HYDROLOGICAL PROCESSESINVITED COMMENTARY

Hydrol. Process. 18, 829–832 (2004)Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hyp.5518

Hydrological effects of soil water repellency: on spatialand temporal uncertainties

Stefan H. Doerr1*John A. Moody2

1 Department of Geography,University of Wales Swansea,Singleton Park, Swansea SA28PP, UK2 US Geological Survey, 3215Marine Street, Suite E-127,Boulder, CO 80303, USA

*Correspondence to:Stefan H. Doerr, Department ofGeography, University of WalesSwansea, Singleton Park, SwanseaSA2 8PP, UK.E-mail: s.doerr@swan.ac.uk

Water repellency is a natural phenomenon in soils that is vari-able in space and time. Based on a recent bibliography on waterrepellency (Dekker et al., 2003), more than 1000 studies to datehave investigated or referred to water repellency (hydrophobicity)in soils. Studies reporting the presence of repellency have comefrom a wide range of soil types and environments, and studies onwildfire-affected regions have been particularly common, followedwith decreasing frequency by those from other types of perma-nently vegetated, tilled arable, and certain types of contaminatedland. However, it is interesting to note that a detailed examinationof the hydrological effects of water repellency is often not includedespecially as spatial scales increase. For example, infiltration andrunoff processes at plot- or hillslope-scales have been investigatedin less than 10% of studies, and at the catchment scale in lessthan 3% studies. Furthermore, these studies have by no meansall unequivocally demonstrated connections between repellency andhydrological effects.

Despite this apparent knowledge gap, it is increasingly commonto promote water repellency as a factor in explaining hydrologicalresponses such as irregular wetting, preferential flow within thesoil matrix, and reduced infiltration and enhanced overland flowfor unsaturated conditions at a range of scales, which in turn areoften quoted as promoting slopewash, and rill and gully erosion.For anyone observing the wetting behaviour (or lack thereof) ofa highly repellent soil in the laboratory or in the field, it is easyto visualize how this phenomenon must certainly bear significanthydrological implications at a range of spatial and temporal scales.The implied connections, however, need to be demonstrated; inattempting this, the key question that arises is at what spatial andtemporal scales the connections between water repellency and thehydrological response are negligible and at what scales they aresignificant.

Spatial UncertaintiesThe volume of soil affected by water repellency can be highly vari-able at a wide range of spatial scales. At the large horizontal scale(i.e. extending across different land-use types), the more extremelevels of repellency, at least, have been shown to be associated withcertain land uses. Within a given land-use type prone to repellency,

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spatial variability does not necessarily decreasewith scale of area investigated. The variability ofrepellency at these different scales appears to belinked to, but not fully determined by, the spatialdistribution of hydrophobic organic matter in thesoil. The distribution of the organic matter gen-erally declines with depth, such that water repel-lency rarely affects soils at depths greater than0·5 m (see reviews by Wallis and Horne (1992)and Doerr et al. (2000)). In areas affected by wild-fire, intense soil heating can lead to the combustionof the organic matter and associated hydrophobiccompounds, rendering the soil surface wettable,but intense heating as a result of wildfire can alsocause enhanced repellency at some depth in the soil(DeBano et al., 1976). Given the spatially variableheat output of such fires, and particularly the vari-able heat impulse into the soil, this superimposesan additional ‘layer’ upon the existing spatial vari-ability.

Apart from the spatial variability of repellentsoil itself, additional spatially variable factors mayinfluence the hydrological consequences of repel-lency. For example, the variability of macropores(e.g. root channels, animal burrows) will affectinfiltration and water movement in repellent ter-rain (Burch et al., 1989), and the variations in thestorage capacity of the canopy, litter and duff lay-ers will determine how much water will be deliv-ered to the soil surface during a rainfall event,which in itself is likely to be spatially variable. Inconsidering the effects of these additional factors, itmay be critical whether or not their spatial vari-ability is organized into patterns or is randomlydistributed. Furthermore, the hydrological effectsof the specific spatial distribution of repellencymay not be a simple linear spatial superposition ofthese additional variables, but they may be linkednonlinearly or have feedback mechanisms. To givea simplistic example: even where large areas ofthe topsoil are repellent, a few large macropores,which connect to a thick layer of wettable sub-soil, may be sufficient to intercept any repellency-induced overland flow after short distances. Onthe other hand, the same number of macropores,if located outside the main pathways for overlandflow, may have little effect, such that overland flowpathways may connect uninterrupted all the wayto a stream channel.

Temporal UncertaintiesWater repellency is a transient phenomenon atmore than one temporal scale, which complicatesthe evaluation of both its presence and associ-ated hydrological effects. Depending on its levelof persistence, which is most commonly examinedusing the water drop penetration time (WDPT)method (Letey et al., 2000), repellency of a soilor soil pore-wall surface tends to disappear sometime after coming in contact with water. Althoughthe underlying physico-chemical processes for thischange in wettability are not fully understood, itis well established that the persistence of repel-lency is variable, ranging from only a few sec-onds to in excess of several hours (see review byDoerr et al. (2000)). It is this type of transient phe-nomenon that causes the infiltration capacity ofsome water-repellent soils to increase during rain-fall, a behaviour that is contrary to what manyof us have learned. Actual persistence measure-ments are rarely conducted for more than 5 h, butlaboratory and field evidence suggest that the soilpore space may, in extreme cases, resist wettingfor weeks or even months (Dekker, 1998; Doerrand Thomas, 2000). It is evident that persistenceof repellency is a key variable in governing itshydrological effects. For example, contact angle orcritical surface tension methods indicate only theinitial severity of the repulsion of liquid water froma surface (Roy and McGill, 2002), and even themeasured delay in infiltration for water dropletson the surface of a soil sample (WDPT test) maynot always be a good indicator for the actual resis-tance time of the bulk soil to wetting (Doerr andThomas, 2000).

Even more uncertainty exists with respect tothe longer term transience of repellency. It isoften stated that repellency disappears after pro-longed wet periods (e.g. wet winter months) onlyto reappear after a dry spell, when soils dry out.Very few studies have actually provided insightinto how long during a season or year the repel-lency may be present, particularly with respectto its re-emergence after drying, and DeJongeet al. (1999) have provided some evidence suggest-ing that repellency may actually decrease again,when soils become very dry. Equally little isknown about how long it takes for repellency to

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INVITED COMMENTARY

become re-established where it has been eliminatedthrough intense soil heating during a wildfire orthrough mixing of wettable and repellent layersduring deep ploughing. Furthermore, the poten-tially important, yet poorly established, role of soilmicrobial activity may be critical in driving someof the temporal variations in repellency (Hallettet al., 2003). We are thus faced with challeng-ing uncertainties in trying to predict the short-and long-term variations in repellency withoutmaking impractically frequent (and often neces-sarily destructive) repeat measurements. Becausethese measurements commonly disturb the systemat small spatial scales, they prevent collection ofreliable measurements over time, restricting theassessment of temporal variability—essentially thehydrological counterpart of the Heisenberg uncer-tainty principle in physics.

How Can We Tackle These Uncertainties?One approach to tackling the problems of spatialand temporal uncertainty would be first to estab-lish and quantify the patterns of the spatial andtemporal distribution of water repellency and theassociated soil properties at all scales, and thento determine how the connections between waterrepellency and hydrological processes change asthe spatial and/or temporal scale of these pat-terns change. In making progress towards thisgoal, it may be useful to look beyond hydrology,avoid reductionism, and incorporate or modify‘tools’ from other disciplines (like statistics, biol-ogy, and ecology) that deal with complex systems.Statistical studies of river patterns (Rodriguez-Iturbe and Rinaldo, 1997; Moody and Troutman,2002) have asked the question “do systems havea unique characteristic scale” (in space, in timeor in both) or is the system scale-free, such thatcommunication of information is possible across allscales? Perhaps we should make more use of sta-tistical analysis to quantify (such as self-similar,isotropic or anisotropic) the spatial distributionof repellency and to unravel the apparent dis-order that characterizes the spatial and tempo-ral distribution of soil properties. To accomplishthis, we need high-resolution field-data sets with anested structure designed to measure soil proper-ties (repellency in particular) across several orders

of magnitude in scale. Then, aggregation of suchdata sets at larger and larger spatial or tempo-ral scales could be utilized to determine prop-erties of uncertainty. This approach, for exam-ple, was successfully applied to a soil moisturedata set by Rodrigues-Iturbe et al. (1995), whoinvestigated the pattern of soil moisture by defin-ing soil moisture ‘islands’ (patches of soil mois-ture at or above a certain level that were allspatially connected). In a similar fashion, Daven-port et al. (1998) used percolation theory in orderto predict the probability that bare soil patchesbetween vegetation patches will become inter-connected as the pattern of vegetation changes.They found a definite nonlinear threshold forerosion as the percentage of space occupied byvegetation patches decreased and the patches ofbare soil became more connected. This propertyof the spatial connectivity of forest patches hasbeen studied using fragmentation analysis (For-man and Godron, 1986; Riemann and Tillman,1999), which can incorporate interruptions (suchas roads bisecting forest stands at the landscapescale and perhaps macropores in soils) in the gen-eral pattern of spatial distributions. Perhaps theconnectiveness of runoff patches in water-repellentterrain can be investigated by defining statisti-cal width functions like those used to investigatethe routing of water from seemingly random andcomplex drainage networks (Troutman and Kar-linger, 1985). Unravelling the spatial patterns ofwater repellency, runoff, and infiltration ‘patches’will suggest how effective infiltration parameterscan be calculated at different scales. Some evi-dence suggests that variance affecting hydrologicalresponse in general (Wood, 1998) and hydrologi-cal effects of repellency in particular (Doerr et al.,2003) may decrease with increasing spatial scale,indicating that the overall hydrological effects ofwater repellency may be simpler to model as thescale increases.

It is important to remember that perceived abi-otic properties such as repellency and processeslike runoff and erosion are inherently linked tobiotic processes through feedback mechanisms.These mechanisms are frequently nonlinear, andthus lead to nonlinearity in the properties and pro-cesses themselves (Davenport et al., 1998; Wood,

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1998). Possible feedback mechanisms between abi-otic and biotic components of the system sug-gest that ecological theories like hierarchy theory(O’Neill et al., 1986), which recognizes that pro-cesses at a finer scale may drive processes at largerscales, may be useful in formulating a conceptualframework to advance the understanding of theconnections between water repellency and hydro-logic processes. Despite the substantial progressmade over the last decades in understanding var-ious aspects of water repellency, further detailedstudies are required to determine its spatial andtemporal distribution at hierarchical scales. It willbe these investigations that will allow us to begin toestablish to what degree water repellency actuallycontributes to unusual hydrological events, suchas the recent runoff and erosion events follow-ing the 2001 fires in the USA and Australia, orthe 2000 and 2002 river floods in the UK. Suchstudies are certainly labour intensive and timeconsuming, but they are necessary if we are todiscover any generalities that are applicable toa wide range of situations, which in turn wouldreduce the need for additional detailed studies inthe future.

AcknowledgementsThis commentary was supported in part by NERCgrant NER/A/S/2002/00143 and NERC AdvancedFellowship NER/J/S2002/00662. We wish to thankDeborah Martin and Cynthia Froyd for their use-ful comments on this contribution.

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