Résumé
The resurgence of water shortages in the autonomous district of Abidjan, a megalopolis of over 5 million inhabitants, has prompted decision-makers to look for new sources of palliative water, such as the Aghien lagoon. The main aim of this study is to estimate the water potential of the Aghien lagoon catchment area for supplying drinking water to the Abidjan district. The data used, consisting of hydroclimatic and suspended matter data, were collected over three years of observations (2015, 2016 and 2017) and satellite images from 2016 and 2017. The methodological approach adopted is to analyse the hydroclimatic data in order to characterise seasonal variability and its impact on runoff, on the one hand, and to quantify the total volume of water passing through the sub-catchments, on the other. The universal soil loss equation was then integrated into the GIS to produce thematic soil loss maps. Finally, ATHYS software and the GR4J model were used to reproduce the hydrological behaviour of the Bété and Djibi sub-catchments. The GR4J model was used to determine the groundwater potential of these basins. Over the study period, the flow coefficient was high at 24.2% in 2015 and 27.76% in 2017 for the Bété and Mé rivers respectively, and at 27.8% in 2015 and 29.6% in 2016 for the Djibi, but low at 19.9% for the Bété and 21.6% for the Mé in 2016 and 23.1% for the Djibi in 2017. This is favoured by a surface condition that has been fairly degraded by the intensification of agricultural activities and urbanisation. The specific flow of the tributaries of the Aghien lagoon shows that the Djibi produces more runoff per unit area than the Bété and Mé. The cumulative flow of the main Bété and Djibi tributaries of the Aghien lagoon was 105.15 Mm3 , 87.33 Mm3 and 136.77 Mm3 respectively in 2015, 2016 and 2017, while the lagoon's storage volume was estimated at 75 Mm3 . The Bété and Djibi rivers can ensure the sustainability of the Aghien lagoon. Annual soil losses were 49871.52 t/yr, 68045.04 t/yr and 98280 t/yr, 124362 t/yr respectively in the Djibi and Bété rivers over the same study period. The results of the suspended solids analysis made it possible to identify the various seasonal inputs. However, annual solid inputs are estimated at 637,07.103 t/year, 348,3.103 t/year and 7063,03.103 t/year with a specific degradation of 2817.98 t/km²/year, 4944.45 t/km²/year and 1784.47 t/km²/year respectively at the Bété, Djibi and Mé rivers from 2015 to 2017. Hydrological modelling using ATHYS software and the GR4J model was used to simulate the flows in the tributaries of the Aghien lagoon. Analysis of the Nash criterion applied to the runoff yields satisfactory values. The hydrological balance shows that more than 34.24% of rainfall infiltrated the Djibi. At the Bété, 29.83% was used to recharge the water table in 2017. This shows that the Aghien lagoon can be used as a source of drinking water to supply the Abidjan district.