Mapping Four Decades of Land Change in West Africa: 1975, 2000 and 2013

The West Africa Land Use Dynamics project, begun in 1999, set out to map land use and land cover across West Africa, trace how they have changed through time and across the region, and understand the effects on the environment. It was carried out through the AGRHYMET Regional Center in Niamey, Niger, with partners from 17 countries, the Sahel Institute and the U.S. Geological Survey, and with major support from the U.S. Agency for International Development in West Africa. Its aim is to make people aware of how landscapes and land resources are changing, and to inform national and regional decision-makers.

Three snapshots: 1975, 2000 and 2013

The resulting dataset maps land use and land cover across sub-Saharan West Africa, including the Cabo Verde islands, for three periods. Most of the region was mapped at 2-kilometre resolution; Chad, being large, at 4 kilometres; and the small countries of The Gambia and Cabo Verde at 1 kilometre and 500 metres. Large parts of northern Mauritania, Mali, Niger and Chad lie in the Sahara, where land cover barely changes, so only the southern parts of those four countries — roughly below 18 degrees north — were mapped.

The classification drew largely on the Yangambi Classification, the standard naming guide for vegetation types in tropical Africa since 1956, especially in West and Central Africa. Because Yangambi describes mainly vegetation, while a land-cover map must also show surfaces without it, the project's 24 classes combine Yangambi's vegetation types with other classes commonly used in West Africa for other kinds of land use.

West Africa's land use and land cover in 1975, 2000 and 2013, at 2-kilometre resolution. USGS.

How it was done

Mapping all of West Africa three times over took hundreds of Landsat images and careful method, with the aim of maps consistent enough from one period to the next to measure change reliably. The land cover was interpreted visually, using a GIS tool built for the purpose, the Rapid Land Cover Mapper, to make accurate maps quickly. Thousands of aerial photographs and high-resolution satellite images were used to check the interpretations. National teams produced the first versions; the USGS then reviewed them independently and in detail, and, in agreement with each country's team, several interpreters revised them to give an accurate, consistent picture of every country and its border areas.

Lake Chad (left) and the Tibesti Mountains (upper right), photographed looking east from the International Space Station on February 12, 2015, with an artist's rendering of Landsat 8 added. NASA.

What changed

The time series gives an unprecedented, systematic basis for analysing land change across the region.

• How much: about 13.7 percent of the mapped area (673,472 square kilometres) changed between 1975 and 2000 (excluding Cabo Verde, which was not mapped for 1975), and 11.8 percent (583,012 square kilometres) between 2000 and 2013. Togo, Burkina Faso, Ghana and Benin changed the most.
• Farming was by far the biggest change. Rainfed agriculture covered 10.7 percent of the land in 1975; by 2013 it was the second largest land cover, more than 1.1 million square kilometres or about 22.4 percent of the land. All four farming classes — rainfed agriculture, agriculture in shallows and recession areas, irrigated agriculture, and plantations — grew, as did open mines and settlements.
• Savanna lost ground: by 2013 savanna had lost 406,432 square kilometres and Sahelian short-grass savanna 119,424.
• Settlements grew by 140 percent, mostly along the coast.
• Forest — forest, degraded forest and gallery forest (the corridors of forest along rivers) — lost a net 100,176 square kilometres from 1975 to 2013, 24.6 percent of its 1975 area.

West African countries have lost, and are still losing, large areas of natural land cover to a landscape shaped heavily by people and dominated by farming.

Land cover change around Ouagadougou, Burkina Faso, 1975–2013: mostly the spread of rainfed farming (yellow) and the growth of the city (red). USGS.

Cropland surrounding a village in central Senegal. Photograph by Gray Tappan, USGS.

Wooded savanna being cleared for cropland, southern Senegal. Photograph by Gray Tappan, USGS.

Forest cover change in the Upper Guinean countries, 1975–2013: forest remaining in 2013, degraded forest remaining, and forest or degraded forest lost. USGS.

What the maps are for

The dataset gives a complete, "wall-to-wall" view of the region's landscapes at three moments, so trends can be analysed — objective information to help decision-makers understand nearly 40 years of change and plan ahead. It can be used to study the effects of deforestation and forest degradation, estimate trends in carbon emissions, and monitor water and farming resources, among many other uses.

The analysis was published as an atlas, Landscapes of West Africa: A Window on a Changing World, which highlights landscapes that have been transformed, such as the Sahelian savannas and the tropical Upper Guinean forest, and examines what drove the changes and their environmental, social and economic effects. The atlas and all the data can be downloaded free from the project website, with no restrictions on later use, sale or redistribution.

Sources

• U.S. Geological Survey, Earth Resources Observation and Science Center (contact W. Matthew Cushing), West Africa Land Use and Land Cover Time Series, Fact Sheet 2017–3004, February 2017. https://doi.org/10.3133/fs20173004
• Project website and data: https://eros.usgs.gov/westafrica