Influence of multimodality soil on their hydrodynamic behavior: case of soils of the unsaturated zone of Allada plateau

Abstract

The derivative function f of the cumulative particle-size distribution curve of certain soils has two (02) local maxima (two modes) in weight percentage for the particle-size ranges of sand and clay (one in the range of clays and the other in the range of sands). Are referred to as bimodal soils, soils with a particle-size distribution function F having two (02) inflection points, the first in the range of clays and the second in the range of sands. Such soils are already part of multiporous soils that have multimodal behavior. Haverkamp and Reggiani (2002), have established for soils whose particle-size have a monomodal behavior a shape similarity between the cumulative particle-size distribution curve and the water retention curve h ( ). A soil whose particle-size distribution has two modes (bimodal distribution of soil particle-size) usually poses enormous difficulties to soil physicists. Indeed, this character, when already achieves two in soil (bimodal soils), results in nine (09) unknown for the same water retention curve model with mathematical-physical basic, making it very difficult if not impossible to determine hydrodynamic parameters. So, monomadal soils facilitate the study of water transfers in the soil. The hydrodynamic models are available for these types of soils and involve more than 4 unknowns. And with the initial and boundary conditions, they allow the indeterminations up without difficulty. Now the work of Tomasella and Hodnett (1998; 2000; 2002) appears to link the modal character of the soil to climate zones to which they belong. They have come to say that the monomodal soils are specific to temperate regions and bimodal soils are specific to tropical or subtropical regions. The objective of the study is to test the hypothesis of bimodality for the case of soils of the unsaturated zone of Allada plateau located in the intertropical zone and to confirm the applicability of Brooks and Corey (1964) and van Genuchten (1980) models considered in this study and which are only valid on monomodal soils. The analysis according to USDA classification of the main soils of the study area namely haplic Acrisols, umbric Fluvisols and ferric Acrisols and their representation according to the soil textural triangle with an associated bimodal zone revealed 66 % of monomodal soils and 34% of bimodal soils in the study area. The comparative analysis of results with those of similar studies of the european databases and the Maheshwaram watershed in South India (subtropical) and the Ouémé watershed (subhumid) in Benin (De Condappa, 2006; Giertz and Diekkrüger, 2003), has validated mainly monomodal soils, especially within the B horizons. Which invalidates the hypothesis of Tomasella and Hodnett for this zone and confirms the validity of hydrodynamic models mentioned in the context of this study.