The integrated assessment of river basins based
on a holistic system analysis approach is of high relevance for
sustainable water resources management. In this study, the climate
variability especially rainfall patterns and land use changes were
analyzed and assessed between 1950-2012 and 1972-2000 respectively in
the Kara River basin (Togo and Benin). Then, the catchment hydrologic
characteristics and its rainfall-runoff dynamics were assessed.
The
methodological approach consisted in fitting statistical distributions
to annual maximum rainfall, cumulative annual rainfall and annual
number of rainy days for climate variability analysis. To assess
changes in land cover, an object-based image analysis approach combined
with GIS techniques were used. Catchment characteristics and its runoff
generation mechanism were assessed using statistical methods and
hydrologic models (the lumped conceptual metric model IHACRES and the
distributed model J2000).The results show no significant trend in annual
maximum rainfall except at two locations with decreasing trend. The
cumulative annual rainfall is decreasing while annual number of rainy
days is increasing except at one station. The results also reveal that
annual maximum and annual total rainfall which depend on West African
monsoon dynamics are highly likely to be also influenced by local
relief and topography. It is also evident that the peak of the rainy
season shifted from September to August since the 1980s. Also there has
been a drought from 1970s which was intensified in 1980s leading to the
shift in isohyets downwards all over the basin. From 1990s to 2000s,
there has been an increasing trend but the amount of rainfall received
still remains less than that of 1960s over the basin.
The results
also indicate that the basin has experienced important changes
with significant decrease in natural vegetation. Agricultural
expansion and deforestation appear to be the dominant driving
forces. In fact, agricultural land has doubled between 1972
and 2000 by increasing from 19% to 43% of the total basin area while
savanna decreased from 63% to 45%. Forest land increased by 1.6%
from1972 to 1987 but showed a decrease of 6% from 1987 to 2000, while
woody savannah decreased by 3.4% and human settlements increased during
the same period. Changes occurred between 1987 and 2000 were found to be
very important in comparison to the period between 1972 and 1987. The
same analysis in the neighbouring Binah v River catchment between 1972
and 2013 revealed the similar changes with agriculture expansion as the
dominant driving force. The assessment of catchment characteristics and
its rainfall-runoff dynamics reveal that the three nested sub-catchments
that constitute the basin have similar rainfall-runoff dynamics and the
runoff generation processes, i.e. volume and timing, are controlled by
topographic gradient,soils,geology and rainfall distribution. The two
models was calibrated and validated for the basin and its
sub-catchments and, modelling results indicate that the two
models simulate discharge volume and timing for the given
period adequately. Results further indicate that runoff generation
mechanism is dominated by inter flow and saturation excess overland
flow and the runoff response to rainfall is very sensitive to climate
and land cover changes. It is also shown that combining IHACRES and
J2000 modelling is efficient in reproducing the basin hydrology, but
also that applying a calibrated process- driven model offers the
potential to assess climate and land management impacts on water
resources
for their sustainable management.
Moreover, a river
basin information system containing available data and the first results
was established in order to facilitate the access and sharing of
information and data for decision making but also support future
research.
This study, which is a first direct assessment
of the basin, provides for not only the basin but also other
sub-catchments of the Volta basin and in general in West Africa, good
information and guidelines for the integrated land and water resources
management (ILWRM),an appropriate approach to strive for sustainable
management of water resources and to adapt to global change impacts.