Soil salinization processes in rice irrigation schemes in the Senegal River Delta

Soil salinization constitutes a major threat to irrigated agriculture (mainly rice, Oryza sativa L.) in the Senegal River Delta. It is generally hypothesized that salinization is caused by (i) capillary rise from a saline water table and (ii) concentration of salts in the field due to lack of adequate drainage facilities. The impact of field water management and rice cropping intensity on salinization in the Delta was determined using an electromagnetic conductivity meter (Geonics EM38). More than 4000 measurements were made in 40 rice fields on a typical heavy clay soil (Vertic Xerofluvent). Thirty EM38 measurements per field (0.25 ha) estimated average field soil salinity with a relative error of 20%. A multiple linear regression model based on EM38 readings explained 60 to 75% of the variability in conductivity of 1:5 saturation extracts at 0- to 5-, 10- to 15-, and 30- to 35-cm depths. Higher cropping intensity limited upward salt transport from the water table. Average horizontal and vertical EM38 measurements increased in the following order: two rice crops per year with drainage: 0.73 and 0.98 dS [m.sup.-1]; one rice crop per year with drainage: 1.26 and 1.76 dS [m.sup.-1]; one rice crop per year without drainage: 2.23 and 2.98 dS [m.sup.-1]; and abandoned fields: 4.77 and 4.29 dS [m.sup.-1]. Results indicate a beneficial effect of flooded rice on salinity for this type of heavy clay soil. Irrigation development in the area needs to be accompanied by monitoring of water table depth.