The Impacts Of Land-Use and Land Cover Changes On Rainfall In West Africa

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.