Modelling water and salt transfer in rice plantations in the Senegal river delta
Soil salinisation is a major hazard of the extension of irrigation in the world. In the Delta of the Senegal River Valley, a highly saline groundwater close to the soil surface increases this hazard. A water and salt modeling by means of the Hydrus code has been performed to evaluate this hazard. Two soil types and two crop systems, a single and a double one, have been considered. The calculations have been performed on the complete agricultural cycle to determine water flows going down during the irrigation phase, and water and salt flows ascending to the soil surface during the irrigation and non irrigation phases. The modeling shows that: 1) the recharge due to the irrigation period induces an elevation of the water table of about 50 cm; 2) an unsaturated zone persists between the flooded surface and the water table; 3) the cumulative infiltration is two times higher in the double crop rice system than in the single crop system; 4) the high evaporation during the dry phase causes an ascension of from 5.5 to 6 t of salts per ha and per year according to the soil type and the crop system. This modeling, which was based on field measurements of hydrodynamic properties, has to be validated by field observations of the fluxes. Some processes such as air trapping should be tested. Piezometer monitoring over the past 10 years indicates a slight rise in the saline water table. All these results demonstrate the relevance of setting up a surficial drainage system for the flooded irrigation fields, to avoid an elevation of the water table and to evacuate salts from the soil surface, in order to perpetuate agriculture in the Delta of the Senegal River Valley.