Temporal dynamics of water repellency and soil ...

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Accepted for publication in Australian Journal of Soil Research

Temporal dynamics of water repellency and soil moisture in eucalypt plantations, Portugal Gemma Leighton-Boyce* A , Stefan H. DoerrA , Richard A. Shakesby A , Rory P.D. WalshA , António J.D. FerreiraB,C, Anne-Karine BouletB , Celeste O.A. CoelhoB A

Department of Geography, University of Wales Swansea, Singleton Park, Swansea SA2 8PP, UK B Centro das Zonas Costeiras e do Mar, Departamento de Ambiente e Ordenamento, Universidade de Aveiro, P-3810-193 Aveiro, Portugal C Environmental Technologies Sector, Department of Pure and Environmental Sciences, ESAC, IPC, Bencanta, P-3040-316 Coimbra, Portugal

*corresponding author Fax: ++44 1792 295955, E-mail: [email protected] Abstract This paper investigates water repellency and soil moisture under four different Eucalyptus globulus plantations in Portugal. On eight occasions over a 16-month period, measurements were made at three depths (surface, 10 cm and 20 cm) at 60 points on four 10 m × 18 m grids. The main results are: (i) at all sites and depths, spatial frequency of repellency (defined as percentage of repellent grid points) followed a moisture-related seasonal cycle, its amplitude being greatest for the longest established site, where surface repellency was contiguous in dry late-summer conditions, but was entirely absent after wet winter conditions; (ii) at a few points at two sites, repellency persisted during winter; (iii) repellency severity was dichotomously distributed regardless of season (i.e. soils were generally either wettable or highly repellent ); and (iv) at the longest established site, when soil moisture was < 14% soils were repellent, and when soil moisture > 27% soils were wettable. This may either support the existence of a ‘transition zone ’, or be an artefact of the different scales of repellency and soil moisture assessments. Reasons for the observed changes in repellency and their relationship with soil moisture and antecedent rainfall are explored and soil hydrological implications discussed. Additional keywords: water repellence, repellency severity, %Ethanol method, hydrophobicity, Eucalyptus globulus, overland flow, Time Domain Reflectometry, transition zone Suggested abridged title Temporal dynamics of water repellency

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Introduction The spatial and temporal dynamics of soil water repellency can be expected to cause considerable variations in soil hydrological response. In particular, the time taken for repellency to break down during rainfall, the speed of its re-establishment in dry weather, and its severity and spatial contiguity are potentially important factors (Burch et al. 1989; Shakesby et al. 2000; Doerr et al. 2003). In climates ranging from temperate to semi-arid, it is generally supposed that repellency has a broadly seasonal cycle, ranging from being most pronounced in dry summer conditions to declining or disappearing in wet winter conditions (e.g. Roberts and Carbon 1971; Jungerius and de Jong 1989; Crockford et al. 1991), but this has rarely been demonstrated by monitoring these changes under field conditions (Shakesby et al. 2000). Although it has long been recognised that these temporal variations in repellency are linked in some way to soil moisture fluctuations (e.g. Gilmour, 1968), the soil moisture content associated with a change in repellency, the manner of any ‘switching’ mechanism and the extent of involvement of factors other than soil moisture are poorly understood (Shakesby et al. 2000). For different soil types, Dekker and Ritsema (1994) and Soto et al. (1994) reported different critical moisture contents at which the soils are expected to change from a wettable to repellent state and vice versa. Under laboratory conditions, however, Doerr and Thomas (2000) found that repellency could not be re-established by air drying previously highly repellent soils that had been rendered wettable by thorough wetting, casting some doubt on the unequivocal control of soil moisture alone on the switch between wettable and repellent conditions. More recently, Dekker et al. (2001) have revised the concept that repellency change occurs at a distinct soil moisture level, and instead have suggested that such a change may occur over a broader soil moisture range , and termed this a ‘transition zone’. The aim of this study is to examine in detail the temporal changes in water repellency and their relationship to soil moisture content and antecedent rainfall for a land use type susceptible to high levels of repellency. Thus, on eight occasions over a period of 16 months in Eucalyptus globulus plantations in north-central Portugal, repeat measurements of repellency severity and soil moisture were made on soil sampled using a grid system at four sites differing in tree age, management practice and fire history. For in situ soil at three depths (surface, 10 cm and 20 cm), the temporal dynamics of (i) the spatial frequency of repellency (SFR; defined as the percentage of points on a grid classified as repellent), (ii) the repellency severity, and (iii) soil moisture are assessed. Relationships between the repellency variables, soil moisture and antecedent rainfall are explored for this period and particular emphasis is placed on examining the applicability of the concepts of moisture thresholds or of a moisture ‘transition zone’ in which the soil ‘switches’ between wettable and repellent states. Study area The four study sites lie in Eucalyptus globulus plantations in the Águeda basin, which is located in the foothills of the Caramulo Mountains east of Águeda (40º35’N, 8º26’W), north-central Portugal. The climate is wet -Mediterranean in character, with a long-term mean annual rainfall of 1379 mm at 200 m a.s.l., and a strong seasonal pattern, which has a generally wet autumn-spring (October-April) period and a mainly dry, warm summer (June September). Long-term average monthly temperatures range from 19.8ºC in August to 5.8ºC in January. Soils are generally shallow (50% Ethanol). Five drops of ethanol solution were applied to the soil using a dropper bottle held immediately above the soil surface. If three or more drops infiltrated within three seconds , a further five drops of the next lowest concentration were applied. Conversely, if less than three drops infiltrated, five drops of the next highest concentration were applied. This method was used to determine the lowest concentration of ethanol at which the majority (at least three of the five) drops infiltrated within three seconds, and this was taken as indicative of the repellency severity at that point. It should be noted here that a repellenc y class =1% Ethanol denotes water repellent conditions. 4. The soil down to 10 cm depth was then excavated and steps 1-3 repeated for soil at that depth. Finally the soil pit was dug down to 20 cm depth and the procedure repeated again. The soil and litter layer were then replaced. Daily rainfall totals were obtained from a raingauge at an altitude of 200 m a.s.l. located within 4 km of all the sites. Walsh et al. (1995) developed an altitudinal correction factor to estimate rainfall amounts at different locations within the study area. Using this factor, estimated rainfall amounts for the ‘mature’ (430 m a.s.l.), established (430 m), young (260 4

m) and newly burnt (180 m) sites were calculated to be respectively 15%, 15%, 4% higher and 1% lower than those measured at the raingauge. The altitudinally-corrected rainfall estimates are used here. Results The frequenc ies of measurement points within each repellency severity category for the four sites at each of the three soil depths for each of the eight sampling occasions are given in Figure 1. The diagram shows: (i) spatial frequency of repellency (SFR), defined as the percentage of measurement points on a grid classified as repellent (=1% Ethanol, see Table 2). To facilitate description and analysis of the large dataset, SFR is given either for all three depths combined (SRF m), or, where specified, for an individual soil depth (SRFi). (ii) The severity of the repellency found. (iii) The mean and standard deviation of soil moisture for each site, depth and sampling occasion. Figure 2 shows the relationship at the ‘mature’ site between soil moisture and repellency severity for measurements made at the surface and at 10 cm depth using data for all grid points on all sampling occasions. Table 3 shows mean soil moisture (volume percent; all depths combined) and the corresponding SFRm, together with the two-week and eight-week cumulative rainfall prior to each of the sampling occasions. Table 4 summarises the percentage of repellent soil samples within each of the descriptive repellency severity classes (low, moderate, severe, extreme, see Table 2 for associated %Ethanol concentrations) for each site. a) Temporal patterns in the spatial frequency of repellency The mean spatial frequency of repellency (SFRm; mean of the three depths) varied considerably over the 16-month measurement period (Table 3). At all four sites, SFRm was comparatively high in August 2000 (‘mature’, 85%; established, 59%; young, 59%; newly burnt, 75%. In contrast, it was very low on the next two sampling occasions (February and April 2001) at all sites. SFRm was also particularly low in May 2001 at the ‘mature’ and established sites (3% and 0% respectively), but intermediate at the young and newly burnt sites in the same month (33% and 48%, respectively). In August 2001 SFRm was similarly high to its value in August 2000 at all four sites. In October, November and June 2001 SFRm was intermediate-high at all sites. Between August 2000 and February 2001, the percentage of points classified as repellent at the surface (SFRi) at the ‘mature’ site dropped from 100% (entirely repellent) to 0% (entirely wettable, Figure 1). It also changed from entirely wettable to entirely repellent over ~2 months from April to June 2001 and from minimal frequency (SFRi=10%) to entirely repellent (SFRi=100%) over the 22 days between the May and June 2001 sampling occasions (Figure 1). Except at the young site, SFRi at the surface tended to be higher than at the two measured depths; this was particularly evident, for example, at the ‘mature’ site in June 2001 (Figure 1). Repellent soil conditions over the entire grid (SFRi 100% ) were recorded at the surface for the ‘mature’ site in August 2000, June, August and November 2001, and at 10 cm depth on both of the August sampling occasions. In contrast, a contiguously repellent layer (i.e. all sampling points exhibiting repellency) was not detected on any measurement date at any of the other sites for the surface or the two depths. The highest SFRi values recorded at individual depths were for the established (78% , August 2001, surface), young 5

(68%, August 2000 and November 2001, 10 cm depth) and newly burnt (97%, August 2000, surface) sites. b) The relationship between spatial frequency of repellency (SFRm), mean soil moisture and antecedent rainfall When SFRm (mean of all depths) was at its lowest, antecedent rainfall and mean soil moisture were both high and vice versa (Figure 1 and Table 3). For example, at the ‘mature’ site, when SFRm was highest in August (85% in August 2000, 91% in August 2001), mean soil moisture (all depths) was low in August both in 2000 (7.3% vol.) and in 2001 (8.8% vol. ) and 2- and 8-week antecedent rainfall totals were also low, with respectively 7 and 109 mm in August 2000, and 6 and 66 mm in August 2001 (Table 3). Conversely, when SFRm was at its lowest in February (0%), April (0%) and May (3%) 2001, mean soil moisture was comparatively high (36.7% vol., 27.2% vol. and 30.7% vol. , respectively) and 2- and 8-week antecedent rainfall totals were also high, with values of 277-351 and 877-879 mm, 49 mm and 616 mm, and 71 and 281-321 mm, respectively, for these three months (Table 3). Also apparent from Figure 1 and Table 3 is that when SFRm was intermediate in value, it varied in an apparently unsystematic manner with mean soil moisture. At all four sites, SFRm was greater in October than in June 2001 despite (i) mean soil moisture in October being similar to or higher than in June, and (ii) a greater 2-week antecedent rainfall for October than in June. For example, at the ‘mature’ site in October 2001, the mean moisture content was 19.9% vol., but SFRm was higher (63%) after a 2-week antecedent rainfall of 83-87 mm than in June 2001 (SFRm = 52%) when only 5 mm fell in the preceding 2-week period but a similar mean soil moisture content (19.4% vol.) was recorded (Table 3). c) Temporal patterns in repellency severity Unlike SFR, repellency severity shows no clear temporal pattern. At all four sites, soils were predominantly either highly repellent or wettable throughout the period of investigation, reflecting a dichotomy in the degree of wettability. Thus, where repellent conditions occurred, repellency was most commonly classed as extreme (=36% Ethanol) rather than severe, moderate or low, with the percentage of extremely repellent measurements increasing with stand age at all but the newly burnt site (Figure 1 and Table 4). For example, at the young and newly burnt sites, even during February and April 2001 when repellent soil conditions were found only at a small percentage of grid points, these relatively few repellent measurements were mostly in the extreme category (Figure 1). d) The relationship between repellency severity and soil moisture at individual points Repellency severity and associated soil moisture data derived at each individual measurement point site at the ‘mature’ site for the eight sampling occasions at the surface and 10 cm depth (n=480 at each depth) are given in Figure 2. These data from the ‘mature’ site are suited to examining the relationship between the severity of repellency and soil moisture since repellency varied here from entirely repellent (SFRi=100%) to entirely wettable (SFRi=0%). In contrast, for all depths at the established, young and newly burnt sites and at the 20 cm depth at the ‘mature’ site, some wettable points were recorded even when soil moisture was very low, and hence favourable for the manifestation of repellenc y. This could mean that soil at these points may not have had the potential to become repellent throughout the measurement period regardless of soil moisture conditions. As pointed out already in c above, it also evident from Figure 2 that repellency severity has a dichotomous distribution, with soils being predominantly either highly 6

repellent or wettable. Thus, the overwhelming majority of measurement results fell either into the three highest severity classes (9-11) or in the wettable class (1). It is also notable that the high repellency values largely correspond to lower soil moisture contents than the wettable ones (Figure 2). Virtually all surface soil samples at the 'mature' site were wettable at soil moisture contents above 27% vol. (Class 1) , and repellent (> Class 1) at moisture contents below 14% vol. (see Table 2 for class definitions). The three exceptions to this pattern in the surface dataset were recorded in May 2001 (moderate repellency, 5% soil moisture; severe repellency, 35% vol. soil moisture) and in October 2001 (low repellency, 31% vol. soil moisture). The tendency for repellency, where present, to be of high severity is also apparent when soil moisture values are between 14% vol. and 27% vol. (Table 4b). This is particularly apparent at the mature and established sites, but less so at the young and burnt sites since there were relatively few repellent measurements at these latter sites between 14 and 27% vol. soil moisture. Discussion Temporal patterns in the spatial frequency of repellency and in repellency severity From Table 3, it is apparent that the spatial frequency of repellency (SFRm, mean of all depths) is broadly linked to antecedent rainfall, such that a low rainfall amount in the preceding 2- and 8-week periods corresponds to relatively high SFRm and vice versa. This relationship, however, is not as good as that between SFRm and mean soil moisture. This can be attributed to the likely importance of storm timing, intensity and amount during the antecedent period rather than the period total, and also to the influence of other factors likely to affect repellency through their impact on soil moisture, such as variations in temperature, relative humidity and evapotranspiration rate. Notwithstanding the potential influence of these factors, the effect of antecedent rainfall is nevertheless clearly illustrated for the ‘mature’ and established sites in the two consecutive autumn months of October 2001 and November 2001. The 2-week antecedent rainfall prior to the October measurements was relatively high (83-87 mm) and SFRm was appreciably lower (‘mature’, 63%; established, 35%) than when measured in the following month (‘mature’, 89%; established, 50%), which had a 2-week antecedent rainfall at both sites of only 3 mm (Table 3). Differences in rainfall between generally wet, cool autumn-winter and dry, warm summer conditions are thought to be the main reason why SFRm shows a general seasonal cycle from being low during wet winter conditions to being at its greatest in late summer (August), which usually follows several weeks of little or no rain with low relative humidity and comparatively high daily temperatures. A seasonal pattern of this nature has been suggested from much smaller datasets than presented here by Walsh et al. (1995) and Doerr and Thomas (2000) for the eucalypt and pine stands in the study area and elsewhere by, for example, Burch et al. (1989) for soils under eucalypts, Jungerius and de Jong (1989) for grass covered dunes and Imeson et al. (1992) for Mediterranean type forests. There are also exceptions, however, to the seasonal loss of repellency. For example, Dekker and Ritsema (1995) found that repellency and thus low soil moisture contents persisted throughout wet winter conditions for a sandy soil under grass cover in the Netherlands. Thus, prolonged contact with water may not result in the breakdown of water repellency and associated increase in soil water content. For a highly repellent (24-36% Ethanol) eucalypt soil from the study region placed into a perforated container, Doerr and Thomas (2000) reported that no sign of infiltration occurred after 30 days of immersion in water in the laboratory. In the present study, repellency persisted throughout the 16-month measurement period for a few 7

points at the young and burnt sites (Figure 1), although the intermittent timing of the sampling does not rule out a change in repellency between the sampling occasions. The lack of any clear link between mean soil moisture and SFR at intermediate soil moisture values (e.g. as in June and October 2001) is thought to reflect the threedimensional distribution of soil moisture and repellency during periods that are neither particularly dry nor wet. Under these conditions, increases in soil moisture resulting from rain falling when SFR is intermediate are likely to be preferentially concentrated in macropores and already wettable parts of the soil. This will tend to result in higher-thananticipated SRF in recently wet periods, whereas lower SRFs would occur in intermediate situations following more prolonged drying. This would tally with the much higher SFR at the mature site in October 2001 compared with June 2001. Averaging the results for the entire grid or for a certain soil depth under such conditions could be expected to encompass a range of mean soil moisture contents and SFRs, even though the relationship between these variables might have prove d more systematic on a grid point by grid point basis if nondestructive repeat measurements for each individual point could have been carried out. It is not possible to give the precise time required for repellency to break down or become re -established at the study sites, but there are some indications from the data presented here. Under natural rainfall conditions, the data in Figure 1 show that for surface soil at the ‘mature’ site, the transition from entirely wettable (SFRi=0%) to entirely repellent (SFRi=100%) occurred within two months (April-June 2001), and from SFRi=10% up to SFRi=100% in only 22 days (May-June 2001). The actual times taken for the transition might in fact have been much less, but the intermittent nature of the sampling would not have allow ed this to be detected. A more rapid transition from wettable to repellent conditions has certainly been previously reported. For example, Crockford et al. (1991) found that surface soil water repellency in an Australian eucalypt forest could become reestablished after only a week of hot dry weather , although it is not clear from their study whethe r the repellency reported was contiguous across the entire study area. With respect to water repellency induced by wildfire, there is little consensus on its ‘lifetime’ (see review by Doerr et al. 2000a). For example, DeBano et al. (1976) reported that water repellency induced by burning of logging residues in summer in California lasted only until the first autumn rains. It is not clear, however, whether this constituted a temporary breakdown associated with wetting or the irreversible disappearance of a short-lived fireinduced phenomenon. At the newly burnt site in the current study, there is certainly no indication for the irreversible disappearance of surface repellency following rainfall. On the contrary, surface SFRi increased to moderate levels by May 2001 and to high levels by June 2001 following its low values in February and April 2001. In support of these observations, Doerr et al. (1998) also found no evidence of fire-induced changes to the repellency in newly burnt forest stands in the study area. They attributed this to the presence of high levels of prefire repellency and to soil temperatures during burning that were insufficient to destroy surface repellency. In accordance with the latter, comparison of SFRi for different depths at the newly burnt site (Figure 1) gives no indication of the development of a subsurface repellent layer overlain by a wettable layer, as might be anticipated if the fire had been sufficiently severe (DeBano et al. 1976). The increase in SFRm and repellency severity with the age of the long unburnt stands, in the order young, established and ‘mature’ (Figure 1 and Table 4), may reflect differences in the amount or the rate of supply of repellent substances. For example, Doerr et al. (1998) suggested that eucalypt litter and root networks could be potential sources of repellent substances, and both of these sources become better developed with stand age 8

(Table 1). The relatively high SFRm at the newly burnt site may reflect a legacy of repellency accrued prior to the fire during 10 years of tree growth and litter accumulation, although repellent substances from burnt vegetative matter might have been added (DeBano and Krammes 1966). The fact that repellency was either generally of high severity or absent with comparatively few measurements representing intermediate levels is particularly notable. To the authors’ knowledge , this has only been reported in one previous study from South Africa, which also focussed on eucalypt stands (Scott 1994). The reasons for this dichotomous distribution are unclear. The few other studies involving repeated sampling have reported broad ranges of repellency severity (e.g. Jungerius and de Jong 1989; Crockford et al. 1991; Dekker 1998), with repellency measurements ranging from extreme to entirely wettable. One possibility is that the substances causing hydrophobic ity in the eucalypt stands in the study are particularly ‘potent’ and, associated with the homogeneous planting patterns and stand ages , are particularly well distributed, thus establishing predomina ntly high levels of repellency where it occurs. Certainly eucalypts have been associated with particularly severe repellency in other studies (e.g. Gilmour 1968; Crockford et al. 1991; Scott 2000). It is also possible, but considered here less likely, that the sampling intervals used in this study were not sufficiently frequent to detect any intermediate levels of repellency severity that may have occurred during transitional phases. Assuming that the latter is not the case, the dichotomy in the degree of soil wettability observed here indicates that the switching from wettable to the highest achievable repellency severity level does not involve a gradual increase, but rather a sudden transition. This view is supported by predominance of high repellency severities found for repellent conditions also at intermediate soil moisture contents (14-27% vol.; Table 4b). The underlying principles and nature of this switch clearly warrant further investigation. Level of repellency severity and soil moisture content at individual sampling points Previous studies have suggested that a volume of soil can hold substantial amounts of water and yet still be repellent. Above a certain moisture content, however, a repellent soil becomes entirely wettable (Dekker and Ritsema 1994; Soto et al. 1994; Doerr et al. 2000b). Dekker and Ritsema (1994) considered that for every repellent soil there was a point of change between repellent and wettable conditions, which they termed the ‘critical soil moisture threshold’. In later study Dekker et al. (2001) replaced the concept of the transition between repellent and wettable states occurring at a distinct soil moisture content, with a moisture ‘transition zone’ concept, within which both wettable and repellent soil conditions may occur, but only repellent conditions exist below and wettable ones above the moisture boundaries of this zone. The transition between repellent and wettable conditions is thus thought to occur at any moisture content within the transition zone. The relationship between soil moisture and water repellency observed in the current study (Figure 2) would seem at first glance to tally with the transition zone concept introduced by Dekker et al. (2001) rather than a narrow moisture threshold dividing repellent and wettable conditions. The plot of soil moisture against repellency severity class for the ‘mature’ site (Figure 2) shows that, with the exception of three outliers, below 14% vol. moisture, the soil was invariably repellent and above 27% vol. moisture invariably wettable. For the intermediate moisture contents between these boundaries, both wettable and repellent conditions were found. The boundaries found here broadly match the limits of the ‘transition zone’ of 23% vol. and 18% vol. reported for a sandy soil in the Netherlands by Dekker et al. (2001), and the upper moisture boundary is very close to the upper 9

threshold (28% vol.) reported by Doerr and Thomas (2000). Furtermore, the 19-20% vol. mean soil moisture contents recorded in June and October 2001 at the ‘mature ’ site, when SFRm differed (52% and 63% respectively) falls within these soil moisture-repellency boundaries found at the point scale. There are a number of possible reasons why both repellent and wettable soils were found at moisture contents between 14% vol. and 27% vol. at individual sampling points. Following the ‘transition zone’ concept of Dekker et al. (2001) the transition between repellent and wettable conditions could occur within a specific range of soil moisture contents. Alternatively, an apparent transition zone might be the result of two potentially interlinked factors: (a) the existence of two separate critical moisture thresholds for the ‘switches’, from wettable to repellent conditions and vice versa, and (b) the short-term wetting history of the soil. Thus, a hysteresis may exist for the expression of water repellency (just as there exists for soil matrix suction), resulting in a different behaviour depending on whether a soil is in a wetting or a drying phase. Another possible contributory or alternative scenario is that the existence of an apparent transition zone is simply an artefact of the different scales at which repellency and soil moisture are usually assessed. In both the present study and that of Dekker et al. (2001), soil moisture was assessed for a volume of ~100 cm3 of soil (using TDR and gravimetric techniques, respectively), whereas repellency was assessed for a thin ‘skin’ of soil in contact with a droplet of water or ethanol solution. During a wetting or drying phase the moisture at the very surface of the soil volume may differ significantly from the value derived from measuring moisture for the ~100 cm3 volume of underlying soil. This discrepancy could then mask any existing closer relationship between soil moisture and repellency, and the resulting wider distribution of points would erroneously suggest a transition zone. This effect might well account also for the plotted positions of the three outlier value s. Clearly, more research is needed into the relationship between soil moisture and repellency and the conditions controlling the transition between repellent and wettable states using soil moisture assessment methods with a higher spatial resolution. Implications Some broad implications of the results of this study are briefly considered here. Firstly, some comment should be made on the broad thresholds concerning the conditions and timescales of the breakdown and re-establishment of repellency at the slope scale. A key question is the duration of the extensive and severe repellency developed during the long dry summer period persists following the onset of autumn rain events. For example, 30-minute rainfall simulations with an intensity of ~100 mm hr -1 carried out at the ‘mature’ site of this study (Leighton-Boyce 2002) suggest that a such an intense event would not result in a decrease in the contiguous surface repellency (SFRi=100%) and high subsurface SFRi (>90%) found following prolonged dry conditions (e.g. August 2000 and 2001). The October 2001 sampling results suggest, however, that rainfall totalling ~140 mm, falling on 14 days of a 26 day period, is likely to result in a small decrease in SFR. Considerable repellency may remain, however (e.g. SFRi=88%, 65% and 37% at the surface , 10 and 20 cm depths , respectively, October 2001). No repellency was found at the ‘mature’ site in February 2001, although some repellent soil conditions were found at the young and newly burnt sites, suggesting that prolonged winter rainfall (~1200 mm distributed over up to five months) is likely to result in the breakdown of repellency to a level at which it is unlikely to be of hydrological significance at the slope scale. These findings support work by Ferreira et al. (2000) , who reported the existence of some repellency under eucalypt plantations within the same study region following 200 mm of rain falling in one week. Similarly, 10

Crockford et al. (1991) found that several weeks of consistently wet weather was required for repellency to break down in a eucalypt forest in Australia. In terms of the reestablishment of repellency, again using the example of the ‘mature’ site, a prolonged period of between three and nine weeks of generally dry weather were required for surface repellency to become completely re -established after the entirely non-repellent and relatively wet (27% vol. soil moisture) conditions found following the long wet winter period. Clearly, however, these broad thresholds for the breakdown and re-establishment of repellency suggested here can only be taken as a general indication of repellency transitions that may occur for soils under similar conditions (vegetation cover, climate, soil type). That the temporal dynamics of repellency extent are likely to be site-specific is supported by the variations in the generally seasonal pattern in SFR between the four sites of this study. Potentially, the data presented here provide some quantitative information that can be used to improve the demarcation between repellent and wettable phases using antecedent rainfall as a proxy for soil water repellency occurrence. This could reduce the need for extensive and high frequency measurements. Accurate demarcation between repellent and wettable phases is essential if the links between water repellency and slope and catchment scale hydrogeomorphic responses are to be understood. Also, the incorporation of the effects of water repellency within hydrological models requires that its temporal dynamics are understood at a range of temporal scales, including the longer-term scales addressed in this study. The soil moisture conditions associated with repellent and wettable conditions found here may also be of importance in the management of repellency, for example, the design of irrigation strategies to prevent the development of repellency. With respect to the study area, the results suggest that spatially contiguous repellency may be less prevalent following prolonged dry conditions within the study region than previously suggested (e.g. Doerr et al. 1998). Conversely, relatively young stands on recently rip-ploughed areas may be more repellent than previously thought (e.g. Shakesby et al. 1993). This emphasises the need to consider stand age when investigating hydrological responses in eucalypt plantations and potentially also other land use types prone to repellency development. Conclusions The detailed approach used in this study at four sites ha s provided a detailed analysis of the temporal changes in in situ surface and subsurface soil water repellency over a 16-month period under E. globulus stands in Portugal. The results demonstrate that variations in the occurrence of repellency and more specifically its spatial frequency (SFR, defined as the percentage of measurements classified as repellent) are inversely related to soil moisture and to antecedent rainfall. The seasonal variations in SFR are in marked contrast to repellency severity; at all times of year the repellency measurements showed a strong dichotomy with soil being either wettable or of high repellency severity, with relatively little soil of low or intermediate repellency being found. Our results add to the debate concerning the nature of the change or ‘switch’ in repellency characteristics (from wettable to repellent and vice versa) and the soil moisture level or levels at which these changes occur. Potentially repellent areas were found to be invariably repellent at soil moistures below 14% and wettable above 27% (volume). This would appear to support the presence of the transition zone concept introduced by Dekker et al. (2001), which suggest s that the transition between repellent and wettable conditions can occur within a range of moisture contents between limiting upper and lower boundaries. Two alternative explanations, however, are proposed in this paper. First, the apparent 11

transition zone could actually comprise two separate transition points; an upper threshold which must be reached for repellency to break down, and a lower threshold which must be reached for repellency to re-establish. Second, the apparent transition zone may simply be an artefact of measuring soil moisture and repellency at different spatial scales. Further investigation is required to ascertain whether a transition zone actually occurs when soil moisture and repellency are measured at the same spatial scales, and, if it is verified, what mechanisms underlie its existence. Whilst this study has shed some light on the longer-term temporal dynamics of soil water repellency, further study is clearly needed. Specifically, higher frequency monitoring of in situ repellency during the critical wetting and drying phases could lead to further improvements in our understanding of the timing of the changes between repellent and wettbale states and the associated conditions. In addition, more accurate prediction of SFRm could be achieved if the distribution of antecedent rainfall, evaporation and evapotranspiration rates, humidity, temperature, and a range of site-specific characteristics (e.g. altitude, aspect, litter cover and antecedent repellency frequency and distribution) are considered. Given that variations in the generally seasonal pattern in SFR were found between the relatively similar sites in this study, it is important to consider that repellency dynamics may be very different for differing vegetation, climate and soil types. Acknowledgements We thank K. Burdett, K. Cooke, K. Emeny, D. Hayward, A. Vater and R. Woodland for assistance with fieldwork. GLB acknowledges the financial support of a Natural Environment Research Council (NERC) Research Studentship (GT/4/99/295/TS) and SHD and RAS of a EU contract (FAIR-CT98-4027). This work does not necessarily reflect the European Commission's views and in no way anticipates its future policy in this area. The authors thank the anonymous referees for their thorough and constructive comments.

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References Burch GJ, Moore ID, Burns J (1989) Soil hydrophobic effects on infiltration and catchment runoff. Hydrological Processes 3: 211-222. Crockford H, Topalidis S, Richardson DP (1991) Water repellency in a dry sclerophyll eucalypt forest – measurements and processes. Hydrological Processes 5: 405-420. DeBano LF, Krammes JS (1966) Water repellent soils and their relation to wildfire temperatures. International Association of Hydrological Sciences 21: 4-19. DeBano LF, Savage SM, Hamilton AD (1976) The transfer of heat and hydrophobic substances during burning. Soil Science Society of America Proceedings 40: 779-782. Dekker LW (1998) ‘Moisture variability resulting from water repellency in Dutch soils’. PhD thesis, Wageningen Agricultural University, the Netherlands. Dekker LW, Ritsema CJ (1994) How water moves in a water-repellent sandy soil. 1. Potential and actual waterrepellency. Water Resources Research 30: 2507-2517. Dekker LW, Ritsema CJ (1995) Fingerlike wetting patterns in two water-repellent loam soils. Journal of Environmental Quality 24: 324-333. Dekker LW, Doerr SH, Oostindie K, Ziogas AK, Ritsema CJ (2001) Water repellency and critical soil water content in a dune sand. Soil Science Society of America Journal 65: 1667-1674. Doerr SH (1998) On standardising the ‘water drop penetration time’ and the ‘molarity of an ethanol droplet’ techniques to classify soil hydrophobicity: A case study using medium textured soils. Earth Surface Processes and Landforms 23: 663-668. Doerr SH, Thomas AD (2000) The role of soil moisture in controlling water repellency: new evidence from forest soils in Portugal. Journal of Hydrology 231-232: 134-147. Doerr SH, Shakesby RA, Walsh RPD (1998) Spatial variability of soil hydrophobicity in fire -prone eucalyptus and pine forests, Portugal. Soil Science 163: 313-324. Doerr SH, Shakesby RA, Walsh RPD (2000a) Soil water repellency: its characteristics, causes and hydrogeomorphological consequences. Earth Science Reviews 51: 33-65. Doerr SH, Walsh RPD, Shakesby RA (2000b) Soil hydrophobicity in north-west Europe: its occurrence and implications for modelling soil hydrological response. British Hydrological Society Occasional Paper 11: 211-218. Doerr SH, Ferreira AJD, Walsh RPD, Shakesby RA, Leighton-Boyce G and Coelho COA (2003). Soil water repellency as a potential parameter in rainfall-runoff modelling: experimental evidence at point to catchment scales from Portugal. Hydrological Processes 17: 363-377 . FAO (Food and Agriculture Organization of the United Nations) (1988) FAO-UNESCO soil map of the world. Revised legend. World Soil Resources Report 60, Rome. Ferreira AJD, Coelho COA, Walsh RPD, Shakesby RA, Ceballos A, Doerr SH (2000) Hydrological implications of soil-water repellency in Eucalyptus globulus forests, north-central Portugal. Journal of Hydrology 231 -232: 165-177. Gilmour DA (1968) Water repellence of soils related to surface dryness. Australian Forestry 32: 143-148. Imeson AC, Verstraten JM, Van Mullingen EJ, Sevink J (1992) The effects of fire and water repellency on infiltration and runoff under Mediterranean type forests. Catena 19: 345-361. 13

Jungerius PD, de Jong JH (1989). Variability of water repellency in the dunes along the Dutch coast. Catena 16: 491-497. Krammes JS, Osborn J (1968) Water-repellent soils and wetting agents as factors influencing erosion. In ‘Proceedings of the Symposium on Water Repellent Soils, May 6-10, 1968’. (Eds LF DeBano, J Letey) pp. 177-187 (Riverside, California). Leighton-Boyce G (2002) ‘Spatio -temporal dynamics and hydrogeomorphic implications of soil water repellency within Eucalyptus forests in north-central Portugal’. PhD thesis , University of Wales Swansea, UK. Letey J. (1969) Measurement of contact angle, water drop penetration time, and critical surface tension. In ‘Water repellent soils. Proceedings of a Symposium on Water Repellent Soils’. (Eds LF DeBano, J Letey) pp. 177-187. (University of California: Riverside, CA) O’Connor, K. M. & Dowding, C. H. (1999) ‘GeoMeasurements by Pulsing TDR Cables and Probes’. CRC Press, London. Pereira V, FitzPatrick EA (1995) Cambisols and related soils in north-central Portugal: their genesis and classification. Geoderma 66: 185-212. Ritsema CJ, Dekker LW, Hendrickx JMH, Hamminga W (1993) Preferential flow mechanisms in a water repellent sandy soil. Water Resources Research 29: 2183-2193. Roberts FJ, Carbon BA (1971) Water repellence in sandy soils of south-western Australia. 1. Some studies related to field occurrence. Division of Plant Industry CSIRO (Australia), Field Station Record 10: 13-20. Scott DF (1994) ‘The hydrological effects of fire in South African catchments’. PhD thesis , University of Natal, Pietermarit zburg, South Africa. Scott DF, Versfeld DB, Lesch W (1998) Erosion and sediment yield in relation to afforestation and fire in the mountains of the western Cape Province, South Africa. South African Geographical Journal 80: 52-59. Scott DF (2000) Soil wettability in forested catchments in South Africa: as measured by different methods and as affected by vegetation cover and soil characteristics. Journal of Hydrology 231-232: 87-104. Soto B, Basanta R, Benito E, Perez R, Diaz-Fierros F (1994) Runoff and erosion from burnt soils in northwest Spain. In ‘Soil erosion as a consequence of forest fires’. (Eds M Sala, JL Rubio), pp. 91-98 (Geoforma Ediciones, Logroño). Shakesby RA, Coelho COA, Ferreira AJD, Terry JP, Walsh RPD (1993) Wildfire impacts on soil erosion and hydrology in wet Mediterranean forest, Portugal. International Journal of Wildland Fire 3, 95-110. Shakesby RA, Doerr SH, Walsh RPD (2000) The erosional impact of soil repellency: current problems and future research directions. Journal of Hydrology 231-232: 178-191. Topp GC, Davis JL, Annan AP (1980) Electromagnetic determination of soil water content: measurements in coaxial transmission lines. Water Resources Research 16:574-582. Walsh RPD, Coelho COA, Shakesby RA, Ferreira AJD, Thomas AD (1995) Post-fire land use and management and runoff responses to rainstorms in northern Portugal. In: ‘Geomorphology and Land Management in a Changing Environment’. (Eds D MC Gregor, D Thompson) pp. 283-308. (Wiley, Chichester). 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. 14

Table 1. Study site characteristics Tree height‡ (m)

Aspect

Slope angle (°)

Altitude (m a.s.l.)

Mean litter depth§ (cm)

NNW

8

430

10.4

NW

7

430

7.8

Seedlings planted in deep-ploughed soil

SSE

11

260

0.6

Burnt; regrowth from coppiced stumps

ESE

7

180

3.9

Site

Age†

Land management history

‘Mature’

10 years

8-10

Seedlings planted in undisturbed soil

Established

5 years

5-8

Regrowth from coppiced stumps

Young

6 months

1-2

Newly burnt

4 months

1-2



Estimated age on 1 August 2000 ‡ Estimated height in January 2001 § n = 480

Table 2. Ethanol concentrations (% volume), respective surface tensions, and associated descriptive water repellency categories and classes used in this study (adapted from Doerr et al. 1998) Ethanol concentration (%) Surface tensi on (Nm10-3 ) Descriptive category Severity class

0

1

2

3

5

8.5

13

18

24

36

50

>50

72.1

66.9

63.9

60.9

56.6

51.2

46.3

42.3

38.6

33.1

31.0

< 31.0

wettable 1

low repellency 2

3

moderate repellency 4

5

6

7

severe repellency 8

9

extreme repellency 10

11

15

12

Table 3. Mean soil moisture of all measurement points and depths, 2-week and 8-week antecedent rainfall, and spatial frequency of repellency (SFRm; mean of all three depths ), for each site and measurement date. The measurement dates ordered by soil moisture. Ranges in rainfall amounts for some measurement dates reflect the fact that rainfall occurred during the measurement process. a)

Mature plantation

Measurement date February 2001 May 2001 April 2001 October 2001 June 2001 November 2001 August 2001 August 2000

Mean soil moisture (% vol.) 36.7 30.7 27.2 19.9 19.4 10.9 8.8 7.3

Antecedent rainfall (mm) 2-week

8-week

SFRm (5)

277-351 71 49 83-87 5 3 6 7

877-879 281-321 616 159 202 273 66 109

0 3 0 63 52 89 91 85

b) Established regrowth plantation Measurement date

Mean soil moisture (% vol.)

May 2001 February 2001 April 2001 October 2001 June 2001 November 2001 August 2001 August 2000

37.2 31.5 31.1 24.7 17.1 12.5 10.6 10.5

Antecedent rainfall (mm) 2-week

8-week

149-159 162 74 86 5 3 17 7

343-346 882 616 156 202 273 77 108

SFRm (%) 0 1 0 35 30 50 58 59

16

c) Young plantation Measurement date February 2001 April 2001 May 2001 October 2001 November 2001 June 2001 August 2001 August 2000

Mean soil moisture (% vol.) 24.4 20.6 13.6 12.1 11.0 10.0 6.7 6.2

Antecedent rainfall (mm) 2-week

8-week

SFRm (%)

287-350 130 32-35 94 0 0 1-12 7-9

773-785 595 261 120-130 252 179 54 100-102

10 3 33 50 54 43 62 59

d) Newly burnt plantation Measurement date

Mean soil moisture (% vol.)

April 2001 February 2001 May 2001 October 2001 June 2001 November 2001 August 2001 August 2000

26.3 18.9 14.3 10.3 9.9 8.6 6.8 5.8

Antecedent rainfall (mm) 2-week

8-week

126 149 30-32 55 0 3 6 8-9

562 768 244-256 116 174 239 58 100

SFRm (%) 1 7 48 83 77 82 85 75

17

Table 4. Frequency distribution of different repellency severities for all depths, sites and sampling occasions for a) all measurements, and b) measurements where soil moisture was between 14% and 27% vol. Excluded are all measurements where soil was classed as wettable. For severity classes see Table 2. Percentage of measurements classed as: Site a)

b)

Low

Moderate

Severe

Extreme

n

‘Mature’ Established

5 9

7 11

16 24

72 56

692 417

Young

15

18

22

45

568

Newly burnt

3

9

26

62

823

‘Mature’ Established

10 11

10 11

23 16

56 63

172 79

Young

30

30

10

30

45

Newly burnt

8

23

31

46

50

FIGURE CAPTIONS Figure 1. Spatial frequency of repellency (SFR) at each site and each of the three depths for each measurement date. Mean values of soil moisture (horizontal bars) and corresponding standard deviations (vertical lines) are superimposed on compound bars showing the proportions of measurements with different repellency severities. Figure 2. Plots of repellency class and soil moisture for all measurements made on soil at the surface and 10 cm depth at the ‘mature’ site. Dots represent individual data points and different numbers of points with the same moisture and severity values are represented by proportional circles.

18

60%

30

40%

20

20%

10

0%

0

100%

50

80%

40

60%

30

40%

20

20%

10

0%

0

0 10 20 0 10 20 0 10 20 0 10 20 0 10 20 0 10 20 0 10 20 0 10 20

0 10 20 0 10 20 0 10 20 0 10 20 0 10 20 0 10 20 0 10 20 0 10 20

Aug 00

Aug 00

Apr 01

May 01

Jun 01

Aug 01

Oct 01

Nov 01

80%

40

60%

30

40%

20

20%

10

0%

0

Proportion of measurements (n=60)

50

Proportion of measurements (n=60)

100%

Apr 01

May 01

Jun 01

Aug 01

Oct 01

Nov 01

100%

50

80%

40

60%

30

40%

20

20%

10

0%

0

0 10 20 0 10 20 0 10 20 0 10 20 0 10 20 0 10 20 0 10 20 0 10 20

0 10 20 0 10 20 0 10 20 0 10 20 0 10 20 0 10 20 0 10 20 0 10 20

Aug 00

Aug 00

Feb 01

Apr 01

May 01

Jun 01

Aug 01

Oct 01

Nov 01

Soil depth (cm) and sampling occasion

Key:

Feb 01

Soil depth (cm) and sampling occasion

Mean % vol. soil moisture

Soil depth (cm) and sampling occasion

Wettable

Low

Moderate

Severe

Mean % vol. soil moisture

Feb 01

Mean % vol. soil moisture

40

Proportion of measurements (n=60)

80%

Proportion of measurements (n=60)

Established 50

Mean % vol. soil moisture

Mature 100%

Feb 01

Apr 01

May 01

Jun 01

Aug 01

Oct 01

Nov 01

Soil depth (cm) and sampling occasion

Extreme

19

12

Figure 2

Surface

10

Repellency class

8 6 4 2

0

10

20

30

40

50

60

12

10 cm depth

10

Repellency class

8 6 4 2

0

10

20

30

40

50

60

Soil moisture (%)

Key:

1

45

90

135

180 points

20