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 km 2 , located in the north of the district of Abidjan (South-East from Côte 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 Pro-grams). 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 R 2 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 R 2 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.


Introduction
The fresh water available on the earth's surface represents a low percentage, around 0.76% [1]. Despite this low availability, it has always been an essential element in human societies. Great civilization flourished when it was abundant, secured its access, or died out because of its absence. Today, human activities are subjecting resources to increasing pressure, until posing real problems of availability [2]. The uses of water for drinking water supply (DWS), industrial or agricultural needs, have led to unsustainable management of water resources.
As proof, the availability of water per inhabitant has steadily decreased over the past decades, with significant inequality between the countries of the north and those of the south [2]. Côte d'Ivoire, like these countries, is facing this decline in the availability of its water resources under the pressures of climate change, human activities and population growth. For illustrative purposes, the Abidjan groundwater which serves as a drinking water supply for the Abidjan population is currently under strong pressure and can no longer meet the water needs of the populations of the Autonomous District of Abidjan. Indeed, a deficit of 58 million m 3 /year of water must be mobilized to meet the pressing needs of this constantly growing population. The Ivorian government, therefore, considered exploiting the Aghien lagoon to make up for this deficit. The Aghien lagoon watershed, with an approximate area of 365 km 2 , is located in the South-East of the Côte d'Ivoire, precisely in the north of the district of Abidjan. The Mé, Djbi and Bété rivers are the main tributaries of the Aghien lagoon ( Figure 1). The Aghien lagoon has great potential for a set of uses. Indeed, it constitutes a living space for the riparian populations, a buffer zone between the continent and the ocean and a water reserve for the addiction of drinking water and agriculture in the medium and long term. It was therefore retained by the Ivorian authorities as an alternative resource to make up for the water deficit estimated at 58 million m 3 /year in the economic capital Abidjan [3]. In order to help Ivorian decision-makers to develop policies for better management of this lagoon in a context of climate change, a study on its quantitative availability was carried out by [4]. According to this study, the flow of the Aghien lagoon would decrease on average from 10% (RCP 4.5) to 17% (RCP 8.5) in the future. In this same dynamic of helping decision-makers to effectively manage the Aghien lagoon, a study on its qualitative availability in this context of climate change is discussed in this paper. The objective of this paper is to assess the potential impact of climate change on sediment yield in the watershed of the Aghien lagoon using the SWAT (Soil and Water Assessment Tools) hydrological model coupled with six (6) climate models of the CORDEX-Africa project.

Data
The data used are described in the manuscript of [5]. These are: the digital Elevation model (DEM) data, the soil map and soil type data, the land use data of the Aghien lagoon watershed, meteorological data (precipitation, temperatures) from the period 1951 to 1998 and future climate data from 6 CORDEX-AFRIQUE climate models for the RCP4.5 and RCP8.5 scenarios. These six (6) climate models are ISPL-CM5, HADGEM2, MIROC, MPI, CNRM and EC-EARTH. The delta method was required to correct for the bias of the six climate models [4]. All of these data were used to set up the hydrological SWAT model and to project the future flow [4]. Monthly sediment measurement data was also required. These measurements were taken (11 sampling points) at Anyama, Abobo debacardère, Akandjé, Aghien and Akouyaté ( Figure 2). The surface

SWAT-CUP (SWAT Calibration Uncertainty Procedures) is a platform that
brings together different calibration and uncertainty analysis procedures for the SWAT model. SWAT-CUP brings together three procedures: Generalized Likelihood Uncertainty Estimation (GLUE [6], "Parameter Solutions" (ParaSol [7], Sequential Uncertainty Fitting (SUFI-2 [8]). This package is used because of its adaptation to SWAT projects, its ergonomic interface, its processing speed and its features which allow to create graphs or compare the different results between them. The SUFI-2 procedure proposes about ten objective-functions to optimize the calibration of the model, among these the Nash-Sutcliffe criteria and the coefficient of determination R2 which are the two objective-functions most used in this field [9]. These two statistical parameters are used to measure the performance of the model. For the SWAT model uncertainty analysis, two bands are compared: the 95PPU for model simulation and the band representing the measured data plus its error. Two indices called "P-factor" and "R-factor" are used for the analysis of the uncertainties of the SWAT model [12]. The P-factor is the fraction of the measured data (plus its error) in the 95PPU band interval and ranges from 0 to 1, where 1 indicates 100% of the measured data within the model prediction uncertainty (1 indicates a perfect model simulation considering the uncertainty).
The quantity (1 -P-factor) could therefore be called the model error. A P-factor value greater than 0.7 or 0.75 could be adequate for this factor according to [12].
This of course depends on the scale of the project and the adequacy of the input data and calibration. The R factor, on the other hand, is the ratio of the average width of the 95PPU band and the standard deviation of the measured variable.
An R-factor value of less than 1.5, still depending on the scale of the project, would be desirable for this clue [8]. These two clues are used to judge the strength of calibration and validation. Bigger P-factor can be obtained at the expense of a bigger R-factor. Therefore, there is often a balance between the two factors. In the final iteration, when the acceptable values of R-factor and P-factor are reached, the parameter ranges are taken as calibrated parameters.

Calibration and Validation of SWAT Model
The calibration is carried out on the flow initially over the period 1960-1981 [5].
The Nash-Sutcliffe (NSE) and R 2 determination coefficients in calibration (1960)(1961)(1962)(1963)(1964)(1965)(1966)(1967)(1968)(1969)) are 0.807 and 0.809, respectively. In validation (1970-1981) NSE = 0.59 and R 2 = 0.64. These values are greater than the guide value (0.5). According to [10] coefficients of NSE and R 2 greater than 0.5 are acceptable and demonstrates the good performance of the SWAT model in reproducing the discharge. Thus, in view of values of the NSE and R 2 coefficients obtained in calibration and validation, the SWAT model reproduced the discharge fairly faithfully [5]. Then, in a second step, the calibration was carried out on the sediments flows. In order to improve the reproduction of the observed point sediment data, the most sensitive model quality parameters ( phase, calibration took place in two steps. First, a sensitivity analysis selects the most sensitive parameters among the eleven. Second, these parameters were adjusted to obtain a good matching between the sediment data observed and those simulated by the SWAT model.

1) Sensitivity analysis
The optimization process which reflects the sensitivity of the 11 parameters of an objective function (the Nash-Sutcliffe coefficient (NS > 0.5) and the coefficient of determination R 2 > 0.5, R-factor and P-factor). P-factor values greater than 0.7 and R-factor values less than 1.5 are recommended to obtain good modeling [13]. It should be noted that the sediment calibration was carried out throughout the 2014-2015 period for which the sediment data were available. The model was therefore not validated for sediment loadings because there was not a long series of sediment data in the basin studied to do it. The sensitivity analysis, calibration and validation procedure is shown schematized in Figure 3 below.

3) Prospective simulation of sediment yield
The outputs of the six corrected climate models are edited in "txt" format and The calibration parameters, the soil map and land use were kept constant.

Sediment Sensitivity Analysis
The sensitivity analysis makes it possible to identify the main parameters of the These are shown in Table 3, ranked from most sensitive to least sensitive.

Evolution of Sediment Yield in the Watershed of the Aghien Lagoon
The

Discussion
The results of modeling the impacts of climate change on the quantitative availability of the Aghien lagoon obtained showed that the average of the 6 (six) CORDEX AFRIQUE climate models gives a decrease in precipitation, the flow of the lagoon and an increase in evapotranspiration in the future, which will accentuate the reduction in flows in the catchment area of the Aghien lagoon [4].
In fact, the flow of the Aghien lagoon would decrease by 8%, 10% and nearly  The study by [14] and [15] indicated that increased temperature can worsen the rate of soil erosion through its influence on vegetation and alteration, which can increase sediment flow in rivers. The study conducted by [15]  ing these months may be due to the significant influence of increased evapotranspiration and crop growth processes under warmer climate, as mentioned by [16]. Increased sediment loads can intensify many problems associated with the accelerated loss of lagoon storage through sedimentation and siltation of water distribution systems. This loss is associated with a reduction in the carrying capacity and increased water turbidity of the lagoon [17]. The increase in water turbidity can also have negative consequences on animal and vegetable biodiversity of the Aghien lagoon. Thus, in view of the evolution of sediment flows, a probable degradation of the water quality of the Aghien lagoon mainly caused by the increase in sediment could occur in the future.

Conclusion
The SWAT model was used to reproduce the sediment yield of the Aghien lagoon watershed located in the southeast of Côte d'Ivoire in the north of the district of Abidjan. There were limited sediment data available. The data available on sediment flows were limited to the period from June 2014 to January 2015.
The model was therefore calibrated over this entire period and therefore was not tested over a validation period. Thus, the calibration of the SWAT model for sediments using SWAT-Cup version 5.1.6 was satisfactory with a Nash-Sutcliffe

Recommendation
Solutions based on "biological engineering" could be implemented. For example, consideration should now be given to reducing sediment flow by using grass strips to limit runoff and prevent soil erosion. Shading in the immediate envi- ronment of the Aghien lagoon could also be ensured by afforestation of the banks in order to limit the increase in temperature which could lead to water stress, which will reduce plant growth and therefore worsen the rate of erosion of the ground. The decision-makers and managers of the Aghien lagoon basin should take into account the trends described by our results in the implementation of adaptation measures to climate change to ensure the sustainability of the said lagoon. Moreover, these results seem to be transposable to other watersheds with comparable geographical characteristics, particularly in the West African zone.