SOIL SALINIZATION PROCESSES IN RICE IRRIGATION SCHEMES IN THE SENEGALRIVER DELTA

Soil salinization constitutes a major threat to irrigated agriculture (mainly rice, Oryza sativa L.) in the Senegal River Delta. It is gener ally hypothesized that salinization is caused by (i) capillary rise fr om a saline water table and (ii) concentration of salts in the field d ue to lack of adequate drainage facilities. The impact of field water management and rice cropping intensity on salinization in the Delta wa s determined using an electromagnetic conductivity meter (Geonics EM38 ). More than 4000 measurements were made in 40 rice fields on a typica l heavy clay soil (Vertic Xerofluvent). Thirty EM38 measurements per f ield (0.25 ha) estimated average field soil salinity with a relative e rror of 20%. A multiple linear regression model based on EM38 readings explained 60 to 75% of the variability in conductivity of 1:5 saturat ion 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 fo llowing order: two rice crops per year with drainage: 0.73 and 0.98 dS m(-1); one rice crop per year with drainage: 1.26 and 1.76 dS m(-1); one rice crop per gear without drainage: 2.23 and 2.98 dS m(-1); and a bandoned fields: 4.77 and 4.29 dS m(-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 monitori ng of water table depth.