Abstract:
This study uses the Regional Climate Model version 3 (RegCM3), a high resolution model
to study the response of rainfall to changing land use/land-cover (LULC) over West Africa. A real
and four hypothetical land use/land-cover scenarios were used in the model which was run from
1991-2000 with the same lateral boundary. The four LULC scenarios considered are real land
cover which is the control, deforestation, afforesting the northern belt, thereby changing the
grassland to evergreen forest, afforesting the south and afforesting Nigerian geo-political domain
The results show that in deforesting from the coast to 14oN, from mixed forest to grass,
there is a rainfall reduction of about 10-30% in the deforested area. It also led to a late onset of
rainfall over the coastal areas and elongates the period of little dry season. This rainfall reduction
in deforestation experiment was due to slight reduction in the speed of the African Easterly Jet.
The core speed was observed in the experiment to reduce by about 0.55m/s. This reduction in the
speed of AEJ may have led to changes in the vertical wind shear between the two dominant West
African summer jets. There is reduction in the leaf area index and consequently evapotranspiration
which led to the weakening of rainfall recycling process in the perturbed area.
Deforestation also reduces frictional convergence and evapo-transpiration, making less moisture
available in the northward flowing monsoon wind but allows more moisture laden wind to reach
the north from south.
On the other hand, afforestation in the northern belt increases rainfall in northern part of
West Africa by about 100% in the extreme northern part of the afforested area between latitude
14o and 18oN while the rainfall between latitude 11o -14oN. Afforestation in the north also leads
to early onset of rainfall and lengthens the little dry season. The rainfall increment in the
experiment was due to increased frictional drag, evapo-transpiration, moisture available for
convection and changes to latent heat and sensible heat flux in the perturbed area.
Afforesting the southern belt of West Africa in experiment AFS leads to modest rainfall
change in the area afforested, but has a significant effect on the onset of rainfall which is observed
to be early in the Guinea coast but late in the middle belt. AFS also leads to reduction in rainfall
between April and May but increment in July and August.
Afforesting Nigerian political domain shows that rainfall increased over the northern part
of the domain and reduces in the coastal belt between May and October.