Management of the Manantali Dam on the Senegal River: quantitative analysis of a conflict of objectives

Located on the Senegal River in West Africa, the Manantali Dam has been given different rival objectives: energy production and low-flow augmentation on the one hand; flood support on the other. This study examines a large range of management strategies and quantifies the results for each objective, depending on the available water resources. The operational management of the dam is numerically simulated over the 1970-2000 period. The low flow augmentation is considered to satisfy the needs of irrigated agriculture (supposed equal, doubled or tripled compared with the actual needs) with a high guaranty. The flood support is considered for different objectives of traditional flood-plain farming areas (translated into flow hydrographs) and for different thresholds conditioning water release. The results obtained for 216 simulated management scenarios show a notable linear relationship between the mean annual energy production and the mean annual area of traditional cropping. In comparison with a base scenario with no flood support, the increase of I ha of mean traditional cultivation area resulting from flood support involves a loss of mean electricity production, which amounts to 614 W in the present situation and would increase to 789 W if the irrigation needs trebled. The high capacity of the Manantali Reservoir makes it possible to reach the following objectives: meeting the present irrigation needs; annual flood support allowing to 52 000 ha of mean flood-plain cultivation area (and a minimum of 45 000 ha); mean annual energy production of 768 GWh (and a minimum of 372 GWh).