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