Excerpt
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.