Rice and Salinity in the Sahel

Rice lands in the Sahel are once again being threatened. This time, however, it is not drought or pests but a more preventable problem, salinization. Since 1990, the results of the structural adjustments recommended by the International Monetary Fund and the World Bank are becoming evident. These agencies have urged the Sahelian countries of Senegal, Mauritania, Niger, Mali and Burkina Faso to transfer rice production from public to private hands. In the salinity-prone Senegal River delta, privatization of the rice sector has resulted in the initiation of cheap irrigation schemes which frequently are so poorly constructed that they sometimes last for only one season. Poor water management, the absence of efficient drainage and the high cost of irrigation have led to rising groundwater tables and soil sodification and alkalinization, leaving areas unsuitable for future cultivation.

Thousands of hectares in the Sahelian floodplains are being used to grow rice. The enormous demand for the crop has led to a doubling of the region's output from 352,000 tons of rice (1980) to approximately 800,000 tons (1992), replacing millet and sorghum as the staple crop in Sahelian cities and some rural areas. About 150,000 hectares of the Sahel are under rice cultivation and an equal area is cultivated with partial water control. Although this represents only 5% of the area under rice cultivation in West Africa, the Sahelian fields yield about 12% of the region's total rice output. Continued increases in rice production were anticipated with the implementation of structural adjustments. However, it now seems that this increase in rice production is unsustainable, and that current cultivation methods could damage the land's production capabilities.

Rising Water

The annual rainfall in the Sahel is between 150 and 600 mm -- insufficient for the cultivation of upland or rainfed-lowland rice. The clay floodplains associated with the major, permanent rivers in the Sahel, however, are suited to grow rice under flooded conditions. A canal irrigation system is used, typically with an elevated principal tributary. Water is pumped from the rivers or diverted from dams.

Less than one-fourth of the more fertile and irrigable clay soils in the Sahel have been developed for irrigation. But wherever water and markets are easily accessible, pressure on the land is high. Correspondingly, farmers grow rice even on sandy soils which are not suited for irrigation. Uncontrolled irrigation of plots with highly permeable soils, particularly since the liberalization of production, and the growing practice of keeping principal canals flooded year-round, have resulted in a dramatic rise in the groundwater table.

In the 1940s, groundwater tables in the largest irrigation schemes in Mali were between 30 and 50 meters deep. They oscillated with the seasons but always returned to their original level. Today, groundwater tables are around 1 meter below the surface at the center of irrigation systems with little to no oscillation. Even 20 kilometers away from the irrigated lands, the water table has been rising at a rate of 0.5 meters annually.

The agronomic consequences of the rising water table are grave for two reasons. First, the groundwater is high in dissolved salts due to leachate, and salinizes the topsoil by capillary rise. This is generally irreversible at sites where topography does not permit lateral drainage. Secondly, alternative crops such as vegetables, maize, sugar cane and cotton can no longer be grown due to a lack of oxygen in the soil. On lands under the Niger Office in Mali, people are largely reduced to planting a rice monoculture.

Saline and Alkaline Soils

In Mauritania and the Sahelian parts of Senegal, the rice fields lie on the banks of the Senegal river. Most of the cultivated surfaces were prone to seasonal transgressions of saltwater from the sea until completion of two dam projects in the late 1980s, the Diama and the Manantali dams. River regulation reduced farmers' opportunities for traditional flood-recession farming, but enabled the growing of irrigated crops throughout the year.

For private rice producers who took advantage of market liberalization, cost of irrigation is roughly proportional to water consumption. Because they operate their own pumps, they hesitate to remove the valuable water from their plots, leaving it to evaporate and saline concentrations to accumulate.

Even highly salinized soils can be fully regenerated with appropriate water management. But a very disturbing development was observed on the irrigated areas in Mali. Previously acid clay soils turned alkaline, almost entirely losing their organic matter content and physical structure through solubilization. Sodification/alkalinization, a type of salinization, is associated with nearly irreversible soil degradation. It is driven by the latent alkalinity and sodium adsorption ratio (SAR) of irrigation water which, over years or decades, replaces the exchangeable bases in the minerals with sodium and precipitates insoluble carbonate.

Water table samples from Mali were found to have SAR values of 10 to 50, and a pH level ranging from just below 8.5 to 10. Over the past 30 years, the porosity of samples collected at Mali's Niger Office has declined by 30%, while the permeability has been halved. The soils have formed a compact concrete-like crust which makes plowing, planting and root penetration difficult. The high pH, due to sodification/alkalinization, also reduces plant nutrient availability and increased nitrogen losses through volatilization.

Both sodification/alkalinization and rising groundwater tables were not anticipated when the huge irrigation schemes in Mali were designed without provision for drainage. The groundwater level seemed too deep and the irrigation water too pure to pose any such risks. It is now clear that the waters of the Niger and Senegal rivers carry substantial alkalinity, and their salt content sometimes increases markedly between the main rivers and the actual irrigation site. Alkalinization and sodification are more advanced at the Niger Office than elsewhere in the Sahel because these are the oldest irrigation systems in the area. The same development, however, is likely to occur in the middle and upper valley of the Senegal River if no preventive measures are taken.


Although the causes of soil degradation in Sahelian rice irrigation systems are diverse, preventive and regenerative solutions generally hinge on improved water and irrigation management. Soil regeneration is economically possible where salinity is not associated with severe alkalinity, and where a saline groundwater table is not interactive with the topsoil. But some highly degraded soils in the Niger Office's areas in Mali, sometimes with a pH of 9 or more, can only be regenerated with large amounts of organic matter and gypsum or the production of halophytes such as amshot, Echinochloa stagninum (Ag-Sieve Vol. 5 No. 5).

Physical measures to prevent soil degradation and improve irrigation management consist of discouraging:

  • rice cultivation on soils with high percolation,
  • rice double cropping where the groundwater level is already high, and
  • farmers from drying plots by evaportion.

    Systematic drainage-irrigation cycles during a cropping season and evacuation of water from irrigation canals during the off season need to be encouraged.

    Farmers need to be educated on the risks of poor water and soil management. More research is needed on decision-making processes and land tenure rights in rice growing communities before technologies will be adopted that bring about long-term economic returns.

    Roger Bertrand, Bassirou Keita, Mamadou Kabirou N'Diaye. 1993. La degradation des sols des perimetres irrigues des grandes vallees sud-sahariennes (cas de l'Office du Niger au Mali), Cahiers Agricultures. 2: 318-29.

    WARDA, Annual Report 1991, 1992 and 1993, West Africa Rice Development Association. West Africa Rice Development Association, 01 Bouaké, B.P. 2551, Ivory Coast.


    Dr. Kouamé Miézan, Sahel Irrigated Rice Program


    Dr. Michael Dingkuhn Senior Physiologist WARDA, B.P. 96 St. Louis, SENEGAL Tel: 221-62-64-93 Fax: 221-62-64-91