Re-greening in the Sahelian landscape an analysis using high resolution 10m satellite data

Abstract

Desertification has become an increasing concern in the Sahelian landscape. Initiatives, such as the Great Green Wall (GGW) and Action Against Desertification (AAD) are aimed at reversing the effects of desertification. Using the AAD geographical regions as a reference, the rate of greening was analysed using high resolution Sentinel-2 satellite image data (10m) and compared to Moderate Resolution Imaging Spectroradiometer (MODIS), MOD13A1 (500m) and MYD13Q1 (250m) data products. Using Google Earth Engine (GEE), a total of 20 regions in 6 different countries were studied to observe the change in Normalized Difference Vegetation Index (NDVI) over two time periods; 2000 to 2022 and 2016 to 2022, representing long term trends and recent trends, respectively. In addition to observing the rate of greening occurring in AAD regions, a total of 51 sample sites were chosen to gain an in-depth perspective as to why any potential greening is occurring.
The findings suggest that re-greening has occurred in recent years (2016-2022) throughout the Sahelian landscape in all satellite data types used. Long-term trends indicate that the Sahel is spatially heterogeneous. For the most part, MOD data indicated that there have been re-greening trends in the majority of the AAD regions; however, MYD data showed opposite findings. The results suggest that browning trends are prominent in the Sahel, notably in the Gambia, Niger, and Nigeria. Vegetation-rainfall dynamics were also analysed for both AAD regions and sample sites. Results indicated that the correlation between NDVI and precipitation became more unclear using higher resolution data. 59% of sites had a strong positive correlation with precipitation, 35% had a positive correlation whilst 6% of all sites had an insignificant relationship. The main explanation for insignificant relationships was due to land use change and because of agricultural practices, such as irrigation.
Whether or not the re-greening trend will continue is up for debate. Results yielded conflicting results, for example MOD data suggested that there has been a long-term re-greening trend, but MYD data suggested that there has been a browning trend. Future research using the sentinel-2 imagery will be able to determine the current state of vegetation in the Sahelian landscape.

Popular Abstract

Desertification is a major issue in the Sahel region of Africa, where once-fertile land is turning into desert. This area faces unpredictable, low rainfall and harsh conditions that make it prone to droughts and soil erosion. To combat this, significant reforestation efforts like the Great Green Wall (GGW) and Action Against Desertification (AAD) have been launched.
The GGW, started by the African Union in 2007, aims to restore 100 million hectares of land by 2030 by planting trees to stabilize soil and improve fertility. Unfortunately, the initiative has struggled, with only 4% of the target achieved and high plant mortality due to poor management. In response, other projects like AAD have focused on smaller, more manageable interventions to improve success rates.
Using satellite data, we can study how ‘green’ the land below is because the land reflects different wavelengths back to the satellite receiver. With this information in mind, the aims of this study were to determine if the Sahel region is getting greener, in terms of vegetation growth. Using advanced satellite imagery from the Sentinel-2 and MODIS satellite data programs, vegetation changes across 20 regions in six Sahelian countries were analysed. Sentinel-2, which is a recent satellite data program launched in 2016, provides high resolution data (10m), whilst MODIS data provides images at a coarser resolution (250m to 500m); however, it has been operational since the 2000’s. Therefore, two time periods were chosen to note changes in recent (2016 to 2022) and long term (2000 to 2022) changes.
The findings reveal some promising trends. Recently, from 2016 to 2022, there has been noticeable positive greening in the Sahel. However, long-term data shows a mixed picture, with some areas, particularly in Gambia, Niger, and Nigeria, experiencing browning (a decrease in vegetation).
Greening was heavily linked to precipitation using the MODIS data, but the link weakened with the use of high-resolution data provided by Sentinel-2. Sentinel-2, as it has a higher spatial resolution can capture more detail. In effect, the high-resolution satellite is able to capture more local changes which are overlooked by coarser resolution data (such as MODIS).
The use of high-resolution satellite data is an extremely useful tool, especially when studying remote parts of the world, where there is little field research occurring. However, when using high-resolution data, more complex relationships are revealed.

• author: Petersson, Leo
• supervisor: Hongxiao Jin; Lanhui Wang
• organization: Dept of Physical Geography and Ecosystem Science
• course: NGEM01 20231
• year: 2024
• type: H2 - Master's Degree (Two Years)
• subject: Earth and Environmental Sciences
• keywords: Physical Geography, Ecosystem Analysis, NDVI, Sentinel-2, MODIS, Sahel, GGW, AAD
• publication/series: Student thesis series INES
• report number: 678
• language: English
• id: 9176154
• date added to LUP: 2024-10-07 16:54:11
• date last changed: 2024-10-07 16:56:45