Potential Impact of Climate Change on the Sediment Fluxes of a Watershed in West Africa: Cas of the Aghien Lagoon, Côte d’Ivoire

The semi-distributed SWAT (Soil and Water Assessment Tools) model was used in this study to model the sediment yield in the watershed of the Aghien lagoon with an area of 365 km2, located in the north of the district of Abidjan (South-East from C?te d’Ivoire). A sensitivity and uncertainty analysis, as well as calibration of the SWAT model, was conducted using the Sequential Uncertainty Adjustment Procedure (SUFI-2) which is one of the programs interfaced with SWAT in the SWAT-Cup package (SWAT-Calibration-Uncertainty Programs). Five parameters of the SWAT model were found to be more sensitive to sediment fluxes. These have been modified (calibration) sparingly in order to improve the reproduction of observed sediments data. Two measures were used to assess the uncertainty analysis of the model: P-factor and R-factor. The R2 and Nash-Sutcliffe (NS) coefficients of determination were used to assess the quality of the calibration. The P-factor obtained is 0.58 and the R-factor is 2.28. The NS and R2 coefficients in calibration over the period from June 2014 to January 2015 are 0.51 and 0.86 respectively. These values indicate correct consideration of uncertainties by the model and satisfactory calibration of the SWAT model for solid fluxes. Then, the model was used to simulate the sediment fluxes at the horizons 2040 (2035-2056), 2060 (2057-2078) and 2080 (2079-2100) in order to assess the impact of climate change on sediments in the watershed of the Aghien lagoon. The results indicate that sediment fluxes could increase in the future under the RCP 4.5 and RCP 8.5 scenarios. With RCP 4.5, sediment fluxes would increase on average by 14.42%. They could increase by 17.95% on average under RCP 8.5.

Estimation of the Impact of Climate Change on Water Resources Using a Deterministic Distributed Hydrological Model in Cote d’Ivoire:Case of the Aghien Lagoon

This work aims to evaluate the impact of climate change on the quantitative availability of the Aghien lagoon located in the north of the Abidjan district in Côte d’Ivoire. In the first step, the semi-distributed SWAT (Soil and Water Assessment Tools) based physical model (Arnold et al., 1998) was calibrated and validated at the monthly time step over the period 1960-1981, in the Me watershed for which data from flow rates are available. SWAT was then applied on the watershed of the lagoon of Aghien which is ungauged but for which the challenges are considerable for the drinking water supply of the Abidjanese population. In the second step, the gross outputs (precipitation, temperatures) of six climate models of the CORDEX-Africa project under the “Representative Concentration Pathways” (RCP 4.5 and RCP 8.5) scenarios were corrected using the delta method. These corrected outputs were used at the SWAT model input to project the impact of climate change on the flow of the Aghien lagoon to horizons 2040 (2035-2056), 2060 (2057-2078) and 2080 (2079-2100). The projections made on these different horizons were compared with the simulated flow over the period 1960-1981. The results show a sensible decrease in the annual flow of the Aghien lagoon compared to the reference period (1960-1981). Under the medium assumption (RCP 4.5), the models predict a decrease in the annual discharge almost 10% on average. Under the pessimistic hypothesis (RCP 8.5), the average annual discharge should decrease by more than 17%. On a monthly basis, flows in August and September would increase by more than 80% and those in October and November would increase by more than 20% in both RCP scenarios.