Effects of soil water repellency on infiltration rate and flow instability

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<ul><li><p>Effects of soil water repellency on infiltration rateand flow instability</p><p>Z. Wanga,*, Q.J. Wua,1, L. Wua, C.J. Ritsemab, L.W. Dekkerb, J. FeyencaDepartment of Environmental Sciences, University of California, Riverside, CA 92521, USA</p><p>bAlterra, Green World Research, P.O. Box 47, 6700 AA Wageningen, The NetherlandscInstitute for Land and Water Management, Katholieke Universiteit Leuven, Vital Decosterstraat 102, 3000 Leuven, Belgium</p><p>Received 25 November 1998; accepted 22 May 1999</p><p>Abstract</p><p>Laboratory infiltration experiments were carried out to quantify the effects of soil water-repellency on infiltration rate and thewetting front instability. A two-dimensional transparent chamber (41.5 cm wide, 50 cm high and 2.8 cm thick) was constructedfor infiltration experiments using three water-repellent Ouddorp sands (The Netherlands) and a wettable silicon sand. Theresults showed that if the water-ponding depth (h0) at the soil surface was lower than the water-entry value (hwe) of repellentsands, infiltration would not start until the water drop penetration time (WDPT) is exceeded; and contrary to infiltration inwettable soils, the infiltration rate increased with time. However, infiltration could immediately start at any time when h0 .hwe: The wetting front was unconditionally unstable for h0 , hwe; resulting in fingered flow. However, the flow was condi-tionally stable for h0 . hwe if the soil was not layered in a fine-over-coarse or wettable-over-repellent configuration, and if soilair was not compressed during infiltration. The occurrence of stable and unstable flow in repellent soils was consistent with theprediction based on a linear instability analysis. The findings can be used to improve irrigation efficiencies in water repellentsoils, e.g. using high-ponding irrigation methods. q 2000 Elsevier Science B.V. All rights reserved.</p><p>Keywords: Soil water repellency; Infiltration rate; Flow instability</p><p>1. Introduction</p><p>Many soils of the world are water repellent. Theyare difficult to manage and pose negative effects onagricultural productivity and environmental sustain-ability (Debano, 1969; Letey, 1969; Bond, 1969;vant Woudt, 1969; Jamison, 1969; Holzhey, 1969;Letey et al., 1975; Ritsema et al., 1993). The effects</p><p>of water repellency on infiltration are not yet fullyunderstood. Field observations have indicated thatthe rates of water infiltration into repellent soils arevery irregular. The fingered by-passing flow is morelikely to occur in repellent soils rather than in wettablesoils (Raats, 1973; Philip, 1975; Parlange and Hill,1976; Glass et al., 1989; Wang et al., 1998b,c).Fingered preferential flow causes uneven distributionof water in the crop root zone, and accelerates thecontaminant transport to ground water.</p><p>The purpose of this paper is to quantify the effectsof soil water repellency on infiltration rate and flowinstability. We apply the unstable flow theory topredict the onset of wetting front instability and the</p><p>Journal of Hydrology 231232 (2000) 265276</p><p>0022-1694/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.PII: S0022-1694(00)00200-6</p><p>www.elsevier.com/locate/jhydrol</p><p>* Corresponding author. Tel.: 11-909-787-6422; fax: 11-909-787-3993.</p><p>E-mail address: wangz@mail.ucr.edu (Z. Wang).1 Present address: Department of Biological Systems Engineer-</p><p>ing, Washington State University, Pullman, WA 99164, USA.</p></li><li><p>occurrence of fingering in the vadose zone of water-repellent soils. Two-dimensional chamber experi-ments were carried out to study the dynamics of infil-tration and fingering in three Ouddorp repellent sandsof The Netherlands. The results are compared withinfiltration and fingering in a wettable sand of thesame texture.</p><p>2. Theoretical analysis</p><p>Little is known from previous publications aboutthe law of infiltration into repellent soils. Experimen-tal studies have indicated that infiltration into repel-lent soils is complicated by the occurrence of fingeringin porous media. Many field and laboratory studieshave shown that fingering occurs in both wettableand repellent soils, not only in structureless sandysoils but also in structured loam and clay soils, andunder both rainfall and irrigation conditions (Parlangeand Hill, 1976; Starr et al., 1978, 1986; Clothier et al.,1981; Diment and Watson, 1985; Jury et al., 1986;Hillel and Baker, 1988; Glass et al., 1988, 1989;Butters et al., 1989; Kung, 1990; Ghodrati and Jury,1990; Baker and Hillel, 1990; Roth et al., 1991;Ghodrati and Jury, 1992; Selker et al., 1992; Juryand Fluhler, 1992; Ritsema et al., 1993; Flury et al.,1994; Liu et al., 1994; Nieber, 1996; Held and Illan-gasekare, 1995; Dekker and Ritsema, 1994, 1995).Many investigators suspect that factors leading tofingering may include vegetation, microtopography,irrigation method, soil water-repellency, soil layering,and soil macropores. However, theoretical analysessuggest that fingering can be induced by the onset offlow instability at the wetting front between two quali-tatively different fluids. Fingering may take place intwo-fluid flow even if there is no porous structure, e.g.in cracks or the HeleShaw cells (Saffman andTaylor, 1958). It is then possible that many of theaforementioned factors can induce wetting frontinstability in different ways.</p><p>The original linear instability analyses of Saffmanand Taylor (1958) considering viscous and gravita-tional forces, and of Chuoke et al. (1959) and Wanget al. (1998c) later including capillary forces, resultedin theoretical criteria for the onset of instability at thewetting front. According to Wang et al. (1998c), thecondition for the onset of instability at the interface of</p><p>two immiscible fluids in a porous medium is</p><p>V 1erw 2 rnwgk cos bmw 2 mnw 2</p><p>eus puka2</p><p>mw 2 mnw . 0 1</p><p>where V is the infiltration rate, the subscript w refersto the wetting fluid and nw the nonwetting fluid; eindicates the wettability of the driving fluid to theporous medium (e 1 for the wetting fluid displacingthe nonwetting fluid, and e 21 for the reverseddisplacement); r is the density and m the viscosityof the fluids, g the acceleration due to gravity, kthe effective permeability of the porous medium,b the angle between the gravitational directionand the direction of the flow, s p the effective macro-scopic interfacial tension, and a the magnitude of adisturbance to the wetting front.</p><p>During vertical infiltration b 0 of water into thevadose zone, the density and viscosity of air are negli-gible. Thus Eq. (1) can be reduced (Wang et al.,1998c) intouhweu</p><p>3</p><p>c,</p><p>VKs</p><p>, 1 2uhweu</p><p>3</p><p>c2</p><p>where Ks is the natural saturated conductivity, hwe thewater-entry value of the porous medium, and c is aconstant indicating the relative effects of the maxi-mum wetting front perturbation and microscopicheterogeneity on flow instability. According to theexperiments by Yao and Hendrickx (1996), c</p></li></ul>

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