Climate and land use change impacts on floods in the Mono River basin of Togo and Benin

Abstract

Floods are prominent in West Africa and are expected to exacerbate due to global changes, including climate change, land use change, as well as industrial and infrastructure development. It is important to evaluate in advance their patterns and potential impact in order to develop proper response and adaptation strategies. The aim of this study is to investigate the impacts of future climate and land use change scenarios on floods in the Mono river catchment of Benin and Togo, while assessing the capacity of the two countries to jointly manage these impacts. The performances of four remotely-sensed precipitation datasets, namely CHIRPS, PERSIANN, TAMSAT and GPCC, were first of all evaluated to address the lack of data in the basin. PERSIANN, GPCC and TAMSAT indicate the best results for flood simulation purposes. In addition, it was found that applying the kriging interpolation method to compute the areal precipitation is enough to deal with gaps in observation data, without any need of filling-in the missing values. <br /> This study also explored the use of a Multicriteria Decision Analysis approach to select the best performing Regional Climate Models (RCMs) in the Mono River catchment. The Technique for Order Preferences by Similarity to an Ideal Solution (TOPSIS) was used to rank RCMs downloaded from the Coordinated Regional Downscaling Experiment (CORDEX) database. Annual mean temperature showed an increasing trend over the period 1966-2070, whereas annual rainfall depicts high variabilities with no statistically significant trend. Moreover, the annual cycle of rainfall is expected to be characterised by rainfall intensification and a delayed start of rainy seasons. <br /> The combined impact of these climate projections and land use/land cover change (LULCC) scenarios was assessed using the Soil and Water Assessment Tool (SWAT) for runoff simulation, and the TELEMAC-2D model for flood mapping. The effect of the planned Adjarala dam was also examined. Results show an increase of the magnitude of flood extremes under future climate and land use change scenarios. Events of 10-years return periods during 1987-2010 are expected to become 2-years return period events under the climate and land use change scenarios considered. The planned Adjarala dam showed potentials for extreme peak and flood extent reduction. However, flow duration curves revealed that the discharge of the river during low flow periods may also reduce if the Adjarala dam was built. In addition, the capacity of Benin and Togo to jointly address the impacts of the projected changes (climate change, LULCC, and the Adjarala dam) was investigated. The transboundary Water Cooperation Quotient (WCQ) was used to assess the current level of water cooperation between the two countries. Results indicate existing grounds for transboundary water cooperation (WCQ = 72/100). This finding updates the results published in 2017 by the Strategic Foresight Group (SFG). Mechanisms of data exchange, alternative dispute resolution, and frameworks for joint and sustainable coordination of flood, drought and ecosystems management are still lacking. <br /> This study contributes to a better understanding of how changes in climate and land use would influence floods in the Mono river catchment, and demonstrates the readiness of Benin and Togo to collaboratively manage the impacts. The findings of this thesis serve as basis for adaptation measures identification and the establishment of an active transboundary water cooperation between Benin and Togo, in order to sustainably manage floods in the Mono river catchment.