Environmental change and flooding in the Gambia River Basin
It is argued in this thesis that the climate signal shows more
strongly in the runoff regime of the Gambia River Basin (GRB)
than the signal from deforestation. Partial and multiple
regression was used to partition the effects on runoff of
rainfall characteristics and deforestation over the GRB since the
turn of this century. The expected shorter, higher more rapidly
responding wet season flood peaks which result from deforestation
have not occurred in the GRB. Rather, peak floods have fluctuated
since the beginning of the century, but showing a clear declining
trend similar to the rainfall regime. The large size (z 7550 km')
of the sub-catchments of the GRB inhibit synchronisation of the
rapid runoff that is associated with deforestation. Furthermore,
deforestation, as it occurs in the GRB, takes place piece-meal
as small plots of land are cleared. The nature of clearance of
vegetation is important; the vegetation cleared is either
replaced with another type of vegetation, for example, groundnuts
or millet, or is soon allowed to recover after a cropping phase.
Surface and sub-surface hydrological processes within the GRB are
therefore not subjected to the severe form of alteration that
characterise massive and total clearance of vegetation schemes
in urban development.
However, deforestation has significantly affected low flowsthere
are now longer periods of lower dry season flows, and these
are ascribed to the diminishing recharge of ground water. By
augmenting overland flow and reducing interception and
infiltration, deforestation causes a reduction in ground water
recharge, which is an important component of dry season flows.
Both climate change and deforestation have worked in parallel to
cause a fluctuating but declining flow regime of the Gambia
River. This, in turn, affects both the agricultural potential and
productivity of the GRB.