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Characterizing Nonrigid Aggregated Soil–Water Medium Using its Shrinkage Curve

^a Institut de Recherche pour le Développement (IRD), Centre de Montpellier, 911 av. Agropolis, BP64501, 34094 Montpellier, France
^b Laboratoire Environnement et Développement (LED), Université Paris VII, 2 place Jussieu, BP7071, 75251 Paris Cedex 5, France
^c Agricultural and Biological Engineering Dep., Purdue Univ., West Lafayette, IN 47906, USA

View larger version (22K): [in a new window]   Fig. 1. A typical continuously measured shrinkage curve (SC) of a reconstructed soil sample using three hundred points of measurement. Points A, B, C, D, E, and F are the transition points of the shrinkage phases as determined by parametric modeling of the SC according to the exponential model (XP) of Braudeau et al. (1999). LN, CV refers to as linear and curvilinear phases, respectively.

View larger version (83K): [in a new window]   Fig. 2. The pedostructure concept is shown taking into consideration the hierarchical functional levels of the soil medium.

View larger version (63K): [in a new window]   Fig. 3. Various configurations of air and water partitioning into the two pore systems, inter and intra primary peds, related to the shrinkage phases of a standard shrinkage curve (SC). wcn represents the condensed water lodged in interstitial pore site Scn, and wsw represents the swelling water lodged in interstitial pore site Ssw that can be interped macropores (m) or matrix micropores (µ). Asterisk means that the site is active and the corresponding numbers of the active sites are nsw and ncn. The various pools of water, wre, wbs, wst, wip, are represented with their domain of variation. The inflection points Ibs and Ist, and by extension Ire and Iip, are placed on the graph for representing the quasi-linear shrinkage phases delimited by the transition points (A, B, C, D, E, and F). Points N', M', and L' are the intersection points of the tangents at those linear phases and Points N, M, and L, the corresponding points of the SC.

View larger version (19K): [in a new window]   Fig. 4. Graphical representation of the specific volumes (Vµ and V), the specific pore volumes (Vpµ and Vpm), the air contents and the water pools (W, wre, wbs, wst, and wip) of the pedostructure starting from a measured SC. Vµ is the specific volume of primary peds, equal to (Vpµ + Vs) = [max (wre)w + wbs w + Vs], and w is the water bulk density.

View larger version (35K): [in a new window]   Fig. 5. Continuously measured and modeled shrinkage curves (SCs) for three alluvial soils in the Lower Majerda Valley, Tunisia, using the retractometry procedure. The graph on the right hand side shows the fit for the whole data range. One out of three points is represented. The parameters determined by software for soil hydrostructural characterization (CARHYS) are shown in Table 4. The cumulative variation curves of the water pools, wre, wbs, wst, and wip, added to Vs are represented referring to as (1), (2), (3), and (4), respectively. Points A, B, C, D, E, and F are the transition points of the shrinkage phases defined by the exponential model (XP) and represented on the corresponding water pools curves.

View larger version (21K): [in a new window]   Fig. 6. Shrinkage curves (SCs) and the corresponding micro and macropore volume (Vpµ and Vpm) of two clayey soils in Senegal: (a) vertisol, and (b) ferralitic soil. For the swelling soil case (Fig. 6a) Curve 1 is the modeled wbs water pool contribution (Kbswbs + VA) to the SC when Kst = 0.4 and Curve 2 when Kst = 0. (Points A, B, C, and D are the initial parameters determined by the exponential model (XP) with taking Kst = 0.4; Points C'', D'', E'', and F'' were obtained by CARHYS using the pedostructure (model for SCs) model with Kst = 0).

View larger version (29K): [in a new window]   Fig. 7. An example of how the exponential functions of the exponential model (Eq. [A1]) simulates the curvilinear part of the curve wbs = f(W) of the vertisol (Eq. [16]), and how it can thus determine the effective beginning and end of variation of the curve. The mean deviations between the two fitted curves in the ranges {WD–WC} and {WB–WA} are 3.5 x 10–4 and 1.4 x 10–4 dm3 kg–1, respectively.