Characterizing Nonrigid Aggregated Soil-Water Medium Using its Shrinkage Curve

The properties of the soil-water medium are presented in the literature independent of its internal organization and operation. The objective of this study is to develop and test a conceptual model that used a continuously measured shrinkage curve (Se) to describe the functional organization of the soil-water medium. In this model, two functional porosities (micro and macro) are delineated and quantified by the Se. In addition, the equilibrium for four functional water pools is represented and parameterized by the Se. A set of eleven parameters was found necessary to model the seven phases of the se and to describe the corresponding soil hydrostructural changes. A method to accurately obtain the parameters of this model by a specific analysis of the continuously measured se is demonstrated. Examples of continuously measured and modeled SCS according to the pedostructure model (PS) are presented and discussed. M ODELlNG THE SC can be classified in three categories: 1. Those considering certain parts of the curve only such as the model of McGarry and Malafant (1987) and that of Giraldez et a1. (1983), which amounts to neglecting or fixing a certain number of parameters; 2. Those modeling its mathematical shape as in the case of Groenevelt and Grant (2001), where they used the sigmoid curve. A limited number of parameters are then necessary (two or three), but the selected parameters do not have any relevance to the processes being modeled; and 3. Those considering the entire SC but making assumptions on the shape of the phases, as is the case of the models of Tariq and Durnford (1993) and Braudeau et al. (J999): a straight line for the quasilinear parts, and polynomial or exponential function for the curvilinear parts. The parameters of the models are, therefore, representative points of the se and, contrary to the preceding case, are in connection with the process being analyzed. These points represent particular hydrostructural (see Appendix 1 for definitions) states of the soil and constitute the boundary conditions (V, W) for each shrinkage phase. However, these models do not provide any explanation for the corresponding configuration of the distribution of air, water, and solids in soil, which remains to be explained. The PS proposed in this paper addresses this issue by modeling the entire SC without any embodied assumptions related to its shape and functionality. E. Braudeau, lnstitut de Recherche pour le Developpernent (IRD), Centre de Montpellier, 911 av. Agropolis, BP645Ul, 34094 MODtpeJlier, France; J.-P. Frangi, Laboratoire Fnvironnernent cl Developpemeot (LED), Universite Paris VII, 2 place Jussieu, BP70~1, 75251 Paris Cedex 5, France; R.H. Mobtar, Agricultural and Biological Engineering Dep., Purdue L'niv., West Lafayette, Ij\; 47906. CSA. Received 23 Nov. 2002. *Corresponding author (erik.braudeauts'ird.fr}. Published in Soil Sci. Soc. Am, J 68:359-370 (2004). © Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA 359 Braudeau et al. (1999) developed a method for continuously measured SCs providing accurate estimate of the soil water content at each transition point of the shrinkage phases. They recognized seven shrinkage phases on a typical curve (Fig. 1), successively linear and curvilinear, and divided into four stages: interpedal, structural, basic, and residual. They suggested that the transition points (A, B, C, D, E, and F of Fig. 1) are characteristic of the hydrostructural behavior of the soil and they supplied a method to determine these points from soil volume change data using optimal fitting techniques. Appendix 2 describes the equations used by their model. However, these equations were empirically established and thus do not supply any information about the hydrostructural processes of the observed shrinkage phases of the curve; neither do they allow a better understanding of the mechanical, agronomic, and hydraulic properties associated with these shrinkage phases, To quantitatively characterize these properties, we need a physically based equation of the soil SC that is an equation linked to a conceptual model of the soil structure and water interaction. The nested soil structure has led morphologists to recognize the soil horizon as the basic typological entity composed of peds or aggregates, arranged in several structural levels (Brewer, 1964). The soil and water interactions act at these small scales and are the agents of soil physical properties that emerge at the macroscopic scale. When water is lost from a moist soil core sample, the sample experiences several mechanical states known as shrinkage phases (Yong and Warkentin, 1966). A few studies modeled the hydrostructural changes of soil samples during their shrinkage (Braudeau, 1988a, 1988b; Perrier et al., 1995; Voltz and Cabidoche, 1995; Chertkov, 2000), each using different hypotheses to represent the soil medium. Two major approaches exist for modeling soil water physical properties in soil matrix. The first views soil structure as an assembly of capillary tubes between and within rigid aggregates, forming only one or two levels of structure. Soil structure is then characterized by a monoor bimodal distribution of pore volume by class size of equivalent pore diameter (Coppola, 2000). The second approach considers soil structure as an arrangement of particles and swelling aggregates at various scales: clayey plasma, primary peds and assembly of primary peds that constitutes the soil fabric at the horiAbbreviations: CARHYS, software for soil hydrostructural characterization; PS, pedostructure (model of shrinkage curve); SC, shrinkage curve; SL, straight lines (method of shrinkage curve parameters determination); XP, exponential (model of shrinkage curve). Horizon Pedostructure Primary ped V=Vp"+V'" V'" =VI' +Vs Fig. 2. The ped ostructure con cept is shown tak ing into con sideration th e hier archical fun ction al levels of the so il med ium . 0.62 -l----~-.------_f----'---'-__1 0.0 0.1 0.2 0.3 Water content W (kg kg") Fig. 1. A typical continuously measured shrinkage curve (Se ) of a recon structed so il sample using thr ee hundred points of measurement. Points A, B, C, D, E, and F ar e the tra nsitio n points of th e shr inkage phases as determined by parametric mod elin g of the se according to th e exponenlialmodel (XP ) of Braudeau et al, (1999 ). LN , CV refers to as linear and curvilinear phases, resp ectivel y. zon le ve l (Braudeau, 1988a, 1988b; Colleui lle and Braudeau , 1996 ). F igure 2 sche matizes th is ap proach an d defines the pedostru cture as the so il-water fabr ic of a soi l horizon. The first poin t of view is the o ldes t and most widely spread. Man y equations arise from it, including the wellkn own ex pressi on re lating soil water po tential to po re dia met e r. Howeve r, this approach great ly restricts reality because it disregards bo th the hierarchy of soil st r uctu re and the specific properties of the water-plasma intera ct ion such as swe lling sh rinkage, rearrangeme nt of particles, and th e presence of swel ling pressure . The second ap proach recogn izes the o rga nized soi l struc ture and the swelling properties of aggregates. Its use has been lim ited in mode ling physical properties ev en tho ugh it better explains and fits observed data. A major obs tacle limiting the use of this approa ch is without a doubt th e lack of a recognized experimenta l me tho d to characterize so il structure scales and their respective con tributions to so il hydraulic pr op erties. This requires a metho d that co nce ptua lly d istinguishes and de limits th e nes ted le ve ls of so il structure as well as ph ysicall y separa tes these functiona l leve ls. Colleuille a nd Brau deau (1996) propose d a meth od of so il fractionation into p rimary aggregates . T he basic concep t used for the dis tinction of th e primary peds stru cture level, an d thus for the fractiona tio n me thod , was rela ted to th e "air en try point into the soil clayey plasm a" (G roeneve lt and Bo lt , ] 972 ; Sposi to , 1973; Sposito and Giraldez, ] 976). By de finit io n the primary pe ds ar e those representing th e first pa rtition ing level of the clay ey plas ma (B rewer, ] 964). At moisture state jus t before air en try in the plasma, the interpeda l porosity is dry while primary peds rema in sa turated , that defines thei r function al pa rtitioning. Braudeau (19R8a, 1988b) and Braudeau and Bniand (1993) hav e shown th at this air e ntry po int th at is the tra nsitio n po int between the basic and the resid ua l shrinkage phases (Point B, Fig. 1) can be obtained precise ly fro m the SC on co nditio n th at it is co ntinu o usly me asured. Braudeau e t al. (1999) develop ed a device and a mod eling method of the SC to precisely determi ne the air en try point in the plasma of the so il corresponding to Point B of the Se. The obj ectiv e of th is study is to develop and test a p hysically based model of the pe dostruc ture fitting th e contin uo usly measured SC during dr ying. The goal is to characterize and paramete rize the soil-water med ium using its measured SC to model its internal hydrostructural co nfig ura tion. We divide the ar ticle into two part s; (i) description of th e shrinkage pro cesses and the ir relationships caused by the removal of th e wa te r pools held by ea ch of the two pore systems, inter and in tra primary ped s, and (ii) applica tion of the PS to the characteriza tio n of four soil types.