape signatures - badlands and deep gully systems. We are collaborating with the University of Cape Town and Rhodes University in South Africa and the Universities of Coventry, Aberdeen and Sheffield in the UK.

The Karoo

The work is focused on the landscape signatures of degradation: heavily eroded 'badland' areas and deep gully systems. For the past eight years we have concentrated on an area of the Klein Seekoei valley in the Sneeuberg uplands about 70 km north of the town of Graff Reinet. The valley is wetter than the lowland Karoo because of its altitude (ca. 1600 m) and is an area of sheep farming. In the past, the valley bottoms ('lands') were used to grow wheat.

There is some evidence in the Karoo as a whole, and in the Klein Seekoei valley, that very high stock numbers (sheep largely) are the cause of vegetation change and soil erosion leading to the formation of badlands and gully systems. But, climate change e.g. an upward trend in the number of intense rainfall events per year, and burning, may contribute.

We are investigating:

• The initiation of badlands and gullies;
• The physical processes associated with formation and development of badlands and gullies;
• Changes in the degraded areas through time;
• The knowledge and attitudes of the farming community;
• The possibilities for rehabilitation of degraded areas;
• The impact of erosion on hydrology, particularly dams and water quality;
• Rainfall patterns and climate change; and
• Recent environmental change using reservoir sediments.

Recent Environmental Change

Farmers in the Karoo have been building small storage reservoirs since the 1840s. The most famous local site, Cranemere, was described by Eve Palmer in her book The Plains of Camdeboo (London, Collins) published in 1966. Cranemere is one of five reservoirs that have been sampled by several members of the research team over the last five years and it contains the longest sedimentary sequence so far recovered (4.3 metres). The reservoirs have trapped sediment delivered from their local catchments and preserve a record of environmental change that can be linked both to climate and to changes in land management.

The age of the sediments is determined using the fallout radionuclides 210Pb and 137Cs. The former is a natural radionuclide which falls onto the reservoir surface, is incorporated into the reservoir sediments, and decays at a half life of ca. 22 years. By contrast, 137Cs is an atmospheric pollutant that first appeared in the Southern Hemisphere in the late 1950s and peaked in fallout in ca. 1965. Here we simply try to match the known fallout record to the pattern of 137Cs activity recorded in the reservoir sediments. Gravel layers are associated with known floods and help date the sediments.

From the dated reservoir sediments, and some additional ground surveying, it is possible to reconstruct the history of erosion and sediment transport over the lives of the reservoirs.

The figure shows this reconstruction for two reservoirs located on the northern flanks of the Compassberg, a single imposing mountain at over 2500 m altitude located some 50 km to the North East of the town of Graaf Reinet. Sediment yields, plotted on the X axis, are high by global standards for such a dry region. However, a notable feature of these graphs is the difference in timing between the increase in sediment yield at each site. The Dam 7 catchment has a short history of cultivation that peaked in the late 1950s but which was totally abandoned by the 1980s. The Dam 10 catchment has a long history of grazing within the catchment that peaked in intensity in the late 1930s.

Our analysis of the weather records for several stations in the region has suggested that the annual rainfall is neither increasing nor decreasing, although the number of rain days has declined in some records over the last 100 years with an increase in the magnitude of individual daily rainfalls. While more extreme daily rainfalls have occurred over the last 40 years than in the previous 60 years, it seems unlikely that rainfall alone is responsible for the increase in the trends in sediment yields plotted above. While rain provides the energy to move sediment, destabilisation of the landscape appears to have been caused by overgrazing (Dam 10) and additionally, cultivation in Dam 7 catchment.

Of major concern is the lack of evidence for any recovery of these landscapes despite evidence to suggest major reductions in stocking density and total abandonment of cereal cultivation.

The reservoir sediments are also analysed for other radionuclides in addition to their geochemical and mineral magnetic signatures. These signatures are compared with similar properties of the possible sources that could be contributing to the reservoir sediments.

Our results for the two reservoirs shown above have told us that for the last ca. 70 years:

• The sidewalls of major gully systems have made little or no contribution to the sediments accumulating in the reservoirs but the gullies have provided improved connectivity between the reservoirs and more remote parts of the catchment;
• Topsoil is the major source of sediment accumulating in the reservoirs;
• Occasional episodes of burning (either natural or controlled as part of a rangeland management system) within the catchment are associated with periods of increased erosion.

Further details of these research projects can be found in the publications listed below.

Methods

The topic requires an interdisciplinary approach using methodologies from natural and social science, including:

• Monitoring of selected sites for change;
• Mapping of degraded areas using air photographs from 1945, 1980 and 2000;
• Simulated rainfall experiments;
• Assembly of database on small dams;
• Dating of sediment infill in small dams (NERC funded project);
• Interviewing of farmers; and
• Coring of willows.

Personnel

This is a co-operative project - people currently involved include:

• Professor John Boardman: Emeritus Fellow, Environmental Change Institute, University of Oxford, and Honorary Professor, University of Cape Town
• Professor Mike Meadows (University of Cape Town)
• Professor Tony Parsons (Leicester University)
• Professor Ian Foster (Coventry University)
• Professor Kate Rowntree (Rhodes University, Grahamstown, South Africa)
• Dr Tim Mighall (Aberdeen University)
• Dr Pippin Anderson (University of Cape Town)

Funding

Funding for the project has been received from the UK Natural Environment Research Council (NERC), the Trapnell Fund (Oxford) and the Oppenheimer Fund (Oxford).

Future Projects

We have several projects that we would like to initiate or develop which are suitable for MSc, PhD students or as research projects:

• Environmental history of rainfed wheat farming in the Sneeuberg. This would involve use of farm diaries, interviews (oral histories) and field assessment using air photographs (John Boardman and William Beinart).
• An extension of Sabrina Chesterman's work (see below) on conversion of traditional stock farms to wildlife farms.
• Botanical surveys of degraded and non-degraded sites in the Sneeuberg (this work is being initiated by Dr Pippin Anderson).
• Rainfall simulation experiments described in Boardman et al (2003) could be extended.
• We know little about weathering processes and rates on badlands: experiments are needed in this area.
• Effects of fire on erosion (Some work by Ian Foster and Tim Mighall on magnetic signatures of sediments and charcoal in dams)