Bart Schultz
Between June and October, the average annual rainfall is approximately 600 millimetres. Rainfall increases significantly towards the south, in some areas exceeding 1,500 millimetres per year. The northernmost part of the country has a desert climate, the central part a semi-arid climate, and the southernmost part a tropical wet and dry climate. The landscape consists primarily of rolling sandy plains of the western Sahel, which extend southeast ward. The northern border is formed by the Senegal River.
The mangrove areas are primarily located along the banks and beaches in the estuaries of the Casamance, Saloum, and Senegal rivers, covering a total area of approximately 500,000 hectares. The average tidal amplitudes in the estuaries range from 1.0 to 1.5 metres, with spring tide amplitudes of up to 2.5 metres. Parts of the mangrove areas have been replaced by tannes, which are essentially traditional polders where the mangroves were cut down for rice paddies or salt pans. Because the soils in these areas often became acidic after reclamation, soil surveys have been conducted since 1972. Based on these surveys, soil maps have been made and improvement measures implemented for specific reclamation projects.
Rice cultivation in the mangrove zone of West Africa began in the mid-18th century. Traditional systems are still the most widespread, in Senegal the Diola system. This system consists of small basins or strips of land surrounded by dikes. Within these polders, rice is grown on ridges. Discharge of excess water during low tide is necessary, among other things, to flush out salts and acids that accumulate in the polders during the dry season. The Diola systems functioned well until persistent dry periods began in 1969. The hardest-hit areas are primarily along the northern and drier coasts of West Africa, including Senegal.
The dikes in traditional polders were typically 1.0 to 1.5 metres high, with a crest level 0.20 metres above the maximum spring tide. For discharge and water intake sluices, hollow tree trunks, preferably palm trees, were placed in the dike body at various levels, allowing for varying water levels in the polders. The lower sluice was constructed below the surface, with its outlet in a trench. Fibrous abutments impregnated with clay and sometimes wooden culverts were used for sealing.
After land reclamation, a framework of ditches was dug, and the excavated clods of topsoil were placed on the intervening areas to form raised beds, each a few square metres. Land reclamation typically began on the landward side of the floodplain and later extended to the riverbank. In areas with erosive tidal currents, the rice polders were constructed behind a fringe of mangrove forests.
Since the middle of the last century, large-scale land reclamation projects for rice cultivation have been implemented. The most important were located in the lower reaches of the Casamance River. Upstream of the tidal areas, the channels were dammed to regulate the supply of freshwater. Concrete dams with wooden sluices were built in the riverbeds to keep out saltwater and create sufficient discharge capacity for excess water.
A 1972 World Bank project report mentions the Dagana (3,000 hectares), Debi (1,025 hectares), and Lampsar (10,000 hectares) polders. The project included the construction of an irrigation system in the Dagana polder, enabling year-round rice cultivation on 2,730 hectares. In the Debi polder, the project involved improving water management and increasing the irrigated area from 260 hectares to 1,025 hectares.
Other publications also provided names and surface areas of polders. These include the 10 hectare Medina Polder and the 30 hectare Dieba Polder in the Casamance delta. Along the Senegal River lies the approximately 1,000 hectare Richard Toll Polder (Casier Polder). The 1,000 hectare Nianga Polder is also mentioned. A 2005 African Development Fund report mentions a project to be implemented in the Lower Casamance region in areas with dikes but without regular drainage. The dikes would be reinforced and equipped with discharge sluices. The sources are listed in the Country Document of Senegal.
At the request of the Senegal River Basin Development Authority, the Netherlands Enterprise Agency (RVO) submitted proposals for the construction of eight polders in the plains along the Senegal River. These polders would consist of approximately 63 kilometres of dikes, 60 kilometres of drainage canals, eight pumping stations, and eight irrigation water inlets. Four of these polders would be constructed on the Senegalese side of the river and four on the Mauritanian side. The polders would be located near the towns of Rosso and Richard Toll, along existing agricultural land.
Another key component of the project was the removal of typha – bulrush, an invasive species in Senegal – and soil improvement in the new polders. Once typha no longer obstructed the watercourses, local residents would have easier access to water. The polders were intended to create new agricultural land on which rice and other crops, such as vegetables, could be grown. As part of this program, the consulting firm Royal HaskoningDHV developed a master plan for the development of the Senegalese delta and designs for eight polders, totalling between 3,000 and 4,000 hectares. It is difficult to determine whether these polders were actually realised.
