Detecting environmental change using time series, high resolution imagery and field work – a case study in the Sahel of Mali
- Author(s)
- Martin Brandt, Cyrus Samimi, Clemens Romankiewicz
- Abstract
Climatic changes and population pressure have caused major environmental change in the Sahel during the last
fifty years. Many studies use coarse resolution NDVI time series such as GIMMS to detect environmental trends;
however explanations for these trends remain largely unknown. We suggest a five-step methodology for the
validation of trends with a case study on the Dogon Plateau, Mali.
The first step is to monitor long-term trends with coarse scale time series. Instead of GIMMS, we use a
combination of LTDR (derived from AVHRR) and SPOT VGT NDVI data, covering the period from 1982-today
with a temporal resolution of 10 days and a spatial resolution of 5.6 km. Areas with significant trends are further
analysed in a second step. Here we use a decomposed MODIS time series with a spatial resolution of 250 m. Due to
the large scaled MODIS dataset, trends can be identified at a local scale / village level. Using very high resolution
imagery (e.g. SPOT, Quickbird) areas of interest can be compared with pre-drought Corona-imagery. This offers
a detailed overview of the environmental change at tree-level. Yet many explanations for the changes identified
remain unclear. On-site field work provides information on the land use systems, vegetation composition and the
current environmental condition. Still many explanations for change can only be speculated and hypothesized. For
this reason, interviews with the local population are vital for providing missing details.
In this case study, an area near Fiko is introduced and analysed, where significant negative NDVI-trends
are observed at a coarse scale. Using the MODIS dataset, the spatial pattern shows areas with both positive and
negative trends within the same area. A comparison of high resolution imagery with the Corona images show
major land use changes over the past fifty years. What used to be dense bush cover has partially been converted
to farmer managed agro-forestry and a significant proportion is now degraded land. Furthermore, an increase
of tree cover on the fields can be detected. An initial field trip validated the suspected soil erosion and loss of
trees and shrubs outside the fields used for agricultural purposes. On the fields surrounding the village many
useful trees of all ages were identified. Interviews with local people showed that good farmer-management using
traditional methods, without outside-influence of projects, led to an increase of tree cover on the fields and healthy
environmental conditions. The land outside of the current agricultural area is highly degraded, which locals
explain by the following points: the extreme droughts in the 1970s and 1980s; lack of protection by farmers;
legal and illegal felling by inhabitants of provincial towns in the region and increased livestock numbers. Due to
the declining vegetation cover and supported by the unfavourable morphology, the susceptibility to soil erosion
increases. Many useful trees and shrubs have become rare in these areas.
This example demonstrates that land use plays a major role and shows the importance of an integrative
approach and input of local inhabitants when interpreting environmental change in the Sahel.
- Organisation(s)
- Department of Geography and Regional Research
- External organisation(s)
- Universität Bayreuth
- Journal
- Geophysical Research Abstracts
- Volume
- 14
- No. of pages
- 1
- ISSN
- 1029-7006
- Publication date
- 2012
- Peer reviewed
- Yes
- Austrian Fields of Science 2012
- 507005 Cultural geography, 105408 Physical geography
- Sustainable Development Goals
- SDG 15 - Life on Land
- Portal url
- https://ucrisportal.univie.ac.at/en/publications/f1278a1a-199b-47ba-adb6-7b99ca8b49e5