soil water repellency: its causes, characteristics and hydro-geomorphological significance

Ž .Earth-Science Reviews 51 2000 33–65www.elsevier.comrlocaterearscirev

Soil water repellency: its causes, characteristics andhydro-geomorphological significance

S.H. Doerr ), R.A. Shakesby, R.P.D. WalshDepartment of Geography, UniÕersity of Wales Swansea, Singleton Park, Swansea SA2 8PP, UK

Received 27 May 1998; accepted 26 January 2000

Abstract

Ž .Water repellency hydrophobicity of soils is a property with major repercussions for plant growth, surface andsubsurface hydrology, and for soil erosion. Important advances have been made since the late 1980s in identifying the rangeof environments affected by water repellency, its characteristics and its hydro-geomorphological impacts. This reviewsummarises earlier work, but focusses particularly on these recent advances and identifies remaining research gaps.

Ž . Ž .The associations of water repellency with a soils other than coarse-textured ones, b an expanding list of plant species,Ž .and c a widening range of climates other than seasonally dry types have been recurrent themes emphasised in recent

literature. Nevertheless, knowledge about the extent of water repellency amongst world soils is still comparatively sparse. Itsorigin by the accumulation of long-chained organic compounds on or between soil particles is now widely accepted, butunderstanding of their exact chemical composition and means of attachment to particle surfaces remains incomplete. Thetransient nature of water repellency has been found to be mainly associated with fluctuations in soil moisture, but the preciseprocesses and required conditions for the changes from hydrophobic to hydrophilic and vice versa are so far only poorlyunderstood.

Significant advances relating to the hydro-geomorphological impacts of hydrophobic layers have been made since the late1980s in identifying and separating the various effects of such layers on surface and subsurface water flow. It has becomeevident that these effects in turn are influenced by variables such as the frequency and effectiveness of flow pathwaysthrough hydrophobic layers as well as their position and transitory behaviour. Recent literature has continued to highlight therole of water repellency in promoting soil erosion and it is now recognised that it can promote rainsplash detachment and

Ž .soil loss not only by water, but also by wind. Major research gaps, however, remain in a isolating the erosional impact ofŽ .water repellency from other factors, and b identifying the exact role of, and the interactions between the different variables

controlling development and effectiveness of flow pathways through hydrophobic soil. Improved understanding of the

) Corresponding author. Tel.: q44-1792-295228; fax: q44-1792-295955.Ž .E-mail address: [email protected] S.H. Doerr .

0012-8252r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.Ž .PII: S0012-8252 00 00011-8

( )S.H. Doerr et al.rEarth-Science ReÕiews 51 2000 33–6534

effects of soil water repellency will enable its overall role in surface and subsurface hydrological and erosional processes tobecome more clearly defined. q 2000 Elsevier Science B.V. All rights reserved.

Keywords: water repellency; hydrophobicity; soil hydrology; soil erosion

1. Introduction

Ž .Soil water repellency hydrophobicity reducesthe affinity of soils to water such that they resistwetting for periods ranging from a few seconds to

Žhours, days or weeks e.g. King, 1981; Doerr and.Thomas, 2000 . In addition to its detrimental and

Žoften costly implications for plant growth e.g. House,.1991; York, 1993 , it has substantial hydrological

and geomorphological repercussions. These includethe reduced infiltration capacity of soils, enhancedoverland flow and accelerated soil erosion, unevenwetting patterns, development of preferential flow

Žand the accelerated leaching of agrichemicals e.g.Imeson et al., 1992; Shakesby et al., 1993; Ritsema

.et al., 1993, 1997 . Instances of water repellencyŽhave been recorded as early as 1917 Schantz and

.Piemeisel, 1917 , but relatively few studies investi-gated this phenomenon prior to the 1960s. Duringthe 1960s and 1970s, research into soil water repel-lency was intensified, particular foci being repel-lency-induced by wildfire, and management andamelioration strategies for water-repellent land, top-ics that were covered in detail by a Astate-of-the-artB

Ž .review by DeBano 1981 . During the followingdecade, research broadened significantly, and it be-came apparent that water repellency was much morewidespread than previously acknowledged. Progressmade during this period was summarised by Wallis

Ž .and Horne 1992 . Subsequently, a considerable bodyof research has been published, which is widelydisseminated amongst the pedological, agricultural,geomorphological, geochemical and hydrological lit-erature. This paper aims to provide a critical review

Ž .of the phenomenon, focussing in particular 1 on thesignificant advances that have been made since the

Ž .late 1980s, and 2 on those topics that have not beenreviewed previously in much detail, such as thephysico-chemical principles and the hydro-geomor-phological consequences of water repellency. Cur-rently used techniques to measure water repellencyand amelioration methods for affected land are dis-

Ž . Ž .cussed by Wallis and Horne 1992 and Doerr 1998 ,Ž .and Wallis and Horne 1992 and Moore and Black-

Ž .well 1998 , respectively, and are therefore not con-sidered in detail here.

2. Physico-chemical principles of water repellencyand its occurrence in soils

2.1. The origin of water repellency

Water repellency is a relative concept: no surfaceactually exerts a repelling force on a liquid. There isalways some attraction between a liquid and anysolid. In practice, therefore, an entirely hydrophobic

Ž .surface does not exist Tschapek, 1984 . A hy-drophilic surface allows water to spread over it in acontinuous film whereas water on a hydrophobicsurface water ‘balls up’ into individual dropletsŽ .Adam, 1963 . If the surface is a porous medium like

Ž .sand or soil, water infiltration is inhibited Fig. 1 .For hydrophobic sand or soil with sufficiently largepore openings, water might occupy the openings butwill not cover the individual grains, whereas hy-drophilic particles will be covered by a film of waterŽ .Anderson, 1986 . The affinity or repellency betweenwater and solid surfaces originates from mutual at-

Fig. 1. Water droplets resisting infiltration into soil due to extremewater repellency. Hypodermic needle for scale.

( )S.H. Doerr et al.rEarth-Science ReÕiews 51 2000 33–65 35

Ž .tractive forces adhesion and the attraction betweenŽ .the water molecules cohesion . To understand these

forces better, some properties of water are brieflyconsidered here.

A water molecule comprises an oxygen atom witha partial negative charge and two hydrogen atomswith a partial positive charge. The hydrogen andoxygen atom bonds are positioned 1058 apart, givingthe water molecule a strongly dipolar structureŽ .Parker, 1987 . The attraction of these positive andnegative ends causes water molecules to form aggre-gates, held together by Ahydrogen bondsB. Wateradheres to most natural surfaces since they consist ofpositively and negatively charged ions attracting thenegative end or the positive ends of a water molecule,respectively. However, the dipole character of wateralso results in a comparatively strong force counter-acting the attraction to charged surfaces. Within aliquid, the net force acting on an individual moleculeis zero as it is surrounded by other molecules andtheir forces. Beyond the surface of a liquid, however,no similar molecules exist to oppose the attractionexerted by the molecules within the liquid. Conse-quently, the surface molecules experience a net at-tractive force towards the interior, thus promotingthe reduction of the surface area of water. Thus, if

opposing forces are minimal, liquids will assume aŽ .spherical shape i.e. that of a droplet . To enlarge the

surface of a liquid, work is necessary. This work isrelated to the surface tension or surface free energyof the liquid and is expressed in Newton per metre.Most liquids have a surface tension of 20 to 40=

10y3 Nrm at 208C, but that of water is exception-ally high at 72.75=10y3 Nrm. With increasingtemperature, the surface tension of liquids is reducedŽ .Parker, 1987 .

The same principle applies to solid surfaces, al-though their nature inhibits deformation into a spher-ical shape. Thus, the surface tension of solids leadsto lateral forces at the surface. Values for hard solidsrange from 500 to 5000=10y3 Nrm, increasing

Ž .with hardness and melting point Zisman, 1964 . Forwater to spread on a solid, the adhesive forcesbetween them must exceed the cohesive forces withinthe body of water. Thus, surfaces with a surface-freeenergy )72.75=10y3 Nrm attract water and aretherefore hydrophilic. The higher the surface tensionof the solid, the stronger is the attraction. All princi-pal soil minerals have a much higher surface freeenergy than water and are therefore hydrophilicŽ .Tschapek, 1984 , whereas soft organic solids, suchas waxes or organic polymers can exhibit surface

Ž . Ž .Fig. 2. Schematic representation of I an amphiphilic molecule and IIra–c changes in orientation of such molecules on a mineral surfaceŽ .while in contact with water based on Tschapek, 1984; Ma’shum and Farmer, 1985; Velmulapalli, 1993 .


free energy values below 72.75=10y3 Nrm andŽ .are thus hydrophobic Zisman, 1964 .

2.2. Water repellency in soils

In theory, a single layer of hydrophobic moleculescan render a hydrophilic mineral surface water-repel-

Ž . Ž Ž ..lent Zisman, 1964 see Section 4 and Fig. 2 IIa . Ithas been suggested, however, that hydrophobic com-pounds tend to be absorbed as small globules and notin uniform monolayers. Thus, an amount equivalentto that of several monolayers may be required toresult in a complete cover on a mineral grainŽ .Ma’shum et al., 1988 . The amount required is still

Ž .relatively small. For example, Ma’shum et al. 1988induced severe water repellency in 1000 g ofmedium-sized sand using only 0.35 g of hydrophobiccompounds.

Attempts to investigate such hydrophobic coatingsusing microscope examination have produced incon-sistent results. They range from the observation of a

Ž .distinctive coating Franco et al., 1995 to no de-Žtectable coating on hydrophobic material Jungerius

.and de Jong, 1989 and from coatings on both hy-Ž .drophobic and hydrophilic particles Jex et al., 1985

to coatings on only some, but not all hydrophobicŽ . Ž .grains Doerr, 1997 Fig. 3 . This inconsistency

may be caused by one or both of the followingŽ .reasons: 1 soil particle surfaces are frequently cov-

Fig. 3. Hydrophobic organic coating on a previously hydrophilicsand grain formed during an experimental burn of Eucalyptus

Žglobulus litter over dry, washed sand SEM image, 180= magni-.fication .

ered with other small particles of various originŽ .Tschapek, 1984 , and thus, the coating observed

Ž .may be unrelated to hydrophobicity; 2 since or-ganic coatings can be as thin as a few molecularmonolayers they may remain undetected even usingScanning Electron Microscopy. Whatever their pre-cise nature, it is now widely accepted that organiccoatings are a common cause of water repellency.

Soil water repellency can also be caused by thepresence of hydrophobic interstitial matter. If hy-drophobic particles are present in the pore spaces ofhydrophilic matrix, the wettability of the compositematerial is reduced. For example, severe water repel-lency has been induced by intermixing as little as2–5% by weight of organic matter to hydrophilic

Ž .sand McGhie and Posner, 1981 . For naturally hy-drophobic sand, it has been suggested that a slight tomoderate repellency can be caused by the presenceof hydrophobic particles in a soil matrix, but thatmore extreme water repellency results from a coating

Ž .on individual soil particles Bisdom et al., 1993 .

3. Severity of water repellency and its classifica-tion

Most techniques for measuring and classifyingsoil water repellency are summarised in TschapekŽ . Ž .1984 and Wallis and Horne 1992 . The two mostcommon methods, the ‘Water Drop Penetration

Ž . Ž .Time’ WDPT test Van’t Woudt, 1959 and theŽ . Ž‘Molarity of an Ethanol Droplet’ MED test also

known as the ‘Percentage Ethanol’ or ‘Critical Sur-. Ž .face Tension’ test Letey, 1969 are referred to in

this review. The former determines how long waterrepellency persists in the contact area of a waterdroplet and the latter measures indirectly the appar-ent surface tension of a soil surface, i.e. how stronglywater is repelled. These properties are somewhat, but

Ž .not always well related Dekker and Ritsema, 1994 .Recent advances relating to these tests are presented

Ž . Ž .in Dekker et al. 1998 and Doerr 1998 .Perception of what constitutes a low or high

degree of water repellency varies widely. To distin-guish between hydrophilic and hydrophobic soils,

Ž .WDPT thresholds of 1 s Roberts and Carbon, 1971 ,Ž . Ž5 s Bisdom et al., 1993 , or 60 s Walsh et al.,


.1994 have been used, making direct comparisonsbetween studies difficult. Furthermore, actual WDPTvalues found in the literature are not always directlycomparable because, for practical reasons, few work-ers have conducted this test for more than 600 s andeven where this has been carried out, tests wereterminated before droplet infiltration had occurredŽ .Table 1 . In most studies to date, soils have exhib-ited WDPTs of less than 600 s, and in comparativelyfew cases have values exceeding 1, or even 5 h beenrecorded. The MED test has been used less widelyand thus fewer data are available. A molarity equiva-lent to 16.6% ethanol was rated as the highest sever-

Ž .ity class by King 1981 , but this value has beenŽ .exceeded more recently Table 1 . As an indication

of the relevance of the hydrophobicity values re-ported, it may be noted that agricultural production isreportedly affected above a molarity equivalent of

Ž5% ethanol for some Australian sandy soils Franco.et al., 1995 . For the purpose of this review, soils

Ž .with a WDPT)1 h Dekker and Ritsema, 1994 ora MED equivalent to )20% ethanol are termed‘extremely hydrophobic’.

Although these tests allow classification of soilsaccording to their persistence and severity in waterrepellency, it has been shown that they are notalways well related to the actual wetting behaviour

Ž .of soils Doerr and Thomas, 2000 and Dekker et al.Ž .1999 recommended the wider use of wetting ratemeasurements in future studies.

4. Origin and characteristics of hydrophobic sub-stances

4.1. Substances responsible for water repellency

It is commonly accepted that soil water repellencyis caused by organic compounds derived from livingor decomposing plants or microorganisms. Hy-

Ždrophobic substances occur in many life forms Ta-.ble 2 . Waxes on plants, for example, not only

increase frost hardiness and drought resistance, butcan also reduce wettability and enhance the self-

Žcleaning ability of leaf surfaces Neinhuis and.Barthlott, 1997 . The identification of the specific

compounds causing water repellency has continued

Table 1Ž .Maximum WDPT and % ethanol values measured in various studies –sno data

Ž . Ž .Author s WDPT s % Ethanol Vegetation type Location

Ž .Carter et al. 1994 – 19.0 blue lupin AustraliaŽ .Chan 1992 60 – cultivated land Australia

Ž .Crockford et al. 1991 2700 40.0 eucalyptus forest New South WalesŽ .Barrett and Slaymaker 1989 )600 – subalpine meadow British Columbia

Ž .Burcar et al. 1994 5400 30.0 pine forest CaliforniaŽ .Brock and DeBano 1990 70 – chaparral CaliforniaŽ .Dekker and Ritsema 1994 21,600 25.0 grass The Netherlands

Ž .Doerr et al. 1996 )18,000 36.0 eucalyptus stands PortugalŽ .Dyrness 1976 1800 – lodgepole pine Oregon

Ž .Giovannini and Lucchesi 1983 480 – chaparral SardiniaŽ .Harper and Gilkes 1994 206 4.3 crop rotation Australia

Ž . Ž .Jex et al. 1985 453 – unspecified FloridaŽ .Jaramillo and Herron 1991 10,800 20.0 pine stand Colombia

Ž .Karnok et al. 1993 – 18.9 turf grass GeorgiaŽ .King 1981 260 10.0 croprpasture rotation AustraliaŽ .King 1981 – 16.6 pasture Australia

Ž .McGhie and Posner 1980 )3600 – eucalyptus forest AustraliaŽ .Richardson and Hole 1978 1800 – red pine stand WisconsinŽ .Roberts and Carbon 1971 7800 – lucerne Australia

Ž .Scholl 1971 540 – juniper trees UtahŽ .Teramura 1980 360 – chaparral CaliforniaŽ .Witter et al. 1991 55 – dune vegetation The Netherlands


Table 2Some naturally occurring hydrophobic substances and their estab-

Žlished sources after Horn et al., 1963; Kolattukudy et al., 1976;.Tulloch, 1976

Ž .Substance Source s

n-alkanes bacteria, fungi, algae,higher plants

Olefines bacteria, fungi, algae,higher plants

Terpenoides many plant waxesMonoketones bacteria, higher plantsb-diketones higher plants

Ž .e.g. eucalyptus, grassesŽ .Polyesters of hydroxy-fatty acids higher plants e.g. pines

Žto be a focus of soil research in the last decade e.g.Franco et al., 1994; Hudson et al., 1994; McIntosh

.and Horne, 1994 . However, despite advances inanalytical techniques, identifying the exact sub-

Ž .stance s responsible in a given soil has yet to beachieved. Furthermore, how these compounds arebonded to soil particles also remains unclear. Acomplicating factor in such studies is the naturalabundance of various, potentially responsible sub-stances in soil. For example, from just one sampled

Ž .soil, Almendros et al. 1988 extracted 93 organiccompounds, many of which were hydrophobic.

The compounds identified from water-repellentsoils can be divided into two main groups. The firstare the aliphatic hydrocarbons, which are substancesconsisting of hydrogen and carbon with the carbonatoms arranged in an elongated chain. They are

Žnon-polar i.e. have no positive or negative charges.at either end of the chain and are consequently

almost insoluble in water. The second group repre-sents polar substances of amphiphilic structure, com-prising a hydrocarbon chain with one end having afunctional group with a positive or negative charge.This end is hydrophilic, whereas the other is hy-

Ž Ž ..drophobic Fig. 2 I . Despite being generally watersoluble, amphiphilic molecules can be highly effec-tive at producing a hydrophobic coating providedtheir polar ends are bonded to a surface as illustrated

Ž .in Fig. 2 IIa . Both groups are thought to cause waterŽ .repellency McIntosh and Horne, 1994 , but the po-

Žlar molecules i.e. fatty acids and certain waxes such.as esters and salts of fatty acids appear to be the

main constituent of the hydrophobic coating on wa-

Žter-repellent sands Ma’shum et al., 1988; Hudson et.al., 1994; Franco et al., 2000 .

4.2. Sources of hydrophobic substances

4.2.1. VegetationIn many studies, the occurrence of water repel-

lency has been associated with particular vegetationŽ .types Table 3 . This list is not exhaustive and it

cannot be assumed that these species always inducewater repellency under natural conditions. In some

Žstudies, fire has been a ‘triggering’ factor e.g. De-Byle, 1973; Reeder and Juergensen, 1979; Mallik

. Ž .and Rahman, 1985 see Section 5.1 . In other stud-ies, the link is based on laboratory experiments withfresh plant material, so that natural decompositionand incorporation into the soil have not been not

Žconsidered e.g. Roberts and Carbon, 1972; Reeder.and Juergensen, 1979; McGhie and Posner, 1981 .

Furthermore, the mechanism of input of these hy-drophobic substances into the soil it is not alwaysclear. Although decaying plant litter has been shownto be a source of these substances in some studiesŽe.g. Reeder and Juergensen, 1979; McGhie and

.Posner, 1981 , other studies have found water repel-lency to be more closely associated with the root

Žactivity of plants Dekker and Ritsema, 1996a; Doerr.et al., 1998 .

Plants most commonly associated with water re-pellency seem to be certain evergreen tree types. Inparticular, trees with a considerable amount of resins,waxes or aromatic oils such as eucalyptus and pinesare well represented, both within and outside their

Ž .native environment Table 3 . Water repellency hasalso been found under shrubs ranging from temper-

Ž .ate heathland Mallik and Rahman, 1985 , orŽ .mediterranean shrubland Giovannini et al., 1987 , to

Ž .semi-desert chaparral DeBano, 1991 .Soils under grassland can also resist wetting, a

problem that has been of considerable concern onareas of high economic value such as golf greens.

Ž .For example, under bentgrass Agrosti spp. , dryspots, which can persist prolonged irrigation, are a

Ž .common feature Karnok et al., 1993; York, 1993 .The association of water repellency with grass hasalso been noted on pasture in, for example, AustraliaŽ . ŽCrockford et al., 1991 , Canada Barrett and Slay-

. Žmaker, 1989 and the Netherlands Dekker and Rit-


.sema, 1994 but the species involved are not alwaysŽ .stated. Furthermore, dune grass Spinifex hisutus is

Žthought to induce hydrophobicity McIntosh and

Table 3Higher plant species reportedly associated with water repellency

Ž .Plant species or vegetation type Author s

Deciduous treesŽ .Populus tremuloides Reeder and Juergensen 1979Ž .Larix occidentalis Reeder and Juergensen 1979

Evergreen treesŽ .Acacia mearnsii Scott 1992

Ž .Banksia speciosa Moore and Blackwell 1998Ž .Citrus spp. Jamison 1942

Ž .Eucalyptus astringens McGhie and Posner 1981Ž .Eucalyptus globulus Doerr et al. 1996

Ž .Eucalyptus marginata Roberts and Carbon 1972Ž .Eucalyptus patens Moore and Blackwell 1998

Ž .Juniperus osteosperma Scholl 1971Ž .Pinus banksiana Richardson and Hole 1978

Ž .Pinus jeffreyii Meeuwig 1971Ž .Pinus monophylla Holzhey 1969

Ž .Pinus patula Jaramillo and Herron 1991Ž .Pinus pinaster Shakesby et al. 1993

Ž .Pinus radiata Scott and Van Wyk 1990Ž .Pinus resinosa DeByle 1973Ž .Pinus strobus Meeuwig 1971Ž .Pseudotsuga macropora Holzhey 1969Ž .Pseudotsuga mentziesii DeByle 1973Ž .Picea engelmanii DeByle 1973

Ž .Quercus ilex Cerda et al. 1998`Ž .Quercus turbinella Holzhey 1969

Ž .Quercus suber Sevink et al. 1989Ž .Tsuga canadensis Richardson and Hole 1978

ShrubsŽ .Adenostoma fascilatum DeBano 1969Ž .Adenostoma sparcifolium Holzhey 1969Ž .Arctostaphylus spp. Holzhey 1969

Ž .Calluna Õulgaris Mallik and Rahman 1985Ž .Chamaespartium spp. Richardson and Hole 1978

Ž .Chrysotamnus spp. DeBano 1969Ž .Cistus monspelliensis Giovannini et al. 1987Ž .Erica arborea Giovannini et al. 1987

Ž .Vaccinium spp. Richardson and Hole 1978Ž .Ulex europaeus Soto et al. 1994

GrassesŽ .Agrostis spp. Wilkinson and Miller 1978

Ž .Erharta calycina McGhie and Posner 1981Ž .Phalaris spp. Bond 1964

Ž .Phragmites spp. Berglund and Persson 1996Ž .Sphagnum spp. Berglund and Persson 1996Ž .Spinifex hisutus McIntosh and Horne 1994

Ž .Table 3 continued

Ž .Plant species or vegetation type Author s

CropsŽ .Chamaecystitus proliferus Carter et al. 1994

Ž .Hordeum Õulgare McGhie and Posner 1981Ž .Lupinus cosentinii Moore and Blackwell 1998

Ž .Medicago satiÕa Bond 1964Ž .Trifolium subterranum Roberts and Carbon 1972

.Horne, 1994 . Water repellency has also been associ-ated with some crops. For example, in Australia, itsdevelopment has become a major concern under blue

Ž . Ž .lupin Lupinus cosentinii Carter et al., 1994 .The production of highly hydrophobic compounds

by plants may not only serve the physiological pur-Žpose outlined earlier. It has been suggested e.g.

.Scott, 1992; Moore and Blackwell, 1998 that therelease of hydrophobic substances in the soil is, in asimilar fashion to allelopathy, used by plants tosuppress the germination of competing vegetationand to improve water conservation by channellingwater deep into the soil profile following preferentialflow pathways, while at the same time reducingevaporation due to the partial dryness of the surface

Ž .soil layer see also Section 8.5 .

4.2.2. Soil fungi and microorganismsThe association of water-repellent with certain

plants may not always be direct. Water repellencyhas also been associated with fungal growth and soilmicroorganisms, which in turn can be associatedwith specific vegetation types. Schantz and PiemeiselŽ . Ž1917 postulated that the ‘fairy rings’ circles ofenhanced grass growth in pastures and crops, fol-

.lowed by a zone of water-repellent soil were due tothe mycelia of the fungus Basidiomycea, a commonspecies that decomposes litter and especially lignin

Ž .in soils Scheffer and Schachtschabel, 1989 . A rangeof fungi and microorganisms has been associatedwith water repellency, for example Penicillium ni-

Ž .gricans and Aspergillus sydowi Savage et al., 1972 ,Žor Actinomycetes microorganisms with fungal and. Ž .bacterial properties Jex et al., 1985 . Effects have

been found to be species-dependent, with somespecies inducing hydrophobicity, and others reducinghydrophobicity in an already hydrophobic material


Ž .McGhie and Posner, 1981; Roper, 1998 . However,reports on the effect of even the same species are notalways consistent. Thus, Aspergillus niger, e.g. hasbeen reported to induce water repellency by Borne-

Ž .misza 1964 , but in a separate study, it had no effectŽ .Savage et al., 1972 .

Compared with the input of organic material fromhigher plants, the biomass input from organismswithin the soil can be considerable. For example, thefungal net biomass in soil alone may be equal to oreven exceed aboveground biomass production, asshown for the Pacific Northwestern forests of the

Ž .USA Fogel and Hunt, 1979 . Clearly, since alsomany species of algae and bacteria can produce

Ž .hydrophobic compounds see Table 2 , their role inthe establishment of water repellency could well be

Ž .significant, though York and Baldwin 1992, p. 11concluded that Ano firm link has been provided,which can unquestionably connect the presence ofmicrobes with the production of water-repellent ma-terialsB. Given that microbes and fungi are alsoinvolved in the decomposition of hydrophobic com-

Ž .pounds Franco et al., 1994; Roper, 1998 , it mayremain very difficult to isolate a particular fungus ormicroorganism as the sole agent responsible for wa-ter repellency in a soil.

4.2.3. Soil organic matter and humusApart from investigating the direct influence of

vegetation and microorganisms on water repellency,research has also attempted to establish general rela-tionships between soil organic matter andror organiccarbon content and the degree of water repellency.The results, however, have been very inconsistent.Apart from the more commonly found positive corre-

Žlation between the variables e.g. Wallis et al., 1990;.Berglund and Persson, 1996; McKissock et al., 1998 ,

Ž .a negative correlation Teramura, 1980 as well as noŽcorrelation Jungerius and de Jong, 1989; DeBano,

.1991; Wallis et al., 1993 have also been reported.The simple explanation for this inconsistency may bethat the small amount of hydrophobic compoundsnecessary to cause water repellency is not propor-tional to the actual amount of organic material pre-sent in soil, particularly if different soil horizons oreven different soils are compared. Thus, a consistentrelationship of water repellency might be expectedwith a type, or a fraction of, the organic material,

rather than the total amount of organic matter orŽ .carbon Wallis and Horne, 1992 . Concerning mate-

rial type, links have been established between litterand humus type and repellency. A more severe soilwater repellency has usually been found under a

Ždeeper litter layer Scott and Van Wyk, 1990; Crock-. Žford et al., 1991 , or a mor-type humus Sevink et

.al., 1989; Imeson et al., 1992 . These findings areŽ .supported by Dinel et al. 1990 , who reported that

Ž .the concentration of hydrophobic lipids decreaseswith the efficiency of the decomposition regime, and

Ž .by Valat et al. 1991 , who found the degree ofhumification in peat to be positively and the degreeof decomposition negatively correlated with waterrepellency.

The above findings indicate that water repellencydevelopment is not only dependent on certain or-ganic substances being released by certain plants or

Ž .microbes into susceptible soil see also Section 5.2 .It seems that a slow natural decomposition regimeandror the excessive accumulation of hydrophobiccompounds under managed vegetation are an addi-tional factor in water repellency development. For

Ž .example, Franco et al. 2000 have suggested that aselective microbial activity is essential in the devel-opment and appearance of repellency, accumulatingpolar waxes from a pool of plant waxes present insoil. Thus, further advances in identifying the sourcesand the exact chemical composition of hydrophobiccompounds may, by themselves, not be sufficient indeveloping our understanding of soil water repel-lency development. More research into water repel-lency in relation to decomposition regime is alsoneeded. Such knowledge would also be useful indeveloping further biological amelioration strategiesfor soil water repellency.

5. Non-biological factors affecting water repel-lency

5.1. Soil temperature and the effect of fire on waterrepellency

By burning plant litter and heating sand and soilin laboratory experiments, DeBano and Krammes


Ž . Ž . Ž .1966 , DeBano et al. 1970 and Savage 1974observed that fire could induce hydrophobicity inpreviously hydrophilic soil, and either enhance orreduce the surface hydrophobicity in an already hy-drophobic soil, depending on fire temperature, theamount and type of litter consumed and pre-fire soilmoisture level. They proposed what subsequentlybecame a widely accepted mechanism in whichheated hydrophobic organic substances in the litterand topsoil become volatilised during burning with aproportion travelling downward following the tem-perature gradient in the litter and soil until they

Ž .condense in a concentrated form see also Fig. 3 .Such fire-induced water repellency became a focusof research during the 1960s and 1970s in NorthAmerica and the view that Athe heat during a firemarkedly changes and intensifies water-repellencyBŽ .DeBano, 1981, p. 5 has since become widely ac-cepted. Subsequently, fire-related water repellency

Žhas been reported from Europe e.g. Mallik and.Rahman, 1985; Imeson et al., 1992 , South Africa

Ž . ŽScott and Van Wyk, 1990 , and Australia Zierholz.et al., 1995 .

Apart from redistributing and concentrating hy-drophobic substances in the soil, the heat during afire is also thought to improve the bonding of these

Ž .substances to soil particles Savage et al., 1972 andmake them chemically more hydrophobic by pyroly-

Ž . Ž .sis Giovannini, 1994 . DeBano 1991 suggestedthat the heating of any hydrophilic soil containing

more than 2–3% organic matter would induce waterrepellency.

The effect of high soil temperature on waterrepellency has been investigated in manylaboratory-based studies and is well established. Wa-ter repellency is generally intensified at temperaturesof 175–2008C. Hydrophobic substances are fixed tothe soil particles around 2508C, but destroyed above

Ž .270–3008C e.g. Savage, 1974; DeBano et al., 1976 .Somewhat different thresholds have been reported by

Ž .Nakaya 1982 , possibly the result of differences inmeasuring methods, length of heating times andtypes of chemical compounds present in the soilsinvestigated. For example, DeBano and KrammesŽ .1966 found that water repellency increased at lowertemperatures if longer heating times were sustained.

Quantifying directly the effect of burning on asoil in field conditions has not always been at-tempted as many studies have been instigated by thepassage of a fire and provide little information on thesoil status before burning. Where surface water re-pellency has been measured separately on burnt andunburnt land, it is usually higher on the former, or,the surface water repellency is destroyed, while a

Žrepellent layer is developed at depth e.g. McNabb etal, 1989; Brock and DeBano, 1990; Scott and Van

. Ž .Wyk, 1990 Table 4 . However, fire may not in-evitably lead to an increase of water repellencysomewhere in the soil profile. A net reduction has

Ž .also been reported Giovannini and Lucchesi, 1983

Table 4Maximum water repellency levels measured in studies investigating fire-affected terrain

Ž .Author s Unburnt land Burnt land Vegetation Location

Ž . Ž .Boelhouwers et al. 1996 -5 s )120 s WDPT eucalyptus forest South AfricaŽ . Ž .Brock and DeBano 1990 70 s 107 s WDPT chaparral California

Ž . Ž .Doerr et al. 1996 )18,000 s )18,000 s WDPT eucalyptus and pine PortugalŽ . Ž .Dyrness 1976 1800 s 7200 s WDPT pine forest Oregon

Ž . Ž .Giovannini and Lucchesi 1983 460 s 240 s WDPT chaparral SardiniaŽ . Ž .McNabb et al. 1989 588 718 contact angle various trees Oregon

Ž .Mallik and Rahman 1985 decrease in hydrophobicity 1 month after fire heather Great BritainŽ .Reeder and Juergensen 1979 increase in number of hydrophobic sites slash deposits Michigan

following fireŽ .Scott and Van Wyk 1990 more than doubling of sites with WDPT pine forest South Africa

)10 s after fireŽ .Scholl 1975 118 increase in contact angle after fire chaparral Arizona


and in areas where soils are ‘naturally’ highly hy-drophobic, fire may have very little effect on waterrepellency, provided that soil temperatures remain

Žbelow the threshold of repellency destruction Zier-.holz et al., 1995; Doerr et al., 1996 .

Comparatively little is known about the longevityof these high temperature effects on water repellencybecause long-term post-fire monitoring is rare andthe existing results vary widely. For example, inconiferous forests in the USA, fire-induced waterrepellency has been found to persist for as long as 6

Ž .years Dyrness, 1976 or as little as a few monthsŽ .DeBano et al., 1976 . On the other hand, for aSardinian chaparral, water repellency that was de-stroyed during a fire became re-established within 3

Ž .years Giovannini et al., 1987 . Because of the manyŽvariables involved e.g. the soil temperature reached,

type of organic compounds, soil type, climatic condi-.tions , the persistence of fire effects on water repel-

lency may be very site-specific and thus difficult topredict.

Water repellency is also affected by heating soilto temperatures not as high as those considered

Ž .above. Crockford et al. 1991 and Dekker et al.Ž .1998 found respectively that drying soil at 438C or708C both increased WDPTs, an effect that might becaused by an increase in the alignment of the hy-

Ž .drophobic molecules see also Section 6 . Anotherpossibility is that during heating, waxes from particu-late organic matter migrate onto soil particle sur-faces, thereby inducing or increasing water repel-

Ž .lency Franco et al., 1995 . The recognition that theheat applied during oven-drying can increase waterrepellency has recently led to the preference of air-drying over oven-drying of samples prior to testing

Ž .water repellency Dekker et al., 1998; Doerr, 1998 .The actual temperatures of the soil and the water

applied also affect their affinity. In Australia, sum-mer rains wet repellent sandy soils more readily thanautumn or winter rains, a feature that has been

Žattributed to higher soil temperatures King, 1981;.Blackwell, 1993 . Similarly, repellency measure-

ments in the field give higher MED values if con-ducted in the shade, rather than on soils exposed to

Ž .the sun Blackwell, personal communication . Thisapparent decrease in water repellency with increas-ing temperature may be caused by the reduction in

Ž .surface tension see Section 3 and thus increased

‘wetting power’ of the warmer water or MED testsolution. In addition, it might be that an amphiphilicmolecule coating on soil particles has a generallyenhanced ‘reactivity’ in warmer conditions and thusbecomes detached more readily during water contact.

5.2. Soil texture and clay content

Soil water repellency has in the past been gener-Žally associated with coarse-textured, sandy soils e.g.

Roberts and Carbon, 1971; Wilkinson and Miller,.1978; McGhie and Posner, 1980; DeBano, 1991 . It

is reasoned that, given a limited supply of hydropho-bic substances to coat soil particles, coarser particlesare more susceptible to developing water repellencybecause of their smaller surface area per unit volume

Žcompared with soils of finer texture Giovannini and.Lucchesi, 1983; Blackwell, 1993 . For example, a

medium-sized sand has a surface area of 0.00772 Ž .m rg DeBano, 1971 , whereas clay can have a

2 Ž .surface area as large as 900 m rg Rowell, 1994 .Ž .Thus, Crockford et al. 1991 found an increase in

water repellency with particle size within a soilŽ .sample. DeBano 1991 concluded that water repel-

lency is most likely to develop in soils with less than10% clay content, and it is now well established thatthe addition of dispersible clay can be very effective

Žin reducing water repellency in sandy soil Cann and.Lewis, 1994; Carter and Hetherington, 1994 .

Notwithstanding the apparent higher susceptibilityof coarse textured soils to develop water repellency,it has become increasingly evident during the lastdecade that even severe water repellency is notuncommon in soils with considerable clay content.Soils with 25% to more than 40% clay have been

Žfound to exhibit extreme water repellency Crock-ford et al., 1991; Chan, 1992; Dekker and Ritsema,

.1996b . It has been suggested that this may occur aslong the clay forms aggregates, thus reducing thesurface area to be covered with a hydrophobic skinŽ .Wallis et al., 1991; Bisdom et al., 1993 . Thisconcept, however, does not explain the occurrence ofhydrophobicity in all cases, as some studies havefound that the finer fractions of water-repellent soilscan exhibit a similar, or even higher degree of

Žrepellency than the coarse ones Doerr et al., 1996;.de Jonge et al., 1999 . It may be that particulate


hydrophobic organic matter is itself relatively fine,enhancing the degree of repellency in the fine sieve

Ž .fraction de Jonge et al., 1999 . Alternatively, insome environments, the supply of hydrophobic mate-rial might be sufficiently high not only to cover thecoarser, but also much of the finer-sized particles

Ž .with an organic coating Doerr et al., 1996 . In suchcases, a fine-grained soil could then be more water-repellent than a coarser one due to its larger totalarea of hydrophobic surface within the soil matrix.This might explain why the degree of water repel-lency, where encountered in finer-textured soil, canbe amongst the highest levels reported anywhereŽe.g. Crockford et al., 1991; Chan, 1992; Doerr et al.,

.1996 , whereas the susceptibility to water repellencydevelopment would be higher in coarse-textured soils,as indicated by the much greater number of incidentsreported.

In conclusion to Sections 4 and 5, it should benoted that it does not seem to be possible, as indi-

Ž .cated by McKissock et al. 1998 for a range ofŽAustralian soils, to use any of the individual soil or

.vegetation characteristics discussed in these sectionson their own to predict accurately the occurrence orthe degree of water repellency that can be expectedin a soil.

6. Temporal variations of water repellency andthe influence of soil moisture

Water repellency is usually a transient soil prop-erty, varying through time. An important factor inthese variations is soil moisture. Until recently, hy-drophobicity has been generally considered to bemost severe in dry soil and to decline as soil mois-ture increases until a critical moisture content isreached, above which a soil becomes hydrophilicŽe.g. DeBano, 1971; Witter et al., 1991; Carter et al.,

. Ž .1994 . Thus, Dekker and Ritsema 1994 consideredit important to distinguish ‘actual repellency’ of a

Žfield moist soil from its ‘potential repellency’ the.maximum repellency measured when soils are dry .

Although water repellency generally disappears whensoils become wet, the soil moisturerwater repellencyrelationship is, nevertheless, more complex thanstated above. This section reviews ideas on how

water absorption in hydrophobic soils occurs, itseffects on the actual repellency of soils, and howhydrophobicity is re-established after wetting.

6.1. Water absorption of soils under water-repellentconditions

Soils can absorb water while being hydrophobicŽ .Crockford et al., 1991; Dekker and Ritsema, 1996a .For example, water repellency was found to bepresent for soil moisture contents of up to 22%Ž . Ž .grav. in sandy loams Doerr and Thomas, 2000 , in

Ž . Ža clayey peat with up to 38% vol. Dekker and. Ž .Ritsema, 1996a , but as little as 2% vol. for dune

Ž .sands Dekker and Ritsema, 1994 . The followingmechanisms are suggested here to explain this seem-ingly paradoxical behaviour:

Ž .1 Water is thought to move freely as vapour in awater-repellent soil allowing soil water to be redis-

Ž .tributed Barrett and Slaymaker, 1989 until the soilhas reached its maximum adsorption capacity for

Ž .individual water molecules Myamoto et al., 1972 .The adsorption capacity of a mineral surface forindividual molecules, however, is small and furthercondensation of water onto surfaces can only takeplace in the form of droplets, a process that is

Ž .constrained in hydrophobic soils Osmet, 1963 .Ž . Ž .2 Imeson et al. 1992 suggested that fine hy-

drophilic material present in the pore spaces of anotherwise hydrophobic soil matrix allowed partialwetting of the soil. This could also take place by

Žvapour condensation and would allow moisture in.addition to what has been adsorbed as vapour onto

hydrophobic particles without affecting the repel-lency of the whole soil matrix.

Ž .3 Similarly, some initially hydrophobic soil par-ticles may have changed to a hydrophilic status

Ž .during vapour adsorption see Section 6.2 , allowinga further water uptake, while enough hydrophobicsurface area remains particularly on the soil surfaceto maintain the hydrophobic reaction measured bythe WDPT or MED tests.

Findings on the actual effect of a moisture in-crease in water-repellent soil are inconsistent. Manyauthors have reported an inverse relationship of soil

Žmoisture with water repellency e.g. King, 1981,.Witter et al., 1991 . However, an initial increase in


water repellency with soil moisture has also beenfound. This was attributed to an enhanced activity ofhydrophobic substances producing microorganisms

Ž .by Jex et al. 1985 , whereas in studies by BerglundŽ . Ž .and Persson 1996 and de Jonge et al. 1999 bio-

logical processes were not considered.

6.2. Water absorption and the loss of water repel-lency

In theory, a material can remain hydrophobic aslong as the organic layer covering its surface remainsunaltered during its contact with water. However, itis well known from textile engineering that watercontact can weaken water repellency, a process mostpeople have experienced themselves while wearing asupposedly water-repellent garment. The WDPT test,for example, takes advantage of the fact that pro-longed contact with water can lead to the loss of soilwater repellency. The following concepts are thoughtto cause this breakdown:

Ž .1 Where water repellency is mainly caused by acoating of amphiphilic molecules on soil particles,the attraction of water to the polar ends of thesemolecules is thought to weaken the soilrmoleculebond, leading eventually to the displacement of theorganic compounds from soil particles and resulting

Žin a wettable soil Tschapek, 1984; Ma’shum and. Ž Ž ..Farmer, 1985 see Fig. 2 IIa–c . This concept,

however, remains poorly understood and it is notclear, which factors determine the length of timenecessary for water repellency to break down. Obser-vations on the timing and conditions of water repel-lency breakdown under field conditions are rare,although several weeks of wet weather were neces-sary for repellency to break down in an Australian

Ž .eucalyptus forest Crockford et al., 1991 .Ž .2 Water repellency can be overcome if surface-

active substances migrate from the soil into the wateruntil the surface tension of the water is sufficiently

Žreduced to allow infiltration Richardson and Hole,.1978; Barrett and Slaymaker, 1989 . Surface-active

substances can be provided by humic and fulvicacids present in the soil. Humic acids in largerquantities are only soluble above pH 6.5, whereasfulvic acids are soluble throughout the pH range, sothat the latter would be more important in reducing

Žthe surface tension of water Chen and Schnitzer,.1978 . Following findings by Tschapek and Wa-

Ž .sowski 1976 , these acids, however, would onlyreduce the surface tension of water enough to allow

Žmoderately repellent soil particles MED equivalent.of -15% ethanol to be wetted, but be insufficient

to wet more severely repellent soils.

6.3. Re-establishment of water repellency after wet-ting

Soil water repellency is largely regarded as aseasonal phenomenon, being usually low or com-pletely absent under prolonged wet conditions and

Žmost severe during extended dry periods e.g. Crock-ford et al., 1991; Imeson et al., 1992; Ritsema and

.Dekker, 1994b . Thus, it is generally assumed thatwater repellency becomes readily re-established upon

Ž .drying e.g. Valat et al., 1991; Walsh et al., 1994 .As an underlying mechanism, it is thought that thepolar ends of amphiphilic compounds associate andinteract through hydrogen bonds if humidity be-comes very low. This forces the molecules to re-adopta position in which their polar ends are attached tothe mineral surface and the non-polar ends are orien-

Ž Ž ..tated outwards as in Fig. 2 IIa , thus re-establishingŽwater repellency during drying Tschapek, 1984;

.Ma’shum and Farmer, 1985; Valat et al., 1991 .However, under field conditions, some usually wa-ter-repellent soils were found to be dry and hy-

Ž .drophilic at times Burch et al., 1989 and CrockfordŽ .et al. 1991 found that it took 6 to 9 days of hot and

dry weather for water repellency to become re-established. Furthermore, in laboratory experimentson fine- and medium-textured soils, water repel-lency, which had disappeared during wetting experi-

Žments, did not reappear after drying Crockford et.al., 1991; Doerr and Thomas, 2000 . This suggests

that the proposed re-orientation of amphiphilicmolecules is not necessarily caused by the drying

Žprocess alone. It has been shown that heating i.e..oven-drying thoroughly wetted samples can restore

water repellency to some extent, although not to itsŽinitial levels Ma’shum and Farmer, 1985; Doerr,

.1997 . This may be associated with the re-attachmentof some organic molecules on soil particles as sug-gested above. This mechanism is also known in the


Žoutdoor fabric industry where the heat treatment e.g..tumble-drying or ironing can be used to restore the

Ž .water repellency of used rainwear Gore et al., 1994 .It is, however, not clear which processes lead to thecomplete re-establishment of water repellency afterwetting in some soils. It might be that waxes presentas interstitial globules in the soil matrix migrate ontomineral surfaces aided by warm temperatures or

Ž .microbiological mechanisms Franco et al., 1995 . Inother cases, a completely new input of hydrophobicsubstances into the soil may be required for repel-

Ž .lency to be restored Doerr and Thomas, 2000 .To conclude this section, it is fair to say that there

is still very little known on the timing of, and theprinciples underlying, the breakdown and re-estab-lishment of water repellency and on the effects ofsoil moisture on repellency. What is clear is that,associated with climatic factors, water repellency canfollow short-term or seasonal variations. The criticalsoil moisture content above which water repellencydisappears varies considerably between soils, and

Ž .seems, at least in part see Section 6.1 , to besoil-texture related. However, this threshold and theexact mechanisms involved in the cessation andrestoration of water repellency have not been investi-gated in much detail, despite their practical implica-tions. For example, an improved ability to predict theseasonal and short-term fluctuations of hydrophobic-ity could lead to more efficient irrigation regimes indry periods, enabling soils to be kept sufficientlymoist to avoid the onset of water-repellent conditionsŽ .Miller and Wilkinson, 1977 .

7. Distribution of soil water repellency

7.1. The global and regional scales

Until the mid-1980s, most studies reporting soilwater repellency had been conducted in areas ofsemi-arid or mediterranean climate like that of the

Žsouthwestern USA e.g. Krammes and Osborn, 1969;. ŽScholl, 1975 , South and Western Australia e.g.

.Bond, 1969; Roberts and Carbon, 1971 , drier re-Žgions in Africa Rietfeld, 1978; Bishay and Bakhati,

. Ž1976 and the Mediterranean Giovannini and Luc-.chesi, 1984 . Thereafter, a considerable number of

studies reported water repellency also from muchŽwetter regions such as Great Britain Mallik and

. ŽRahmann, 1985 , British Columbia Barrett and

. ŽSlaymaker, 1989 , the Netherlands Jungerius and de. Ž .Jong, 1989 , Colombia Jaramillo and Herron, 1991 ,

Ž .north-central Portugal Shakesby et al., 1993 andŽ .Sweden Berglund and Persson, 1996 . This suggests

that water repellency is not largely confined to rela-tively dry climates.

Mapping of water-repellent soils is usually notincorporated in general soil surveys although, wheresystematic surveys have been conducted, water repel-lency has been shown to affect large areas. Forexample, in the Netherlands 75% of the crop- and

Žgrass-land exhibit water repellency Dekker and Rit-.sema, 1994 and in Australia, 5 million ha of landŽ .are affected House, 1991 , leading to production

Ž .losses of up to 80% in agriculture Blackwell, 1993 .The effects of water repellency are not surprisinglymost pronounced in environments with long dryperiods, as is the case for parts of Australia. Theycan, however, also be of economic significance inclimates with less seasonal precipitation patterns such

Ž . Žas Great Britain York, 1993 or Sweden Berglund.and Persson, 1996 , provided that a dry spell reduces

soil moisture sufficiently to allow the onset ofwater-repellent conditions.

7.2. Small-scale Õariations

Water repellency has often been reported to bediscontinuous within the soil both spatially and verti-cally. This has been found to be particularly preva-lent in fire-affected soils. The pattern of a surfacelayer rendered hydrophilic by the intense heat of achaparral fire followed by a layer made distinctlywater-repellent by condensed hydrophobic sub-stances has first been described in detail by DeBanoŽ .1969 in California. This pattern has often beenreferred to in later studies of fire-affected terrainŽe.g. Dyrness, 1976; Scott and Van Wyk, 1990;

.Boelhouwers et al., 1996 . In some cases, however, adistinct hydrophilic surface soil layer has not been

Ž .found Sevink et al., 1989; Soto et al., 1994 , thesurface water repellency was destroyed without re-

Žpellency being induced at depth in the soil Giovan-.nini and Lucchesi, 1983 , or fire had very little effect


Žon an already highly hydrophobic soil Doerr et al.,. Ž .1996 see also Fig. 8 . This contrast in findings may

be in part attributed to different soil temperaturesŽ .reached during burning. DeBano 1981 argued that

chaparral fires are usually hotter than forest fires dueboth to the entire consumption of both living anddead fuel and to the lower soil moisture levels. Themaximum soil temperature reached during a firevaries with the soil moisture status, thickness of theinsulating litter layer, burn duration and post-fire

Ž .smouldering DeBano, 1991 . The spatial diversityof these variables could also explain the usuallypatchy distribution of water repellency found at the

Žsurface of burnt soil e.g. Brock and DeBano, 1990;.Scott and Van Wyk, 1990; Imeson et al., 1992 .

Spatial variations seem also to be typical forŽwater repellency in unburnt soils e.g. Jungerius and

de Jong, 1989; Karnok et al., 1993; Ritsema and.Dekker, 1994b although under certain conditions

water repellency can be spatially continuous. Crock-Ž .ford et al. 1991 investigating natural eucalyptus

forest found this to be the case provided a thick litterŽ .layer was present see also Section 4.2 and Doerr et

Ž .al. 1998 attributed spatial homogeneity of waterrepellency in commercial eucalyptus forest to an

exceptionally high release and thorough distributionof hydrophobic substances combined with the partic-ularly uniform vegetation and litter cover of the area.

Variations of water repellency with depth in un-burnt soils may be less pronounced than in theirfire-affected counterparts, but layering also seems

Ž .typical. For example, Barrett and Slaymaker 1989Ž .and Crockford et al. 1991 working in forests andŽ .Brock and DeBano 1990 investigating chaparral

soils reported that water repellency was confined to,or most severe in a layer few centimetres in thick-

Ž . Ž .ness see also Fig. 4 . Dekker and Ritsema 1994 , inan extensive study investigating the spatial variabil-ity of water repellency in Dutch dunes, found thatwater repellency showed considerable variation

Ž .within the top 50 cm. Roberts and Carbon 1971observed that hydrophobic sandy soils in Australiacommonly had a surface hydrophilic layer a fewmillimetres thick above the repellent layer. In con-

Ž .trast, Doerr et al. 1996 found that water repellencywas common from the mineral surface down to nearthe bedrock for both burnt and unburnt forest soils inPortugal, and attributed this to a high release ofhydrophobic substances into the relatively shallowsoils.

Ž .Fig. 4. Vertical extent of the water-repellent layer as indicated by the hand in sandy soil, near Geraldton, Western Australia. Thedevelopment of water repellency here is associated with the cultivation of Lupinus cosentinii.


Variations in the distribution pattern of waterrepellency can be of major importance for the hydro-logical and geomorphological effects of soil waterrepellency as discussed in Section 8.

8. The hydrological and geomorphological conse-quences of water repellency

8.1. OÕerÕiew

The main hydrological impacts of soil water re-Ž .pellency reported in the literature are a reduced

Ž . Ž .infiltration capacity; b increased overland flow; cspatially localised infiltration andror percolation, of-

Ž .ten with fingered flow development; d effects onthe three-dimensional distribution and dynamics of

Ž .soil moisture; e enhanced streamflow responses toŽ .rainstorms; and f enhanced total streamflow. It is

also normally argued that because of enhanced over-land flow on and increased erodibility of water-repel-lent soil, slopewash, and sometimes the formation ofrills and gullies, may be promoted. This sectionhighlights new developments since previous reviews

Žin this field e.g. DeBano, 1981; White and Wells,.1982; Wallis and Horne, 1992 , focussing also on

studies that question some of the generalisations thathave previously been made.

8.2. Infiltration and oÕerland flow

The most frequently reported impacts of water areŽthose of reduced soil infiltration capacity e.g. Van

. Ž .Dam et al., 1990, Imeson et al., 1992 Fig. 5 andŽthus increased overland flow e.g. McGhie and Pos-

ner, 1980; Crockford et al., 1991; Witter et al.,.1991 . For example, the infiltration capacity of a

water-repellent soil was found to be 25 times lowerthan for a similar soil rendered hydrophilic by heat-

Ž . Ž .ing DeBano, 1971 . Wallis et al. 1990 found thatthe infiltration capacity was six times lower on awater-repellent dry sand than on adjacent moist, less

Ž .repellent sands and, in a separate study 1991 , re-ported that, for the first 5 min of measurement, ahydrophobic soil had only 1% of the potential infil-tration capacity when hydrophilic.

A water-repellent surface layer causes rainwaterto pond and, if rainfall is sufficient and surface

Ž .detention is exceeded, Hortonian infiltration-excessŽ .overland flow will occur Fig. 6 . The frequency of

Fig. 5. Hydraulic conductivity curves from a wettable and aŽwater-repellent sandy soil from the Netherlands modified from

.Van Dam et al., 1990 .

Žgaps through this layer such as structural or dryingcracks, root holes and burrows, and patches of hy-

.drophilic or less hydrophobic soil will then deter-mine whether overland flow is widespread or only

Ž .local Fig. 7A . As outlined in Section 7.2, a water-repellent layer also frequently underlies a hy-drophilic and often highly permeable topsoil or ashlayer. Rainfall infiltrating such a topsoil may pond

Ž .above the water-repellent layer Fig. 7B and cansubsequently:

Ž .1 be stored in the hydrophilic layer and laterevaporated or used in transpiration;

Ž .2 run off as saturation overland flow when thehydrophilic layer becomes saturated;

Ž .3 spread out as ‘distribution flow’ and movevertically downwards as ‘preferential flow’ eitherthrough structural or other gaps in the water-repel-lent layer or as ‘fingered flow’ through verticalcylinders of hydrophilic or less water-repellent soilŽ .see Section 8.3 ;

Ž .4 move laterally downslope as throughflowabove the water-repellent layer;

Ž . Ž5 enter the matrix of the water-repellent layer ifentry-pressure is sufficient, or water repellency un-dergoes a phase-change to a hydrophilic condition as

.outlined in Section 6.2 .


Fig. 6. Water repellency causing ponding of rainwater and overland flow on sandy soil near Geraldton, Western Australia, following a largeŽ .rainfall event 75 mm in 3–4 h in March 1999 .

Where overland flow is considered the key effectof water repellency, it is often not clear whetherHortonian or saturation overland flow, or a combina-tion of the two, is involved, particularly if the hy-drophilic layer is thin, below the surface, or discon-tinuous. Thus, in an Australian eucalyptus forest,

Ž .Burch et al. 1989 reported a threefold increase inoverland flow from 5% to 15% after drought hadenhanced water repellency. In burnt pine forest inSouth Africa, saturation overland flow promoted bya subsurface water-repellent layer led to an increasein the stormflow response to 7.5% compared with

Žjust 2.2% on unburnt terrain Scott and Van Wyk,. Ž .1990 , and Jungerius and de Jong 1989 attributed

the lack of any simple relationship between rainfalland overland flow in Dutch sand dunes to spatialvariations in water repellency.

Some studies, particularly those investigatingburnt terrain, highlight water repellency as only oneof several factors enhancing overland flow re-

Ž .sponses. Thus, Dyrness 1976 reported a threefoldincrease in overland flow after a fire in a pine forest

Ž .in Oregon and Walsh et al. 1994 found a 5–25%higher overland flow response on Portuguese burntcompared with unburnt eucalyptus and pine forest.Rather than invoking only water repellency, post-fire

increases in overland flow have also been attributedŽto removal of a protective vegetation cover with

resulting increases in rainbeat compaction, inwash offines into cracks and rootholes and reduction in

.infiltration capacity , reduction in soil particle size,erosion of permeable topsoil, stone lag development

Žand organic matter losses White and Wells, 1982;.Imeson et al., 1992; Shakesby et al., 1996 . Such

increases in overland flow were demonstrated usingrainfall simulation plot experiments carried out inPortugal during dry conditions on unburnt Pinuspinaster slopes and a nearby area burnt two years

Ž .before Walsh et al., 1998 . In both areas, mostsurface soil remained dry and intensely hydrophobicthroughout the 1-h 40–46 mm artificial rainstorms.However, whereas in the unburnt soil overland flow

Ž .was modest 4% and most water infiltrated throughcracks and rootholes, on the burnt soil overland flow

Ž .was more substantial 8–20% .Thus, depending on pre-fire conditions, post-fire

increases in overland flow can be associated with:Ž .1 fire inducing or significantly increasing water

repellency, thus enhancing overland flow responsesuntil soil hydrophobicity declines to pre-fire levels;

Ž .2 increased effectiveness of pre-existing waterrepellency, where a return to pre-fire conditions of a


Ž . Ž .Fig. 7. Schematic illustration of possible hydrological responses of soil with A a hydrophobic layer located on the surface, and B ahydrophobic layer sandwiched between hydrophilic soil.

prevailing, but less effective soil hydrophobicitywould await re-vegetation, root development and there-formation of a litter mulch;

Ž .3 soils that are hydrophilic before and after thefire, but where other fire-related changes than hy-

Ždrophobicity enhance overland flow Zierholz et al.,.1995 .

Ž . Ž .Scenarios 1 and 2 are summarised in Fig. 8,which contrasts the more conventional view in whichoverland flow is enhanced mainly due to fire-in-

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Ž .Fig. 8. Schematic illustration of the effects of burning and water repellency on overland flow generation exemplified by two contrasting scenarios. A and B CalifornianŽ . Ž .chaparral where a slight hydrophobicity in the surface layer is destroyed and an intensely hydrophobic layer is generated below the surface based on DeBano, 1969 ; C and D

Portuguese eucalyptus forest, where fire has little effect on the extreme hydrophobicity already present in the soil.


Ž .duced water repellency Fig. 8A and B with ascenario in which overland flow is enhanced due tothe increased effectiveness of pre-existing water re-

Ž .pellency Fig. 8C and D . Some of the changesŽ .involved in Scenario 3 may also promote overland

flow after clearfelling rather than burning, as re-ported for a Pinus radiata catchment in South AfricaŽ .Scott and Lesch, 1997 .

That Hortonian overland flow tends to be mostpronounced where soils have an uninterrupted hy-

Ž .drophobic layer Fig. 7Ar1 provides an explanationfor the contrasts in overland flow response onwater-repellent terrain reported in the literature. Sev-eral studies have pointed towards the localisation ofoverland flow on water-repellent soils. For example,only 25% of the soil following a fire in southwestOregon was water-repellent, resulting in a low im-

Žpact on infiltration and overland flow McNabb et. Ž .al., 1989 . Also Meeuwig 1971 and Imeson et al.

Ž .1992 , working in pine forests in North Americaand northeast Spain, respectively, found that Horto-nian overland flow generated on water-repellent soilaround trees tended to infiltrate on adjacent hy-

Ž .drophilic soil around shrubs Fig. 7Ar2 . This pat-tern was also found using simulated rainstorms in

Ž .southwest Spain Cerda et al., 1998`The short-term temporal variations of water repel-

lency outlined in Section 6 also need to be consid-ered here. Reductions in infiltration capacity andincreases in overland flow can be expected to bemost pronounced following prolonged dry periods,when water repellency tends to become most severeŽ . Ž .see also Fig. 9 . For example, Burch et al. 1989recorded infiltration capacities in Australian eucalyp-tus forest of 0.75–1.9 mmrh when dry, but 7.9–14.0mmrh when wet. In many areas, hydrophobicity-lin-ked overland flow is therefore confined to storm

Ževents following dry weather Sevink et al., 1989;.Walsh et al., 1994 . In burnt Portuguese pine and

eucalyptus forests, the enhanced Hortonian overlandflow responses following dry weather when the soilsare most hydrophobic contrast sharply with the mutedoverland flow in moderately wet weather, when soils

Ž . Žare generally hydrophilic Table 5 Walsh et al.,.1994; Shakesby et al., in press . The high responses

in very wet weather are caused by saturation over-land flow rather than water repellency. Since fewstudies address this issue and the understanding of

Fig. 9. Relative frequency of actual water repellency in a Dutchsilt loam soil on three different occasions. The volume of water-

Ž .repellent soil in the topsoil 0–15 cm decreases considerablyŽbetween October 1992 and January 1993 modified from Dekker

.and Ritsema, 1995 .

the processes involved in the breakdown of waterŽ .repellency is poor Section 6.2 , it is often not

known how long hydrophobicity-linked low infil-tration capacities may persist during wet conditions.

Ž .O’Loughlin et al. 1982 found that the effectivenessof post-fire hydrophobicity was short-lived and

Ž .Zwolinski 1971 also found water repellency rapidlydisappearing during simulated rainfall; in both areasoverland flow was insignificant. Whereas in hy-drophilic soils infiltration capacity declines duringrainstorms, in hydrophobic soils the infiltration ca-

Žpacity often increases as soils become wet Burch et.al., 1989; Imeson et al., 1992 . For extremely water-

repellent soils, however, wetting may not occur at allduring irrigation or rainstorms. This has, for exam-ple, been reported from golf greens in the USAŽ .Karnok et al., 1993 and for some Portuguese forest

Žsoils during 40–46 mm of simulated rain Walsh et.al., 1998 . Water repellency for some of these Por-

tuguese soils has proved so persistent that soils haveremained dry beneath a water layer for more than 3

Ž .weeks Doerr and Thomas, 2000 .Finally, the effects of water repellency on over-

land flow generation should also be seen in relationŽ .to the scale of measurements. Imeson et al. 1992


Table 5Hydrophobic- and hydrophilic-phase overland flow responses at Portuguese pine and eucalyptus forest plots following fire in July 1991 and

Ž .August 1992 after Walsh et al., 1994

Ž . Ž . Ž .Date Rain mm Peak intensity mmrh Antecedent weather Overland flow %

Plot A Plot B

( )a Lourizela : regenerating pine plots after fire in July 199112r3r93 18.9 6.8 dry 19.3 7.030r4r93 46.1 11.9 wet 7.8 4.017r9r93 11.2 4.2 very wet 40.3 21.5

( )b Falgorosa: regenerating eucalyptus plots after fire in August 199212r3r93 18.9 7.5 dry 14.1 11.530r4r93 23.5 7.9 wet 4.7 3.117r9r93 28.4 5.0 very wet 19.7 10.9

argued that although water-repellent soils can pro-duce high overland flow rates locally, the effects atslope or catchment scales can be more subduedbecause of the high spatial variations in infiltration.Similar findings were reported by Roberts and Car-

Ž . Ž .bon 1971 and Pradas et al. 1994 .

( )8.3. Preferential flow including fingered flow

Preferential flow is the concentrated verticalmovement of water via preferred pathways throughthe soil matrix. It may originate for a variety ofreasons such as cracks and macropores, textural dis-

Žcontinuities and unstable wetting fronts which may. Žresult from soil layering or air entrapment Ritsema

.et al., 1993 . Although not restricted to hydrophobicŽ .soils Kung, 1990; Ritsema and Dekker, 1994a ,

hydrophobicity can be particularly effective at pre-venting or hindering downward water movement,directing it into structural or textural preferential

Ž .flow paths Fig. 7Ar2 and B or creating an unstableŽ .irregular wetting front Fig. 10 .

Consequently soils may not wet completely withŽ .the passage of a wetting front DeBano, 1971 , and

water may be channelled via macropores and cracks,Ž .by-passing the soil matrix Burch et al., 1989 . Root

channels and rodent burrows are thought to representparticularly effective bypass route-ways through wa-

Žter-repellent soil Garkaklis et al., 1998; Ferreira et. Ž .al., 2000 . Walsh et al. 1995 considered that such

by-pass routes explained why even large storms pro-duced little overland flow for highly hydrophobicmature pine and eucalyptus forest soils in Portugal.

Where a water-repellent layer is overlain by hy-drophilic soil, infiltrating water tends to pond above

Fig. 10. Uneven wetting patterns caused by water repellency insandy soil near Geraldton, Western Australia, following a large

Ž .rainfall event 75 mm in 3–4 h in March 1999 .


the former, and is then directed as lateral flow tovertical preferential flow routeways through the un-

Ž .derlying water repellent layer Fig. 7B . This phe-nomenon has been described for many burnt soils by

Ž .DeBano 1981 and has been investigated further byŽ . Ž .Ritsema and Dekker 1995 and Ritsema et al. 1993 ,

who have termed the lateral spreading process ‘dis-tribution flow’. On a grass-covered sandy dune in

Ž .Holland, Ritsema et al. 1993 used tracers to recordŽ .distribution flow within a thin - 2.5cm hy-

drophilic, relatively moist humus topsoil, which sup-plied water via columns of less hydrophobic soilŽ .preferential flow paths in an otherwise extremelyhydrophobic layer to a second hydrophilic zone be-low 45 cm depth, where the water spread laterally.

ŽThis has been termed ‘fingered flow’ Ritsema and.Dekker, 1994a,b . The fingers formed only after dry

weather when soil moisture levels in the sandy,water repellency-prone layer were below a ‘critical’

Ž .value of 4.75% vol. . The fingers ranged from 10 toŽ .50 cm in width expanding in wetter weather and

were the sole means of water transport for severalhours during sustained rainfalls until the soil be-tween fingers became wet and hydrophilic. Suchfingers have been shown to recur at the same placesin successive storms following intervening dryweather, possibly aided by preferential leaching ofwater-repellent substances from the finger pathwaysŽ .Ritsema et al., 1998a,b . In contrast, instead of anyfingered flow, a uniform wetting front may developin a water-repellent layer if the overlying hydrophilic

Ž .top-layer is very thick Van den Bosch et al., 1999 .Ž .Bauters et al. 1998 found that sandy soils with

different degrees of water repellency all exhibitedfingered flow, whereas a uniform, broadly horizontalwetting front developed in non-repellent soil. Infiltra-tion of the former occurred only when ponding depthexceeded water-entry pressure, the critical value of

Ž .which was found together with finger flow velocityto increase with water repellency severity. Wetting

Ž .patterns in terms of finger width were found toconform to unstable flow theory and depended onthe characteristic soil water curve of the soil inquestion.

Preferential flow in general is thought to be rein-forced by soil water hysteresis between wetting anddrying phases, a feature of most hydrophilic soils but

Žexaggerated in hydrophobic ones Ritsema et al.,

.1998a,b . The retarding impact of hysteresis andenhancing impact of preferential flow have beenincorporated into a model for unsaturated soil watermovement in water-repellent soil and tested with

Žfield data from the Netherlands Van Dam et al.,.1996 . The model demonstrates clearly how prefer-

ential flow has the effect of reducing residence timesof solutes in the unsaturated zone.

The consequences of hydrophobicity-related pa-rameters on surface and subsurface hydrology dis-cussed in Sections 8.2 and 8.3 have often beenconsidered separately and an attempt is made in Fig.11 to provide a synthesis. The model suggests thatimpacts will vary not only with the degree of hy-drophobicity, but also with the location of the hy-drophobic layer in the soil profile, the thickness ofany overlying hydrophilic layer, the extent and spa-tial contiguity of water repellency per unit area, andas with the effectiveness of preferential flow path-ways. In addition, the temporal regime of water

Ž .repellency see Section 6 is crucial, as it determinesthe proportion of storm events in which water repel-lency exerts an influence.

8.4. Impacts on the three-dimensional distributionand dynamics of soil moisture, eÕapotranspirationand plant growth

As is evident from the previous sections, waterrepellency influences the three-dimensional distribu-tion and dynamics of soil moisture, including evapo-ration patterns. The impact will vary with the verticalposition of the water-repellent layer, the frequency ofpreferential flow routeways through the water-repel-lent layer and the temporal persistence and regime ofwater repellency. For example, a strongly water-re-pellent surface layer with preferential flow route-ways can lead to dry surface soil and higher soil

Žmoisture in the subsoil e.g. Meeuwig, 1971; Burch.et al., 1989; Imeson et al., 1992 . In a study in

Ž .northeast Spain, Imeson et al. 1992 described howa surface water-repellent layer not only trapped wa-ter in the BrC horizon, but also prevented evapora-tion and upward capillary movement of water.

Hydrophobicity-induced fingered flow can lead toconsiderable variations in water content in an ini-tially water-repellent soil. For example, Dekker and

Ž .Ritsema 1996a found differences in soil moistureŽ .of up to 28% vol. between closely spaced samples


ŽFig. 11. Schematic model of the influence of hydrophobicity characteristics on slope runoff processes and erosion risk SOF and HOFw x w x w xabbreviate saturation-and Hortonian overland flow, respectively, and q , qrs and s indicate a strong, low to negligible and

negligible erosion risk.

Ž .in both clay and sandy soils Fig. 12 . Such differ-ences do not only result in the widely reported poor

Žseed germination and plant growth see Wallis and.Horne, 1992 . Any type of preferential flow path

formation can also lead to accelerated leaching ofsurface-applied agrichemicals and an increased riskof surface and groundwater contaminationŽ .Hendrickx et al., 1993; Ritsema et al., 1993 .

8.5. Effects on streamflow generation and patterns

The tendency for fire-related water repellency andfire effects to increase both total streamflow and the

Fig. 12. Contour plot showing a finger-like moisture distributionin a water-repellent sandy soil. The driest areas are associated

Žwith the highest degree of repellency modified from Ritsema and.Dekker, 1997 .

magnitude of storm peaks as well as reducing peak-Žflow response times has been well established White

.and Wells, 1982 . For example, increases of 800%for streamflow and 450% for catchment runoff effi-ciency during the first post-fire wet season wereattributed in large part to hydrophobicity-enhancedoverland flow in a pine forest catchment in ArizonaŽ .Campbell et al., 1977 . More recent studies havesuggested that impacts may be more complex. Scott

Ž .and Lesch 1997 attributed the lack of streamflow 9years after the afforestation of grassland catchmentswith Eucalyptus grandis and Pinus patula in SouthAfrica to an enhanced deep drainage through thewater-repellent soil via preferential flow along theeucalyptus root channels. Instead of promoting over-land flow, water repellency associated with afforesta-tion enhanced water storage at greater depths in thesoil, permitting its later use in transpiration.

Variations in water repellency, its response to fireand its interaction with other factors were thought by

Ž .Scott 1993 to be responsible for differences incatchment response to fire between Fynbos vegeta-tion, Pinus radiata and Eucalyptus festigata inmountain catchments in South Africa. In the nativeFynbos catchments, only a modest increase in total

Ž .discharge 16% and no increase in stormflow oc-curred following both prescribed fire and wildfire.This low response was attributed to only a minor andspatially patchy increase in water repellency, causing


little overland flow to reach the base of slopes. Theincrease in total flow was thought to be due toreduced evapotranspiration. In contrast, in the Euca-lyptus festigata and Pinus radiata catchments, thestormflow component increased by 92% and 201%respectively, the latter being attributed to fire-in-duced hydrophobicity. For the former, water repel-lency had been a pre-fire feature, which becamemore effective in promoting overland flow followingfire.

8.6. Soil erosion

Because of the reduction in infiltration capacitytypical of hydrophobic soils and resulting tendencyfor increased overland flow during rainfall eventsdescribed in Section 8.2, enhanced erosion has oftenbeen attributed to this soil hydrophobicity. Studieswhere such a connection has been made have, how-ever, tended to infer rather than demonstrate a directcausal link between erosion and water repellency.

Ž .For example, Megahan and Molitor 1975 , workingin a pine and fir forest in Idaho, noted a tendencytowards increased soil loss on water-repellent land.Also increases in soil losses immediately after wild-fire have been linked to fire-induced or fire-en-

Žhanced water repellency e.g. Wells et al., 1979;.Morris and Moses, 1987; Shakesby et al., 1993 ,

including the development of rills and gullies onŽburnt land e.g. White and Wells, 1982; Giovannini,

.1994 . Establishing how much of the increased ero-sion is due to water repellency, however, is difficult,as burning also leads to other changes that canenhance overland flow generation as discussed inSection 8.2. Fire can directly promote the erodibilityof soil by the removal of the protective vegetationand litter cover, the loss of organic matter, thebreakdown of aggregates and the reduction of soil

Ž .particle size Scott, 1993 .Few studies have been successful in isolating the

impact of water repellency on erosion from othereffects. A notable exception has been the Califor-

Ž .nian-based study by Osborn et al. 1964 who moni-tored soil losses at bounded plots in burnt, water-re-pellent terrain, some of which had been made lessrepellent by applying wetting agent prior to the firstrainfall event. The amount of sediment removedfrom the untreated plots was more than thirteentimes as much as that removed from the treated

Table 6Erosion, rainfall and maximum rainfall intensity for rainfall events

Ž . Ž . Žon untreated water-repellent and treated hydrophilic plots ap-2 . Žprox. 3=12 m in burnt forest, California modified from Os-.born et al., 1964

No. of days Rainfall Maximum intensity Eroded material3Ž . Ž .on which mm of rainfall cm

Ž .rainfall mmrhoccurred Ž . Ž .Untreated Treated

2 35.8 34.3 for 20 min 515.4 5.71 24.9 62.3 62.31 156.5 962.8 45.32 33.5 10.2 general 110.4 17.02 15.7 11.3 2.82 23.1 Trace Trace2 44.4 91.4 for 2 min 184.1 TraceTotal 333.9 1846.3 133.1

Ž . Ž .low-repellency plots Table 6 . By the end of themonitoring period, all untreated plots had rills up to100-mm deep extending almost their entire lengths,whereas the treated plots showed either minor or norilling.

In a series of studies carried out in sand dunesunder the more humid conditions of the Dutch coastŽe.g. Jungerius and van der Meulen, 1988; Witter et

.al., 1991; Jungerius and ten Harkel, 1994 , the im-pact of water repellency was also demonstrated.These studies highlighted short-term temporal vari-ability in water repellency and the impact of over-land flow on erosion. In winter, when the sand tendsto be moist and hydrophilic, infiltration capacity isextremely high such that heavy rainfall leads to verylittle erosion. After dry spells in summer, however,water repellency is restored and comparatively littlerainfall causes substantial erosion. The impact ismost pronounced on quicker-drying, south-facingslopes with an incomplete cover of moss and algaewhere shallow rills become incised between shrubsand lead downslope to fan-shaped sand deposits. Onslopes of more than 68, overland flow entrains sandgrains in mudflow-like tongues that extend to the

Ž .foot of the slope Jungerius and ten Harkel, 1994 .As a result of the difference in aspect causing differ-ences in the time taken to re-establish water-repellentconditions and thus differences in erosion suscepti-bility, dunes with east–west aligned crests haveasymmetrical cross-profiles with lower-angled south-


than north-facing slopes. Jungerius and van derŽ .Meulen 1988 reported more than half of the ero-

sional loss in one summer on the dunes occurring injust two storms. The subdued topography of thedunes has been attributed to this seasonal water

Ž .repellency-enhanced erosion Witter et al., 1991 .Erosion caused by enhanced overland flow on

water-repellent soil was also suspected by BridgeŽ . Ž .and Ross 1983 and Thompson 1983 on coastal

sand dunes in southeast Queensland and by Soto etŽ .al. 1994 for burnt scrub in northwest Spain, al-

though no figures were provided in support of thisinterpretation. Water repellency was viewed by

Ž . Ž .Shakesby et al. 1994, 1996 and Walsh et al. 1994as the main factor explaining consistently high ratesof erosion on small plots in newly burnt Eucalyptusglobulus and Pinus pinaster forest soils in Portugalin storm events following prolonged dry periodsŽ .Table 7 . In one case, soil losses per millimetre ofoverland flow in a summer storm event of 20 mmwere consistently higher than during wet winterphases, reaching up to 35 times the value calculatedfor the eucalyptus plots during the preceding winterperiod when more than 200 mm of rain fell. It shouldbe noted, however, that the total amounts of sedi-ment removed were nevertheless higher during win-ter rainfall because of much higher volumes of over-land flow.

Rainsplash erosion may also be of particular im-portance on water-repellent soils. Although at theDutch coastal dune sites, overland flow erosion wasviewed as the main erosion process with rainsplashproducing only 2% of the annual sediment yieldŽJungerius and van der Meulen, 1988; Jungerius and

.ten Harkel, 1994 , work on Portuguese forest soilsreferred to above indicated that the contribution ofthe latter process to soil loss may be substantial.Under simulated rainfall conditions, Terry and

Ž .Shakesby 1993 found that rainsplash detachmentamounts recorded for hydrophilic soils were only52–58% on flat surfaces and 51–72% on slopingsurfaces of amounts recorded for water-repellentsoils. Observations of individual drops falling on thetwo types of soil indicated a difference in the actualsplash mechanism. For the hydrophilic soil, a cohe-sive surface crust of closely packed grains quicklydeveloped during simulated rainfall, whereas on thehydrophobic soil, despite a surface water film form-ing over the surface, a continual supply of readilyavailable dry soil was available for splash ejectionfrom both above and below the film. Some questionsmight be raised about the magnitude of its impact,but these results and others reported from northern

Ž .Portuguese forests e.g. Shakesby et al., 1993, 1994provide strong support for the view that water repel-lency plays an important role in promoting soildetachment. Although water repellency was found tobe equally severe in both unburnt and burnt forests

Ž .in these studies Doerr et al., 1998 , it seems that fireŽ .created the conditions of bare soils for water repel-

lency to enhance erosion processes.The erosional impact of hydrophobicity in soils

depends strongly on the degree of contiguity of theŽ .hydrophobic surface Shakesby et al., 2000 . For

example, following a fire in California, Booker et al.Ž .1993 noted that overland flow and slopewashŽrainsplash and overland flow entrainment and trans-

.port were promoted where cracks and other route-

Table 7Mean erosion per millimetre of overland flow for six 8=2 m plots in areas of both newly burnt Eucalyptus globulus and Pinus pinaster,

Ž .northern Portugal modified from Walsh et al., 1994

Period Eucalyptus burnt August 1992 Pine burnt July 1991

Rainfall Mean erosion per millimeter of overland Rainfall Mean erosion per millimeter of overlandŽ . Ž . Ž . Ž .mm flow g mm flow g

a bNov. 22–Dec. 12 1992 33.5 37.8 56.0 45.9Dec. 12–Mar. 5 1993 205.4 8.2 320.3 18.7Mar. 5–Apr. 29 1993 163.9 10.8 – –Apr. 29–Jun. 14 1993 255.7 83.3 – –Jun. 14–Jul. 4 1993 19.9 292.3 30.8 393.7

a Based on values for two plots only.b Based on values for four plots only.


ways through the hydrophobic layer were absent. OnŽ .the other hand, Booker et al. 1993 observed that

Ž .where preferential subsurface flow rather thanoverland flow was present through the existence ofcracks and other routeways, the slopes were prone tolandsliding, because of the enhanced transfer of wa-ter to the subsoil.

Water repellency can also have an indirect impacton erosion processes. In a study on sandy loam soils

Ž .in California, Krammes and Osborn 1969 notedthat following wildfire, material accumulating in theform of debris cones through a process of dry-creepŽ .dry ravel at the foot of slopes was usually water-re-pellent. On inspection, the water-repellent soil wasfound to have a lower bulk density than its hy-drophilic counterpart. Similar conclusions were

Ž .drawn by DeBano et al. 1979 about the relativedensities of non-repellent and water-repellent soils.

Ž .Krammes and Osborn 1969 reasoned that, at leastfor these sandy loams, hydrophilic particles tend todraw together when drying out because of cohesiveand adhesive forces between the particles. Forwater-repellent soils, however, these forces are low,so that paradoxically rainfall may encourage dry-creep because of this failure of the particles toadhere together. Over a 6-month period, they foundthat soil losses by dry-creep from plots treated withwetting agents were 54–56% lower than thoserecorded from untreated hydrophobic plots.

Although most erosion studies on hydrophobicsoils suggest enhanced sediment yield, resistance toerosion may be improved for water-repellent soils

Žwith well-developed aggregates Wallis and Horne,. Ž .1992 . For example, DeBano 1981 argued that

improved aggregate stability was caused by hy-drophobic organic material in the aggregates reduc-ing the swelling and the destructive forces of trappedair. The stabilising effect was found to be greatestfor aggregates 0.5–5.0 mm in size. For larger aggre-gates, the impact of water repellency became lessand plant roots became the most important aggregat-ing mechanism. Other authors have also noted im-proved water stability of water-repellent soil aggre-

Žgates Rawitz and Hazan, 1978; Giovannini and.Lucchesi, 1983; Capriel et al., 1995 .

Not only water erosion, but also wind erosion canbe influenced by soil water repellency. The erodibil-ity by wind may not differ much between non-repel-

lent and repellent soils when dry. However, as em-Ž .phasised in an Australian study by Carter 1990 , the

periods when soils are bare and dry, and thus mostsusceptible to wind erosion, are likely to be longerandror more frequent for the latter. More indirecteffects of water repellency on soil erodibility havebeen found in the studies on Dutch dunes referred toabove. The water-repellent upper and non-repellentlower sands have different degrees of resistance withrespect to wind erosion. The surficial ‘grey’ sands,although susceptible to erosion by overland flow, arecomparatively resistant to aeolian action. Once thegrey sand is eroded sufficiently by overland flowfollowing summer storms, the underlying yellow,hydrophilic sands can become exposed and these aresusceptible to entrainment by wind and to the devel-

Žopment of blowouts e.g. Jungerius and de Jong,.1989; Jungerius and ten Harkel, 1994 .

9. Conclusion

Since the late 1980s, the study of soil hydropho-bicity has expanded both geographically as well as inspecific research directions. Current understanding ofthe causes and some of the key factors affecting

Ž .water repellency Fig. 13 confirm that water repel-lency in soils is caused partly or entirely by hy-drophobic, long-chained organic molecules, releasedfrom decomposing or burning plant litter. Recently,the root zone and the leaf surfaces of living plantshave also been acknowledged as possible sources ofhydrophobic compounds. These organic compoundscan cause hydrophobicity by their presence as acoating on individual soil grains or aggregates, or asinterstitial particles between soil grains. The role offire in affecting water repellency has been firmlyestablished over previous decades and the most no-table recent advance seems to be the increasingrealisation that burning may, in some cases, causelittle change in already hydrophobic soil. The en-hancing effect of soil heating on water repellency tolower temperatures than those associated with burn-ing, however, has only in the past few years beenwidely acknowledged. Similarly, the greater suscep-tibility of coarse-textured soils to hydrophobicitydevelopment has long been known, but increasinglyin more recent studies, it has been demonstrated that


Fig. 13. A summary of current ideas of how water repellency develops in soils and of the factors controlling its occurrence.


when water repellency becomes established in finer-textured soils, it can be equally or even more severethan in the former.

There are clear research gaps that relate to thecauses and characteristics of water repellency. Theyconcern particularly the poor understanding of theexact chemical composition of the compounds caus-ing hydrophobicity and their mechanisms of attach-ment to soil particles. In addition, the roles of soilfungi and microorganisms and thus of the decompo-sition regime of organic matter in general are stillrelatively unclear, for both the establishment anddestruction of water repellency. This uncertainty is,for example reflected in the number of conflictingreports on the relationship of organic matter to waterrepellency. It appears, however, that the relationshipsof general soil and vegetation parameters to waterrepellency vary so widely between soils that it mayremain very difficult to establish firm links that areapplicable to a wide range of soils and environments.

Major advances since the late 1980s have beenmade in understanding the impacts of water repel-lency on hydro-geomorphological processes. Its abil-ity to cause or enhance uneven wetting and preferen-tial flow in soils is now widely acknowledged andearlier, relatively simple views of how hydrophobic-ity enhances overland flow have had to be refined.The impact of water repellency on water movementover and within the soil, and thus on soil erosion riskby water has often not been sufficiently isolated

Ž .from other factors particularly after burning , butthe range of impacts are now known to be far morecomplex than previously acknowledged. Variablesdetermining these impacts are not only the frequencyand effectiveness of flow pathways through any hy-

Ž .drophobic layer Fig. 11 , themselves influenced byvegetation type, land use, and soil structure, but alsothe position, intensity and temporal regime of the

Ž .hydrophobic layer s in the soil.Two important research gaps can be identified

regarding the hydro-geomorphological significanceof soil water repellency. First, little is still under-stood about the spatial contiguity of hydrophobicityand the frequency and effectiveness of preferentialflow pathways, and their overall influence on runoffprocesses and streamflow generation. For example,the role of hydrophobicity in the development of

Ž .tunnelling piping and the development of pipeflow

responses to rainstorms are poorly understood. Sec-ond, there is only a poor understanding of the tempo-ral regime of hydrophobicity and its hydrologicalimpact. Although it is well established that soils canlose their water-repellent character during long wetperiods, little has been achieved in identifying theexact wetting mechanisms involved, the threshold

Žconditions needed for this change e.g. size, durationor frequency of storm events, or critical soil moisture

. Žcontent and the mechanisms and conditions e.g..temperature and length of dry period for hydropho-

bicity to become re-established. This information isnecessary for establishing the overall role of hy-drophobicity in terms of the percentage and timingof rainstorm events in which it can be expected toaffect runoff processes in different types of environ-ments. Such knowledge is not only crucial for under-standing and predicting slope and catchment hydro-logical responses, but also for optimising plantgrowth and reducing groundwater contamination riskon managed land. Furthermore, it is important forestablishing and understanding the overall role ofwater repellency in influencing surface and subsur-face erosion processes.

In the last decade or so, it has become increas-ingly evident that soil water repellency is a wide-ranging phenomenon and not just a pedological curiorestricted to some very specific environments. Itsactual extent amongst soils world-wide, however,remains unclear. If water repellency is shown, as

Ž .suggested by Wallis et al. 1991, p. 360 , Ato be thenorm rather than the exceptionB, then its importanceparticularly for managed land will prove far greaterthan acknowledged at present, and it should thereforereceive much more attention than it currently at-tracts.

Acknowledgements

The authors wish to thank the anonymous review-ers for their valuable comments on the manuscriptand A. Ratcliffe and N. Jones for drawing manyof the figures. We also wish to acknowledge fun-

Ž Ž .ding from the EU contacts EV4V-0106-C TT ,EV5V-0041, ENV4-CT97-0686 and FAIR 6CT98-

. Ž .4027 , and Nato contract CRG.CRG.960704 , whichhas enabled us to pursue research into water repel-lency.


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STEFAN H. DOERR is a Research Officer in the GeographyDepartment of the University of Wales Swansea. Together withhis co-authors, he belongs to the Land Surface Processes andManagement Group and also to the University’s EnvironmentalDynamics Institute. His PhD thesis concerned the origin, charac-teristics and effects of soil water repellency in commercial forestsin Portugal. He has since been involved in a series of researchprojects related to water repellency and soil erosion and is cur-rently Scientist-in-Charge at Swansea of a global research projectconcerned with developing amelioration strategies to reduce envi-ronmental deterioration and agricultural production losses in waterrepellent regions. Dr. Doerr’s publications also lie in the field ofkarst and pseudokarst geomorphology.


RICHARD A. SHAKESBY is a Senior Lecturer in Geography atSwansea. His research interests include soil erosion and landdegradation in the Mediterranean, the Sudan, southern Africa andGreat Britain. Since the late 1980s, he has worked in Portugalinvestigating the measurement, modelling and combating of soilerosion in relation to land management changes, forest fire andwater repellency. Dr. Shakesby’s publications and research inter-ests also include Quaternary geomorphology, especially sedimentsand landforms associated with Holocene glacier fluctuations. He isa Council Member of the European Society for Soil ConservationŽ .ESSC .

RORY P.D. WALSH is a Reader in Geography at Swansea. Hisresearch interests and expertise lie in the fields of Mediterraneanand tropical hydrology and erosion. He has worked on the impactsof forest fires and land management on hydrology and soil erosionin Portugal. He has also longstanding experience of investigatingrunoff processes, erosion and climatic change in the West Indies,the Sudan and Borneo. He had conferred on him the Back Awardin 1996 by the Royal Geographical Society for his tropicalresearch and is currently the Research Co-ordinator of the RoyalGeographical Society’s South-East Asia Rainforest Research Pro-gramme based at Danum Valley in Sabah.

soil water repellency: its causes, characteristics and hydro-geomorphological significance

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