Abstract
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 savannah 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 extension 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 interflow 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.
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