4. Vegetation Phenology

Explaining Time Series of Vegetation Phenology

What causes the changes in Phenology we have observed?

We said on the previous page that water availability was the primary determinant of growth in the Sahel. So we have imposed are the rainfall in 1982 and 1999 on the phenology graphs, for Bobo Dioulasso and Tougere Koumbe, for you to look at.

Time series phenology with rainfall superimposed for Bobo Dioulasso in 1982 and 1999.

The rainfall at Bobo Dioulasso (shown above) is much greater than at Tougere Koumbe (shown right) and covers a longer period of the year. As you can see the rainfall at Tougere Koumbe is much more variable.

From the first exercise we know that the greening at Tougere Koumbe occurs on the flood plain of the Niger River, primarily due to flooding, as such so the low and irregular rainfall at Tougere Koumbe and has little impact on the NDVI profile.

By contrast, at Bobo Dioulasso, the vegetative response correlates directly with the rainfall. The changes in the phenology hint towards a change in the vegetative type and when we look more closely we discover that this is due to increased levels of agriculture in 1999 vs. 1982.

Time series phenology with rainfall superimposed for Tougere Koumbe in 1982 and 1999.

The increased agriculture explains the changes in the shape of the phenological profiles. However, both the agricultural crops and the natural vegetation are largely dependent upon rainfall for growth and hence the overall pattern has remained relativly unchanged.