World Agroforestry Centre - Transforming Lives and Landscapes

Surveying and Restoring Degraded Land

Large areas of agricultural land in sub-Saharan Africa are degraded. This is one reason why crop yields have barely risen during recent decades. In Tanzania, soil erosion is also causing serious environmental problems by increasing the rate of sedimentation in Lake Tanganyika. Scientists from the World Agroforestry Centre have helped to identify the cause of the problem and possible solutions.

Some 10 million people live around Lake Tanganyika, located in eastern Africa.However, the livelihoods of most of the population is threatened by pollution, overfishing and – arguably most important of all – erosion and sedimentation. Erosion is a problem for farmers, as they are losing soil fertility. Sedimentation causes eutrophication, a process of nutrient enrichment which could pose a threat to many aquatic species, and therefore to local fishing communities. To tackle the problem, the United Nations Development Programme (UNDP) and the Global Environment Facility (GEF) established the Strategic Action Programme for Lake Tanganyika.

“The main aim of the project is to reduce sedimentation, and in order to do that we first had to identify degradation hotspots,” explains World Agroforestry Centre scientist, Fergus Sinclair. The researchers began by mapping the topography, using satellite imagery and aerial photography. They then looked at vegetation changes over time by ‘back processing’ satellite imagery from the past 30 years.

Remote sensing expert, Thomas Gumbricht, found that about 40% of the lake basin had lost some of its vegetation, and there had been a particularly significant decrease between 2001 and 2008. “By combining information from the long-term series of satellite images, and a higher-quality short-term series, we have been able to develop an index of land degradation,” explains Gumbricht. “This has helped us to identify the 5% of the basin most likely to be suffering from serious degradation.”

Most of the degradation hotspots are in Burundi and Tanzania, although certain critical areas have also been identified in the Democratic Republic of Congo and Zambia. Besides tracking vegetation changes over time, Gumbricht has been able to show where sedimentation has changed the nature of the shoreline, and by analysing sediment colour, he has been able to establish where it has come from.

Once the project had identified the degradation hotspots, researchers were able to focus on planning remedial activities. Two teams of students conducted detailed surveys, which involve mapping and interviews with local communities.

“We now have a better understanding of the factors that led to degradation and the sort of interventions farmers are prepared to adopt. The basic assumption is that planting the right trees in the right place will help to improve soils and restore degraded landscapes,” explains Sinclair. Various strategies are now being promoted by national and local extension services, including the planting of fertiliser trees to improve soil fertility, establishing woodlots and fruit orchards, and introducing farming practices which help conserve the soil. There has been a strong emphasis on capacity building, with regional workshops and training exercises involving a wide range of partners.

“Planting the right trees in the right
place will help to improve soils and
restore degraded landscapes.”

Interpreting past and present

Mapping the vegetation of the whole of Africa is one thing; doing it in such a way that you can track changes over time, quite another. Yet this is precisely what researchers at the World Agroforestry Centre are currently doing. “Remote sensing, using data provided by satellites, presents the best way for monitoring and surveying ecosystem health,” explains Thomas Gumbricht. “However, there are all sorts of problems to overcome when using satellite imagery taken in earlier times.”
With the support of the United Nations Environment Programme (UNEP), Gumbricht had processed satellite images for the whole of Africa going back to the year 2000. He is currently analysing data taken from images going back to the 1970s. This will provide a picture of how vegetation, and soil health, has changed over the past four decades.
Processing satellite imagery over a period of time is much more demanding than the analysis of single-date images. Among other things, Gumbricht has had to ‘correct’ data to take into account different atmospheric conditions, clouds and the cloud shadow, variations in satellite reflectance and other factors. “It is a bit like taking lots of different accents and ending up with the Queen’s English,” he says.
Gumbricht has developed techniques for ‘getting rid’ of clouds and is now able to distinguish between bare soil and harvested fields – both of which were impossible before. He has developed highly sophisticated techniques that are able to track the annual vegetation growth by measuring the amount of chlorophyll in images provided by Moderate Resolution Imaging Spectroradiometer (MODIS) satellites.
The remote sensing methodologies developed by Gumbricht and his colleagues are helping to support a number of research projects. They have played a crucial role in identifying degradation hotspots around Lake Tanganyika. The information is being used by the Africa Soil Information Service, in conjunction with ground sampling, to analyse soil health at 60 randomly selected sites in sub-Saharan Africa. Most of the research conducted by the World Agroforestry Centre is conducted on small sites scattered across the tropics. The remote sensing data will help scientists to identify other areas where there are similar conditions, which might benefit from some of the interventions which have been tried and tested by the Centre.

High-end science this may be, but that does not mean it is of interest to just the chosen few. “We are making all the data from our remote sensing research as widely available as possible so that people without the relevant technical skills can use the information,” says Keith Shepherd, a leading soil scientist. This means that researchers and extension workers will be able to use the data generated in Gumbricht’s Nairobi office to improve soils – and crop yields – in remote parts of Africa.