Modeling the Impacts of Climate Change, Land Use Change and Dam Management on Water Resource in West Africa: Case of the Mono River Basin, Togo-Benin

Abstract

A good understanding of climate change, land-use change and dam management impacts on water balance components must enable the development of sustainable water resources strategies in West Africa. The Mono River Basin (MRB) in West Africa is a river basin shared with Togo and Benin Republics. Its contributions to the socio economic development of the region are needless to emphasize. But its course has been lately subject to several human activities such as dam construction that may have modified its functions. The present study objectives are to analyze the accuracy of CILSS, ESA and Globeland30 land cover datasets between 1975 and 2013, to investigate climate change detection via trend analysis on the hydro-climatic datasets over the period of 1961 to 2016, to simulate and compare discharge using empirical lumped, and to assess water balance component changes using semi- distributed hydrological models over two baseline periods in the Mono River Basin. The methodological approaches consist in land cover reclassification and accuracy evaluation. The three datasets were used to predict future LULC changes between 2020 and 2027 using the Terrset Land Change Modeler. Afterward, the non-parametric Mann Kendall (MK) trend analysis of historical hydro-climatic data was applied, and these data sets were used as inputs for the lumped models, GR4J (Génie Rural à 4 paramètres Journaliers), IHACRES (Identification of unit Hydrographs and Component flows from Rainfall, Evapotranspiration and Stream data) and SWAT (Soil, and Water Assessment Tool) simulations are undertaken. The results indicate for the accurate CILSS data set, there are an increase of 30.97% of cropland area, the losses of (6.91%) of forest area and the decrease of (25.59%) of savanna between 1975 to 2013 and are explained by the increase in population and their food demand. The climate change detection analysis reveals positive and negative trends of hydro-climatic data over MRB from 1961 to 2016. Mean temperatures increase at α = 0.01 and 0.05 significance levels in the three stations investigated whereas a negative non-significant trend is noticed for average rainfall. Meanwhile, the discharge presents a significant seasonal and annual trend for three gauge stations investigated. An acceptable accuracy (R2 ≥ 0.9) of validated ensemble climate models allow the computation of extreme climate indices under RCP4.5 and RCP8.5 scenarios which shows a significant annual trend of some climate extreme indices of rainfall and temperature at three selected stations between 2020 and 2045. The hydrological modeling analysis indicates that the two lumped models discharge predictions are acceptable with evaluation efficiencies over pre-dam period (1964 – 1986) and more and less acceptable during post-dam period (1988-2010). IHACRES model was found to underestimating extreme high runoff in the downstream of MRB (1964-1986). Finally, the simulation with SWAT semi distributed model performances and uncertainty analysis show that there are good model performances (Calibration_1964-1975; R2> 0.60; KGE ≥ 0.70 et PBIAS ≤ ± 4.5; validation_1976 - 1986: KGE ≥ 0.50 and PBIAS ≤ ± 3.40) and acceptable parameters values range between 1964 and 1986. Conversely, there are poor model performances (calibration_1988-2000: KGE ≥ 0.60 and PBIAS ≤ ± 20); validation_2001-2011: KGE ≥0.24 and PBIAS ≤ ± 17.20) during the second period (1988-2010). An individual assessment of surface runoff, evapotranspiration and water yield components shows that its seasonal and annual variability depends on different land-use type change, climate conditions and also on the presence or not of reservoir in the watershed. This indicates that the implementation of the dam on the MRB in 1987 has affected the hydrological system of the river. Land use land cover change with the amplification of climate change are the others drivers accelerating this change. The study has proposed effective strategies for better planning and management of water resources in MRB such as land use management, climate change adaptation basin and Nangbéto reservoir reliable managements.