-
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
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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.
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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
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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
L.F. DeBano / Journal of Hydrology 231–232 (2000) 4–32 7
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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
L.F. DeBano / Journal of Hydrology 231–232 (2000) 4–328
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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
L.F. DeBano / Journal of Hydrology 231–232 (2000) 4–32 9
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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
L.F. DeBano / Journal of Hydrology 231–232 (2000) 4–3210
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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
L.F. DeBano / Journal of Hydrology 231–232 (2000) 4–32 11
-
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
L.F. DeBano / Journal of Hydrology 231–232 (2000) 4–3212
-
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
L.F. DeBano / Journal of Hydrology 231–232 (2000) 4–32 13
-
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
L.F. DeBano / Journal of Hydrology 231–232 (2000) 4–3214
-
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
L.F. DeBano / Journal of Hydrology 231–232 (2000) 4–32 15
-
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
L.F. DeBano / Journal of Hydrology 231–232 (2000) 4–3216
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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
L.F. DeBano / Journal of Hydrology 231–232 (2000) 4–32 17
-
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
L.F. DeBano / Journal of Hydrology 231–232 (2000) 4–3218
-
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
L.F. DeBano / Journal of Hydrology 231–232 (2000) 4–32 19
-
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
L.F. DeBano / Journal of Hydrology 231–232 (2000) 4–3220
-
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
L.F. DeBano / Journal of Hydrology 231–232 (2000) 4–32 21
-
repellency in the management of agricultural andwildland
environments.
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