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Modélisation hydrogéochimique : une méthode adaptée à la prévision de la salinité, de l'alcalinité et de la sodicité des sols sous irrigation

Marlet S., Vallès V., Lafolie F., Condom N.. 1998. In : AISS; AFES. Congrès mondial des sciences du sol = World congress of soil science = Congreso mundial de la ciencia del suelo. Montpellier : CIRAD, p. 1-9. Congrès Mondial de Science du Sol. 16, 1998-08-20/1998-08-26, Montpellier (France).

Irrigation water quality is currently evaluated and classified from indicators whereas models can predict the effect of irrigation on soil salinity, alkalinity and sodicity. The geochemical model IRRICHEM is presented. It accounts for speciation of the major chemical components, precipitation or dissolution of few minerals and cation exchange. This model was calibrated in Niger and Pakistan. The simulation of irrigation water evaporation shows that the composition of the soil solution and exchange complex results from their ability to neutralize alkalinity through mineral precipitation during the concentration process. Residual alkalinity concept accounts for this mechanism and is a good indicator to predict if water turns saline or alkaline. Cationic exchange capacity acts as a buffer with respect to alkalinization. This geochemical model was coupled with two hydrological models to predict the time changes in salinity, alkalinity and sodicity according to water quality and irrigation management. The first one is a simple water balance model including irrigation amount and leaching fraction. It provides informations on the trend of soil chemical properties and can be easily performed at different scales when irrigation and drainage performances are known. The second one is a solute transport model based on Richard's equation and the convection dispersion equation. It allows to analyze the chemical alterations with respect to time and location in the profile. These approaches are illustrated by examples in Niger and Pakistan.

Mots-clés : salinisation du sol; irrigation; ph du sol; eau d'irrigation; modèle de simulation; géochimie; niger; pakistan

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