Soil water repellency: its causes, characteristics and ...

Earth-Science Reviews 51 Ž2000. 33–65 www.elsevier.comrlocaterearscirev

Soil water repellency: its causes, characteristics and hydro-geomorphological significance S.H. Doerr ) , R.A. Shakesby, R.P.D. Walsh Department of Geography, UniÕersity of Wales Swansea, Singleton Park, Swansea SA2 8PP, UK Received 27 May 1998; accepted 26 January 2000

Abstract Water repellency Žhydrophobicity. of soils is a property with major repercussions for plant growth, surface and subsurface hydrology, and for soil erosion. Important advances have been made since the late 1980s in identifying the range of environments affected by water repellency, its characteristics and its hydro-geomorphological impacts. This review summarises earlier work, but focusses particularly on these recent advances and identifies remaining research gaps. The associations of water repellency with Ža. soils other than coarse-textured ones, Žb. an expanding list of plant species, and Žc. 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. Its origin by the accumulation of long-chained organic compounds on or between soil particles is now widely accepted, but understanding of their exact chemical composition and means of attachment to particle surfaces remains incomplete. The transient nature of water repellency has been found to be mainly associated with fluctuations in soil moisture, but the precise processes and required conditions for the changes from hydrophobic to hydrophilic and vice versa are so far only poorly understood. Significant advances relating to the hydro-geomorphological impacts of hydrophobic layers have been made since the late 1980s in identifying and separating the various effects of such layers on surface and subsurface water flow. It has become evident that these effects in turn are influenced by variables such as the frequency and effectiveness of flow pathways through hydrophobic layers as well as their position and transitory behaviour. Recent literature has continued to highlight the role of water repellency in promoting soil erosion and it is now recognised that it can promote rainsplash detachment and soil loss not only by water, but also by wind. Major research gaps, however, remain in Ža. isolating the erosional impact of water repellency from other factors, and Žb. 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. E-mail address: [email protected] ŽS.H. Doerr..

0012-8252r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 5 2 Ž 0 0 . 0 0 0 1 1 - 8

34

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

effects of soil water repellency will enable its overall role in surface and subsurface hydrological and erosional processes to become more clearly defined. q 2000 Elsevier Science B.V. All rights reserved. Keywords: water repellency; hydrophobicity; soil hydrology; soil erosion

1. Introduction Soil water repellency Žhydrophobicity. reduces the affinity of soils to water such that they resist wetting for periods ranging from a few seconds to hours, days or weeks Že.g. King, 1981; Doerr and Thomas, 2000.. In addition to its detrimental and often costly implications for plant growth Že.g. House, 1991; York, 1993., it has substantial hydrological and geomorphological repercussions. These include the reduced infiltration capacity of soils, enhanced overland flow and accelerated soil erosion, uneven wetting patterns, development of preferential flow and the accelerated leaching of agrichemicals Že.g. Imeson et al., 1992; Shakesby et al., 1993; Ritsema et al., 1993, 1997.. Instances of water repellency have been recorded as early as 1917 ŽSchantz and Piemeisel, 1917., but relatively few studies investigated this phenomenon prior to the 1960s. During the 1960s and 1970s, research into soil water repellency was intensified, particular foci being repellency-induced by wildfire, and management and amelioration strategies for water-repellent land, topics that were covered in detail by a Astate-of-the-artB review by DeBano Ž1981.. During the following decade, research broadened significantly, and it became apparent that water repellency was much more widespread than previously acknowledged. Progress made during this period was summarised by Wallis and Horne Ž1992.. Subsequently, a considerable body of research has been published, which is widely disseminated amongst the pedological, agricultural, geomorphological, geochemical and hydrological literature. This paper aims to provide a critical review of the phenomenon, focussing in particular Ž1. on the significant advances that have been made since the late 1980s, and Ž2. on those topics that have not been reviewed previously in much detail, such as the physico-chemical principles and the hydro-geomorphological consequences of water repellency. Currently used techniques to measure water repellency and amelioration methods for affected land are dis-

cussed by Wallis and Horne Ž1992. and Doerr Ž1998., and Wallis and Horne Ž1992. and Moore and Blackwell Ž1998., respectively, and are therefore not considered in detail here.

2. Physico-chemical principles of water repellency and its occurrence in soils 2.1. The origin of water repellency Water repellency is a relative concept: no surface actually exerts a repelling force on a liquid. There is always some attraction between a liquid and any solid. In practice, therefore, an entirely hydrophobic surface does not exist ŽTschapek, 1984.. A hydrophilic surface allows water to spread over it in a continuous film whereas water on a hydrophobic surface water ‘balls up’ into individual droplets ŽAdam, 1963.. If the surface is a porous medium like sand or soil, water infiltration is inhibited ŽFig. 1.. For hydrophobic sand or soil with sufficiently large pore openings, water might occupy the openings but will not cover the individual grains, whereas hydrophilic particles will be covered by a film of water ŽAnderson, 1986.. The affinity or repellency between water and solid surfaces originates from mutual at-

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


tractive forces Žadhesion. and the attraction between the water molecules Žcohesion.. To understand these forces better, some properties of water are briefly considered here. A water molecule comprises an oxygen atom with a partial negative charge and two hydrogen atoms with a partial positive charge. The hydrogen and oxygen atom bonds are positioned 1058 apart, giving the water molecule a strongly dipolar structure ŽParker, 1987.. The attraction of these positive and negative ends causes water molecules to form aggregates, held together by Ahydrogen bondsB. Water adheres to most natural surfaces since they consist of positively and negatively charged ions attracting the negative end or the positive ends of a water molecule, respectively. However, the dipole character of water also results in a comparatively strong force counteracting the attraction to charged surfaces. Within a liquid, the net force acting on an individual molecule is zero as it is surrounded by other molecules and their forces. Beyond the surface of a liquid, however, no similar molecules exist to oppose the attraction exerted by the molecules within the liquid. Consequently, the surface molecules experience a net attractive force towards the interior, thus promoting the reduction of the surface area of water. Thus, if

35

opposing forces are minimal, liquids will assume a spherical shape Ži.e. that of a droplet.. To enlarge the surface of a liquid, work is necessary. This work is related to the surface tension or surface free energy of 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 exceptionally high at 72.75 = 10y3 Nrm. With increasing temperature, the surface tension of liquids is reduced ŽParker, 1987.. The same principle applies to solid surfaces, although their nature inhibits deformation into a spherical shape. Thus, the surface tension of solids leads to lateral forces at the surface. Values for hard solids range from 500 to 5000 = 10y3 Nrm, increasing with hardness and melting point ŽZisman, 1964.. For water to spread on a solid, the adhesive forces between them must exceed the cohesive forces within the body of water. Thus, surfaces with a surface-free energy ) 72.75 = 10y3 Nrm attract water and are therefore hydrophilic. The higher the surface tension of the solid, the stronger is the attraction. All principal soil minerals have a much higher surface free energy than water and are therefore hydrophilic ŽTschapek, 1984., whereas soft organic solids, such as waxes or organic polymers can exhibit surface

Fig. 2. Schematic representation of ŽI. an amphiphilic molecule and ŽIIra–c. changes in orientation of such molecules on a mineral surface while in contact with water Žbased on Tschapek, 1984; Ma’shum and Farmer, 1985; Velmulapalli, 1993..

36


free energy values below 72.75 = 10y3 Nrm and are thus hydrophobic ŽZisman, 1964.. 2.2. Water repellency in soils In theory, a single layer of hydrophobic molecules can render a hydrophilic mineral surface water-repellent ŽZisman, 1964. Žsee Section 4 and Fig. 2ŽIIa... It has been suggested, however, that hydrophobic compounds tend to be absorbed as small globules and not in uniform monolayers. Thus, an amount equivalent to that of several monolayers may be required to result in a complete cover on a mineral grain ŽMa’shum et al., 1988.. The amount required is still relatively small. For example, Ma’shum et al. Ž1988. induced severe water repellency in 1000 g of medium-sized sand using only 0.35 g of hydrophobic compounds. Attempts to investigate such hydrophobic coatings using microscope examination have produced inconsistent results. They range from the observation of a distinctive coating ŽFranco et al., 1995. to no detectable coating on hydrophobic material ŽJungerius and de Jong, 1989. and from coatings on both hydrophobic and hydrophilic particles ŽJex et al., 1985. to coatings on only some, but not all hydrophobic grains ŽDoerr, 1997. ŽFig. 3.. This inconsistency may be caused by one or both of the following reasons: Ž1. soil particle surfaces are frequently cov-

Fig. 3. Hydrophobic organic coating on a previously hydrophilic sand grain formed during an experimental burn of Eucalyptus globulus litter over dry, washed sand ŽSEM image, 180= magnification..

ered with other small particles of various origin ŽTschapek, 1984., and thus, the coating observed may be unrelated to hydrophobicity; Ž2. since organic coatings can be as thin as a few molecular monolayers they may remain undetected even using Scanning Electron Microscopy. Whatever their precise nature, it is now widely accepted that organic coatings are a common cause of water repellency. Soil water repellency can also be caused by the presence of hydrophobic interstitial matter. If hydrophobic particles are present in the pore spaces of hydrophilic matrix, the wettability of the composite material is reduced. For example, severe water repellency has been induced by intermixing as little as 2–5% by weight of organic matter to hydrophilic sand ŽMcGhie and Posner, 1981.. For naturally hydrophobic sand, it has been suggested that a slight to moderate repellency can be caused by the presence of hydrophobic particles in a soil matrix, but that more extreme water repellency results from a coating on individual soil particles ŽBisdom et al., 1993..

3. Severity of water repellency and its classification Most techniques for measuring and classifying soil water repellency are summarised in Tschapek Ž1984. and Wallis and Horne Ž1992.. The two most common methods, the ‘Water Drop Penetration Time’ ŽWDPT. test ŽVan’t Woudt, 1959. and the ‘Molarity of an Ethanol Droplet’ ŽMED. test Žalso known as the ‘Percentage Ethanol’ or ‘Critical Surface Tension’ test. ŽLetey, 1969. are referred to in this review. The former determines how long water repellency persists in the contact area of a water droplet and the latter measures indirectly the apparent surface tension of a soil surface, i.e. how strongly water is repelled. These properties are somewhat, but not always well related ŽDekker and Ritsema, 1994.. Recent advances relating to these tests are presented in Dekker et al. Ž1998. and Doerr Ž1998.. Perception of what constitutes a low or high degree of water repellency varies widely. To distinguish between hydrophilic and hydrophobic soils, WDPT thresholds of 1 s ŽRoberts and Carbon, 1971., 5 s ŽBisdom et al., 1993., or 60 s ŽWalsh et al.,


1994. have been used, making direct comparisons between studies difficult. Furthermore, actual WDPT values found in the literature are not always directly comparable because, for practical reasons, few workers have conducted this test for more than 600 s and even where this has been carried out, tests were terminated before droplet infiltration had occurred ŽTable 1.. In most studies to date, soils have exhibited WDPTs of less than 600 s, and in comparatively few cases have values exceeding 1, or even 5 h been recorded. The MED test has been used less widely and thus fewer data are available. A molarity equivalent to 16.6% ethanol was rated as the highest severity class by King Ž1981., but this value has been exceeded more recently ŽTable 1.. As an indication of the relevance of the hydrophobicity values reported, it may be noted that agricultural production is reportedly affected above a molarity equivalent of 5% ethanol for some Australian sandy soils ŽFranco et al., 1995.. For the purpose of this review, soils with a WDPT) 1 h ŽDekker and Ritsema, 1994. or a MED equivalent to ) 20% ethanol are termed ‘extremely hydrophobic’.

37

Although these tests allow classification of soils according to their persistence and severity in water repellency, it has been shown that they are not always well related to the actual wetting behaviour of soils ŽDoerr and Thomas, 2000. and Dekker et al. Ž1999. recommended the wider use of wetting rate measurements in future studies. 4. Origin and characteristics of hydrophobic substances 4.1. Substances responsible for water repellency It is commonly accepted that soil water repellency is caused by organic compounds derived from living or decomposing plants or microorganisms. Hydrophobic substances occur in many life forms ŽTable 2.. Waxes on plants, for example, not only increase frost hardiness and drought resistance, but can also reduce wettability and enhance the selfcleaning ability of leaf surfaces ŽNeinhuis and Barthlott, 1997.. The identification of the specific compounds causing water repellency has continued

Table 1 Maximum WDPT and % ethanol values measured in various studies Ž –s no data. Author Žs.

WDPT Žs.

% Ethanol

Vegetation type

Location

Carter et al. Ž1994. Chan Ž1992. Crockford et al. Ž1991. Barrett and Slaymaker Ž1989. Burcar et al. Ž1994. Brock and DeBano Ž1990. Dekker and Ritsema Ž1994. Doerr et al. Ž1996. Dyrness Ž1976. Giovannini and Lucchesi Ž1983. Harper and Gilkes Ž1994. Jex et al. Ž1985. Jaramillo and Herron Ž1991. Karnok et al. Ž1993. King Ž1981. King Ž1981. McGhie and Posner Ž1980. Richardson and Hole Ž1978. Roberts and Carbon Ž1971. Scholl Ž1971. Teramura Ž1980. Witter et al. Ž1991.

– 60 2700 ) 600 5400 70 21,600 ) 18,000 1800 480 206 453 10,800 – 260 – ) 3600 1800 7800 540 360 55

19.0 – 40.0 – 30.0 – 25.0 36.0 – – 4.3 – 20.0 18.9 10.0 16.6 – – – – – –

blue lupin cultivated land eucalyptus forest subalpine meadow pine forest chaparral grass eucalyptus stands lodgepole pine chaparral crop rotation Žunspecified. pine stand turf grass croprpasture rotation pasture eucalyptus forest red pine stand lucerne juniper trees chaparral dune vegetation

Australia Australia New South Wales British Columbia California California The Netherlands Portugal Oregon Sardinia Australia Florida Colombia Georgia Australia Australia Australia Wisconsin Australia Utah California The Netherlands

38


Table 2 Some naturally occurring hydrophobic substances and their established sources Žafter Horn et al., 1963; Kolattukudy et al., 1976; Tulloch, 1976. Substance

SourceŽs.

n-alkanes

bacteria, fungi, algae, higher plants bacteria, fungi, algae, higher plants many plant waxes bacteria, higher plants higher plants Že.g. eucalyptus, grasses. higher plants Že.g. pines.

Olefines Terpenoides Monoketones b-diketones Polyesters of hydroxy-fatty acids

to be a focus of soil research in the last decade Že.g. Franco et al., 1994; Hudson et al., 1994; McIntosh and Horne, 1994.. However, despite advances in analytical techniques, identifying the exact substanceŽs. responsible in a given soil has yet to be achieved. Furthermore, how these compounds are bonded to soil particles also remains unclear. A complicating factor in such studies is the natural abundance of various, potentially responsible substances in soil. For example, from just one sampled soil, Almendros et al. Ž1988. extracted 93 organic compounds, many of which were hydrophobic. The compounds identified from water-repellent soils can be divided into two main groups. The first are the aliphatic hydrocarbons, which are substances consisting of hydrogen and carbon with the carbon atoms arranged in an elongated chain. They are non-polar Ži.e. have no positive or negative charges at either end of the chain. and are consequently almost insoluble in water. The second group represents polar substances of amphiphilic structure, comprising a hydrocarbon chain with one end having a functional group with a positive or negative charge. This end is hydrophilic, whereas the other is hydrophobic ŽFig. 2ŽI... Despite being generally water soluble, amphiphilic molecules can be highly effective at producing a hydrophobic coating provided their polar ends are bonded to a surface as illustrated in Fig. 2ŽIIa.. Both groups are thought to cause water repellency ŽMcIntosh and Horne, 1994., but the polar molecules Ži.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-

ter-repellent sands ŽMa’shum et al., 1988; Hudson et al., 1994; Franco et al., 2000.. 4.2. Sources of hydrophobic substances 4.2.1. Vegetation In many studies, the occurrence of water repellency has been associated with particular vegetation types ŽTable 3.. This list is not exhaustive and it cannot be assumed that these species always induce water repellency under natural conditions. In some studies, fire has been a ‘triggering’ factor Že.g. DeByle, 1973; Reeder and Juergensen, 1979; Mallik and Rahman, 1985. Žsee Section 5.1.. In other studies, the link is based on laboratory experiments with fresh plant material, so that natural decomposition and incorporation into the soil have not been not considered Že.g. Roberts and Carbon, 1972; Reeder and Juergensen, 1979; McGhie and Posner, 1981.. Furthermore, the mechanism of input of these hydrophobic substances into the soil it is not always clear. Although decaying plant litter has been shown to be a source of these substances in some studies Že.g. Reeder and Juergensen, 1979; McGhie and Posner, 1981., other studies have found water repellency to be more closely associated with the root activity of plants ŽDekker and Ritsema, 1996a; Doerr et al., 1998.. Plants most commonly associated with water repellency seem to be certain evergreen tree types. In particular, trees with a considerable amount of resins, waxes or aromatic oils such as eucalyptus and pines are well represented, both within and outside their native environment ŽTable 3.. Water repellency has also been found under shrubs ranging from temperate heathland ŽMallik and Rahman, 1985., or mediterranean shrubland ŽGiovannini et al., 1987., to semi-desert chaparral ŽDeBano, 1991.. Soils under grassland can also resist wetting, a problem that has been of considerable concern on areas of high economic value such as golf greens. For example, under bentgrass Ž Agrosti spp.., dry spots, which can persist prolonged irrigation, are a common feature ŽKarnok et al., 1993; York, 1993.. The association of water repellency with grass has also been noted on pasture in, for example, Australia ŽCrockford et al., 1991., Canada ŽBarrett and Slaymaker, 1989. and the Netherlands ŽDekker and Rit-


sema, 1994. but the species involved are not always stated. Furthermore, dune grass Ž Spinifex hisutus . is thought to induce hydrophobicity ŽMcIntosh and Table 3 Higher plant species reportedly associated with water repellency Plant species or vegetation type AuthorŽs. Deciduous trees Populus tremuloides Larix occidentalis

Reeder and Juergensen Ž1979. Reeder and Juergensen Ž1979.

Evergreen trees Acacia mearnsii Banksia speciosa Citrus spp. Eucalyptus astringens Eucalyptus globulus Eucalyptus marginata Eucalyptus patens Juniperus osteosperma Pinus banksiana Pinus jeffreyii Pinus monophylla Pinus patula Pinus pinaster Pinus radiata Pinus resinosa Pinus strobus Pseudotsuga macropora Pseudotsuga mentziesii Picea engelmanii Quercus ilex Quercus turbinella Quercus suber Tsuga canadensis

Scott Ž1992. Moore and Blackwell Ž1998. Jamison Ž1942. McGhie and Posner Ž1981. Doerr et al. Ž1996. Roberts and Carbon Ž1972. Moore and Blackwell Ž1998. Scholl Ž1971. Richardson and Hole Ž1978. Meeuwig Ž1971. Holzhey Ž1969. Jaramillo and Herron Ž1991. Shakesby et al. Ž1993. Scott and Van Wyk Ž1990. DeByle Ž1973. Meeuwig Ž1971. Holzhey Ž1969. DeByle Ž1973. DeByle Ž1973. Cerda` et al. Ž1998. Holzhey Ž1969. Sevink et al. Ž1989. Richardson and Hole Ž1978.

Shrubs Adenostoma fascilatum Adenostoma sparcifolium Arctostaphylus spp. Calluna Õulgaris Chamaespartium spp. Chrysotamnus spp. Cistus monspelliensis Erica arborea Vaccinium spp. Ulex europaeus

DeBano Ž1969. Holzhey Ž1969. Holzhey Ž1969. Mallik and Rahman Ž1985. Richardson and Hole Ž1978. DeBano Ž1969. Giovannini et al. Ž1987. Giovannini et al. Ž1987. Richardson and Hole Ž1978. Soto et al. Ž1994.

Grasses Agrostis spp. Erharta calycina Phalaris spp. Phragmites spp. Sphagnum spp. Spinifex hisutus

Wilkinson and Miller Ž1978. McGhie and Posner Ž1981. Bond Ž1964. Berglund and Persson Ž1996. Berglund and Persson Ž1996. McIntosh and Horne Ž1994.

39

Table 3 Ž continued . Plant species or vegetation type

AuthorŽs.

Crops Chamaecystitus proliferus Hordeum Õulgare Lupinus cosentinii Medicago satiÕa Trifolium subterranum

Carter et al. Ž1994. McGhie and Posner Ž1981. Moore and Blackwell Ž1998. Bond Ž1964. Roberts and Carbon Ž1972.

Horne, 1994.. Water repellency has also been associated with some crops. For example, in Australia, its development has become a major concern under blue lupin Ž Lupinus cosentinii . ŽCarter et al., 1994.. The production of highly hydrophobic compounds by plants may not only serve the physiological purpose outlined earlier. It has been suggested Že.g. Scott, 1992; Moore and Blackwell, 1998. that the release of hydrophobic substances in the soil is, in a similar fashion to allelopathy, used by plants to suppress the germination of competing vegetation and to improve water conservation by channelling water deep into the soil profile following preferential flow pathways, while at the same time reducing evaporation due to the partial dryness of the surface soil layer Žsee also Section 8.5.. 4.2.2. Soil fungi and microorganisms The association of water-repellent with certain plants may not always be direct. Water repellency has also been associated with fungal growth and soil microorganisms, which in turn can be associated with specific vegetation types. Schantz and Piemeisel Ž1917. postulated that the ‘fairy rings’ Žcircles of enhanced grass growth in pastures and crops, followed by a zone of water-repellent soil. were due to the mycelia of the fungus Basidiomycea, a common species that decomposes litter and especially lignin in soils ŽScheffer and Schachtschabel, 1989.. A range of fungi and microorganisms has been associated with water repellency, for example Penicillium nigricans and Aspergillus sydowi ŽSavage et al., 1972., or Actinomycetes Žmicroorganisms with fungal and bacterial properties. ŽJex et al., 1985.. Effects have been found to be species-dependent, with some species inducing hydrophobicity, and others reducing hydrophobicity in an already hydrophobic material

40


ŽMcGhie and Posner, 1981; Roper, 1998.. However, reports on the effect of even the same species are not always consistent. Thus, Aspergillus niger, e.g. has been reported to induce water repellency by Bornemisza Ž1964., but in a separate study, it had no effect ŽSavage et al., 1972.. Compared with the input of organic material from higher plants, the biomass input from organisms within the soil can be considerable. For example, the fungal net biomass in soil alone may be equal to or even exceed aboveground biomass production, as shown for the Pacific Northwestern forests of the USA ŽFogel and Hunt, 1979.. Clearly, since also many species of algae and bacteria can produce hydrophobic compounds Žsee Table 2., their role in the establishment of water repellency could well be significant, though York and Baldwin Ž1992, p. 11. concluded that Ano firm link has been provided, which can unquestionably connect the presence of microbes with the production of water-repellent materialsB. Given that microbes and fungi are also involved in the decomposition of hydrophobic compounds ŽFranco et al., 1994; Roper, 1998., it may remain very difficult to isolate a particular fungus or microorganism as the sole agent responsible for water repellency in a soil. 4.2.3. Soil organic matter and humus Apart from investigating the direct influence of vegetation and microorganisms on water repellency, research has also attempted to establish general relationships between soil organic matter andror organic carbon content and the degree of water repellency. The results, however, have been very inconsistent. Apart from the more commonly found positive correlation between the variables Že.g. Wallis et al., 1990; Berglund and Persson, 1996; McKissock et al., 1998., a negative correlation ŽTeramura, 1980. as well as no correlation ŽJungerius and de Jong, 1989; DeBano, 1991; Wallis et al., 1993. have also been reported. The simple explanation for this inconsistency may be that the small amount of hydrophobic compounds necessary to cause water repellency is not proportional to the actual amount of organic material present in soil, particularly if different soil horizons or even different soils are compared. Thus, a consistent relationship of water repellency might be expected with a type, or a fraction of, the organic material,

rather than the total amount of organic matter or carbon ŽWallis and Horne, 1992.. Concerning material type, links have been established between litter and humus type and repellency. A more severe soil water repellency has usually been found under a deeper litter layer ŽScott and Van Wyk, 1990; Crockford et al., 1991., or a mor-type humus ŽSevink et al., 1989; Imeson et al., 1992.. These findings are supported by Dinel et al. Ž1990., who reported that the concentration of Žhydrophobic. lipids decreases with the efficiency of the decomposition regime, and by Valat et al. Ž1991., who found the degree of humification in peat to be positively and the degree of decomposition negatively correlated with water repellency. The above findings indicate that water repellency development is not only dependent on certain organic substances being released by certain plants or microbes into susceptible soil Žsee also Section 5.2.. It seems that a slow natural decomposition regime andror the excessive accumulation of hydrophobic compounds under managed vegetation are an additional factor in water repellency development. For example, Franco et al. Ž2000. have suggested that a selective microbial activity is essential in the development and appearance of repellency, accumulating polar waxes from a pool of plant waxes present in soil. Thus, further advances in identifying the sources and the exact chemical composition of hydrophobic compounds may, by themselves, not be sufficient in developing our understanding of soil water repellency development. More research into water repellency in relation to decomposition regime is also needed. Such knowledge would also be useful in developing further biological amelioration strategies for soil water repellency.

5. Non-biological factors affecting water repellency 5.1. Soil temperature and the effect of fire on water repellency By burning plant litter and heating sand and soil in laboratory experiments, DeBano and Krammes


Ž1966., DeBano et al. Ž1970. and Savage Ž1974. observed that fire could induce hydrophobicity in previously hydrophilic soil, and either enhance or reduce the surface hydrophobicity in an already hydrophobic soil, depending on fire temperature, the amount and type of litter consumed and pre-fire soil moisture level. They proposed what subsequently became a widely accepted mechanism in which heated hydrophobic organic substances in the litter and topsoil become volatilised during burning with a proportion travelling downward following the temperature gradient in the litter and soil until they condense in a concentrated form Žsee also Fig. 3.. Such fire-induced water repellency became a focus of research during the 1960s and 1970s in North America and the view that Athe heat during a fire markedly changes and intensifies water-repellencyB ŽDeBano, 1981, p. 5. has since become widely accepted. Subsequently, fire-related water repellency has been reported from Europe Že.g. Mallik and Rahman, 1985; Imeson et al., 1992., South Africa ŽScott and Van Wyk, 1990., and Australia ŽZierholz et al., 1995.. Apart from redistributing and concentrating hydrophobic substances in the soil, the heat during a fire is also thought to improve the bonding of these substances to soil particles ŽSavage et al., 1972. and make them chemically more hydrophobic by pyrolysis ŽGiovannini, 1994.. DeBano Ž1991. suggested that the heating of any hydrophilic soil containing

41

more than 2–3% organic matter would induce water repellency. The effect of high soil temperature on water repellency has been investigated in many laboratory-based studies and is well established. Water repellency is generally intensified at temperatures of 175–2008C. Hydrophobic substances are fixed to the soil particles around 2508C, but destroyed above 270–3008C Že.g. Savage, 1974; DeBano et al., 1976.. Somewhat different thresholds have been reported by Nakaya Ž1982., possibly the result of differences in measuring methods, length of heating times and types of chemical compounds present in the soils investigated. For example, DeBano and Krammes Ž1966. found that water repellency increased at lower temperatures if longer heating times were sustained. Quantifying directly the effect of burning on a soil in field conditions has not always been attempted as many studies have been instigated by the passage of a fire and provide little information on the soil status before burning. Where surface water repellency has been measured separately on burnt and unburnt land, it is usually higher on the former, or, the surface water repellency is destroyed, while a repellent layer is developed at depth Že.g. McNabb et al, 1989; Brock and DeBano, 1990; Scott and Van Wyk, 1990. ŽTable 4.. However, fire may not inevitably lead to an increase of water repellency somewhere in the soil profile. A net reduction has also been reported ŽGiovannini and Lucchesi, 1983.

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

Unburnt land

Burnt land

Vegetation

Location

Boelhouwers et al. Ž1996. Brock and DeBano Ž1990. Doerr et al. Ž1996. Dyrness Ž1976. Giovannini and Lucchesi Ž1983. McNabb et al. Ž1989.

-5 s 70 s ) 18,000 s 1800 s 460 s 588

) 120 s ŽWDPT. 107 s ŽWDPT. ) 18,000 s ŽWDPT. 7200 s ŽWDPT. 240 s ŽWDPT. 718 Žcontact angle.

eucalyptus forest chaparral eucalyptus and pine pine forest chaparral various trees

South Africa California Portugal Oregon Sardinia Oregon

Mallik and Rahman Ž1985. Reeder and Juergensen Ž1979.

decrease in hydrophobicity 1 month after fire increase in number of hydrophobic sites following fire more than doubling of sites with WDPT ) 10 s after fire 118 increase in contact angle after fire

heather slash deposits

Great Britain Michigan

pine forest

South Africa

chaparral

Arizona

Scott and Van Wyk Ž1990. Scholl Ž1975.

42


and in areas where soils are ‘naturally’ highly hydrophobic, fire may have very little effect on water repellency, provided that soil temperatures remain below the threshold of repellency destruction ŽZierholz et al., 1995; Doerr et al., 1996.. Comparatively little is known about the longevity of these high temperature effects on water repellency because long-term post-fire monitoring is rare and the existing results vary widely. For example, in coniferous forests in the USA, fire-induced water repellency has been found to persist for as long as 6 years ŽDyrness, 1976. or as little as a few months ŽDeBano et al., 1976.. On the other hand, for a Sardinian chaparral, water repellency that was destroyed during a fire became re-established within 3 years ŽGiovannini et al., 1987.. Because of the many variables involved Že.g. the soil temperature reached, type of organic compounds, soil type, climatic conditions., the persistence of fire effects on water repellency may be very site-specific and thus difficult to predict. Water repellency is also affected by heating soil to temperatures not as high as those considered above. Crockford et al. Ž1991. and Dekker et al. Ž1998. found respectively that drying soil at 438C or 708C both increased WDPTs, an effect that might be caused by an increase in the alignment of the hydrophobic molecules Žsee also Section 6.. Another possibility is that during heating, waxes from particulate organic matter migrate onto soil particle surfaces, thereby inducing or increasing water repellency ŽFranco et al., 1995.. The recognition that the heat applied during oven-drying can increase water repellency has recently led to the preference of airdrying over oven-drying of samples prior to testing water repellency ŽDekker et al., 1998; Doerr, 1998.. The actual temperatures of the soil and the water applied also affect their affinity. In Australia, summer rains wet repellent sandy soils more readily than autumn or winter rains, a feature that has been attributed to higher soil temperatures ŽKing, 1981; Blackwell, 1993.. Similarly, repellency measurements in the field give higher MED values if conducted in the shade, rather than on soils exposed to the sun ŽBlackwell, personal communication.. This apparent decrease in water repellency with increasing temperature may be caused by the reduction in surface tension Žsee Section 3. and thus increased

‘wetting power’ of the warmer water or MED test solution. In addition, it might be that an amphiphilic molecule coating on soil particles has a generally enhanced ‘reactivity’ in warmer conditions and thus becomes detached more readily during water contact. 5.2. Soil texture and clay content Soil water repellency has in the past been generally associated with coarse-textured, sandy soils Že.g. Roberts and Carbon, 1971; Wilkinson and Miller, 1978; McGhie and Posner, 1980; DeBano, 1991.. It is reasoned that, given a limited supply of hydrophobic substances to coat soil particles, coarser particles are more susceptible to developing water repellency because of their smaller surface area per unit volume compared with soils of finer texture ŽGiovannini and Lucchesi, 1983; Blackwell, 1993.. For example, a medium-sized sand has a surface area of 0.0077 m2rg ŽDeBano, 1971., whereas clay can have a surface area as large as 900 m2rg ŽRowell, 1994.. Thus, Crockford et al. Ž1991. found an increase in water repellency with particle size within a soil sample. DeBano Ž1991. concluded that water repellency is most likely to develop in soils with less than 10% clay content, and it is now well established that the addition of dispersible clay can be very effective in reducing water repellency in sandy soil ŽCann and Lewis, 1994; Carter and Hetherington, 1994.. Notwithstanding the apparent higher susceptibility of coarse textured soils to develop water repellency, it has become increasingly evident during the last decade that even severe water repellency is not uncommon in soils with considerable clay content. Soils with 25% to more than 40% clay have been found to exhibit extreme water repellency ŽCrockford et al., 1991; Chan, 1992; Dekker and Ritsema, 1996b.. It has been suggested that this may occur as long the clay forms aggregates, thus reducing the surface area to be covered with a hydrophobic skin ŽWallis et al., 1991; Bisdom et al., 1993.. This concept, however, does not explain the occurrence of hydrophobicity in all cases, as some studies have found that the finer fractions of water-repellent soils can exhibit a similar, or even higher degree of repellency than the coarse ones ŽDoerr 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 fraction Žde Jonge et al., 1999.. Alternatively, in some environments, the supply of hydrophobic material might be sufficiently high not only to cover the coarser, but also much of the finer-sized particles with an organic coating ŽDoerr et al., 1996.. In such cases, a fine-grained soil could then be more waterrepellent than a coarser one due to its larger total area of hydrophobic surface within the soil matrix. This might explain why the degree of water repellency, where encountered in finer-textured soil, can be amongst the highest levels reported anywhere Že.g. Crockford et al., 1991; Chan, 1992; Doerr et al., 1996., whereas the susceptibility to water repellency development would be higher in coarse-textured soils, as indicated by the much greater number of incidents reported. In conclusion to Sections 4 and 5, it should be noted that it does not seem to be possible, as indicated by McKissock et al. Ž1998. for a range of Australian soils, to use any of the individual soil Žor vegetation. characteristics discussed in these sections on their own to predict accurately the occurrence or the degree of water repellency that can be expected in a soil.

6. Temporal variations of water repellency and the influence of soil moisture Water repellency is usually a transient soil property, varying through time. An important factor in these variations is soil moisture. Until recently, hydrophobicity has been generally considered to be most severe in dry soil and to decline as soil moisture increases until a critical moisture content is reached, above which a soil becomes hydrophilic Že.g. DeBano, 1971; Witter et al., 1991; Carter et al., 1994.. Thus, Dekker and Ritsema Ž1994. considered it important to distinguish ‘actual repellency’ of a field moist soil from its ‘potential repellency’ Žthe maximum repellency measured when soils are dry.. Although water repellency generally disappears when soils become wet, the soil moisturerwater repellency relationship is, nevertheless, more complex than stated above. This section reviews ideas on how

43

water absorption in hydrophobic soils occurs, its effects on the actual repellency of soils, and how hydrophobicity is re-established after wetting. 6.1. Water absorption of soils under water-repellent conditions Soils can absorb water while being hydrophobic ŽCrockford et al., 1991; Dekker and Ritsema, 1996a.. For example, water repellency was found to be present for soil moisture contents of up to 22% Žgrav.. in sandy loams ŽDoerr and Thomas, 2000., in a clayey peat with up to 38% Žvol.. ŽDekker and Ritsema, 1996a., but as little as 2% Žvol.. for dune sands ŽDekker and Ritsema, 1994.. The following mechanisms are suggested here to explain this seemingly paradoxical behaviour: Ž1. Water is thought to move freely as vapour in a water-repellent soil allowing soil water to be redistributed ŽBarrett and Slaymaker, 1989. until the soil has reached its maximum adsorption capacity for individual water molecules ŽMyamoto et al., 1972.. The adsorption capacity of a mineral surface for individual molecules, however, is small and further condensation of water onto surfaces can only take place in the form of droplets, a process that is constrained in hydrophobic soils ŽOsmet, 1963.. Ž2. Imeson et al. Ž1992. suggested that fine hydrophilic material present in the pore spaces of an otherwise hydrophobic soil matrix allowed partial wetting of the soil. This could also take place by vapour condensation and would allow moisture Žin addition to what has been adsorbed as vapour. onto hydrophobic particles without affecting the repellency of the whole soil matrix. Ž3. Similarly, some initially hydrophobic soil particles may have changed to a hydrophilic status during vapour adsorption Žsee Section 6.2., allowing a further water uptake, while enough hydrophobic surface area remains particularly on the soil surface to maintain the hydrophobic reaction measured by the WDPT or MED tests. Findings on the actual effect of a moisture increase in water-repellent soil are inconsistent. Many authors have reported an inverse relationship of soil moisture with water repellency Že.g. King, 1981, Witter et al., 1991.. However, an initial increase in

44


water repellency with soil moisture has also been found. This was attributed to an enhanced activity of hydrophobic substances producing microorganisms by Jex et al. Ž1985., whereas in studies by Berglund and Persson Ž1996. and de Jonge et al. Ž1999. biological processes were not considered. 6.2. Water absorption and the loss of water repellency In theory, a material can remain hydrophobic as long as the organic layer covering its surface remains unaltered during its contact with water. However, it is well known from textile engineering that water contact can weaken water repellency, a process most people have experienced themselves while wearing a supposedly water-repellent garment. The WDPT test, for example, takes advantage of the fact that prolonged contact with water can lead to the loss of soil water repellency. The following concepts are thought to cause this breakdown: Ž1. Where water repellency is mainly caused by a coating of amphiphilic molecules on soil particles, the attraction of water to the polar ends of these molecules is thought to weaken the soilrmolecule bond, leading eventually to the displacement of the organic compounds from soil particles and resulting in a wettable soil ŽTschapek, 1984; Ma’shum and Farmer, 1985. Žsee Fig. 2ŽIIa–c... This concept, however, remains poorly understood and it is not clear, which factors determine the length of time necessary for water repellency to break down. Observations on the timing and conditions of water repellency breakdown under field conditions are rare, although several weeks of wet weather were necessary for repellency to break down in an Australian eucalyptus forest ŽCrockford et al., 1991.. Ž2. Water repellency can be overcome if surfaceactive substances migrate from the soil into the water until the surface tension of the water is sufficiently reduced to allow infiltration ŽRichardson and Hole, 1978; Barrett and Slaymaker, 1989.. Surface-active substances can be provided by humic and fulvic acids present in the soil. Humic acids in larger quantities are only soluble above pH 6.5, whereas fulvic acids are soluble throughout the pH range, so that the latter would be more important in reducing

the surface tension of water ŽChen and Schnitzer, 1978.. Following findings by Tschapek and Wasowski Ž1976., these acids, however, would only reduce the surface tension of water enough to allow moderately repellent soil particles ŽMED equivalent of - 15% ethanol. to be wetted, but be insufficient to wet more severely repellent soils. 6.3. Re-establishment of water repellency after wetting Soil water repellency is largely regarded as a seasonal phenomenon, being usually low or completely absent under prolonged wet conditions and most severe during extended dry periods Že.g. Crockford et al., 1991; Imeson et al., 1992; Ritsema and Dekker, 1994b.. Thus, it is generally assumed that water repellency becomes readily re-established upon drying Že.g. Valat et al., 1991; Walsh et al., 1994.. As an underlying mechanism, it is thought that the polar ends of amphiphilic compounds associate and interact through hydrogen bonds if humidity becomes very low. This forces the molecules to re-adopt a position in which their polar ends are attached to the mineral surface and the non-polar ends are orientated outwards Žas in Fig. 2ŽIIa.., thus re-establishing water repellency during drying ŽTschapek, 1984; Ma’shum and Farmer, 1985; Valat et al., 1991.. However, under field conditions, some usually water-repellent soils were found to be dry and hydrophilic at times ŽBurch et al., 1989. and Crockford et al. Ž1991. found that it took 6 to 9 days of hot and dry weather for water repellency to become reestablished. Furthermore, in laboratory experiments on fine- and medium-textured soils, water repellency, which had disappeared during wetting experiments, did not reappear after drying ŽCrockford et al., 1991; Doerr and Thomas, 2000.. This suggests that the proposed re-orientation of amphiphilic molecules is not necessarily caused by the drying process alone. It has been shown that heating Ži.e. oven-drying. thoroughly wetted samples can restore water repellency to some extent, although not to its initial levels ŽMa’shum and Farmer, 1985; Doerr, 1997.. This may be associated with the re-attachment of some organic molecules on soil particles as suggested above. This mechanism is also known in the


outdoor fabric industry where the heat treatment Že.g. tumble-drying or ironing. can be used to restore the water repellency of used rainwear ŽGore et al., 1994.. It is, however, not clear which processes lead to the complete re-establishment of water repellency after wetting in some soils. It might be that waxes present as interstitial globules in the soil matrix migrate onto mineral surfaces aided by warm temperatures or microbiological mechanisms ŽFranco et al., 1995.. In other cases, a completely new input of hydrophobic substances into the soil may be required for repellency to be restored ŽDoerr and Thomas, 2000.. To conclude this section, it is fair to say that there is still very little known on the timing of, and the principles underlying, the breakdown and re-establishment of water repellency and on the effects of soil moisture on repellency. What is clear is that, associated with climatic factors, water repellency can follow short-term or seasonal variations. The critical soil moisture content above which water repellency disappears varies considerably between soils, and seems, at least in part Žsee Section 6.1., to be soil-texture related. However, this threshold and the exact mechanisms involved in the cessation and restoration of water repellency have not been investigated in much detail, despite their practical implications. For example, an improved ability to predict the seasonal and short-term fluctuations of hydrophobicity could lead to more efficient irrigation regimes in dry periods, enabling soils to be kept sufficiently moist to avoid the onset of water-repellent conditions ŽMiller and Wilkinson, 1977..

7. Distribution of soil water repellency 7.1. The global and regional scales Until the mid-1980s, most studies reporting soil water repellency had been conducted in areas of semi-arid or mediterranean climate like that of the southwestern USA Že.g. Krammes and Osborn, 1969; Scholl, 1975., South and Western Australia Že.g. Bond, 1969; Roberts and Carbon, 1971., drier regions in Africa ŽRietfeld, 1978; Bishay and Bakhati, 1976. and the Mediterranean ŽGiovannini and Lucchesi, 1984.. Thereafter, a considerable number of

45

studies reported water repellency also from much wetter regions such as Great Britain ŽMallik and Rahmann, 1985., British Columbia ŽBarrett and Slaymaker, 1989., the Netherlands ŽJungerius and de Jong, 1989., Colombia ŽJaramillo and Herron, 1991., north-central Portugal ŽShakesby et al., 1993. and Sweden ŽBerglund and Persson, 1996.. This suggests that water repellency is not largely confined to relatively dry climates. Mapping of water-repellent soils is usually not incorporated in general soil surveys although, where systematic surveys have been conducted, water repellency has been shown to affect large areas. For example, in the Netherlands 75% of the crop- and grass-land exhibit water repellency ŽDekker and Ritsema, 1994. and in Australia, 5 million ha of land are affected ŽHouse, 1991., leading to production losses of up to 80% in agriculture ŽBlackwell, 1993.. The effects of water repellency are not surprisingly most pronounced in environments with long dry periods, as is the case for parts of Australia. They can, however, also be of economic significance in climates with less seasonal precipitation patterns such as Great Britain ŽYork, 1993. or Sweden ŽBerglund and Persson, 1996., provided that a dry spell reduces soil moisture sufficiently to allow the onset of water-repellent conditions. 7.2. Small-scale Õariations Water repellency has often been reported to be discontinuous within the soil both spatially and vertically. This has been found to be particularly prevalent in fire-affected soils. The pattern of a surface layer rendered hydrophilic by the intense heat of a chaparral fire followed by a layer made distinctly water-repellent by condensed hydrophobic substances has first been described in detail by DeBano Ž1969. in California. This pattern has often been referred to in later studies of fire-affected terrain Že.g. Dyrness, 1976; Scott and Van Wyk, 1990; Boelhouwers et al., 1996.. In some cases, however, a distinct hydrophilic surface soil layer has not been found ŽSevink et al., 1989; Soto et al., 1994., the surface water repellency was destroyed without repellency being induced at depth in the soil ŽGiovannini and Lucchesi, 1983., or fire had very little effect

46


on an already highly hydrophobic soil ŽDoerr et al., 1996. Žsee also Fig. 8.. This contrast in findings may be in part attributed to different soil temperatures reached during burning. DeBano Ž1981. argued that chaparral fires are usually hotter than forest fires due both to the entire consumption of both living and dead fuel and to the lower soil moisture levels. The maximum soil temperature reached during a fire varies with the soil moisture status, thickness of the insulating litter layer, burn duration and post-fire smouldering ŽDeBano, 1991.. The spatial diversity of these variables could also explain the usually patchy distribution of water repellency found at the surface of burnt soil Že.g. Brock and DeBano, 1990; Scott and Van Wyk, 1990; Imeson et al., 1992.. Spatial variations seem also to be typical for water repellency in unburnt soils Že.g. Jungerius and de Jong, 1989; Karnok et al., 1993; Ritsema and Dekker, 1994b. although under certain conditions water repellency can be spatially continuous. Crockford et al. Ž1991. investigating natural eucalyptus forest found this to be the case provided a thick litter layer was present Žsee also Section 4.2. and Doerr et al. Ž1998. attributed spatial homogeneity of water repellency in commercial eucalyptus forest to an

exceptionally high release and thorough distribution of hydrophobic substances combined with the particularly uniform vegetation and litter cover of the area. Variations of water repellency with depth in unburnt soils may be less pronounced than in their fire-affected counterparts, but layering also seems typical. For example, Barrett and Slaymaker Ž1989. and Crockford et al. Ž1991. working in forests and Brock and DeBano Ž1990. investigating chaparral soils reported that water repellency was confined to, or most severe in a layer few centimetres in thickness Žsee also Fig. 4.. Dekker and Ritsema Ž1994., in an extensive study investigating the spatial variability of water repellency in Dutch dunes, found that water repellency showed considerable variation within the top 50 cm. Roberts and Carbon Ž1971. observed that hydrophobic sandy soils in Australia commonly had a surface hydrophilic layer a few millimetres thick above the repellent layer. In contrast, Doerr et al. Ž1996. found that water repellency was common from the mineral surface down to near the bedrock for both burnt and unburnt forest soils in Portugal, and attributed this to a high release of hydrophobic substances into the relatively shallow soils.

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


47

Variations in the distribution pattern of water repellency can be of major importance for the hydrological and geomorphological effects of soil water repellency as discussed in Section 8. 8. The hydrological and geomorphological consequences of water repellency 8.1. OÕerÕiew The main hydrological impacts of soil water repellency reported in the literature are Ža. reduced infiltration capacity; Žb. increased overland flow; Žc. spatially localised infiltration andror percolation, often with fingered flow development; Žd. effects on the three-dimensional distribution and dynamics of soil moisture; Že. enhanced streamflow responses to rainstorms; and Žf. enhanced total streamflow. It is also normally argued that because of enhanced overland flow on and increased erodibility of water-repellent soil, slopewash, and sometimes the formation of rills and gullies, may be promoted. This section highlights new developments since previous reviews in this field Že.g. DeBano, 1981; White and Wells, 1982; Wallis and Horne, 1992., focussing also on studies that question some of the generalisations that have previously been made. 8.2. Infiltration and oÕerland flow The most frequently reported impacts of water are those of reduced soil infiltration capacity Že.g. Van Dam et al., 1990, Imeson et al., 1992. ŽFig. 5. and thus increased overland flow Že.g. McGhie and Posner, 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 lower than for a similar soil rendered hydrophilic by heating ŽDeBano, 1971.. Wallis et al. Ž1990. found that the infiltration capacity was six times lower on a water-repellent dry sand than on adjacent moist, less repellent sands and, in a separate study Ž1991., reported that, for the first 5 min of measurement, a hydrophobic soil had only 1% of the potential infiltration capacity when hydrophilic. A water-repellent surface layer causes rainwater to pond and, if rainfall is sufficient and surface detention is exceeded, Hortonian Žinfiltration-excess. overland flow will occur ŽFig. 6.. The frequency of

Fig. 5. Hydraulic conductivity curves from a wettable and a water-repellent sandy soil from the Netherlands Žmodified from Van Dam et al., 1990..

gaps through this layer Žsuch as structural or drying cracks, root holes and burrows, and patches of hydrophilic or less hydrophobic soil. will then determine whether overland flow is widespread or only local ŽFig. 7A.. As outlined in Section 7.2, a waterrepellent layer also frequently underlies a hydrophilic and often highly permeable topsoil or ash layer. Rainfall infiltrating such a topsoil may pond above the water-repellent layer ŽFig. 7B. and can subsequently: Ž1. be stored in the hydrophilic layer and later evaporated or used in transpiration; Ž2. run off as saturation overland flow when the hydrophilic layer becomes saturated; Ž3. spread out as ‘distribution flow’ and move vertically downwards as ‘preferential flow’ either through structural or other gaps in the water-repellent layer or as ‘fingered flow’ through vertical cylinders of hydrophilic or less water-repellent soil Žsee Section 8.3.; Ž4. move laterally downslope as throughflow above the water-repellent layer; Ž5. enter the matrix of the water-repellent layer Žif entry-pressure is sufficient, or water repellency undergoes a phase-change to a hydrophilic condition as outlined in Section 6.2..

48


Fig. 6. Water repellency causing ponding of rainwater and overland flow on sandy soil near Geraldton, Western Australia, following a large rainfall event Ž75 mm in 3–4 h in March 1999..

Where overland flow is considered the key effect of water repellency, it is often not clear whether Hortonian or saturation overland flow, or a combination of the two, is involved, particularly if the hydrophilic layer is thin, below the surface, or discontinuous. Thus, in an Australian eucalyptus forest, Burch et al. Ž1989. reported a threefold increase in overland flow from 5% to 15% after drought had enhanced water repellency. In burnt pine forest in South Africa, saturation overland flow promoted by a subsurface water-repellent layer led to an increase in the stormflow response to 7.5% compared with just 2.2% on unburnt terrain ŽScott and Van Wyk, 1990., and Jungerius and de Jong Ž1989. attributed the lack of any simple relationship between rainfall and overland flow in Dutch sand dunes to spatial variations in water repellency. Some studies, particularly those investigating burnt terrain, highlight water repellency as only one of several factors enhancing overland flow responses. Thus, Dyrness Ž1976. reported a threefold increase in overland flow after a fire in a pine forest in Oregon and Walsh et al. Ž1994. found a 5–25% higher overland flow response on Portuguese burnt compared with unburnt eucalyptus and pine forest. Rather than invoking only water repellency, post-fire

increases in overland flow have also been attributed to removal of a protective vegetation cover Žwith resulting increases in rainbeat compaction, inwash of fines into cracks and rootholes and reduction in infiltration capacity., reduction in soil particle size, erosion of permeable topsoil, stone lag development and organic matter losses ŽWhite and Wells, 1982; Imeson et al., 1992; Shakesby et al., 1996.. Such increases in overland flow were demonstrated using rainfall simulation plot experiments carried out in Portugal during dry conditions on unburnt Pinus pinaster slopes and a nearby area burnt two years before ŽWalsh et al., 1998.. In both areas, most surface soil remained dry and intensely hydrophobic throughout the 1-h 40–46 mm artificial rainstorms. However, whereas in the unburnt soil overland flow was modest Ž4%. and most water infiltrated through cracks and rootholes, on the burnt soil overland flow was more substantial Ž8–20%.. Thus, depending on pre-fire conditions, post-fire increases in overland flow can be associated with: Ž1. fire inducing or significantly increasing water repellency, thus enhancing overland flow responses until soil hydrophobicity declines to pre-fire levels; Ž2. increased effectiveness of pre-existing water repellency, where a return to pre-fire conditions of a


49

Fig. 7. Schematic illustration of possible hydrological responses of soil with ŽA. a hydrophobic layer located on the surface, and ŽB. a hydrophobic layer sandwiched between hydrophilic soil.

prevailing, but less effective soil hydrophobicity would await re-vegetation, root development and the re-formation of a litter mulch; Ž3. soils that are hydrophilic before and after the fire, but where other fire-related changes than hy-

drophobicity enhance overland flow ŽZierholz et al., 1995.. Scenarios Ž1. and Ž2. are summarised in Fig. 8, which contrasts the more conventional view in which overland flow is enhanced mainly due to fire-in-

50 S.H. Doerr et al.r Earth-Science ReÕiews 51 (2000) 33–65 Fig. 8. Schematic illustration of the effects of burning and water repellency on overland flow generation exemplified by two contrasting scenarios. ŽA and B. Californian chaparral where a slight hydrophobicity in the surface layer is destroyed and an intensely hydrophobic layer is generated below the surface Žbased on DeBano, 1969.; ŽC and D. Portuguese eucalyptus forest, where fire has little effect on the extreme hydrophobicity already present in the soil.


duced water repellency ŽFig. 8A and B. with a scenario in which overland flow is enhanced due to the increased effectiveness of pre-existing water repellency ŽFig. 8C and D.. Some of the changes involved in Scenario Ž3. may also promote overland flow after clearfelling rather than burning, as reported for a Pinus radiata catchment in South Africa ŽScott and Lesch, 1997.. That Hortonian overland flow tends to be most pronounced where soils have an uninterrupted hydrophobic layer ŽFig. 7Ar1. provides an explanation for the contrasts in overland flow response on water-repellent terrain reported in the literature. Several studies have pointed towards the localisation of overland flow on water-repellent soils. For example, only 25% of the soil following a fire in southwest Oregon was water-repellent, resulting in a low impact on infiltration and overland flow ŽMcNabb et al., 1989.. Also Meeuwig Ž1971. and Imeson et al. Ž1992., working in pine forests in North America and northeast Spain, respectively, found that Hortonian overland flow generated on water-repellent soil around trees tended to infiltrate on adjacent hydrophilic soil around shrubs ŽFig. 7Ar2.. This pattern was also found using simulated rainstorms in southwest Spain ŽCerda` et al., 1998. The short-term temporal variations of water repellency outlined in Section 6 also need to be considered here. Reductions in infiltration capacity and increases in overland flow can be expected to be most pronounced following prolonged dry periods, when water repellency tends to become most severe Žsee also Fig. 9.. For example, Burch et al. Ž1989. recorded infiltration capacities in Australian eucalyptus forest of 0.75–1.9 mmrh when dry, but 7.9–14.0 mmrh when wet. In many areas, hydrophobicity-linked overland flow is therefore confined to storm events following dry weather ŽSevink et al., 1989; Walsh et al., 1994.. In burnt Portuguese pine and eucalyptus forests, the enhanced Hortonian overland flow responses following dry weather when the soils are most hydrophobic contrast sharply with the muted overland flow in moderately wet weather, when soils are generally hydrophilic ŽTable 5. ŽWalsh et al., 1994; Shakesby et al., in press.. The high responses in very wet weather are caused by saturation overland flow rather than water repellency. Since few studies address this issue and the understanding of

51

Fig. 9. Relative frequency of actual water repellency in a Dutch silt loam soil on three different occasions. The volume of waterrepellent soil in the topsoil Ž0–15 cm. decreases considerably between October 1992 and January 1993 Žmodified from Dekker and Ritsema, 1995..

the processes involved in the breakdown of water repellency is poor ŽSection 6.2., it is often not known how long hydrophobicity-linked low infiltration capacities may persist during wet conditions. O’Loughlin et al. Ž1982. found that the effectiveness of post-fire hydrophobicity was short-lived and Zwolinski Ž1971. also found water repellency rapidly disappearing during simulated rainfall; in both areas overland flow was insignificant. Whereas in hydrophilic soils infiltration capacity declines during rainstorms, in hydrophobic soils the infiltration capacity often increases as soils become wet ŽBurch et al., 1989; Imeson et al., 1992.. For extremely waterrepellent soils, however, wetting may not occur at all during irrigation or rainstorms. This has, for example, been reported from golf greens in the USA ŽKarnok et al., 1993. and for some Portuguese forest soils during 40–46 mm of simulated rain ŽWalsh et al., 1998.. Water repellency for some of these Portuguese soils has proved so persistent that soils have remained dry beneath a water layer for more than 3 weeks ŽDoerr and Thomas, 2000.. Finally, the effects of water repellency on overland flow generation should also be seen in relation to the scale of measurements. Imeson et al. Ž1992.


52

Table 5 Hydrophobic- and hydrophilic-phase overland flow responses at Portuguese pine and eucalyptus forest plots following fire in July 1991 and August 1992 Žafter Walsh et al., 1994. Date

Rain Žmm.

Peak intensity Žmmrh.

(a) Lourizela : regenerating pine plots after fire in July 1991 12r3r93 18.9 6.8 30r4r93 46.1 11.9 17r9r93 11.2 4.2 (b) Falgorosa: regenerating eucalyptus plots after fire in August 1992 12r3r93 18.9 7.5 30r4r93 23.5 7.9 17r9r93 28.4 5.0

argued that although water-repellent soils can produce high overland flow rates locally, the effects at slope or catchment scales can be more subdued because of the high spatial variations in infiltration. Similar findings were reported by Roberts and Carbon Ž1971. and Pradas et al. Ž1994..

Antecedent weather

Overland flow Ž%. Plot A

Plot B

dry wet very wet

19.3 7.8 40.3

7.0 4.0 21.5

dry wet very wet

14.1 4.7 19.7

11.5 3.1 10.9

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

8.3. Preferential flow (including fingered flow) Preferential flow is the concentrated vertical movement of water via preferred pathways through the soil matrix. It may originate for a variety of reasons such as cracks and macropores, textural discontinuities and unstable wetting fronts Žwhich may result from soil layering or air entrapment. ŽRitsema et al., 1993.. Although not restricted to hydrophobic soils ŽKung, 1990; Ritsema and Dekker, 1994a., hydrophobicity can be particularly effective at preventing or hindering downward water movement, directing it into structural or textural preferential flow paths ŽFig. 7Ar2 and B. or creating an unstable irregular wetting front ŽFig. 10.. Consequently soils may not wet completely with the passage of a wetting front ŽDeBano, 1971., and water may be channelled via macropores and cracks, by-passing the soil matrix ŽBurch et al., 1989.. Root channels and rodent burrows are thought to represent particularly effective bypass route-ways through water-repellent soil ŽGarkaklis et al., 1998; Ferreira et al., 2000.. Walsh et al. Ž1995. considered that such by-pass routes explained why even large storms produced little overland flow for highly hydrophobic mature pine and eucalyptus forest soils in Portugal.

Fig. 10. Uneven wetting patterns caused by water repellency in sandy soil near Geraldton, Western Australia, following a large rainfall event Ž75 mm in 3–4 h in March 1999..


the former, and is then directed as lateral flow to vertical preferential flow routeways through the underlying water repellent layer ŽFig. 7B.. This phenomenon has been described for many burnt soils by DeBano Ž1981. and has been investigated further by Ritsema and Dekker Ž1995. and Ritsema et al. Ž1993., who have termed the lateral spreading process ‘distribution flow’. On a grass-covered sandy dune in Holland, Ritsema et al. Ž1993. used tracers to record distribution flow within a thin Ž- 2.5cm. hydrophilic, relatively moist humus topsoil, which supplied water via columns of less hydrophobic soil Žpreferential flow paths. in an otherwise extremely hydrophobic layer to a second hydrophilic zone below 45 cm depth, where the water spread laterally. This has been termed ‘fingered flow’ ŽRitsema 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’ value of 4.75% Žvol... The fingers ranged from 10 to 50 cm in width Žexpanding in wetter weather. and were the sole means of water transport for several hours during sustained rainfalls until the soil between fingers became wet and hydrophilic. Such fingers have been shown to recur at the same places in successive storms following intervening dry weather, possibly aided by preferential leaching of water-repellent substances from the finger pathways ŽRitsema et al., 1998a,b.. In contrast, instead of any fingered flow, a uniform wetting front may develop in a water-repellent layer if the overlying hydrophilic top-layer is very thick ŽVan den Bosch et al., 1999.. Bauters et al. Ž1998. found that sandy soils with different degrees of water repellency all exhibited fingered flow, whereas a uniform, broadly horizontal wetting front developed in non-repellent soil. Infiltration of the former occurred only when ponding depth exceeded water-entry pressure, the critical value of which was found Žtogether with finger flow velocity. to increase with water repellency severity. Wetting patterns Žin terms of finger width. were found to conform to unstable flow theory and depended on the characteristic soil water curve of the soil in question. Preferential flow in general is thought to be reinforced by soil water hysteresis between wetting and drying phases, a feature of most hydrophilic soils but exaggerated in hydrophobic ones ŽRitsema et al.,

53

1998a,b.. The retarding impact of hysteresis and enhancing impact of preferential flow have been incorporated into a model for unsaturated soil water movement in water-repellent soil and tested with field data from the Netherlands ŽVan Dam et al., 1996.. The model demonstrates clearly how preferential flow has the effect of reducing residence times of solutes in the unsaturated zone. The consequences of hydrophobicity-related parameters on surface and subsurface hydrology discussed in Sections 8.2 and 8.3 have often been considered separately and an attempt is made in Fig. 11 to provide a synthesis. The model suggests that impacts will vary not only with the degree of hydrophobicity, but also with the location of the hydrophobic layer in the soil profile, the thickness of any overlying hydrophilic layer, the extent and spatial contiguity of water repellency per unit area, and as with the effectiveness of preferential flow pathways. In addition, the temporal regime of water repellency Žsee Section 6. is crucial, as it determines the proportion of storm events in which water repellency exerts an influence. 8.4. Impacts on the three-dimensional distribution and dynamics of soil moisture, eÕapotranspiration and plant growth As is evident from the previous sections, water repellency influences the three-dimensional distribution and dynamics of soil moisture, including evaporation patterns. The impact will vary with the vertical position of the water-repellent layer, the frequency of preferential flow routeways through the water-repellent layer and the temporal persistence and regime of water repellency. For example, a strongly water-repellent surface layer with preferential flow routeways can lead to dry surface soil and higher soil moisture in the subsoil Že.g. Meeuwig, 1971; Burch et al., 1989; Imeson et al., 1992.. In a study in northeast Spain, Imeson et al. Ž1992. described how a surface water-repellent layer not only trapped water in the BrC horizon, but also prevented evaporation and upward capillary movement of water. Hydrophobicity-induced fingered flow can lead to considerable variations in water content in an initially water-repellent soil. For example, Dekker and Ritsema Ž1996a. found differences in soil moisture of up to 28% Žvol.. between closely spaced samples

54


Fig. 11. Schematic model of the influence of hydrophobicity characteristics on slope runoff processes and erosion risk ŽSOF and HOF abbreviate saturation-and Hortonian overland flow, respectively, and wqx, wqrsx and wsx indicate a strong, low to negligible and negligible erosion risk.

in both clay and sandy soils ŽFig. 12.. Such differences do not only result in the widely reported poor seed germination and plant growth Žsee Wallis and Horne, 1992.. Any type of preferential flow path formation can also lead to accelerated leaching of surface-applied agrichemicals and an increased risk of surface and groundwater contamination ŽHendrickx et al., 1993; Ritsema et al., 1993.. 8.5. Effects on streamflow generation and patterns The tendency for fire-related water repellency and fire effects to increase both total streamflow and the

Fig. 12. Contour plot showing a finger-like moisture distribution in a water-repellent sandy soil. The driest areas are associated with the highest degree of repellency Žmodified from Ritsema and Dekker, 1997..

magnitude of storm peaks as well as reducing peakflow response times has been well established ŽWhite and Wells, 1982.. For example, increases of 800% for streamflow and 450% for catchment runoff efficiency during the first post-fire wet season were attributed in large part to hydrophobicity-enhanced overland flow in a pine forest catchment in Arizona ŽCampbell et al., 1977.. More recent studies have suggested that impacts may be more complex. Scott and Lesch Ž1997. attributed the lack of streamflow 9 years after the afforestation of grassland catchments with Eucalyptus grandis and Pinus patula in South Africa to an enhanced deep drainage through the water-repellent soil via preferential flow along the eucalyptus root channels. Instead of promoting overland flow, water repellency associated with afforestation enhanced water storage at greater depths in the soil, permitting its later use in transpiration. Variations in water repellency, its response to fire and its interaction with other factors were thought by Scott Ž1993. to be responsible for differences in catchment response to fire between Fynbos vegetation, Pinus radiata and Eucalyptus festigata in mountain catchments in South Africa. In the native Fynbos catchments, only a modest increase in total discharge Ž16%. and no increase in stormflow occurred following both prescribed fire and wildfire. This low response was attributed to only a minor and spatially patchy increase in water repellency, causing


little overland flow to reach the base of slopes. The increase in total flow was thought to be due to reduced evapotranspiration. In contrast, in the Eucalyptus festigata and Pinus radiata catchments, the stormflow component increased by 92% and 201% respectively, the latter being attributed to fire-induced hydrophobicity. For the former, water repellency had been a pre-fire feature, which became more effective in promoting overland flow following fire. 8.6. Soil erosion Because of the reduction in infiltration capacity typical of hydrophobic soils and resulting tendency for increased overland flow during rainfall events described in Section 8.2, enhanced erosion has often been attributed to this soil hydrophobicity. Studies where such a connection has been made have, however, tended to infer rather than demonstrate a direct causal link between erosion and water repellency. For example, Megahan and Molitor Ž1975., working in a pine and fir forest in Idaho, noted a tendency towards increased soil loss on water-repellent land. Also increases in soil losses immediately after wildfire have been linked to fire-induced or fire-enhanced water repellency Že.g. Wells et al., 1979; Morris and Moses, 1987; Shakesby et al., 1993., including the development of rills and gullies on burnt land Že.g. White and Wells, 1982; Giovannini, 1994.. Establishing how much of the increased erosion is due to water repellency, however, is difficult, as burning also leads to other changes that can enhance overland flow generation as discussed in Section 8.2. Fire can directly promote the erodibility of soil by the removal of the protective vegetation and litter cover, the loss of organic matter, the breakdown of aggregates and the reduction of soil particle size ŽScott, 1993.. Few studies have been successful in isolating the impact of water repellency on erosion from other effects. A notable exception has been the Californian-based study by Osborn et al. Ž1964. who monitored soil losses at bounded plots in burnt, water-repellent terrain, some of which had been made less repellent by applying wetting agent prior to the first rainfall event. The amount of sediment removed from the untreated plots was more than thirteen times as much as that removed from the treated

55

Table 6 Erosion, rainfall and maximum rainfall intensity for rainfall events on untreated Žwater-repellent. and treated Žhydrophilic. plots Žapprox. 3=12 m2 . in burnt forest, California Žmodified from Osborn et al., 1964. No. of days Rainfall Maximum intensity Žmm. on which of rainfall Žmmrh. rainfall occurred

Eroded material Žcm3 .

2 1 1 2 2 2 2 Total

515.4 62.3 962.8 110.4 11.3 Trace 184.1 1846.3

35.8 24.9 156.5 33.5 15.7 23.1 44.4 333.9

34.3 for 20 min

10.2 general

91.4 for 2 min

ŽUntreated. ŽTreated. 5.7 62.3 45.3 17.0 2.8 Trace Trace 133.1

Žlow-repellency. plots ŽTable 6.. By the end of the monitoring period, all untreated plots had rills up to 100-mm deep extending almost their entire lengths, whereas the treated plots showed either minor or no rilling. In a series of studies carried out in sand dunes under the more humid conditions of the Dutch coast Že.g. Jungerius and van der Meulen, 1988; Witter et al., 1991; Jungerius and ten Harkel, 1994., the impact of water repellency was also demonstrated. These studies highlighted short-term temporal variability in water repellency and the impact of overland flow on erosion. In winter, when the sand tends to be moist and hydrophilic, infiltration capacity is extremely high such that heavy rainfall leads to very little erosion. After dry spells in summer, however, water repellency is restored and comparatively little rainfall causes substantial erosion. The impact is most pronounced on quicker-drying, south-facing slopes with an incomplete cover of moss and algae where shallow rills become incised between shrubs and lead downslope to fan-shaped sand deposits. On slopes of more than 68, overland flow entrains sand grains in mudflow-like tongues that extend to the foot of the slope ŽJungerius and ten Harkel, 1994.. As a result of the difference in aspect causing differences in the time taken to re-establish water-repellent conditions and thus differences in erosion susceptibility, dunes with east–west aligned crests have asymmetrical cross-profiles with lower-angled south-


56

than north-facing slopes. Jungerius and van der Meulen Ž1988. reported more than half of the erosional loss in one summer on the dunes occurring in just two storms. The subdued topography of the dunes has been attributed to this seasonal water repellency-enhanced erosion ŽWitter et al., 1991.. Erosion caused by enhanced overland flow on water-repellent soil was also suspected by Bridge and Ross Ž1983. and Thompson Ž1983. on coastal sand dunes in southeast Queensland and by Soto et al. Ž1994. for burnt scrub in northwest Spain, although no figures were provided in support of this interpretation. Water repellency was viewed by Shakesby et al. Ž1994, 1996. and Walsh et al. Ž1994. as the main factor explaining consistently high rates of erosion on small plots in newly burnt Eucalyptus globulus and Pinus pinaster forest soils in Portugal in storm events following prolonged dry periods ŽTable 7.. In one case, soil losses per millimetre of overland flow in a summer storm event of 20 mm were consistently higher than during wet winter phases, reaching up to 35 times the value calculated for the eucalyptus plots during the preceding winter period when more than 200 mm of rain fell. It should be noted, however, that the total amounts of sediment removed were nevertheless higher during winter rainfall because of much higher volumes of overland flow. Rainsplash erosion may also be of particular importance on water-repellent soils. Although at the Dutch coastal dune sites, overland flow erosion was viewed as the main erosion process with rainsplash producing only 2% of the annual sediment yield ŽJungerius and van der Meulen, 1988; Jungerius and

ten Harkel, 1994., work on Portuguese forest soils referred to above indicated that the contribution of the latter process to soil loss may be substantial. Under simulated rainfall conditions, Terry and Shakesby Ž1993. found that rainsplash detachment amounts recorded for hydrophilic soils were only 52–58% on flat surfaces and 51–72% on sloping surfaces of amounts recorded for water-repellent soils. Observations of individual drops falling on the two types of soil indicated a difference in the actual splash mechanism. For the hydrophilic soil, a cohesive surface crust of closely packed grains quickly developed during simulated rainfall, whereas on the hydrophobic soil, despite a surface water film forming over the surface, a continual supply of readily available dry soil was available for splash ejection from both above and below the film. Some questions might be raised about the magnitude of its impact, but these results and others reported from northern Portuguese forests Že.g. Shakesby et al., 1993, 1994. provide strong support for the view that water repellency plays an important role in promoting soil detachment. Although water repellency was found to be equally severe in both unburnt and burnt forests in these studies ŽDoerr et al., 1998., it seems that fire created the conditions Žof bare soils. for water repellency to enhance erosion processes. The erosional impact of hydrophobicity in soils depends strongly on the degree of contiguity of the hydrophobic surface ŽShakesby et al., 2000.. For example, following a fire in California, Booker et al. Ž1993. noted that overland flow and slopewash Žrainsplash and overland flow entrainment and transport. were promoted where cracks and other route-

Table 7 Mean erosion per millimetre of overland flow for six 8 = 2 m plots in areas of both newly burnt Eucalyptus globulus and Pinus pinaster, northern Portugal Žmodified from Walsh et al., 1994. Period

Nov. 22–Dec. 12 1992 Dec. 12–Mar. 5 1993 Mar. 5–Apr. 29 1993 Apr. 29–Jun. 14 1993 Jun. 14–Jul. 4 1993 a b

Eucalyptus burnt August 1992

Pine burnt July 1991

Rainfall Žmm.

Mean erosion per millimeter of overland flow Žg.

Rainfall Žmm.

Mean erosion per millimeter of overland flow Žg.

33.5 205.4 163.9 255.7 19.9

37.8 a 8.2 10.8 83.3 292.3

56.0 320.3 – – 30.8

45.9 b 18.7 – – 393.7

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


ways through the hydrophobic layer were absent. On the other hand, Booker et al. Ž1993. observed that where preferential Žsubsurface. flow rather than overland flow was present through the existence of cracks and other routeways, the slopes were prone to landsliding, because of the enhanced transfer of water to the subsoil. Water repellency can also have an indirect impact on erosion processes. In a study on sandy loam soils in California, Krammes and Osborn Ž1969. noted that following wildfire, material accumulating in the form of debris cones through a process of dry-creep Ždry ravel. at the foot of slopes was usually water-repellent. On inspection, the water-repellent soil was found to have a lower bulk density than its hydrophilic counterpart. Similar conclusions were drawn by DeBano et al. Ž1979. about the relative densities of non-repellent and water-repellent soils. Krammes and Osborn Ž1969. reasoned that, at least for these sandy loams, hydrophilic particles tend to draw together when drying out because of cohesive and adhesive forces between the particles. For water-repellent soils, however, these forces are low, so that paradoxically rainfall may encourage drycreep because of this failure of the particles to adhere together. Over a 6-month period, they found that soil losses by dry-creep from plots treated with wetting agents were 54–56% lower than those recorded from untreated hydrophobic plots. Although most erosion studies on hydrophobic soils suggest enhanced sediment yield, resistance to erosion may be improved for water-repellent soils with well-developed aggregates ŽWallis and Horne, 1992.. For example, DeBano Ž1981. argued that improved aggregate stability was caused by hydrophobic organic material in the aggregates reducing the swelling and the destructive forces of trapped air. The stabilising effect was found to be greatest for aggregates 0.5–5.0 mm in size. For larger aggregates, the impact of water repellency became less and plant roots became the most important aggregating mechanism. Other authors have also noted improved water stability of water-repellent soil aggregates ŽRawitz and Hazan, 1978; Giovannini and Lucchesi, 1983; Capriel et al., 1995.. Not only water erosion, but also wind erosion can be influenced by soil water repellency. The erodibility by wind may not differ much between non-repel-

57

lent and repellent soils when dry. However, as emphasised in an Australian study by Carter Ž1990., the periods when soils are bare and dry, and thus most susceptible to wind erosion, are likely to be longer andror more frequent for the latter. More indirect effects of water repellency on soil erodibility have been found in the studies on Dutch dunes referred to above. The water-repellent upper and non-repellent lower sands have different degrees of resistance with respect to wind erosion. The surficial ‘grey’ sands, although susceptible to erosion by overland flow, are comparatively resistant to aeolian action. Once the grey sand is eroded sufficiently by overland flow following summer storms, the underlying yellow, hydrophilic sands can become exposed and these are susceptible to entrainment by wind and to the development of blowouts Že.g. Jungerius and de Jong, 1989; Jungerius and ten Harkel, 1994..

9. Conclusion Since the late 1980s, the study of soil hydrophobicity has expanded both geographically as well as in specific research directions. Current understanding of the causes and some of the key factors affecting water repellency ŽFig. 13. confirm that water repellency in soils is caused partly or entirely by hydrophobic, long-chained organic molecules, released from decomposing or burning plant litter. Recently, the root zone and the leaf surfaces of living plants have also been acknowledged as possible sources of hydrophobic compounds. These organic compounds can cause hydrophobicity by their presence as a coating on individual soil grains or aggregates, or as interstitial particles between soil grains. The role of fire in affecting water repellency has been firmly established over previous decades and the most notable recent advance seems to be the increasing realisation that burning may, in some cases, cause little change in already hydrophobic soil. The enhancing effect of soil heating on water repellency to lower temperatures than those associated with burning, however, has only in the past few years been widely acknowledged. Similarly, the greater susceptibility of coarse-textured soils to hydrophobicity development has long been known, but increasingly in more recent studies, it has been demonstrated that

58


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 finertextured soils, it can be equally or even more severe than in the former. There are clear research gaps that relate to the causes and characteristics of water repellency. They concern particularly the poor understanding of the exact chemical composition of the compounds causing hydrophobicity and their mechanisms of attachment to soil particles. In addition, the roles of soil fungi and microorganisms and thus of the decomposition regime of organic matter in general are still relatively unclear, for both the establishment and destruction of water repellency. This uncertainty is, for example reflected in the number of conflicting reports on the relationship of organic matter to water repellency. It appears, however, that the relationships of general soil and vegetation parameters to water repellency vary so widely between soils that it may remain very difficult to establish firm links that are applicable to a wide range of soils and environments. Major advances since the late 1980s have been made in understanding the impacts of water repellency on hydro-geomorphological processes. Its ability to cause or enhance uneven wetting and preferential flow in soils is now widely acknowledged and earlier, relatively simple views of how hydrophobicity enhances overland flow have had to be refined. The impact of water repellency on water movement over and within the soil, and thus on soil erosion risk by water has often not been sufficiently isolated from other factors Žparticularly after burning., but the range of impacts are now known to be far more complex than previously acknowledged. Variables determining these impacts are not only the frequency and effectiveness of flow pathways through any hydrophobic layer ŽFig. 11., themselves influenced by vegetation type, land use, and soil structure, but also the position, intensity and temporal regime of the hydrophobic layerŽs. in the soil. Two important research gaps can be identified regarding the hydro-geomorphological significance of soil water repellency. First, little is still understood about the spatial contiguity of hydrophobicity and the frequency and effectiveness of preferential flow pathways, and their overall influence on runoff processes and streamflow generation. For example, the role of hydrophobicity in the development of tunnelling Žpiping. and the development of pipeflow

59

responses to rainstorms are poorly understood. Second, there is only a poor understanding of the temporal regime of hydrophobicity and its hydrological impact. Although it is well established that soils can lose their water-repellent character during long wet periods, little has been achieved in identifying the exact wetting mechanisms involved, the threshold conditions needed for this change Že.g. size, duration or frequency of storm events, or critical soil moisture content. and the mechanisms and conditions Že.g. temperature and length of dry period. for hydrophobicity to become re-established. This information is necessary for establishing the overall role of hydrophobicity in terms of the percentage and timing of rainstorm events in which it can be expected to affect runoff processes in different types of environments. Such knowledge is not only crucial for understanding and predicting slope and catchment hydrological responses, but also for optimising plant growth and reducing groundwater contamination risk on managed land. Furthermore, it is important for establishing and understanding the overall role of water repellency in influencing surface and subsurface erosion processes. In the last decade or so, it has become increasingly evident that soil water repellency is a wideranging phenomenon and not just a pedological curio restricted to some very specific environments. Its actual extent amongst soils world-wide, however, remains unclear. If water repellency is shown, as suggested by Wallis et al. Ž1991, p. 360., Ato be the norm rather than the exceptionB, then its importance particularly for managed land will prove far greater than acknowledged at present, and it should therefore receive much more attention than it currently attracts. Acknowledgements The authors wish to thank the anonymous reviewers for their valuable comments on the manuscript and A. Ratcliffe and N. Jones for drawing many of the figures. We also wish to acknowledge funding from the EU Žcontacts EV4V-0106-CŽTT., EV5V-0041, ENV4-CT97-0686 and FAIR 6CT984027., and Nato Žcontract CRG.CRG.960704., which has enabled us to pursue research into water repellency.

60


References Adam, N.K., 1963. Principles of water-repellency. In: Moillet, J.L. ŽEd.., Water Proofing and Water-Repellency. Elsevier, London, pp. 1–23. Almendros, G., Martın, F.J., 1988. Effects of ´ F., Gonzales-Vila, ` fire on humic and lipid fractions in a dystric xerochrept in Spain. Geoderma 42, 115–127. Anderson, W.G., 1986. Wettability literature survey: Part 3. The effects of wettability on the electrical properties of porous media. Journal of Petroleum Technology, 1371–1378, Dec. Barrett, G., Slaymaker, O., 1989. Identification, characterisation and hydrological implications of water repellency in mountain soils, Southern British Columbia. Catena 16, 477–489. Bauters, T.W.J., DiCarlo, D.A., Steenhuis, T.S., Parlange, J.Y., 1998. Preferential flow in water-repellent sands. Soil Science Society of America Journal 62, 1185–1190. Berglund, K., Persson, L., 1996. Water repellence of cultivated organic soils. Acta Agriculturae Scandinavica 46, 145–152. Bisdom, E.B.A., Dekker, L.W., Schoute, J.F., 1993. Water repellency of sieve fractions from sandy soils and relationships with organic material and soil structure. Geoderma 56, 105– 118. Bishay, B.G., Bakhati, H.K., 1976. Water repellency of soils under citrus trees in Egypt and means of improvement. Agricultural Resources Review ŽCairo. 54, 63–74. Blackwell, P.S., 1993. Improving sustainable production from water repellent sands. Western Australia Journal of Agriculture 34, 160–167. Boelhouwers, J.C., De Graaf, P.J., Samsodien, M.A., 1996. The influence of wildfire on soil properties and hydrological response at Devils Peak, Cape Town, South Africa. Zeitschrift fur ¨ Geomorphologie, Supplementband 107, 1–10. Bond, R.D., 1964. The influence of the microflora on the physical properties of soils: II. Field studies on water repellent soils. Australian Journal of Soil Research 2, 123–131. Bond, R.D., 1969. The occurrence of water-repellent soils in Australia. In: Proceedings of the Symposium on Water-Repellent Soils, University of California, May 1968. pp. 1–6. Booker, F.A., Dietrich, W.E., Collins, L.M., 1993. Runoff and erosion after the Oakland firestorm. Expectations and observations. California Geology 46, 159–173. Bornemisza, E., 1964. Wettability of soils in relation to their physio-chemical properties. Unpublished PhD thesis. University of Florida. Dissertation Abstracts, 64, 3771. Bridge, B.J., Ross, P.J., 1983. Water erosion in vegetated sand dunes at Cooloola, southeast Queensland. Zeitschrift fur ¨ Geomorphologie, Supplementband 45, 227–244. Brock, J.H., DeBano, L.F., 1990. Wettability of an Arizona chaparral soil influenced by prescribed burning. United States Department of Agriculture, Forest Service, General Technical Report, RM-191, 206–209. Burcar, W.W., Tyler, S.W., Johnson, D.W., 1994. Seasonal preferential flow in two Sierra Nevada soils under forested and meadow cover. Soil Science Society of America Journal 58, 1555–1561. Burch, G.J., Moore, I.D., Burns, J., 1989. Soil hydrophobic effects

on infiltration and catchment runoff. Hydrological Processes 3, 211–222. Campbell, R.E., Baker, M.B., Folliot, P.F., Larson, R.F., Arvey, C.C., 1977. Wildfire effects on a ponderosa pine ecosystem: an Arizona case study. United States Department of Agriculture, Forest Service, Research Paper RM-191, Rocky Mountain Forest and Range Experimental Station, Fort Collins, Colorado, 21 pp. Cann, M., Lewis, D., 1994. The use of dispersible sodic clay to overcome water repellence in sandy soils in the South East of South Australia. Proceedings of the 2nd National Water Repellency Workshop, 1–5 August, Perth, Western Australia. pp. 49–57. Capriel, P., Borchert, Beck., Gronholz, H., Zachmann, J., 1995. Water repellency of the organic matter in arable soils. Soil Biology and Biochemistry 27 Ž11., 1453–1458. Carter, D.J., 1990. Water repellence in soils and its effect on wind erodibility. Proceedings of the National Workshop on Water Repellency in Soils, April, Adelaide, South Australia. pp. 56–59. Carter, D.J., Hetherington, R.E., 1994. Claying of water repellent soils on the South Coast of Western Australia. Proceedings of the 2nd National Water Repellency Workshop, 1–5 August, Perth, Western Australia. pp. 49–57. Carter, D.J., Hetherington, R.E., Morrow, G., Nicholson, D., 1994. Trends in water repellency measurements from soils sampled at different soil moisture and land use. Proceedings of the 2nd National Water Repellency Workshop, 1–5 August 1994, Perth, Western Australia. pp. 49–57. Cerda, ` A., Schnabel, S., Ceballos, A., Gomez-Amelia, D., 1998. Soil hydrological response under simulated rainfall in the Dehesa land system ŽExtremadura, SW Spain. under drought conditions. Earth Surface Processes and Landforms 23, 195– 209. Chan, K.Y., 1992. Development of seasonal water-repellence under direct drilling. Journal of the Soil Science Society of America 56, 326–329. Chen, Y., Schnitzer, M., 1978. The surface tension of aqueous solutions of soil humic substances. Soil Science 125, 7–15. Crockford, S., Topalidis, S., Richardson, D.P., 1991. Water repellency in a dry sclerophyll forest — measurements and processes. Hydrological Processes 5, 405–420. DeBano, L.F., 1969. Water movement in water-repellent soils. Proceedings Symposium Water-Repellent Soils, University of California, May 1968. pp. 61–89. DeBano, L.F., 1971. The effect of hydrophobic substances on water movement in soil during infiltration. Proceedings of the Soil Science Society of America 35, 340–343. DeBano, L.F., 1981. Water repellent soils: a state-of-the-art. United States Department of Agriculture, Forest Service, General Technical Report, PSW-46, Berkeley, California, 21 pp. DeBano, L.F., 1991. The effects of fire on soil properties. United States Department of Agriculture, Forest Service, General Technical Report, INT-280, 151–156. DeBano, L.F., Krammes, J.S., 1966. Water repellent soils and their relation to wildfire temperatures. International Bulletin of the Association of Hydrological Scientists 2, 14–19.

S.H. Doerr et al.r Earth-Science ReÕiews 51 (2000) 33–65 DeBano, L.F., Mann, L.D., Hamilton, D.A., 1970. Translocation of hydrophobic substances into soil by burning organic litter. Proceedings of the Soil Science Society of America 34, 130– 133. DeBano, L.F., Savage, S.M., Hamilton, A.D., 1976. The transfer of heat and hydrophobic substances during burning. Proceedings of the Soil Science Society of America 40, 779–782. DeBano, L.F., Rice, R.M., Conrad, C.E., 1979. Soil heating in chaparral fires: effects on soil properties, plant nutrients, erosion and runoff. United States Department of Agriculture, Forest Service, Research Paper PSW-145, Pacific Southwest Forest and Range Experimental Station, Berkeley, California, 21 pp. DeByle, N.V., 1973. Broadcast burning of logging residues and the water repellency of soils. Northwest Science 47, 77–87. de Jonge, L.W., Jacobsen, O.H., Moldrup, P., 1999. Soil water repellency: effects of water content, temperature and particle size. Journal of the Soil Science Society of America 63, 437–442. Dekker, L.W., Ritsema, C.J., 1994. How water moves in a water-repellent sandy soil: 1. Potential and actual water-repellency. Water Resources Research 30, 2507–2517. Dekker, L.W., Ritsema, C.J., 1995. Fingerlike wetting patterns in two water-repellent loam soils. Journal of Environmental Quality 24, 324–333. Dekker, L.W., Ritsema, C.J., 1996a. Variation in water content and wetting patterns in Dutch water repellent peaty clay and clayey peat soils. Catena 28, 89–105. Dekker, L.W., Ritsema, C.J., 1996b. Preferential flow paths in a water repellent clay soil with grass cover. Water Resources Research 32 Ž5., 1239–1294. Dekker, L.W., Ritsema, C.J., Oostindie, K., Boersma, O.H., 1998. Effect of drying temperature on the severity of soil water repellency. Soil Science 163 Ž10., 780–796. Dekker, L.W., Ritsema, C.J., Wendroth, O., Jarvis, N., Oostindie, K., Pohl, W., Larsson, M., Gaudet, J., 1999. Moisture distributions and wetting rates of soils at experimental fields in the Netherlands, France, Sweden and Germany. Journal of Hydrology 215, 4–22. Dinel, H., Schnitzer, M., Mehuys, G.R., 1990. Soil lipids: origin, nature, content, decomposition, and effect on soil physical properties. In: Bollag, J.M., Stotzky, G. ŽEds.., Soil Biochemistry, vol. 6. Marcel Dekker, New York, pp. 397–429. Doerr, S.H., 1997. Soil water repellency in wet mediterranean pine and eucalyptus forests, Agueda basin, north-central Portugal. Unpublished PhD thesis, University of Wales Swansea, UK. Doerr, S.H., 1998. On standardising the AWater Drop Penetration TimeB and the AMolarity of an Ethanol DropletB techniques to classify soil water repellency: a case study using medium textured soils. Earth Surface Processes and Landforms 23, 663–668. Doerr, S.H. and Thomas, A.D., in press. The role of soil moisture in controlling water repellency: new evidence from forest soils in Portugal. Journal of Hydrology. Doerr, S.H., Shakesby, R.A., Walsh, R.P.D., 1996. Soil water repellency variations with depth and particle size fraction in

61

burned and unburned Eucalyptus globulus and Pinus pinaster ´ forest terrain in the Agueda basin, Portugal. Catena 27 Ž1., 25–47. Doerr, S.H., Shakesby, R.A., Walsh, R.P.D., 1998. Spatial variability of soil water repellency in fire-prone eucalyptus and pine forests, Portugal. Soil Science 163, 313–324. Dyrness, C.T., 1976. Effect of wildfire on soil wettability in the High Cascade of Oregon. United States Department of Agriculture, Forest Service, Research Paper PNW-202, Pacific Northwest Forest and Range Experimental Station, Portland, Oregon, 444–447. Ferreira, A.J.D., Coelho, C.O.A., Walsh, R.P.D., Shakesby, R.A., Ceballos, A., Doerr, S.H., 2000. Hydrological implications of soil water repellency in Eucalyptus globulus and Pinus pinaster forests, north-central Portugal. Journal of Hydrology, Žin press.. Fogel, R., Hunt, G., 1979. Fungal and arboreal biomass in a western Oregon Douglas fir ecosystem: distribution patterns and turnover. Canadian Journal of Forest Research 9, 245–256. Franco, C.M.M., Tate, M.E., Oades, J.M., 1994. The development of water repellency in sands: studies on the physico-chemical and biological mechanisms. Proceedings of the 2nd National Water Repellency Workshop, 1–5 August, Perth, Western Australia. pp. 18–30. Franco, C.M.M., Tate, M.E., Oades, J.M., 1995. Studies on non-wetting sands: I. The role of intrinsic particulate organic matter in the development of water repellency in non-wetting sands. Australian Journal of Soil Research 33, 253–263. Franco, C.M.M., Clarke, P.J., Tate, M.E., Oades, J.M., 2000. Studies on non-wetting sands: II. Hydrophobic properties and chemical characterisation of natural water-repellent materials. Journal of Hydrology. Garkaklis, M.J., Bradley, J.S., Wooler, R.D., 1998. The effects of Woylie Ž Bettongia penicillata. foraging on soil water repellency and water infiltration in heavy textured soils in southwestern Australia. Australian Journal of Ecology 23, 492–496. Giovannini, G., 1994. The effect of fire on soil quality. In: Sala, M., Rubio, J.L. ŽEds.., Soil Erosion as a Consequence of Forest Fires. Geoforma Ediciones, Logrono, ˜ pp. 15–27. Giovannini, G., Lucchesi, S., 1983. Effect of fire on hydrophobic and cementing substances of soil aggregates. Soil Science 136, 231–236. Giovannini, G., Lucchesi, S., 1984. Differential thermal analysis and infrared investigations on soil hydrophobic substances. Soil Science 137, 457–463. Giovannini, G., Lucchesi, S., Cervelly, S., 1987. The natural evolution of a burnt soil: a three-year investigation. Soil Science 143, 220–226. Gore, W.L. et al., 1994. Your Questions Answered. Livingstone, 9 pp. Harper, R.J., Gilkes, R.J., 1994. Soil attributes related to water-repellency and the utility of soil survey for predicting its occurrence. Australian Journal of Soil Research 32, 1109–1124. Hendrickx, J.M.H., Dekker, L.W., Boersma, O.H., 1993. Unstable wetting fronts in water repellent field soils. Journal of Environmental Quality 22, 109–118. Holzhey, C.S., 1969. Water-repellent soils in southern California.

62


In: Proceedings Symposium Water-Repellent Soils, University of California, May 1968. pp. 31–42. Horn, D.H.S., Kranz, Z.H., Lamberton, J.A., 1963. The composition of eucalyptus and other leaf waxes. Australian Journal of Chemistry 17, 464–476. House, M.G., 1991. Select Committee Enquiry into Land Conservation, Legislative Assembly, Perth, Western Australia. Hudson, R.A., Traina, S.J., Shane, W.W., 1994. Organic matter comparison of wettable and non-wettable soils from bentgrass sand greens. Journal of the Soil Science Society of America 58, 361–367. Imeson, A.C., Verstraten, J.M., Van Mullingen, E.J., Sevink, J., 1992. The effects of fire and water repellency on infiltration and runoff under Mediterranean type forests. Catena 19, 345– 361. Jamison, V.C., 1942. The slow reversible drying of surface sandy soils beneath citrus trees in central Florida. Proceedings of the Soil Science Society of America 10, 25–29. Jaramillo, D.F., Herron, F.E., 1991. Evaluacion de la repellencia de algunos andisols de Antioqia Bajo cobertura de Pinus patula. Acta Agronomica 41, 79–85. Jex, G.W., Bleakley, B.H., Hubbel, D.H., Munro, L.L., 1985. High humidity-induced increase in water-repellency in some sandy soils. Journal of the Soil Science Society of America 49, 1177–1182. Jungerius, P.D., de Jong, J.H., 1989. Variability of water repellency in the dunes along the Dutch coast. Catena 16, 491–497. Jungerius, P.D., ten Harkel, M.J., 1994. The effect of rainfall intensity on surface runoff and sediment yield in the grey dunes along the Dutch coast under conditions of limited rainfall acceptance. Catena 23, 253–268. Jungerius, P.D., van der Meulen, F., 1988. Erosion processes in a dune landscape along the Dutch coast. Catena 15, 217–228. Karnok, K.A., Everett, J.R., Tan, K.H., 1993. High pH treatments and the alleviation of soil water repellency on golf greens. Agronomy Journal 85 Ž5., 983–986. King, P.M., 1981. Comparison of methods for measuring severity of water repellence of sandy soils and assessment of some factors that affect its measurement. Australian Journal of Soil Research 19, 275–285. Kolattukudy, P.E., Croteau, R., Buckner, J.S., 1976. Biochemistry of plant waxes. In: Kolattukudy, P.E. ŽEd.., Chemistry and Biochemistry of Natural Waxes. Elsevier, Amsterdam, pp. 289–347. Krammes, J.S., Osborn, J., 1969. Water-repellent soils and wetting agents as factors influencing erosion. Proceedings of the Symposium on Water-Repellent Soils, University of California, May 1968, pp. 177–187. Kung, K.J.S., 1990. Preferential flow in a sandy vadose zone: field observations. Geoderma 46, 51–58. Letey, J., 1969. Measurement of contact angle, water drop penetration time, and critical surface tension. Proceedings of the Symposium on Water-Repellent Soils, University of California, May 1968, pp. 43–47. Mallik, A.U., Rahman, A.A., 1985. Soil water repellency in regularly burned Calluna heathlands: comparison of three measuring techniques. Environmental Management 20, 207–218.

Ma’shum, M., Farmer, V.C., 1985. Origin and assessment of water repellency of a sandy Australian soil. Australian Journal of Soil Research 23, 623–626. Ma’shum, M., Tate, M.E., Jones, G.P., Oades, J.M., 1988. Extraction and characterisation of water-repellent material from Australian soils. Soil Science 39, 99–110. McGhie, D.A., Posner, A.M., 1980. Water repellence of a heavytextured western Australia surface soil. Australian Journal of Soil Research 18, 309–323. McGhie, D.A., Posner, A.M., 1981. The effect of plant top material on the water repellence of fired sands and water-repellent soils. Australian Journal of Agricultural Research 32, 609–620. McIntosh, J.C., Horne, D.J., 1994. Causes of repellency: I. The nature of the hydrophobic compounds found in a New Zealand development sequence of yellow–brown sands. Proceedings of the 2nd National Water Repellency Workshop, 1–5 August, Perth, Western Australia. pp. 8–12. McKissock, I., Gilkes, R.J., Harper, R.J., Carter, D.J., 1998. Relationships of water repellency to soil properties for different spatial scales of study. Australian Journal of Soil Research 36, 495–507. McNabb, D.H., Gaweda, F., Frohlich, H.A., 1989. Infiltration, ¨ water repellency, and soil moisture content after broadcast burning a forest site in southwest Oregon. Journal of Soil and Water Conservation 44 Ž1., 87–90. Meeuwig, R.O., 1971. Infiltration and water repellency in granitic soils. United States Department of Agriculture, Forest Service, Research Paper INT-111, Ogden, Utah, 20 pp. Megahan, W.F., Molitor, D.C., 1975. Erosional effects of wildfire and logging in Idaho. Proceedings of the Symposium on Watershed Management, American Society of Civil Engineers, Cogan, Utah, Aug.. pp. 423–444. Miller, R.D., Wilkinson, J.F., 1977. Nature of the organic coating on non-wettable golf greens. Proceedings of the Soil Science Society of America 41, 1203–1204. Moore, G., Blackwell, P., 1998. Water repellence. In: Moore, G. ŽEd.., Soil Guide. Agriculture Western Australia, pp. 53–63. Morris, S.E., Moses, T.A., 1987. Forest fire and the natural soil erosion regime in the Colorado Front Range. Annals of the Association of American Geographers 77, 245–254. Miyamoto, S., Letey, J., Osborn, J., 1972. Water vapor adsorption by water-repellent soils at equilibrium. Soil Science 114 Ž3., 180–184. Nakaya, N., 1982. Water repellency of soils. Japan Agricultural Research Quarterly 16, 24–28. Neinhuis, C., Barthlott, W., 1997. Characterisation and distribution of water repellent, self-cleaning plant surfaces. Annals of Botany 79, 667–677. O’Loughlin, E.M., Cheney, N.P., Burns, J., 1982. The bushrangers experiment: hydrological responses of a eucalypt catchment to fire. First National Symposium on Forest Hydrology Vol. 86, 132–138, Australian National Conference Publication. Osborn, J.R., Pelishek, R.E., Krammes, J.S., Letey, J., 1964. Soil wettability as a factor in erodibility. Proceedings of the Soil Science Society of America 28, 294–295. Osmet, B.D.J., 1963. The dropwise condensation of steam. In:

S.H. Doerr et al.r Earth-Science ReÕiews 51 (2000) 33–65 Moillet, J.L. ŽEd.., Water Proofing and Water-Repellency. pp. 384–413. Parker, S.D., 1987. Encyclopedia of Science and Technology. McGraw-Hill, New York. Pradas, M., Imeson, A., Van Mulligen, E., 1994. The infiltration and runoff characteristics of burnt soils in northeast Catalonia and the implications for erosion. In: Sala, M., Rubio, J.L. ŽEds.., Soil Erosion as a Consequence of Forest Fires. Geoforma Ediciones, Logrono, ˜ pp. 229–240. Rawitz, E., Hazan, A., 1978. The effect of stabilized, hydrophobic aggregate layer properties on soil water regime and seedling emergence. Journal of the Soil Science Society of America 42, 787–793. Reeder, C.J., Juergensen, M.F., 1979. Fire-induced water repellency in forest soils of upper Michigan. Canadian Journal of Forest Research 9, 369–373. Richardson, J.L., Hole, F.D., 1978. Influence of vegetation on water-repellency in selected western Wisconsin soils. Journal of the Soil Science Society of America 42, 465–467. Rietfeld, J., 1978. Soil Non-Wettability and its Relevance as a Contributing Factor to Surface Runoff on Sandy Soils in Mali. An ‘Ingenious’ Research. Landbouw, Hogeschool, The Netherlands. Ritsema, C.J., Dekker, L.W., 1994a. Soil moisture and dry bulk density patterns in bare dune sands. Journal of Hydrology 154, 107–131. Ritsema, C.J., Dekker, L.W., 1994b. How water moves in a water repellent sandy soil: 2. Dynamics of fingered flow. Water Resources Research 30, 2519–2531. Ritsema, C.J., Dekker, L.W., 1995. Distribution flow: a general process in the top layer of water repellent soils. Water Resources Research 31, 1187–1200. Ritsema, C.J., Dekker, L.W., Hendrickx, J.M.H., Hamminga, W., 1993. Preferential flow mechanism in a water repellent sandy soil. Water Resources Research 29, 2183–2193. Ritsema, C.J., Dekker, L.W., van den Elsen, E.G.M., Oostindie, K., Nieber, J.L., 1997. Recurring fingered flow pathways in a water repellent sandy field soil. Hydrology and Earth System Sciences 4, 777–786. Ritsema, C.J., Nieber, J.L., Dekker, L.W., Steenhuis, T.S., 1998a. Modelling and field evidence of finger formation and finger recurrence in a water repellent sandy soil. Water Resources Research 34, 555–567. Ritsema, C.J., Nieber, J.L., Dekker, L.W., Steenhuis, T.S., 1998b. Stable or unstable wetting fronts in water repellent soils — effect of antecedent soil moisture content. Soil and Tillage Research 47, 111–123. Roberts, F.J., Carbon, B.A., 1971. Water repellence in sandy soils of south-western Australia: 1. Some studies related to field occurrence. Division of Plant Industry CSIRO ŽAustralia.. Field Station Record 10, 13–20. Roberts, F.J., Carbon, B.A., 1972. Water repellence in sandy soils of south-western Australia: 2. Some chemical characteristics of the hydrophobic skins. Australian Journal of Soil Research 10, 35–42. Roper, M., 1998. Sorting out sandy soils. Microbiology Australia 19 Ž1., 6–7.

63

Rowell, D., 1994. Soil Science. Methods and Applications. Longman, Harlow, 350 pp. Savage, S.M., 1974. Mechanism of fire-induced water repellency in soils. Proceedings of the Soil Science Society of America 38, 652–657. Savage, S.M., Osborn, J., Letey, J., Heton, C., 1972. Substances contributing to fire induced water repellency in soils. Proceedings of the Soil Science Society of America 36, 674–678. Schantz, H.L., Piemeisel, R.L., 1917. Fungus fairy rings in eastern Colorado and their effect on vegetation. Agricultural Research 11, 191. Scheffer, F., Schachtschabel, P., 1989. Lehrbuch der Bodenkunde. Enke, Stuttgart, 491 pp. Scholl, D.G., 1971. Soil wettability in Utah juniper stands. Proceedings of the Soil Science Society of America 35, 356–361. Scholl, D.G., 1975. Soil wettability and fire in Arizona chaparral. Proceedings of the Soil Science Society of America 39, 356– 361. Scott, D.F., 1992. The influence of vegetation type on soil wettability. Proceedings of the 17th Congress of the Soil Science Society of South Africa, University of Stellenbosch, 28–30th Jan. 1992. pp. 10B21–10B26. Scott, D.F., 1993. The hydrological effects of fire in South African mountain catchments. Journal of Hydrology 150, 409–432. Scott, D.F., Lesch, W., 1997. Streamflow responses to afforestation with Eucalyptus grandis and Pinus patula and to felling in the Mokubulaan experimental catchments, South Africa. Journal of Hydrology 199, 360–377. Scott, D.F., Van Wyk, D.B., 1990. The effects of wildfire on soil wettability and hydrological behaviour of an afforested catchment. Journal of Hydrology 121, 239–256. Sevink, J., Imeson, A.C., Verstraten, J.M., 1989. Humus form development and hillslope runoff, and the effects of fire and management, under Mediterranean forest in N.E. Spain. Catena 16, 461–475. Shakesby, R.A., Boakes, D.J., Coelho, C.O.A., Gonçalves, A.J.B., Walsh, R.P.D., 1996. Limiting the soil degradational impacts of wildfire in pine and eucalyptus forests, Portugal: comparison of alternative post-fire management practices. Applied Geography 16, 337–355. Shakesby, R.A., Coelho, C.O.A., Ferreira, A.D., Terry, J.P., Walsh, R.P.D., 1993. Wildfire impacts on soil erosion and hydrology in wet Mediterranean forest, Portugal. International Journal of Wildland Fire 3, 95–110. Shakesby, R.A., Coelho, C.O.A., Ferreira, A.D., Terry, J.P., Walsh, R.P.D., 1994. Fire, post-burn land management practice and soil erosion response curves in eucalyptus and pine forests, north-central Portugal. In: Sala, M., Rubio, J.L. ŽEds.., Soil Erosion as a Consequence of Forest Fires. Geoforma Ediciones, Logrono, ˜ pp. 15–27. Shakesby, R.A., Doerr, S.H., Walsh, R.P.D., in press. The erosional impact of soil hydrophobicity: current problems and future research directions. Journal of Hydrology. Soto, B., Basanta, R., Benito, E., Perez, R., Diaz-Fierros, F., 1994. Runoff and erosion from burnt soils in northwest Spain. In: Sala, M., Rubio, J.L. ŽEds.., Soil Erosion as a Conse-

64


quence of Forest Fires. Geoforma Ediciones, Logrono, ˜ pp. 91–98. Teramura, H.A., 1980. Relationship between stand age and water repellency of chaparral soils. Bulletin of the Torrey Botanical Club 104, 42–46. Terry, J.P., Shakesby, R.A., 1993. Soil water repellency effects on rainsplash: simulated rainfall and photographic evidence. Earth Surface Processes and Landforms 18, 519–525. Thompson, C.H., 1983. Development and weathering of large parabolic dune systems along the subtropical coast of eastern Australia. Zeitschrift fur ¨ Geomorphologie, Supplementband 45, 205–225. Tschapek, M., 1984. Criteria for determining the hydrophilicity– hydropobicity of soils. Zeitschrift fur ¨ Pflanzenernaehrung und Bodenkunde 147, 137–149. Tschapek, M., Wasowski, C., 1976. The surface activity of humic acid. Geochimica Cosmochimica Acta 40, 1343–1345. Tulloch, A.P., 1976. Chemistry of waxes of higher plants. In: Kolattukudy, P.E. ŽEd.., Chemistry and Biochemistry of Natural Waxes. Elsevier, Amsterdam, pp. 289–347. Valat, B., Jouany, C., Riviere, L.M., 1991. Characterisation of the wetting properties of air-dried peats and composts. Soil Science 152, 100–107. Van Dam, J.C., Hendrickx, J.M.H., Van Ommen, H.C., Bammink, M.H., Van Genuchten, M.T., Dekker, L.W., 1990. Water and solute movement in a coarse-textured water repellent field soil. Journal of Hydrology 120, 359–379. Van Dam, J.C., Wosten, J.H.M., Nemes, A., 1996. Unsaturated soil water movement in hysteretic and water repellent field soils. Journal of Hydrology 184, 153–173. Van den Bosch, H., Ritsema, C.J., Boesten, J.J.T.I., Dekker, L.W., Hammiga, W., 1999. Modeling solute transport in a water repellent soil. Journal of Hydrology 215, 172–187. Van’t Woudt, B.D., 1959. Particle coatings affecting the wettability of soils. Journal of Geophysical Research 64, 263–267. Velmulapalli, G.K., 1993. Physical Chemistry. Prentice-Hall, London, 991 pp. Wallis, M.G., Horne, D.J., 1992. Soil water repellency. In: Stewart, B.A. ŽEd.., Advances in Soil Science Vol. 20 Springer, New York, pp. 91–146. Wallis, M.G., Horne, D.J., McAuliffe, K.W., 1990. A study of water repellency and its amelioration in a yellow brown sand: 1. Severity of water repellency and the effects of wetting and abrasion. New Zealand Journal of Agriculture Research 33, 139–144. Wallis, M.G., Horne, D.J., Palmer, A.S., 1993. Water repellency in a New Zealand development sequence of yellow–brown sands. Australian Journal of Soil Research 31, 641–645. Wallis, M.G., Scotter, D.R., Horne, D.J., 1991. An evaluation of the intrinsic sorptivity water repellency index on a range of New Zealand soils. Australian Journal of Soil Research 29, 353–362. Walsh, R.P.D., Boakes, D., Coelho, C.O.A., Gonçalves, A.J.B., Shakesby, R.A., Thomas, A.D., 1994. Impact of fire-induced water repellency and post-fire forest litter on overland flow in northern and central Portugal. Proceedings of the Second

International Conference on Forest Fire Research, Nov. 1994, Coimbra, Portugal Vol. II, pp. 1149–1159. Walsh, R.P.D., Coelho, C.O.A., Elmes, A., Ferreira, A.J.D., Gonçalves, A.J.B., Shakesby, R.A., Ternan, J.L., Williams, A.G., 1998. Rainfall simulation plot experiments as a tool in overland flow and soil erosion assessment, north-central Portugal. Geookodynamik 19, 139–152. ¨ Walsh, R.P.D., Coelho, C.O.A., Shakesby, R.A., Ferreira, A.D.J., Thomas, A.D., 1995. Post-fire land use and management and runoff responses to rainstorms in northern Portugal. In: McGregor, D., Thompson, D. ŽEds.., Geomorphology and Land Management in a Changing Environment. Wiley, Chichester, pp. 283–308. Wells, C.G., Campbell, R.E., DeBano, L.F., Lewis, C.E., Fredriksen, R.L., 1979. Effects of fire on soil — a state of knowledge review. United States Department of Agriculture, Forest Service, General Technical Report WO-7. White, W.D., Wells, S.G., 1982. Forest-fire devegetation and drainage basin adjustments in mountainous terrain. Proceedings of the 10th Geomorphology Symposium, 199–223, Binghamton, Boston. Wilkinson, J.F., Miller, R.H., 1978. Investigation and treatment of localized dry spots on sand golf greens. Agronomy Journal 66, 299–304. Witter, J.V., Jungerius, P.D., ten Harkel, M.J., 1991. Modelling water erosion and the impact of water-repellency. Catena 18, 115–124. York, C.A., 1993. A questionnaire survey of dry patch on golf courses in the United Kingdom. Journal of Sports Turf Research 69, 20–26. York, C.A., Baldwin, N.A., 1992. Dry patch on golf greens: a review. Journal of Sports Turf Research 68, 7–19. Zierholz, C., Hairsine, P., Booker, F., 1995. Runoff and soil erosion in bushland following the Sydney bushfires. Australian Journal of Soil and Water Conservation 8, 28–37. Zisman, W.A., 1964. Relation of the equilibrium contact angle to liquid and solid constitution. In: Gould, R.F. ŽEd.., American Chemical Society. Advances in Chemistry Series Vol. 43, pp. 1–51. Zwolinski, M.J., 1971. Effects of fire on water infiltration rates in a ponderosa pine stand. Hydrology and Water Resources in Arizona and the Southwest 1, 107–112. STEFAN H. DOERR is a Research Officer in the Geography Department of the University of Wales Swansea. Together with his co-authors, he belongs to the Land Surface Processes and Management Group and also to the University’s Environmental Dynamics Institute. His PhD thesis concerned the origin, characteristics and effects of soil water repellency in commercial forests in Portugal. He has since been involved in a series of research projects related to water repellency and soil erosion and is currently Scientist-in-Charge at Swansea of a global research project concerned with developing amelioration strategies to reduce environmental deterioration and agricultural production losses in water repellent regions. Dr. Doerr’s publications also lie in the field of karst and pseudokarst geomorphology.

S.H. Doerr et al.r Earth-Science ReÕiews 51 (2000) 33–65 RICHARD A. SHAKESBY is a Senior Lecturer in Geography at Swansea. His research interests include soil erosion and land degradation in the Mediterranean, the Sudan, southern Africa and Great Britain. Since the late 1980s, he has worked in Portugal investigating the measurement, modelling and combating of soil erosion in relation to land management changes, forest fire and water repellency. Dr. Shakesby’s publications and research interests also include Quaternary geomorphology, especially sediments and landforms associated with Holocene glacier fluctuations. He is a Council Member of the European Society for Soil Conservation ŽESSC..

65

RORY P.D. WALSH is a Reader in Geography at Swansea. His research interests and expertise lie in the fields of Mediterranean and tropical hydrology and erosion. He has worked on the impacts of forest fires and land management on hydrology and soil erosion in Portugal. He has also longstanding experience of investigating runoff processes, erosion and climatic change in the West Indies, the Sudan and Borneo. He had conferred on him the Back Award in 1996 by the Royal Geographical Society for his tropical research and is currently the Research Co-ordinator of the Royal Geographical Society’s South-East Asia Rainforest Research Programme based at Danum Valley in Sabah.