water repellency in soils: a historical overvie...review water repellency in soils: a historical...

Review

Water repellency in soils: a historical overview

L.F. DeBano

Watershed Management, School of Renewable Natural Resources, University of Arizona, Tucson, AZ 85721, USA

Received 19 October 1998; accepted 17 April 1999

Abstract

The purpose of this paper is to document some of the more important highlights of the research and historical aspectsconcerning soil water-repellency. This effort traces the evolution of interests and concerns in water repellency from basicstudies in the nineteenth century to the earlier part of the 20th century and up to our current-day understanding of this subject.The interactions among different scientific disciplines, various manager-scientists efforts, and specific scientific and manage-ment concerns are presented chronologically. This growing interest in water repellency generated an earlier conference in 1968which was devoted exclusively to water repellency and has since initiated productive discussions and debate on waterrepellency during several peripherally related national and international conferences. The 1968 conference held in Riverside,California (USA), mainly involved scientists from the United States and Australia. Since this early conference, a large body ofinformation has been published in a wide range of scientific disciplines throughout the world. This worldwide attention hasproduced many recent research findings, which have improved the understanding of water-repellent soils, particularly of thedynamics of the water movement and redistribution in these unique systems. Intermingled with the effort in water repellency isa related, although somewhat separate, body of information dealing with soil aggregation and water harvesting, which areimportant for improving the productivity of fragile arid ecosystems. A summary is presented of the literature on waterrepellency, showing changes in subject areas and national interests over time.q 2000 Elsevier Science B.V. All rights reserved.

Keywords: Water repellency in soils; Historical highlights of research; Manager-scientists efforts

1. Introduction

Water repellency has been a concern of both scien-tists and land managers for well over a century.During this time, the interest in water repellency hasevolved from an isolated scientific curiosity to anestablished field of science that is recognized world-wide. The wide range of topics discussed on this issueexemplify the range of interest in water repellency.The purpose of this paper is to present a detailed over-view of the research and institutional history of thefield of water repellency. This effort traces the evo-lution of knowledge and concerns about water repel-lency from the beginning of this century to our

current-day understanding of this subject. The inter-actions among different scientific disciplines, variousmanager-scientists efforts, and specific scientific andmanagement concerns are presented both chrono-logically and by subject area content.

2. Information base

The information used as the basis for this paperconsisted of: (1) an extensive bibliography of over500 published papers reporting on various aspects ofwater repellency; (2) a bibliography of over 200published papers which contributed information

Journal of Hydrology 231–232 (2000) 4–32www.elsevier.com/locate/jhydrol

0022-1694/00/$ - see front matterq 2000 Elsevier Science B.V. All rights reserved.PII: S0022-1694(00)00180-3

directly related to the understanding of some of thebasic physical, biological, and chemical processes—the kind of knowledge essential to the present level ofunderstanding of water repellency phenomena; and(3) the personal knowledge gained by this author inover 30 years of interest and research in the field ofwater repellency. The bibliography of related litera-ture was derived mainly from important citations inpublished papers on water repellency. This list cannotbe claimed to be complete, but reflects the author’sevaluation of the importance of individual citationsand their contributions to the field. The papers citedin this paper represent only a sample of the entirebibliography used and the cited references wereselected at the discretion of the author. A comprehen-sive bibliography is being published separately forthose interested in a more complete list of citations.

The bibliography described above was first exam-ined chronologically in order to identify changes inemphasis over time and to track the rise and fall ofinterest in different scientific and applied aspects ofwater repellency. This review of literature was alsoused to identify the evolution of regional emphases ondifferent research topics pertaining to water repel-

lency. The final section of this paper summarizes thechronological development of the knowledge and theregional centers that were involved in water repel-lency research at different times.

3. A global perspective

As during the evolution of many sciences, theearlier years produced only a few publications andthe numbers remained low for several decades untilinterest and fundamental understanding accumulated,after which the numbers of publications mushroomed.Fig. 1 illustrates the number of papers publishedduring different time periods on water repellency perse and in areas that contributed directly to the under-standing of water repellency. The total numbers them-selves are not too informative, but when examined indetail they reveal some noteworthy landmarks andstages of development. The database provided thebasis for identifying the overall flow of information,the emphasis of different topic areas, the reasons forthe increased number of publications, and the overallevolution of the science of water repellency. A

L.F. DeBano / Journal of Hydrology 231–232 (2000) 4–32 5

Fig. 1. The number of publications concerning soil water-repellency and related fields during the 20th century.

more detailed review of the chronology of thesepublications and examples of important publicationsare presented below within the framework of differentdecades.

4. Chronological highlights

The disciplines that provide the scientific and utili-tarian basis for our current understanding of waterrepellency are: firstly, the study of the importantrole of organic matter in agricultural systems, andsecondly, the knowledge of soil–water–plant rela-tionships, particularly the physics of soil water move-ment. The accumulation of knowledge in these twoareas evolved both from academic curiosity and fromthe importance of organic matter in the productivity ofagricultural systems. These two areas of scientificinquiry continued to be keystone sciences underlyingthe study of water repellency phenomena over theyears and also appear as an integral part of manypapers included in this issue.

4.1. The roots (pre-20th century)

Interest in water repellency phenomena began wellbefore the 20th century, although it was not identifiedas such. It is not the purpose of this paper to establishan irrefutable beginning point for the study of waterrepellency, but instead to identify selected referencesfound in the literature before 1900, which increasedthe awareness of organic matter (humus) and provideda basis for later studies of water repellency that beganin the early 20th century. An examination of the litera-ture before 1900 indicates that water repellency wasmostly associated with observations on organic matterand its decomposition, particularly where fungi wereinvolved.

Humic substances were first investigated during thelater part of the 18th century when Achard attemptedto isolate humic substances in 1786 (Stevensen,1994). DeSaussure introduced the word “humus” in1804 and humic acids were designated by Dobereinerin 1822. The first comprehensive reports on thechemical nature of humic substances were writtenbetween 1826 and 1862 (Stevensen, 1994). In thesecond half of the nineteenth century, most of thereports were concerned with classifying productsproduced during the decomposition of organic

substances (Kononova, 1961). By the end of the nine-teenth century, it was well established that humus wasa complex mixture of organic substances that weremostly colloidal and had weakly acidic properties(Stevensen, 1994).

Studies on the fungal decomposition of organicmatter were first reported by Waring in 1837 (asreviewed by Bayliss, 1911) and these reports wereprobably the first publications that discussed the effectof mycelium growth on the rate of absorption of waterby soil. These studies described a phenomenon knownas “fairy rings”. The term “fairy ring” was used byearly investigators to describe the arrangement ofplants (usually grass or crop plants) in an approxi-mately circular form, where plant growth on the insideof the circle was stimulated. Circles of bare ground orconcentric zones of withered plants surrounded thisinner circle of healthy plants. These concentric ringswere attributed to various natural and supernaturalsources such as the paths created by dancing fairies,thunder, lightning, whirlwinds, ants, moles, hay-stacks, urine of animals, and so on. In many casesthe fairy ring phenomena was so abundant locallythat it materially affected the yield of crops. Almosta half century later, quantitative data were reported atRothamsted indicating that more soil moisture waspresent in the healthy ring of plants than either outsideor inside it (Lawes et al., 1883). Although none ofthese pre-20th century publications used the term“water repellency”, it was obvious that many ofthese earlier scientists were observing the phenom-enon of water repellency as we know it today.

Another building block, which would contribute tothe understanding of water repellency, was the disci-pline of soil physics, which was just starting to appearat the end of the nineteenth century. Two papers writ-ten by German scientists during the last part of thenineteenth century described the physics of air andwater relationships in soils (Puchner, 1896) and therelationships between rainfall and soil–plant systems(Wollny, 1890). Physical relationships describing thecohesive properties of water had been published muchearlier (Young, 1805).

4.2. Decades of awareness (from 1900 to 1919)

Interest in organic matter, particularly humicsubstances, continued into the earlier part of the

L.F. DeBano / Journal of Hydrology 231–232 (2000) 4–326

20th century. Starting in 1908, Schreiner and Shorey(1910) initiated a series of studies to identify organicchemicals contained in a California soil. During theirinvestigations of humic substances, they reportedstudying a soil that “could not be wetted, either byman, by rain, irrigation or movement of water fromthe subsoil” (Schreiner and Shorey, 1910, p. 9).

During the earlier part of the 20th century the inter-est in “fairy ring” phenomena continued. Bayliss(1911) reported on the fairy ring phenomena andcited measurements reported earlier by Molliard(1910) that showed that the soil proliferated withfungal mycelium was comparatively dry. The areaoccupied by the mycelium contained only 5–7%moisture, compared to 21% in the areas inside andoutside the ring, which were not occupied by myce-lium. Bayliss (1911) validated that soils containingmycelium were difficult to wet and cited an examplewhere rain did not penetrate the soil in mycelia-infested areas but penetrated to a depth of 10 cm inthe adjacent non-mycelial areas. Measurements of soilwater content, associated with rings formed by afleshy fungus (Agaricus tabularis) that had infectedgrasslands in eastern Colorado (Shantz and Piemeisel,1917), indicated that during the spring there were nodifferences in moisture content between the mycelialand non-mycelial zones. After the soil had dried out inlater summer, however, the mycelia-infested soil ringsdid not permit penetration of water. As a result, largedifferences in soil water contents were found betweenthe bare areas and the inner and outer vegetated rings,particularly in the upper foot and early in the growingseason.

During these first two decades of the 20th century,soil physics and water use by plants began emergingas important sciences. A review of the few studies onsoil water movement was published (Buckingham,1907), as was a report on the importance of transpira-tion in crop production (Kiesselbach, 1916).

4.3. Decades of contemplation (from 1920 to 1939)

The period between 1920 and 1939 witnessed thedevelopment of scientific knowledge in peripheraldisciplines that later provided the basis for betterdescribing soil water movement and the physical–chemical nature of wetting. Soil physicists (Zunker,1930; Richards, 1931) began quantifying the concept

of water movement and the importance of capillaryforces on water in the soils.

Interest was also developing in the methods ofquantifying aggregate stability for erosion controlstudies (Middleton, 1930). One of the earlier methodsof quantifying erosion potential was based on thestability of soil aggregates to slaking when exposedto excess water (Yoder, 1936). The interest in thestability of aggregates to wetting continues today,although more sophisticated procedures are availablefor assessing this characteristic. More detailed studieson the stability of soil aggregates to wetting were alsoreported during these two decades, particularly asrelated to organic matter and microbial processes(Kanivetz and Korneva, 1937; Waksman, 1938).

During these early decades of the 20th century, thephysical–chemical nature and the wetting of lowsurface tension solids (e.g. talcs, waxes and resins)were being investigated from an industrial engineer-ing perspective (Bartell and Zuidema, 1936; Wenzel,1936).

Only two publications between 1920 and 1939 werefound that discussed water repellency. These were: areport of resistance to wetting in sands (Albert andKöhn, 1926), and a second report describing thecreation of “ironclad” or artificial catchments(Kenyon, 1929).

4.4. Decades of recognition (from 1940 to 1959)

Between 1940 and 1959, published papers report-ing observations on water-repellent soils beganappearing in several scientific journals. Studies byJamison (1947) showed that resistance to wettingwas affecting the productivity of citrus orchards inFlorida, USA. Elsewhere in the world, Van’t Woudt(1959) reported that organic particle coatings wereaffecting the wettability of soils in New Zealand.The results of an investigation on difficult-to-wetsoils was also reported in the Netherlands (Domingo,1950). Finally, in 1959, detailed microscopic exami-nations of the aggregating effect of microbiologicalfilaments on the aggregation of sand grains werereported in Australia (Bond, 1959). Although waterrepellency was not specifically mentioned as being afactor in aggregate stability, this initial publicationwas the beginning of a series of fruitful researchreports about water repellency that was published


later by a group of Australian scientists (Bond,Emerson and others) during the 1960s and 1970s.

During these two decades, interest in soil aggrega-tion increased (Robinson and Page, 1950; Martin etal., 1955). This interest included increasing the stabi-lity of clay soils (Childs, 1942) using synthetic poly-electrolytes to improve aggregation (Hedrick andMowry, 1952), and improving aggregation todecrease wind erosion (Chepil, 1958). The role ofmicroorganisms in enhancing aggregation was gain-ing interest (Martin and Waksman, 1940; Swaby,1949). Molds and algae were found to be particularlyeffective agents for soil crusting (Fletcher and Martin,1948) and aggregation (Gilmour et al., 1948).

An interest in characterizing contact angles alsobegan emerging (Bikerman, 1941) along with a conti-nuing interest in the physical–chemical process ofwetting from an chemical engineering perspective(Barr et al., 1948). A book on surface-active agentsand detergents was published near the end of thesetwo decades (Schwartz et al., 1958).

Both the theoretical and applied dimensions ofwater movement in soils were gaining closer atten-tion. Theoretical concepts being developed in soilsincluded: describing capillarity (Miller and Miller,1956); recognition of contact angles’ role during infil-tration (Fletcher, 1949); soil water energetics duringinfiltration (Bodman and Colman, 1943); and thenumerical solution of concentration dependent diffu-sion equations (Philip, 1957). During this sameperiod, viscous flow was described in porous media(Chouke et al., 1959) and in the Hele–Shaw cell (Saff-man and Taylor, 1958). These theoretical develop-ments served as the basis for a more comprehensiveapproach to describing water movement in hard-to-wet soil systems during the following decades.Applied research was done on the effect of plants oninterception, stemflow, and ground rainfall (Specht,1957) and the effect of profile characteristics (Hurshand Hoover, 1941) and roots (Gaiser, 1952) on hydro-logic processes in forest soils. Infiltration into soilsfound in wildland environments was also attractingattention, particularly in soils that had been exposedto wildfires (Scott and Burgy, 1956).

Water repellency was also starting to be utilized forbeneficial uses, including its use for water harvestingwhere paved drainage basins provided a source ofwater for livestock or game (Humphrey and Shaw,

1957), its application as moisture, thermal and electricinsulator during highway construction (Kolyasev andHolodov, 1958), and its potential for decreasing soilwater evaporation (Lemon, 1956).

4.5. Decade of renewed interest (from 1960 to 1969)

The decade of the 1960s witnessed a flurry of inter-est in soil water-repellency and in related fields. As aresult, several milestone publications appeared duringthis decade, in addition to a substantial increase in theknowledge about water repellency in soils and relatedfields.

4.5.1. Significant milestonesThe first milestone was represented by a surge in

the number of scientific papers, primarily by scientistsin Australia and the United States, on a wide range oftopics concerning water repellency. Between 1960and 1970, over 90 publications dealing with variousaspects of water repellency were published (Fig. 1),with about one-third of these publications appearingin the proceedings of the first international conferenceat Riverside, CA in 1968 (DeBano and Letey, 1969).An addition of 31 publications reporting scientificfindings related to water repellency were alsopublished during this decade.

A second significant milestone during this decadewas the development of physical methods for charac-terizing soil water-repellency using contact anglemethodology. In 1962, Letey and coworkers at theUniversity of California, Los Angeles published twosignificant papers, one describing the measurement ofliquid–solid contact angles in soil and sand (Letey etal., 1962a), and a second describing the influence ofwater–solid contact angles on water movement in soil(Letey et al., 1962b). These publications were closelyfollowed in 1963 by a publication by Emerson andBond (1963), working in Australia, who described atechnique of using the rate of water entry into dry sandto calculate the advancing contact angle.

A third milestone was the summary and synthesisof all available knowledge of water repellencyconducted during the 1960s along with earlier find-ings. This formed the basis for discussion amonginterested scientists at the first international confer-ence on water repellency held in May 1968 at theUniversity of California, Riverside, USA (DeBano


and Letey, 1969). Thirty-one presentations covering awide range of topics concerning water repellencywere discussed at this conference. Specific presenta-tions included: physics of water movement throughsoil, distribution of water repellency in differentecosystems, theoretical and practical implications ofsurface-active agents (particularly wetting agents),factors responsible for water repellency (microorgan-isms and wildland fires), water harvesting, methods ofmeasuring water repellency, and soil erosionprocesses.

4.5.2. Other advances in water repellencyThe accumulation of knowledge about water repel-

lency and its treatment had accumulated to such anextent during the 1960s that synthesis papers werebeginning to be published. Important summary papersincluded a state-of-the art publication on soil wettabil-ity and wetting agents (DeBano et al., 1967), and aseparate review describing the chemistry of surface-active agents (Black, 1969). In addition to the synth-esis papers, significant papers appeared describing:use of wetting agents to ameliorate water repellency;identification of fire-induced water repellency as acontributor to postfire erosion; interrelationshipsamong organic matter, soil microorganisms andwater repellency; and better definition of the role ofliquid–solid contact angles in water movement.

The use of wetting agents to increase infiltrationand enhance water movement in water-repellentsoils attracted considerable attention during the1960s (Letey et al., 1961; Watson et al., 1969). Parti-cularly noteworthy was a group of scientists and coop-erators at the University of Riverside who studied theusefulness of wetting agents for irrigating water-repellent soils (Letey et al., 1962c), established guide-lines and techniques for using nonionic wetting agents(Letey et al., 1963), and evaluated the longevity ofwetting agents (Osborn et al., 1969). This interest inwetting agents expanded to their use for reducingpostfire erosion (Osborn et al., 1964) and enhancingturfgrass growth (Morgan et al., 1967). Also, severalstudies were published about the effects of surfactantson plant growth (Parr and Norman, 1965) and seedgermination (Osborn et al., 1967). The increased useof surfactants also prompted an evaluation of theireffect on soil aggregation (Mustafa and Letey, 1969).

Large increases in water erosion following wild-

fires had been a long-standing concern in southernCalifornia, USA, particularly in the Los AnglesBasin. Research showed that water repellency onthese erosive watersheds was intensified by the soilheating occurring during a fire (DeBano andKrammes, 1966). The decrease in infiltration due towater repellency had been overlooked previously bythese watershed investigators (Krammes and DeBano,1965), because it was assumed that the decreasedinfiltration after fire resulted primarily from the lossof a protective plant cover and the plugging of soilpores with ashy residue remaining on the soil surface.Awareness of fire-induced water repellency in otherwildland environments in the United States wasquickly reported by other investigators: in forestedenvironments of the Sierra Nevada of Nevada andCalifornia (Hussain et al., 1969) and in many vegeta-tion types throughout the western United States(DeBano, 1969a). The relationships among soilfungi, soil heating, and water repellency were alsodemonstrated (Savage et al., 1969b).

A keen interest in the relationships among organicmatter, soil microorganisms, and water repellencyalso developed during the 1960s. In Australia,research inquiries into the effect of microbial fila-ments on soil properties were gaining momentum(Bond, 1962). Field studies on water repellent sandysoils (Bond, 1964) revealed that filamentous algae andfungi were responsible for the water repellency (Bondand Harris, 1964). In the United States, the productionof water repellency by fungi was also confirmed(Savage et al., 1969b), and the roles of humic acidsand polysaccharides were evaluated (Savage et al.,1969a).

The use of hydrophobic materials for harvestingrainfall (water harvesting) attracted substantial inter-est, particularly in arid regions of the western UnitedStates. Water harvesting interests were summarized inreviews, including a description of waterproofing soilto collect precipitation (Myers and Frasier, 1969) anda comprehensive book on waterproofing and waterrepellency (Moilliet, 1963). A better understandingwas also evolving of the chemistry of a variety ofsynthetic substances that could make soils hydropho-bic and of their effect on soil physical properties(Bozer et al., 1969).

Characterizing water repellency and the effect ofhydrophobic substances on water movement was the


focus of several studies. In addition to the pioneeringpublications on contact angle methodology describedabove, fundamental relationships between contactangles of water and saturated hydrocarbons andexchangeable cations were reported (Cervenka et al.,1968). Additional techniques for characterizing waterrepellency in soils were also reported, includingmeasurements of liquid–solid contact angles (Emer-son and Bond, 1963; Yuan and Hammond, 1968).Studies on soil water movement in hydrophobicsoils consisted of determining the influence of wettingon the liquid water movement in sand (Vladychenskiyand Rybina, 1965), the role of capillary movement insoils (Wladitchensky, 1966; Rybina, 1967), and theprocesses of water movement through water-repellentsoils (DeBano, 1969b), including layered systems(Mansell, 1969).

4.5.3. Related scientific inquiryNumerous publications also appeared during this

decade that contributed fundamental knowledge infields related to water repellency. A comprehensivesynthesis of information on the dynamics of aggrega-tion was published (Harris et al., 1966) in addition to aclassification of aggregates based on their coherencein water (Emerson, 1967). The role of microorganismsand their byproducts on soil structure stabilizationwas the focus of some papers (Bond and Harris,1964; Harris et al., 1964). Polysaccharides werefound to play an important role in stabilizing naturalaggregates (Acton et al., 1963). Synthetic substancessuch as 4-tert-butylpyrocatechol were also evaluatedfor their effectiveness in enhancing soil structuralstability (Hemwall and Bozer, 1964). Synthetic soilconditioners were also used to enhance infiltration andreduce erosion (Kijne, 1967).

Basic information that later contributed to under-standing water movement in soils began emergingduring the 1960s, and provided the basis for describ-ing better the water movement in water repellentsystems during the following decades. The theoreticalbasis for describing infiltration that was developedduring the 1950s was summarized (Philip, 1969).Fundamental information on water movement inlayered systems was being acquired (Miller and Gard-ner, 1962). There was also continued interest in themore theoretical aspects of growth by fingers inHele–Shaw cells (Wooding, 1969). Other important

theoretical topics studied during this decade included:development of stability theory for miscible liquid–liquid displacements (Elrick and French, 1966); abetter understanding of capillary flow (Waldron etal., 1961); contact angle hysteresis (Johnson andDettre, 1964) and equlibria (Zisman, 1964); and thelinkage between infiltration in sand and ground waterrecharge (Smith, 1967). The use of surface-activematerials (nonionic surfactants, fatty alcohols, hexa-deconal) was found to suppress evaporation (Law,1964) and to alter soil water diffusivity (Gardner,1969). Two comprehensive reviews were publishedduring this decade, one on soil water theory (Childs,1967) and a second on contact angle wettability andadhesion (Gould, 1964).

4.6. Decade of spinoffs (from 1970 to 1979)

During the 1970s, over 130 papers were publishedon various aspects of water repellency and anadditional 55 publications on closely related subjectmatter (Fig. 1). Many of the publications of thisdecade clearly reflected the spinoffs arising from theinformation reported at the 1968 conference.

4.6.1. Understanding water repellencyThe interest in water repellency and its manage-

ment implications began to attract worldwide atten-tion. In the United States water-repellent soils werereported in: desert shrub communities in the Ameri-can Southwest (Adams et al., 1970); granitic forestsoils in the Sierra Mountains of the western UnitedStates (Meeuwig, 1971); pinyon–juniper woodlands(Scholl, 1971), chaparral (Scholl, 1975), and ponder-osa pine forests (Zwolinski, 1971; Campbell et al.,1977) in Arizona; mixed conifer forests in California(Agee, 1979); sagebrush-grass communities in thewestern United States (Salih et al., 1973); the highCascade Mountains in the northwestern United States(Dyrness, 1976); coal mine spoils of New Mexico(Miyamoto et al., 1977); and forest soils in upperMichigan (Reeder and Jurgensen, 1979) and inWisconsin (Richardson and Hole, 1978). Elsewherein the world, water repellency was reported in:Australia (Roberts and Carbon, 1971), Egypt (Bishayand Bakhati, 1976), India (Das and Das, 1972), Japan(Nakaya et al., 1977), Nepal (Chakrabarti, 1971), Mali(Rietveld, 1978), and New Zealand (John, 1978). The


management concerns focused primarily on the effectwater repellency had on plant growth, including: the“fairy ring” phenomenon (Stone and Thorp, 1971),impacts on the production of barley (Bond, 1972),and non-wettable spots on gold greens (Miller andWilkinson, 1977).

Other interests in water repellency during the 1970swere similar to those during the 1960s and additionalresearch continued to be conducted on: using wettingagents for remedial treatments, fire-induced waterrepellency, water harvesting, characterizing waterrepellency, and soil water movement.

4.6.2. Remedial treatmentsThe use of wetting agents and other remedial tech-

niques continued to capture the interest of scientistsstudying techniques for ameliorating water repellencyduring this decade. The addition of cores containing aloam soil to water repellent sands was found toincrease the overall infiltration rates into sandy soilsin Australia (Bond, 1978). Chemical remedial treat-ments, however, continued to receive most of theattention in treating water-repellent soils. The under-standing of wetting agents and/or surfactants chal-lenged both scientists and managers. Noteworthypublications describing the basic functioning ofwetting agents in soils included the following topics:factors responsible for increasing the effectiveness ofwetting agents (Mustafa and Letey, 1970), quantify-ing their effect on penetrability and diffusivity rela-tionships in soils (Mustafa and Letey, 1971),evaluating their movement and leaching throughwettable and water-repellent soils (Miller et al.,1975), and assessing their effect on pesticide mobilityin the soil (Huggenberger et al., 1973). A majorconcern was the effect of surfactants on plant physiol-ogy which led to studies that addressed their effectson: seed germination (Burridge and Jorgensen, 1971);plant cells (Haapala, 1970); growth, porosity, anduptake by barley roots (Valoras et al., 1974a); andthe germination and shoot growth of grasses (Miya-moto and Bird, 1978).

Concurrent with the basic studies on wetting agentsdescribed above were several reports which empha-sized their overall application (Letey, 1975) and theirspecialized uses in forestry (DeBano and Rice, 1973),including erosion reduction (Valoras et al., 1974b).The success of using wetting agents to remedy water

repellency encountered in field situations was vari-able. In one study, the use of wetting agents improvedinfiltration into water-repellent coal mine spoils to alimited degree (Miyamoto, 1978). An effort to useoperational-level wetting agent treatments to reducesoil erosion on burned watersheds was not successful,however (Rice and Osborn, 1970).

Two important synthesis publications on surfac-tants were also published during this decade: a state-of-the-art review of soil water-repellency and the useof nonionic wetting agents (Letey et al., 1975) and abook describing the fundamental relationships ofsurfactants to interfacial phenomena (Rosen, 1978).

4.6.3. Fire-induced water repellencyA better understanding of fire-induced water repel-

lency (Savage, 1974; DeBano et al., 1976) and itsimportance in postfire erosion on watersheds (Mega-han and Molitor, 1975) were subjects of activeresearch during the 1970s. Fire-induced water repel-lency was also reported in different situations, includ-ing: under campfires (Fenn et al., 1976); under piles ofburned logs (DeByle, 1973), and in the upper soillayers during prescribed fires in mixed conifer forests(Agee, 1979). The author and others present a moredetailed discussion of fire-induced water repellencyand its erosional consequences elsewhere in this issue.

4.6.4. Water harvestingThe use of water repellency principles provided the

basis for the rapid expansion of water harvesting tech-nology during the 1970s. Over a dozen publishedpapers dealt with different aspects of water harvesting,including: developing the technology of bondingwater repellent films to soil particles (Frasier andMeyers, 1972), assessing the resistance of organo-film-coated soils to infiltration (Fink, 1970), utilizingwax-treated soils for water harvesting (Fink, 1977),developing laboratory evaluation techniques (Fink,1976), assessing freeze-thaw effects on soils treatedfor water repellency (Fink and Mitchell, 1975), estab-lishing water harvesting efficiencies for differentsurface treatments (Rauzi et al., 1973), utilizingwater harvesting as a reforestation tool (Mehdizadehet al., 1978).

Two major efforts to synthesize information onwater harvesting occurred during the 1970s. First, astate-of-the-art synthesis on water harvesting was


published (Cooley et al., 1975). The second effort wasthe convening of an international water harvestingconference held in Phoenix, Arizona in 1974 (Frasier,1975). This conference produced numerous papers onall aspects of water harvesting.

4.6.5. Characterizing water repellencyMajor advances in characterizing both the physical

and chemical nature of water repellency occurredduring the 1970s. Investigation of physical effectswas concerned with assessing those factors affectingwetting phenomena, while the studies involvingchemical characterization of water repellency focusedon humic acids and their interactions with varioussubstances, including soils.

Detailed studies were reported on proposed tech-niques for physically characterizing water repellencyin terms of: wetting coefficients (Bahrani et al., 1970),surface roughness (Bond and Hammond, 1970), andliquid-surface tension and liquid–solid contact anglesduring liquid entry into porous media (Watson et al.,1971). Indices for characterizing water repellencywere developed using contact angle–surface tensionrelationships (Watson and Letey, 1970) and solid–airsurface tensions of porous media (Miyamoto andLetey, 1971). Concerns about the limitations ofscaling when using contact angles also emerged(Philip, 1971; Parlange, 1974).

A better understanding of the chemistry of hydro-phobic substances responsible for water repellencyand their interactions with other substances wasgained during this decade. A comprehensive bookon the chemistry of natural waxes appeared (Kolattu-kady, 1976). Detailed studies reported on the adsorp-tion of water by soil humic substances (Chen andSchnitzer, 1976) and on the surface tensions ofaqueous solutions of soil humic substances (Chenand Schnitzer, 1978). A method was developed toquantify hydrophobic sites on soil minerals usingaliphatic alcohols (Tschapek and Wasowki, 1976).The wettability of different natural substances wasbeing characterized, including that of humic acidand its salts (Tschapek et al., 1973) and of zeolites(Chen, 1976).

4.6.6. Water movementDuring this decade more was being learned about

the effect of hydrophobic substances on soil water

movement during infiltration and evaporation(DeBano, 1975). The beneficial use of water repel-lency to save water (Hillel and Berliner, 1974), parti-cularly by reducing the capillary rise of water to thesoil surface where it was evaporated (Hergenhan,1972), and to reduce fertilizer leaching (Snyder andOzaki, 1974) also gained attention during the 1970s.

4.6.7. Related researchInterest in the interrelationships among aggrega-

tion, soil structure, and water repellency was galva-nized during the 1970s, when a conference was held inLas Vegas, Nevada, USA, under the sponsorship ofthe Soil Science Society of America, dealing specifi-cally with “Experimental Methods and Uses of SoilConditioners” (Stewart et al., 1975). This conferenceconsidered soil stabilization to control wind and watererosion, structural improvement of sodic and claysoils, water harvesting, soil conditioning with bento-nite, water repellency (water movement, fire-inducedwater repellency), the role of organic matter and othernatural mulches (e.g. bark) to improve soil structure,and the use of synthetic material such as bitumenemulsion and polyacrylamide for soil stabilization.This conference did much to link strongly an indepen-dent line of investigations on soil structure and condi-tioners to those being conducted on water repellencyphenomena.

Other important publications on aggregation alsowere published during the 1970s. Fundamental workon cementing substances of iron and aluminum on soilaggregates was being published in Italy by Giovanniniand Sequi (1976) and interest continued about the roleof microorganisms in stabilizing aggregates (Aspiraset al., 1971). A book on modification of soil structure,including chapters on aggregate formation and stabi-lization, was also published (Emerson et al., 1978).

Important theoretical efforts by soil physicistsbegan identifying more realistic models for describingsoil water movement in water repellent systems.Foundations were established for describing thehydrodynamic instability of miscible fluids in porousmedia (Bachmat and Elrick, 1970), solving flowequations for unsaturated soils (Parlange, 1975), anddetermining the extent of equilibrium vapor adsorp-tion by water-repellent soils (Miyamoto et al., 1972).The complications arising from wetting front instabil-ity and unstable flow in layered soils were beginning


to be more fully recognized (Hillel, 1972; Hill andParlange, 1972) and a theoretical framework foranalyzing wetting front instability was proposed(Parlange and Hill, 1976). Experimental studieswere also conducted on the effect of sudden changesin pressure gradients on wetting front instability inheterogeneous media (White et al., 1977); spatialvariability in field-measured water properties (Nielsenet al., 1973); and unstable flow during infiltration(Raats, 1973; Philip, 1975). Preferential flow throughlarge pores was also receiving attention (Ehlers, 1975;Scotter, 1978). A conceptual model for hysteresis wasdescribed (Mualem, 1974).

4.7. Decade of enrichment (from 1980 to 1989)

The decade starting in 1980 was characterized notonly by continuing strong interest in water repellencyper se, but also saw the development of knowledge inrelated areas which would be used during the 1990s asthe basis for describing water movement in hard-to-wet soils. Over 150 publications on water repellencyappeared during this decade; in addition, more than55 papers describing related theory were published(Fig. 1).

The decade began with a state-of-the-art publi-cation, which synthesized much of the publishedinformation on water-repellent soils up through themid-1970s (DeBano, 1981). The managementconcerns enlarged from a limited focus on the effectsof water-repellent soils on plant productivity duringthe 1960s and 1970s, to more complicated andbroader environmental issues in the 1980s. Theseemerging management concerns prompted the initia-tion of a wide spectrum of new research.

4.7.1. Understanding water repellencyWater repellency continued to be reported as a

problem when managing sandy and heavy texturedsoils in: Australia (McGhie and Posner, 1980;Ma’shum and Farmer, 1985), Japan (Nakaya, 1982),Poland (Prusinkiewicz and Kosakowski, 1986), andthe United States (Hubbell, 1988). Numerous publica-tions continued to address the concerns with the wett-ability of golf greens as related to thatch and dry patchin: Australia (Charters, 1980), Great Britain (Shiels,1982), New Zealand (Wallis et al., 1989), and theUnited States (Taylor and Blake, 1982). The effect

of plant shoot material on the development of waterrepellency was also examined (McGhie and Posner,1981).

The wettability of humus and peat materials wasfound to be an important factor in managing forestedecosystems. Humus wettability had to be consideredwhen managing forests in Poland (Grelewicz andPlichta, 1985). Interest in the management of peatbogs was highlighted by a book on peat and water(Fuchsman, 1986). Of special interest were studieson the effect of mineralization on the hydrophilicproperties of peat soils when managing peat bogs(Lishtvan and Zuyev, 1983).

Several areas of water repellency studied during the1960s and 1970s continued to attract interest duringthe 1980s. These include: remedial treatments, fire-induced water repellency, characterizing water repel-lency, soil structure and aggregation, and the effect ofwater repellency on soil water movement.

4.7.2. Remedial treatmentsRemedial treatments captured the interest of

several authors. Application of surfactants was byfar the most popular treatment (Rieke, 1981; Sawadaet al., 1989). The use of dispersible clays, however,was emerging as a promising technique for reducingwater repellency in sandy soils in Australia (Ma’shumet al., 1989). Physical methods of ameliorating waterrepellency other than claying were to emerge laterduring the 1990s when they would become efficientand widespread methods for treating water repellency,particularly in Australia and New Zealand.

4.7.3. Fire-induced water repellencyThe interest in the effect of fire-induced water

repellency continued and in the 1980s it was reportedin: California (Wells, 1987), Canada (Henderson andGolding, 1983), southern Chile (Ellies, 1983); Callunaheathlands in England (Mallik and Rahman, 1985),Italy (Giovannini and Lucchesi, 1983), Oregon(McNabb et al., 1989), the Pacific Northwest of theUSA (Poff, 1989), South Africa (Scott, 1988), Spain(Almendros et al., 1988), and Turkey (Sengonul,1984).

The effect of water-repellent soils on erosionfrom burned watersheds continued to capture theinterest of several investigators. A conceptual modelrelating hillside rill erosion to the formation of a


water-repellent soil layer during fire was developedfor chaparral areas in southern California (Wells,1987). The Universal Soil Loss Equation (USLE)was evaluated on burned forest areas in northwesternSpain (Diaz-Fierros et al., 1987).

4.7.4. Characterizing water repellencyAssessing and quantifying water repellency also

remained a continuing interest. Publications duringthis decade included those on the methods for measur-ing severity of water repellency (King, 1981), fieldtechniques for quantifying water repellency usingsoil survey information (Richardson, 1984), the useof effective contact angle and water drop penetrationtime to classify water repellency (Wessell, 1988), andmeasurement of water repellency using intrinsic sorp-tivity measurements (Tillman et al., 1989). The effectof humidity on water repellency was also assessed(Jex et al., 1985).

4.7.5. Soil aggregation and structureSoil aggregation and its stability remained a popu-

lar subject of many studies. An energy-based indexwas developed for assessing the stability of soil struc-ture (Skidmore and Powers, 1982). The importanceand overall role of organic matter in the structuralstability of soil aggregates was discussed by someauthors (Chaney and Swift, 1984; Oades, 1984).Still other authors focused specifically on the effectof humic substances on the stability of soil aggregates(Chaney and Swift, 1986; Piccolo and Mbagwu,1989).

Significant research results continued to bepublished on water repellency and aggregation stabi-lity (Giovannini and Lucchesi, 1983) and on theidentification of substances responsible for hydro-phobicity in soils (Giovannini and Lucchesi, 1984;Ma’shum et al., 1988). Lastly, a comprehensivereview was published in a book describing the inter-actions between microorganisms and soil minerals(Huang and Schintzer, 1986).

4.7.6. Water movement in soilsDuring the decade starting in 1980, substantial

progress was made in establishing the theoreticalbasis for describing water flow through water repel-lent systems. This theoretical framework provided thebasis for making substantial progress in describing

water movement in hydrophobic soils later duringthe 1990s.

During the 1980s, the theoretical developments inthe field of fluid dynamics which described fingeringphenomena in the Hele–Shaw cells were published(Saffman, 1986). These theoretical developmentsprovided the basis for extending the concept of finger-ing to describe water movement in soils. Hillel andBaker (1988) presented a descriptive theory of finger-ing during infiltration in layered soils, which wasexpanded to describe fingering phenomena in two-dimensional, homogeneous, unsaturated porousmedia (Tamai et al., 1987). Preferential flow patternswere being reported regularly in agricultural soils(Van Ommen et al., 1988).

Concurrent with the interest in fingering phenom-ena described above were reports on the stability ofwater movement through unsaturated porous media(Diment and Watson, 1985), particularly in the vadosezone (Glass et al., 1988), which led to a suggestedmechanism for finger persistence in homogeneous,unsaturated porous media (Glass et al., 1989). Funda-mental studies were also conducted to assess penetra-tion coefficients in porous media (Malik et al., 1981).Characterization of sorptivity and soil waterdiffusivity was being expanded to describe theseprocesses under field conditions (Clothier andWhite, 1981). Subsurface flow processes were alsobeing evaluated above fragipan horizons (Parlangeet al., 1989) and in forest soils (Mosely, 1982).

A successful climax to the research efforts on soilmovement in unsaturated soil was the convening of aninternational conference in New Mexico that wasdevoted entirely to examining flow and transportmodels used to characterize water flow in unsaturatedzones (Wieringa and Bachelet, 1998). One of thepapers at this conference extended recent soil watertheory to describe water and solute movement throughwater repellent sands (Hendrickx et al., 1988).

4.7.7. Miscellaneous researchIn addition to the areas of interest discussed above,

some miscellaneous information on water repellencywas also published during the 1980s. One suchstudy described relationship of vegetation age to soilwater-repellency in California chaparral ecosystems(Teramura, 1980). Water repellency was also reportedto be an important factor in the reclamation of soils


containing degraded lignite (Richardson and Wollen-haupt, 1983). Water repellency was reported incoastal sand dunes of the Netherlands where itspresence affected the erosion processes on sanddune landscapes (Jungerius and Van Der Meulen,1988). Finally, the role of hydrophobic substanceswas evaluated in the adaptation of leaves to periodicsubmersion by tidal water in a mangrove ecosystem(Misra et al., 1984).

4.7.8. Related researchThe increased environmental awareness during the

1980s began to capture the interest of several soilscientists. Of particular concern was the rapid move-ment of contaminants into the groundwater via prefer-ential flow patterns. Although most of the early effortswere concentrated on developing new theoreticalapproaches to infiltration and water movementthrough soils, this effort evolved into the theoreticaland operational frameworks necessary for quantifyingthe effect of water repellency on groundwater con-tamination later during the 1990’s. This approach,therefore, provided an initial stimulus for attackingthe complex process of ultimately merging surfacehydrologic theory with models describing integratedsoil–water systems.

4.7.8.1. Surface hydrology.Surface hydrology andwatershed performance began emerging as majorinterests during the 1980s (Beven, 1989); some ofthis work had a direct bearing on the effect of waterrepellency on catchment responses. Theoretical workon surface hydrology focused on lateral flow throughlayered soils on sloping topography was beginning toappear in the literature (Zaslavsky and Sinai, 1981;Selim, 1987; Miyazaki, 1988). Detailed evaluationswere also made of unsaturated and saturated flowthrough a thin porous layer on hillslopes (Hurleyand Pantelis, 1985). Concurrent with this interest insurface hydrology was the interest in extending theprinciples of water repellency to erosion andhydrologic performance on a watershed level(Topalidis, 1984; Burch et al., 1989).

4.7.8.2. Integrated soil water systems.Concernsintensified during the 1980s about the rapidtransport of pollutants from the soil surface throughthe soil into the underlying ground water. Stemflow

was quickly recognized as an important mechanismcapable of delivering rainfall rapidly to the soilsurface (Van Elewijck, 1989). Once the water hadbeen delivered to the soil surface, macropores andother discontinuities in the soil provided pathwaysthat quickly moved water, containing dissolved andsuspended matter along with adsorbed contaminants,through the soil into the underlying water table(Beven and Germann, 1982; Bouma, 1982). Specificexamples of such processes included nitrogenleaching during sprinkler irrigation (Dekker andBouma, 1984); chloride movement through alayered field soil (Starr et al., 1986); and themovement of water and solute pollutants throughunsaturated zones (Raats, 1984). The rapidlygrowing interest in the storage and movement ofmaterials in soils, particularly pollutants, resulted ina special publication describing the reaction andmovement of organic chemicals in soils (Sawhneyand Brown, 1989). Fundamental studies were alsoconducted on developing models to describeadsorption and transport of hydrophobic organicchemicals in aqueous and mixed solvent systems(Rao et al., 1985) and in natural sediments and soils(Karickhoff, 1981).

4.8. Decade of maturity (from 1990 to 1998)

Between 1990 and 1998, a record breaking 150, ormore, papers on water repellency and over 60 addi-tional papers on related subjects were published (Fig.1). Substantial progress was made in all aspects ofwater repellency, although the increased understand-ing of water movement through these hard-to-wetsystems was particularly noteworthy. The publica-tions concerned with water movement reflected aclose cooperative effort between scientists workingon the cutting edge of soil physics and scientistsconcerned with water repellency phenomena.

4.8.1. Occurrence and amelioration of waterrepellency

The 1990s also witnessed a more profound recog-nition of the implications of water-repellent soils onthe productivity of both cultivated and natural ecosys-tems, as well as recognition of their role in emergingenvironmental issues. This increased awareness led to


a concerted effort to manage and/or ameliorate theadverse effects of water repellency.

4.8.1.1. Occurrence.During the 1990s, waterrepellency continued to be reported to occur in awide range of natural and agricultural environmentsand as a result was fast becoming an importantcomponent in the management and productivity ofsoils worldwide during the 1990s. The most frequentreports described problems on agricultural lands. Theeffect of water repellent sands on crop and pastureproduction was particularly acute on thousands ofhectares in Australia and New Zealand, where theimportance of this problem stimulated intensiveresearch on the problem and intensified a search foreconomic methods of ameliorating this soil condition.(Blackwell, 1993; Carter and Howes, 1994). As aresult, studies on the physiochemical and biologicalmechanisms responsible for water repellency in sandswere initiated (Franco et al., 1994), as were studies onthe role of particulate organic matter (Franco et al.,1995). In the Netherlands, special concerns aroseabout ground water contamination with fertilizersand pesticides that had been transport rapidlydownward through wettable fingers in an otherwisewater-repellent soil (Van Dam et al., 1990).

Other, less widespread, interest on water repellencywas also reported. Localized areas of highly water-repellent soils were created by oil spills, thus requir-ing intensive remedial efforts (Roy and McGill,1998). Furthermore, water repellency was alsobecoming recognized as a key mechanism responsiblefor the self-cleaning features of plant surfaces (Nein-huis and Barthlott, 1997). The diminished aestheticsand playing qualities of golf greens that were afflictedby the age-old problem of “dry patch” still concernedgolf green supervisors (Tucker et al., 1990; York andBaldwin, 1992; Hudson et al., 1994).

Water repellency was also reported in wildlandsoils (i.e. uncultivated soils supporting natural standsof trees, shrubs, and grass), both in fire and non-fireenvironments. Water-repellent soils were reported inseveral wildland environments, including: dry sclero-phyll eucalyptus in Australia (Crockford et al., 1991),eucalyptus and pine forests in Portugal (Doerr et al.,1996), eucalyptus forests in South Africa (Scott,1991), and under windbreaks in Taiwan (Lin et al.,1996). The fire-induced water repellency described

above continued to have its primary impact on wild-land ecosystems.

4.8.1.2. Amelioration.Coping with water-repellentsoils continued to present a challenge to managersof agricultural and pasture lands worldwide (Abadi,1994; Capriel, 1997). Interest in the usefulness ofwetting agents as a remedial treatment for waterrepellency continued (Effron et al., 1990), althoughit did not capture as much interest as during the1960s and 1970s. Wetting agents were also used toenhance irrigation (Wallis et al., 1990) and drainage(Zartman and Bartsch, 1990). Their effects werestudied on aggregation and colloidal stability intropical soils (Mbagwu et al., 1993).

Remedial treatments other than wetting agents werebeginning to be tested and used more extensively,particularly in Australia and New Zealand (Carterand Howes, 1994). These treatments included: directdrilling (Chan, 1992), wide furrow sowing (Blackwellet al., 1994b; Blackwell and Morrow, 1997), and theuse of microorganisms (Roper, 1994) and fertilizers tostimulate microbial breakdown of water repellency(Michelson and Franco, 1994). Soil claying, a treat-ment involving mixing large amounts of clay in theupper water repellent layer, received widespread usein Australia (Blackwell et al., 1994c; Carter andHetherington, 1994; Dellar et al., 1994). Intensiveexaminations were made of the effect of clay miner-alogy and exchangeable cations on water-repellentsoils that had been amended with clay (Ward andOades, 1993). High pH soil treatments offered somealleviation of the hydrophobic condition found on golfgreens (Karnok et al., 1993).

4.8.2. Water movement in soilsWater movement into and through water-repellent

soils was a major focus of research during the 1990s.This effort involved the creation of a forum forexchanging and assessing relevant informationamong scientists (e.g. workshops, conferences), utiliz-ing cutting-edge soil water theory for describing watermovement in water-repellent soils, and identifying therole of water repellency in present-day environmentalissues.

4.8.3. Information exchangeA significant landmark for this decade was the


bringing together of the theoretical and practicaldimensions of distribution flow and fingeringphenomena, as studied during the early 1990s, the1980s, and earlier, during a one-day workshop heldat the Winand Staring Centre in Wageningen in April1994 (Steenhuis et al., 1996). During this workshop,14 papers were presented on various aspects of waterrepellency and supporting theoretical sciences. Thiscollection of papers included several on characteriz-ing field moisture patterns in water-repellent soils, onthe three-dimensional portrayal of preferential flowpatterns, and on the use of ground-penetrating radarfor identifying structural layers that affect finger flowphenomena. Theoretical papers reviewed the instabil-ity of fingered flow and the heterogeneity of thesefingers. Another paper described the field validationof laboratory-based equations for determining fingerdimensions. The use of laboratory theory to predictthe risk of ground water contamination was alsopresented. A final set of papers presented variousmodels describing: instability-driven fingers, fingerformation based on laboratory findings, and hetero-geneity-driven fingers. These presentations werecombined into one document and were published in1996, vol. 70, no. 2–4, of Geoderma.

Numerous additional papers were published on thetheoretical and applied aspects of water movement perse. The application of such theoretical analyses todescribing water movement in hard-to-wet soilsystems both in laboratory and field environmentswas particularly useful. Preferential flow was ofsuch importance that a national symposium wasdevoted to this subject at Chicago, Illinois in 1991(Gish and Shirmohammadi, 1991).

4.8.4. Theoretical developmentIn addition to the papers presented at the Wagenin-

gen and Chicago conferences, numerous other papersappeared which described theoretical studies done onthe transport of water and associated solutes throughthe soil. These reports described the instability ofwetting fronts during infiltration into unsaturatedporous media (Glass et al., 1990; Selker et al.,1992a), infiltration into layered soils (Baker andHillel, 1990; Steenhuis et al., 1991), preferential andlateral flow (Kung, 1990a,b; Heijs et al., 1996),fingered flow (Selker et al., 1992b,c; Glass andNicholl, 1996; Nieber, 1996), and the complications

arising from hysteresis in soil–water systems(McCord et al., 1991). Other studies focused on theeffect of different moisture contents on the formationand persistence of fingered flow in coarse-grainedsoils (Liu et al., 1994a) and outlined closed-formsolutions for predicting finger width development inthese soils (Liu et al., 1994b).

A better understanding of the theoretical basis ofdistribution flow, unstable moisture movement, andfingering phenomena was quickly extended todescribe water movement through water repellentsystems. Detailed field measurements had establishedthat uneven moisture patterns developed as a result ofwater repellent sites throughout the soil profile(Dekker and Ritsema, 1994a). The theoretical modelsdescribing fingering and instability of wetting frontsbegan to be applied intensively to describe the watermovement in the water-repellent soils, particularly byscientists in the Netherlands or their cooperators(Hendrickx et al., 1993; Dekker and Ritsema, 1995,1996a; Ritsema et al., 1998), the dynamics of finger(or preferential) flow in water repellent systems(Ritsema et al., 1993, 1996, 1997a,b; Ritsema andDekker, 1994; Dekker and Ritsema, 1996b; Bauterset al., 1998), the contribution of finger flow to solutemovement through the soil (Van Dam et al., 1990),and water movement through macropores in soils(Mallants et al., 1996). Distribution flow was demon-strated to be an important process in the top layer ofwater-repellent soils (Ritsema and Dekker, 1995) thatdirected surface-applied water to preferrential flowpaths within the soil profile (Ritsema and Dekker,1996a). Studies were designed to predict the occur-rence and diameters of fingers occurring in field soils(Ritsema et al., 1996) and to examine the recurrenceof fingered flow pathways through water repellentfield soils (Ritsema and Dekker, 1998a). The effectof cover type on preferential flow and resulting moist-ure patterns in water-repellent soils was studied underfield conditions (Dekker and Ritsema, 1995, 1996b,1997). The information gained by the above studiesserved as the basis for modeling finger formation andrecurrence (Ritsema et al., 1998), and three-dimen-sional fingered flow patterns (Ritsema et al., 1997a;Ritsema and Dekker, 1998b). The role of hysteresiswhen describing unsaturated flow in water-repellentsoils was reported (Van Dam et al., 1996). A numericalmodel was developed for describing the movement of


heat and water in water repellent sands that werefurrowed for remedial purposes (Yang et al., 1996).

4.8.5. Environmental implicationsThe awareness of environmental issues emerging

during the previous two decades lead to severalstudies during the 1990s that examined the role ofwater repellency within the context of environmentalmanagement. The most apparent need was fordeveloping models describing the transport of field-applied chemicals through the soil (Flu¨hler et al.,1996). Estimates of pollutant loading via fingeredflow were reported (Selker et al., 1991). Informationon water and chemical transport was used as the basisfor developing a soil classification system thatreflected these processes (Quisenberry et al., 1993).The role of solute leaching was beginning to bemodeled for water-repellent soils (De Rooij and DeVries, 1996). The movement of contaminants viafinger flow (Selker et al., 1991) was particularlyimportant in water repellent sandy soils (Van Damet al., 1990).

Also, adsorption of hydrophobic substances on soiland sediments was receiving attention for at least tworeasons. First, the adsorption of hydrophobicsubstances was necessary for inducing permanentwater repellency for water harvesting purposes(Blackwell et al., 1994a), which had received consid-erable attention in the 1970s. Secondly, the adsorptionand transport of hydrophobic contaminants by soilsand sediments was emerging as a sensitive environ-mental concern (Ghosh and Keinath, 1994; Huang etal., 1998; Weber et al., 1998).

4.8.6. Surface hydrology and watershed responsesDuring the 1990s, initial efforts were made to extend

the soil water theory described above to surface hydrol-ogy of hillslopes and even further to describe entirewatershed responses.

4.8.6.1. Surface hydrology.Surface hydrology drewthe interest of some investigators. Models weredeveloped to describe infiltration, soil moisture, andunsaturated flow (Beven, 1991a) and were expandedinto a conceptual approach for predicting runoff(Beven, 1991b). The concepts of hysteresis andstate-dependent anisotrophy were being incorporatedinto the modeling of unsaturated hillslope hydrologic

processes (McCord et al., 1991). Important hydrologicrelationships in unsaturated air systems and their rolein contaminant transport were also receiving attention(Scanlon et al., 1997).

4.8.6.2. Watershed responses.Hydrologic responsesof watersheds, particularly after fire, gained specialattention during the 1990s. Theoretical work onpredicting the relationships among infiltration,unsaturated flow, and runoff was reported forunburned watersheds (Beven, 1991a,b). There wasparticular interest in quantifying the hydrologicprocesses involved in subsurface transport from anupper subcatchment during storm events (Wilson etal., 1991), including hillslope infiltration and lateraldownslope unsaturated flow (Jackson, 1992). Recentadvances in modeling of hydrologic systems alsoserved as the theme of a book (Bowles andO’Connell, 1991).

Watershed behavior and hydrologic responses towildfire and changes in soil wettability receivedconsiderable attention by investigators in Africa(Scott, 1997), Portugal (Shakesby et al., 1993;Walsh et al., 1994), Spain (Imeson et al., 1992) andthe United States (Robichaud, 1996).

Water repellency was recognized as playing anincreasingly important role in erosional processes. Innon-fire environments, water repellency inducederosion in dunes along the coast of the Netherlands(Jungerius and Dekker, 1990; Witter et al., 1991).Raindrop splash was recognized as an importantprocess during the erosion of both hydrophobic andwettable soils (Terry and Shakesby, 1993). Specialattention was also directed toward modeling thespatial variability in hillslope erosion followingtimber harvesting and prescribed burning (Robichaud,1996). Comparisons of experimental results withthose predicted by the Water Erosion PredictionProgram (WEPP) models showed reasonable accu-racy, although the WEPP model showed a consistenttendency to underestimate runoff and erosion (Sotoand Diaz-Fierros, 1998).

4.8.7. Fire-induced water repellencyThere was a continuing interest in determining the

heat-induced changes that occur in soils during fire innatural ecosystems (Soto et al., 1991; Giovannini,1994; Sala and Rubio, 1994) and in the associated


creation of water repellency (bibliography byKalendovsky and Cannon, 1997). Changes in soilwater-repellency in response to soil heating duringfire were reported for several ecosystems, including:pinyon–juniper woodlands in the United States(Everett et al., 1995), forests in Spain (Almendros etal., 1990), eucalyptus forests in Portugal (Walsh et al.,1994; Doerr et al., 1998), and chaparral in the UnitedSates (DeBano et al., 1998). Fire effects studies onclosely related changes included studies on the effectof fire intensity on soil changes (Giovannini andLucchesi, 1997), the overall changes in soil qualityproduced by fire (Giovannini, 1994), and fire-inducedchanges in aggregate stability (Molina et al., 1991).

4.8.8. Characterizing water repellencyA wide range of approaches were used to charac-

terize water repellency, including: evaluatingtraditional methodologies; utilizing new analytictechniques; and designing statistical sampling designsto better describe and assess overall water repellencyunder field-scale conditions.

4.8.8.1. Traditional and new methodologies.Specifictechniques were evaluated, included using intrinsicsorptivity as an index for assessing water repellency(Wallis et al., 1991) and standardizing the “water droppenetration time” and the “molarity of an ethanoldroplet” techniques to classify soil hydrophobicity(Doerr, 1998). A method of characterizing dis-aggregated nonwettable surface soils found at oldcrude oil spill sites was also reported (Roy andMcGill, 1998). A detailed examination was made ofthe relationship between water repellency, measuredin the sieve fractions of sandy soils containing organicmatter, and soil structure (Bisdom et al., 1993).

A need to better designate the differences between“potential” and “actual” water repellency and toestablish a “critical soil water content” when asses-sing water repellency was identified (Dekker andRitsema, 1994a). The water repellency of an oven-dried sample is designated as “potential”, in contrastto the water repellency of a field moist sample whichis referred to as “actual”. The need to distinguishbetween the two arises because water repellency is atime-dependent soil property and wetting resistancedecreases with time, particularly when the soil isexposed to high humidity or water. These changes

make static measurements of water repellency inade-quate (Dekker and Ritsema, 1994b).

Several new analytical and visualization techniqueswere reported which could potentially improve theassessment of water repellency and its effect on soilwater movement. One such technique is the use ofcomputed tomographs as a tool for non-destructiveanalysis of flow patterns in macroporous clay (Heijset al., 1995). Time Domain Reflectometry (TDR)employing standard three-rod probes was also usedto measure volumetric water contents at differenttimes and positions in the soil profile (Ritsema etal., 1997a). A visualization techniques, useful formore vividly portraying three-dimensional fingerflow patterns, was generated using modular visualiza-tion software and associated computer hardware(Heijs et al., 1996; Ritsema et al., 1997a). Anotherpromising technique was the use of NMR (nuclearmagnetic resonance) to quantify and study organicmatter in whole soils (Preston, 1996). Furthermore,a more detailed chemical analysis of organic matterusing reflectance Fourier transform infrared spectro-scopy (DRIFT) was tested as a potential tool for char-acterizing water repellency by providing aliphatic C–H-to-organic C ratios (Capriel et al., 1995). A higherratio indicated greater water repellency.

4.8.8.2. Sampling and landscape characterization.The importance of sampling, characterizing, andportraying the spatial distribution of waterrepellency under field conditions received theattention of several investigators. Spatial variabilityof soil hydrophobicity was studied in fire-proneeucalyptus and pine forests in Portugal (Doerr et al.,1996, 1998). A detailed study was conducted on theinfluence of sampling strategy on detectingpreferential flow paths in water-repellent sand(Ritsema and Dekker, 1996b). It was found that asthe physical size of the soil sample increase, thedetection of detailed preferential flow patternsdiminished until they were eventually unobservable.Therefore, because preferential flow patterns vary inspace and time, the optimal number of samples todetect these paths varied, indicating that samplingstrategies needed to be flexible in design.

There is an emerging interest in characterizingwater repellency on a landscape basis. Such a relation-ship would involve first relating specific soil properties


to water repellency (McKissock et al., 1997), and thenusing the spatial distribution of these diagnostic soilparameters to predict the occurrence of water repel-lency over large landscapes (Harper and Gilkes,1994).

4.8.9. Summary publicationsThe 1990s also were highlighted by several

summary publications on water repellency, or closelyrelated subjects. These included a state-of-the-artpublication on water repellency (Wallis and Horne,1992). The proceedings for two conferences inAustralia also appeared, one held in 1990 in Adelaide(Oades and Blackwell, 1990) and the Second NationalWater Repellency Workshop held at Perth in 1994(Carter and Howes, 1994). In addition, an interna-tional conference on soil erosion and degradation asa consequence of forest fires was held in Barcelona,Spain in 1991, with the proceedings published in 1994(Sala and Rubio, 1994).

The two regional conferences in Australia broughttogether an excellent review of the ongoing researchand application of knowledge being conducted there.Topics discussed at the 1990 conference included areview of the historical problems created by waterrepellency; the physics of water-repellent soils; thechemistry of water repellency; the effect of water-repellent soils on wind erosion of sand soils; and theuse of a furrow sowing press wheel, wetting agents,additions of clays, and plants to ameliorate waterrepellency. The second conference in 1994 expandedon the earlier conference and the topics discussedwere: causes and extent of water repellency, biologi-cal control of water repellency, the use of furrowsowing techniques to improve infiltration, farmerexperiences when managing water-repellent soils,amelioration of water repellency with soil additives(e.g. claying), economic impact of water repellencyon farm management, and establishment of perennialpastures on water repellent sands (Carter and Howes,1994).

The Barcelona conference was concerned with fireeffects, particularly on erosion and soil degradation.Many of the papers discussed fire-induced waterrepellency and erosion processes following the fire.The important topics that were discussed were theaction of forest fires on: vegetative cover and soilerosion, soil quality, physical and chemical properties

of the soil, changes in aggregate stability, and overallpost-fire management and erosion response.

5. Summary

The body of knowledge concerning water repel-lency has evolved from a little known academiccuriosity arising during vegetation studies in theearly 1900s to a highly complex field of study duringthe 1990s. Early in the 1900s, scientists began report-ing hard-to-wet soils, especially those associated withthe “fairy ring” or “dry patch” phenomenon.Restricted water infiltration and redistribution inwater-repellent soils and their effect on the productiv-ity of horticultural crops, citrus trees, pasture produc-tion, and turf management were of concern tomanagers, starting in the 1940s and continuing tothe present. Concerns over runoff and erosion fromwildlands, particularly following fires, attractedconcentrated interest in the 1960s and 1970s. Thisstimulated interest in studying the entire soil–watersystem and led to methods of characterizing waterrepellency in terms of water penetration, liquid-solidcontact angles, and eventually the dynamics of bothlaboratory and field soil water systems. The concernwith restricted water movement in soil led to a concur-rent effort to develop mitigating techniques that wouldimprove plant productivity and reduce runoff anderosion.

The past 30 years have witnessed an ever-increas-ing interest in water repellency. This evolving interesthas shifted the centers of research and the areas ofinterest concerning water repellency in soil. Duringthe 1940s and 1950s, the primary pragmatic interestwas in citrus production in Florida, USA, although“bare patch” disease was reported in clover pasturesin south Australia. In the 1960s, water-repellent soilsbecame a high profile concern both in southern Cali-fornia, USA and in Australia. In the United States, asubstantial research effort on water repellency byfaculty and graduate students at the University ofCalifornia, Riverside (UCR) focused on developingcontact angle theory to characterize water repellencyand also on applying remedial wetting agent treat-ments to increase infiltration rates into water-repellentsoils. Working closely with the UCR group wasa small group of USDA Forest Service scientists


working on fire-induced water repellency and post-fire erosion on the San Dimas Experiment Forest insouthern California. Simultaneously, during the1960s, a concerted effort was being conducted onthe other side of the world by Australian scientists,who were concentrating on understanding microbialeffects on soil physical properties, including waterrepellency in sandy soils and is effect on pastureproductivity. The worldwide interest at this point intime was reviewed and synthesized at the first inter-national conference on water repellency at Riverside,CA in June of 1968.

The 1970s witnessed an awakening to the world-wide occurrence of water repellency and as such wastitled the decade of “spinoffs” Water repellency wasreported in a wide range of agricultural and naturalenvironments. During this decade there was a concen-trated interest in the use of wetting agents as a reme-dial treatment for water-repellent soils. The researchand technology necessary to implement water harvest-ing techniques were well established during thisdecade and their application continues to occur world-wide up to the present. Another significant occur-rences during the 1970s was the bridging of interestsof scientists working on aggregation and on water-repellent soils during a special conference on soilconditioners. Significant advances in soil physicsrelationships describing the instability of flow inhomogeneous and layered systems provided the back-ground for implementing these concepts in the follow-ing decades. The study of hysteresis and preferentialflow through macropores was in its infancy and justbeginning to attract the serious attention of scientistsinterested in water movement in soils.

The decade of the 1980s experienced a combinationof the old and the new. The age-old concerns with“dry patch” and fire-induced water repellencyremained. However, some new aspects of water repel-lency began emerging, including an interest in thewetting of peat soils and organic forest soils. Mostnoteworthy were the advances that were being madein understanding surface hydrology and water move-ment through soils. A great deal of theoretical work onlateral flow in sloping and layered soils was beginningto make its appearance. These developments hadimportant implications in describing catchmentresponses. New approaches for describing soil watermovement in non-uniform systems were rapidly being

developed. The first reports on the concept of watermovement through porous media by fingering werebeginning to appear. The concept of preferentialflow was to become the “center” of interest in describ-ing soil water movement in the following decade.Another important concept that emerged during the1980s was the role of the integrated soil–water systemin the rapid transport of pollutants from the surfaceinto the underlying groundwater. These interests inintegrated systems and preferential water flow wereto blossom and provide the basis for describing waterand pollutant movement through hydrophobic soilsystems in the 1990s. Thus, it seems appropriate todesignate this decade as one of “enrichment” in that itwas setting the stage for immense progress in under-standing water-repellent soil systems in the followingdecade.

The 1990s started at full acceleration with a work-shop, at Wageningen in the Netherlands, on the theo-retical and practical dimensions of distribution flowand fingering phenomena. Many of the paperspresented at this workshop represented a strong coop-erative effort by scientists from the Netherlands andthe United States. This workshop set the stage for afull frontal attack on the process of water movementinto and through hydrophobic soils by lateral flow onthe surface and finger flow through the soil profile.The practical dimension of this effort was the envir-onmental concern with the rapid movement of chemi-cals and other pollutants through the soils into theunderlying groundwater aquifers. Meanwhile, on theother side of the world, in Australia and New Zealand,a strong interest in understanding and amelioratingwater repellency was maturing. Two regional confer-ences in Australia, one in 1990 and a second in 1994,permitted significant discussions of new and inno-vative techniques for ameliorating water-repellentsoils. The treatments to improve water-repellentsoils that were discussed at these two conferencesinclude: claying, wide furrowing, use of press wheels,direct seeding, promoting microbial decompositionof hydrophobic substances, and high pH soil treat-ments.

The remainder of this decade promises to continueto be highly productive, as is indicated by the papersincluded in this issue. The information presented hereis truly a rewarding and significant platform forfuture endeavors in understanding the role of water


repellency in the management of agricultural andwildland environments.

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