surface runoff as affected by soil water repellency in a japanese cypress forest

HYDROLOGICAL PROCESSESHydrol. Process. 21, 23652376 (2007)Published online in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/hyp.6749

Surface runoff as affected by soil water repellency in aJapanese cypress forest

Shusuke Miyata,1* Kenichirou Kosugi,1 Takashi Gomi,2 Yuichi Onda3 and Takahisa Mizuyama11 Laboratory of Erosion Control, Division of Forest and Biomaterials Sciences, Graduate School of Agriculture, Kyoto University, Kyoto 606-8502,

Japan2 Japan Science and Technology Agency, Disaster Prevention Research Institute, Kyoto University, Uji, Kyoto 611-0011, Japan

3 Graduate School of Life and Environmental Science, University of Tsukuba, Tsukuba 305-8502, Japan

Abstract:Recent studies have suggested that soil water repellency can be one of the important factors affecting hydrological processes onheadwater catchments. In Japan, water repellency is known to occur under Japanese cypress (Chamaecyparis obtusa) forests,a typical plantation type in Japan, however, previous studies have not evaluated the severity of water repellency and its effectson surface runoff generation. To quantify water repellency and its effects, this study combined the critical surface tension(CST) test with a new spraying experiment in which the infiltration rates of water and ethanol solutions sprayed over 009-m2plots were compared. Long-term intensive hydrological observations of surface runoff from 2-m2 plots, soil water potential,and soil water content were conducted concurrently.

The spraying experiment revealed that strong water repellency in surface soils, as quantified by the CST test, causedHortonian overland flow despite the high conductivity measured under saturated hydrophilic conditions. Generally, the surfacerunoff coefficient for a storm event was negatively correlated with initial soil moisture conditions. However, during a periodof successive storm events separated by short intervals, the coefficient decreased gradually even when the initial moistureconditions were similar, indicating a weakening of water repellency by repeated wetting. On the other hand, a drying periodwith long inter-rainfall intervals and increasing air temperature was associated with increases in the surface runoff coefficient.These results suggest that the water repellency and the resultant surface runoff depended on the history of rainfall at thesite. Relationships between soil water potential and soil water content indicate that changes in the soil water repellency andconsequently surface runoff coefficient could occur during a single storm event. Copyright 2007 John Wiley & Sons, Ltd.

KEY WORDS overland flow; hydrophobicity; rainfall simulation

Received 31 December 2005; Accepted 17 July 2006

INTRODUCTION

Water repellency in soils, which is caused by compoundsoriginating from fungi, microorganisms, soil organic mat-ter, or humus (DeBano, 2000; Doerr et al., 2000), hasbeen reported worldwide in grasslands (Dekker and Rit-sema, 1994), shrublands (Ferreira et al., 2005), croplands(Bisdom et al., 1993), areas burned by wildfires (Huff-man et al., 2001; MacDonald and Huffman, 2004), andforests. The forest vegetation types most commonly asso-ciated with water repellency are evergreen tree speciessuch as pine (Wahl et al., 2003; Keizer et al., 2005), aswell as oak (Cerda` et al., 1998) and eucalypt (Crockfordet al., 1991; Doerr et al., 2002) species. In Japan, soilsassociated with Japanese cypress plantations (Chamaecy-paris obtusa [Sieb. et Zucc.] Endl.) exhibit water repel-lency (Kobayashi, 2000; Kobayashi and Shimizu, 2007).However, quantitative analyses on the severity of thewater repellency and its effects on hydrological processesare still lacking.

* Correspondence to: Shusuke Miyata, Laboratory of Erosion Control,Division of Forest and Biomaterials Sciences, Graduate School of Agri-culture, Kyoto University, Kitashirakawaoiwake-cho, Sakyo-ku, Kyoto606-8502, Japan. E-mail: [email protected]

Forests cover approximately 65% of all land in Japan,and cypress plantations occupy about 10% of this forestcover. In Mie Prefecture, where this study was conducted,cypress plantations comprise 50% of the forested area.Most of the Japanese cypress trees were planted forcommercial use in headwater areas, which also serveas source areas for groundwater recharge and sometimescause floods. Because of these two important functions,it is critical to clarify the hydrological processes inheadwater catchments consisting of Japanese cypressplantations.

Storm runoff processes in headwater catchments aregenerally governed by a number of factors, including cli-mate, topography, landscape, vegetation type, soil depthand soil properties. Several studies conducted at sitesburned by wildfire have suggested that runoff responseswere accelerated after the fire because the heat pro-duced strong water repellency in the soils (Scott, 1997;Nishimune et al., 2003). However, some unburned catch-ments have also developed more rapid storm runoffthought to result from preferential flow enhanced by soilwater repellency (Doerr et al., 2003). Soil water repel-lency is thus an important factor affecting hydrologi-cal processes and potentially a key parameter in hydro-logical modelling (Doerr et al., 2003). Previous studies

Copyright 2007 John Wiley & Sons, Ltd.

2366 S. MIYATA ET AL.

conducted at forested catchments with hydrophilic soilshave concluded that infiltration-excess overland flow (i.e.Horton-type overland flow) seldom occurs in forestedcatchments (e.g. Tsukamoto, 1961; Dunne and Black,1970) because forest soils usually have sufficient infil-tration capacity to accommodate rainfall intensity rates.In such catchments, subsurface flow has been emphasizedas the process promoting streamflow response (Rice andHornburger, 1998; McGlynn et al., 2002). However, incontrast, recent studies have suggested that surface runoffcan occur in forested catchments when soils are repellent(e.g. Bonell and Williams, 1986; Prosser and Williams,1998; Doerr et al., 2003). The tendency for dry soils toexhibit the strongest water repellency (Witter et al., 1991;Ritsema and Dekker, 1994) implies that for water repel-lent soils, the surface runoff coefficient tends to increaseas antecedent soil moisture decreases. At the same time,the water repellency of field soils depends on the his-tory of rainfall at the site (Crockford et al., 1991; Keizeret al., 2007), which may affect temporal variations in thesurface runoff coefficient.

These findings regarding the effects of soil water repel-lency on the occurrence of surface runoff were valuable,but most of the studies depended on the critical surfacetension (CST) test and the water drop penetration time(WDPT) test for evaluating soil water repellency. Thesetests are suitable for measuring the initial severity andpersistence, respectively, of soil water repellency but pro-vide no direct information on the impact of water repel-lency on surface runoff generation. Several studies con-ducted paired in situ rainfall experiments at hydrophobicand hydrophilic sites (e.g. Cerda` et al., 1998; Benitoet al., 2003), but the impact of water repellency on sur-face runoff generation was not fully explained becausethe effects of factors other than soil water repellencythat might have affected surface runoff generation couldnot be eliminated. These factors include the soil physicalcondition and initial moisture content. Although Osbornet al. (1964), Krammes and Osborn (1969) and recentlyLeighton-Boyce et al. (2007) have shown that soil waterrepellency can cause surface runoff by comparing resultsobtained in untreated plots with plots treated with wet-ting agent, few studies have involved long-term intensivemonitoring of surface runoff, and field observations ofthe effects of changes in soil water repellency on surfacerunoff are insufficient.

Given this background, this study was designed toquantify the water repellency of soils at Japanese cypressplantations and examine the effects of repellency onsurface runoff events by conducting long-term intensiveobservations of hydrological variables including surfacerunoff, soil water potential, and soil water content.Evaluations of soil water repellency based on the CST testwere reinforced by a new spraying experiment introducedhere, which provided direct information on the extent towhich soil water repellency produced Hortonian overlandflow. This study consisted of three components: the CSTtest, an in situ spraying experiment, and the monitoringof surface runoff and soil water properties. Our approach

aims to provide more comprehensive information on themechanism of surface runoff from water repellent soils.

METHODSStudy site

This study took place at the Hinotani-ike watershed(34210N, 136250E; 48 ha) located in Mie Prefecture,central Japan (Figures 1(ab)). Average temperature,mean annual precipitation, average maximal daily rain-fall, and average maximal hourly rainfall from 1979 to2004 were 144 C, 2094 mm, 211 mm, and 425 mm,respectively, at Kayumi meteorological station (34270N,136230E), which is located approximately 9 km fromthe watershed. The region including the study site hasa Cfa climate (i.e. humid subtropical with hot summers;Koppen climate classification) typically with a rainy sea-son from late June to mid-July (Baiu season) and atyphoon season from September to October. Typhoonscan produce high precipitation and sometimes cause dis-astrous landslides and/or debris flows in the region. Themain tree types in the watershed are Japanese cypress andJapanese cedar (Cryptomeria japoneca) planted approx-imately 40 years ago for commercial uses. The standdensity ranges from 1500 to 4000 stems ha1. Japanesecedar was planted only along the stream channels and isnot dominant in the watershed.

We focused on the area which had high densityJapanese cypress for this study because surface runoff andsurface soil erosion in dense cypress forests are one ofthe serious concerns for forest management in Japan. Insuch dense cypress forest, no understory vegetation wasfound because of the limited light conditions producedby a dense cypress canopy. Moreover, the forest floorwas poorly covered by litter because litter of Japanesecypress separates easily into small pieces (Sakai et al.,1987), and, then, is likely carried away by surface runoff(Hattori et al., 1992) or mixed with surface soils (Sakaiet al., 1987). All of our experiments and observationswere conducted on a planar hillslope (Figures 1(bc)).The soil is classified as Cambisol with light clay texture(Tamura, personal communication), and its density andorganic matter content within top 10 cm was 078 g cm3and 020 g g1, respectively. The soil had little or noA0-horizon, a 1-cm thick A-horizon, a 25-cm thick B-horizon, and an approximately 35-cm thick C-horizon,overlying well-cleaved schist.

Soil property measurementsSaturated hydraulic conductivity Ks, the water reten-

tion curve, and the severity of water repellency weremeasured using soil samples collected from a pit(Figure 1(c)). Three undisturbed soil core samples werecollected at depths of 5, 10, 30, and 60 cm for measuringKs and the water retention curve, and at depths of 0, 5,15, 25, 35, and 45 cm for the water repellency test. Coreswere taken using 100 cm3 steel cylinders with a cross-sectional area of 20 cm2 and height of 51 cm. The falling

Copyright 2007 John Wiley & Sons, Ltd. Hydrol. Process. 21, 23652376 (2007)DOI: 10.1002/hyp

WATER REPELLENCY EFFECTS ON SURFACE RUNOFF 2367

40N

45N

145E

135E140E

130E

35N

0 25 50 100 Meters

N

12

3

32

1

4 5 6

0 1 2 4Meters

145

150

120140

160180

200

220

240

LegendSurface runoff observation plot

Location for soil sampling

Field spraying experiment

Tensiometer and soil moisture probe

(a) (b)

(c)

Figure 1. (a) Location and (b) topography of the Hinotani-ike watershed, and (c) monitoring area showing locations of the surface runoff monitoringplots, soil sampling pit, field spraying experiments, and tensiometers and soil moisture probe

head test (Reynolds et al., 2002) was used for measuringKs. The same soil samples were also used for estimatingwater retention curves using the pressure plate method(Dane and Hopmans, 2002). Before the experiments, thesamples were saturated by raising the water table fromthe bottom for more than 24 h until water repellency wasremoved and the soils attained a completely saturatedcondition.

The soil samples for the water repellency test werecollected on 21 December 2004. The samples were sealedand taken to the laboratory immediately; samples werethen tested within one day because it was thought that theseverity of water repellency might vary rapidly becauseof sample drying or chemical reactions involving thehydrophobic substances. The severity of water repellencywas examined using the CST test, in which dropsof ethanol solutions with different concentrations wereplaced on the soil sample surface and the time requiredfor the drops to infiltrate was measured (Watson andLetey, 1970). The surface tension of an ethanol solutiondecreases as the ethanol concentration increases, so thatan ethanol solution with high concentration infiltratesquickly into water repellent soil. In the present study,ethanol solutions with volumetric ethanol concentrationsof 0, 1, 3, 5, 85, 13, 18, 24, and 36% were prepared,corresponding to surface tension increments of 005 0015 N m1 (Doerr, 1998). Five drops of solution wereplaced on a soil sample using a micropipette. If all dropsdid not infiltrate the sample within 5 s, solutions withsuccessively higher concentrations were then applied.When all the drops infiltrated within 5 s, the ethanolconcentration was taken as the resultant score. The

test was conducted in ambient laboratory conditions(20 C, 40% relative humidity [RH]) because it has beensuggested that water repellency of soil samples is affectedby air temperature and relative humidity (Doerr et al.,2002).

Field spraying experimentsA field spraying experiment was carried out at six

plots (solid diamonds in Figure 1(c)) of 30 cm width and30 cm slope-length. The whole six plots had gradientsof approximately 40, which corresponded to the averageslope gradient, and soil conditions of the site (e.g. waterrepellency, soil physical properties, moisture conditions,and micro-topography). Artificial rainfall was suppliedfrom spray nozzles attached to manual compressorswhich supplied mist at an intensity of 182335 mm h1.Because the equipment reproduced drizzling rainfall froma height of approximately 30 cm, we assumed thatcrust development by rain drop impact was minimizedallowing to evaluate the effect of soil water repellencywithout the interference of crusting. To reduce thevariation of initial soil moisture conditions among plots,artificial rainfall was initially sprayed for 40 s. After5 min, another 40-s spray was applied, and the surfacerunoff from the plot was measured. These experimentswere carried out on 24 February 2005 and 10 May 2005.On 24 February 2005, we sprayed pure water over plots1 through 3, and 36% ethanol solution (by volume) overplots 4 through 6 (Figure 1(c)). We hypothesized thatspraying with the ethanol solution would produce lesssurface runoff than spraying with pure water because the



ethanol solution has a lower surface tension and thereforeinfiltrates more readily into water repellent soils. Thus,the effects of soil water repellency on surface runoffgenerations were quantitatively evaluated by comparingthe surface runoff coefficient between the plots sprayedwith pure water and those sprayed with ethanol solution.On 10 May 2005, we interchanged the spraying liquidsused; the 36% ethanol solution and pure water weresprayed over plots 1 through 3 and plots 4 through 6,respectively.

Surface runoff monitoringSurface runoff was monitored at three bounded plots

approximately 1 m wide by 2 m in slope length (plotsA-1, 2, and 3 in Figure 1(c)). Each plot was situated sothat a cypress tree was located at the upslope end withinthe plot; both stemflow and throughfall are importantinput processes on a forested hillslope though stem den-sity in the plots, one stem per approximately 2 m2, maygreater than ambient condition in which stand density wasapproximately 4000 stem ha1. At the downslope end ofeach plot, a plastic sheet was inserted at the boundarybetween the A0 and A horizons; the sheet led the surfacerunoff through a gutter to a tipping bucket. Details foreach plot are described in Table I.

A capacitance probe for soil moisture measurement(Easy AG; Sentek Pty. Ltd., Stepney, Australia) andtensiometers were installed adjacent to the surfacerunoff observation plots (indicated by a solid cross inFigure 1(c)) for monitoring soil water content at 5 cmdepth and soil water potential at 5 and 60 cm depths. Thedepth of 60 cm corresponded to the soilbedrock inter-face. Precipitation was measured using a 02 mm tippingbucket rain gauge at an open site about 150 m from themonitoring plots. Air temperature was measured in thewatershed about 15 m from the plots. All meteorologi-cal and hydrometric data were automatically measuredwith a recording interval of 5 min. We analyzed the dataobtained from the beginning of October 2004 to the endof May 2005. During winter (from 20 December 2004 to23 February 2005), the measurements of soil water poten-tials were interrupted to avoid the tensiometer freezing.After 23 February 2005, a malfunction of one of the ten-siometers resulted in missing potential data at the depthof 5 cm.

Table I. Characteristics of the surface runoff monitoring plots

Plot Plot aream2

Slope gradient(degree)

Cypress stemdiameter at

breast height (cm)

A-1 146 407 232A-2 142 433 188A-3 208 399 146

RESULTSSoil properties

As shown in Figure 2(a), estimated Ks values rangedfrom 540 to 7090 mm h1 (corresponding to 0015 to020 cm s1), with relatively low values in the surfacehorizons (5 and 10 cm deep) and higher values in thesub-surface horizons (30 and 575 cm deep). Saturatedwater content values for soil samples collected at the fourdepths were similar, ranging from 050 to 056 cm3 cm3(Figure 2(b)). Soil water retention curves for the shal-lower horizons had slightly greater reductions in watercontent () in the range of 3 < soil water potential < 05 kPa, indicating that more macropores existedin the shallower horizon (Figure 2(b)).

The severity of water repellency expressed as a per-centage of ethanol concentration is shown in Figure 3(a).Volumetric water contents of the samples used for therepellency measurements are summarized in Figure 3(b).Figure 3(a) includes the rating criterion defined by Doerr(1998). All resultant values at depths of 15, 25, 35, and45 cm were zero, indicating that soil below 15 cm washydrophilic (i.e. wettable), whereas resultant values at

Soil water potential (y) (kPa)

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ric w

ater

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cm3 /c

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0.1

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(a)

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-1

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1000 10000

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th (c

m)

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Figure 2. (a) Saturated hydraulic conductivity (Ks) and (b) water reten-tion curves of three soil samples at each of 5, 10, 30, and 575 cm depths.In Figure 2(a), all measurement values for the three samples are shownat each depth. In Figure 2(b), the average curves for the three samples

are shown



Ethanol concentration (%)0 10 20 30 40

Dep

th (c

m)0

10

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Sample 1Sample 2Sample 3moderate

none slight strong very strong extreme

Severity rating of water repellency

Water content (cm3/cm3)0.10 0.12 0.14 0.16 0.18 0.20

Dep

th (c

m)

0

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20

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60

(a) (b)

Figure 3. Vertical distribution of (a) soil water repellency indicated by volumetric ethanol concentrations obtained by the CST test for three soilsamples at each of 5, 15, 25, 35, and 45 cm depths, and (b) volumetric water content of soil samples used for the CST test. The severity rating of

water repellency is as defined by Doerr (1998)

the soil surface corresponded to 536% ethanol concen-trations, indicating moderate to extreme water repel-lency. At the 5 cm depth, all measurements indicatedextreme water repellency.

Field spraying experimentFigure 4 shows how the precipitation applied for

40 s was separated into infiltration and surface runofffor each spraying experiment using the pure water(W) and the 36% ethanol solution (E). On 24 Febru-ary 2005, results showed higher surface runoff coef-ficients when the pure water was supplied to plots 1through 3 (i.e. W-1 through W-3 in Figure 4) than

when the ethanol solution was supplied to plots 4through 6 (E-4 through E-6 in Figure 4). When thesprays were interchanged on May 10, 2005, sprayingthe pure water to plots 4 through 6 (W-4 throughW-6) produced greater surface runoff coefficients thanspraying the ethanol solution to plots 1 through 3(E-1 through E-3). These results indicate that the dif-ference in the surface runoff coefficient did not stemfrom spatial variation of infiltration rates among theplots but from differences in the application liquids;the application of the ethanol solution reduced thesurface runoff coefficient. Thus, Figure 4 suggests thatsoil water repellency plays an important role in the gen-eration of surface runoff.

W-1

W-2

W-3 E-4

E-5

E-6

W-4

W-5

W-6 E-1

E-2

E-3

Infilt

ratio

n an

d su

rface

runo

ff am

ount

(mm

/40

sec)

0

1

2

3

4Infiltration Surface runoff

Water 36 % ethanol Water 36 % ethanol

February 24 2005 May 10 2005

Experiment

Apllied solution

Date

Plot number 1 2 3 5 6 4 5 6 1 2 34

Figure 4. Surface runoff coefficient, which were calculated by dividing surface runoff amounts by sprayed water (W) and 36% ethanol solution(E) amounts, in the field spraying experiment



Figure 4 also suggests that, when the same kind of liq-uid was applied, the surface runoff coefficient was greateron 10 May 2005 than on 24 February 2005. This differ-ence may be attributable to stronger repellency, perhapsresulting in part from the lower moisture content on 10May. Detailed analyses are presented in the Discussionsection.

Surface runoff and soil water response to rainfallIn this study, storm events were classified by rain-

fall amounts exceeding 2 mm and separated by no-rainperiods of 12 consecutive hours or more. During theobservation period from 1 October 2004 to 31 May 2005,32 storm events were recorded (Figure 5(a)). The entiremonitoring period was divided into the following fourperiods according to the measured precipitation and tem-perature (Figure 5(a)). 1 October 2004 to 10 December2004 (Period 1) was characterized by many heavy stormsand moderate air temperature. 10 December 2004 to 10February 2005 (Period 2) had little rainfall (total precipi-tation was 54 mm) and low air temperature. 10 February

2005 to 31 March 2005 (Period 3) was characterized byintermittent small storm events and low air temperature.Finally, 1 April 2005 to 31May 2005 (Period 4) had lowrainfall (total precipitation was 96 mm) and moderateair temperature. These precipitation and air temperatureregimes are typical in south central Japan. It may be rea-sonable to assume that air temperature relates positivelyto plant photosynthetic activity, and hence can be a goodindex of water consumption by evapotranspiration.

In Figure 5(b), the soil water content at 5 cm depthresponded to rainfall and decreased immediately afterstorm events. During Period 1, soil moisture at 5 cmdepth ranged from 22 to 48%. The water content waslow during Period 2 and increased a little during Period3. Owing to the low precipitation and relatively highevapotranspiration, the water content was lowest duringPeriod 4. The soil water potential at 5 cm depth wasvery responsive to rainfall and decreased rapidly afterstorms, while that at 60 cm depth it exhibited lessvigorous responses and had no positive values during themonitoring period.

10/1 11/1 12/1 1/1 2/1 3/1 4/1 5/1 6/1

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40 DischargeRunoff coefficient

Surfa

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noff

(mm/

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ater

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(c) Plot A-1

(d) Plot A-2

(e) Plot A-3

(a)

(b)

Soil water potential at 5 cm depth

Water content at 5 cm depth


Period 1 Period 2 Period 3 Period 4

Event 3 Event 10

Event 18

Mea

n da

ilyair

tem

pera

ture

(C

)

Event 4

Figure 5. Time series of (a) rainfall and air temperature, (b) soil water potential at 5 and 60 cm depths and volumetric water content at 5 cm depth,and (ce) surface runoff and the surface runoff coefficient observed at Plot A-1, 2, and 3, respectively. Two thick arrows in Figure 5b indicate the

initial moisture conditions of the field spraying experiments on 24 February and 10 May 2005



Surface runoff was recorded at every plot for 25 outof the 32 storm events (Figures 5(ce)). The amount ofsurface runoff was greatest at Plot A-1 and lowest atPlot A-3 for most of the storm events and positivelyrelated to the stem diameters of trees within the plots(Table I), implying that larger trees may gathered morerainwater around their stems. The surface runoff ratefrom Plot A-1 often exceeded rainfall intensity. Plot A-2sometimes generated a surface runoff rate greater than therainfall intensity. These results can be explained by thediversion of rainwater into the observation plots by thecypress stand. Field observations during a heavy stormevent revealed that rainwater accumulated as stemflowand dripped from tree leaves and branches. Thus, theamount of rain applied to the plots became greater thanthe incident precipitation observed at the open site, evenafter allowing for canopy interception and evaporation.

DISCUSSION

Cause of surface runoffA large amount of surface runoff was observed for

most of the storm events at all of the surface runoffobservation plots (Figures 5(ce)). The soil water poten-tial at 60 cm depth, just above the soilbedrock interface,did not show a positive value even during the heavieststorm event that occurred from 18 October to 20 October2004 (i.e. Event 4 in Figure 5(b)). Hence, the observedsurface runoff must have been Hortonian overland flow(infiltration-excess overland flow) rather than saturationoverland flow. However, saturated hydraulic conductivityin the soil profile was far higher than the observed rain-fall intensity (Figures 2(a) and 5(a)). At the same time,the resultant values of the CST test indicated that the soilsurface was moderately to extremely water repellent andthat the soil at 5 cm depth was extremely water repellent(Figure 3(a)). These results suggest that, because of waterrepellency, the soil could not attain its inherent infiltra-tion capacity when wettable, resulting in surface runoffeven when the rainfall intensity was much lower than thesaturated hydraulic conductivity of the soil.

The near-surface soil (5 cm depth) of the Japanesecypress forest exhibited water repellency similar to thatobserved at burned terrains known for their extreme waterrepellency; the concentration of the ethanol solution asan indicator of water repellency ranged from 14 to 24%at burned pine forests (MacDonald and Huffman, 2004)and from 13 to 36% at burned pine and eucalypt forests(Doerr et al., 1998). The resultant value at our studysite was higher than or equal to ethanol concentrationsfound in previous studies; 030% (Scott, 2000), 520%(Buczko et al., 2002), and 1336% (Doerr et al., 1998)for pine forest soils, 135% (Scott, 2000) and 420%(Buczko et al., 2002) for beech forest soils, and 2436%(Doerr et al., 1998) for eucalypt forest soil.

To measure the relative severity and persistence of soilwater repellency, the CST test and WDPT test (e.g. Letey

et al., 2000) are widely applied because of their sim-plicity (e.g. Gerke et al., 2001; Shakesby et al., 2003;Mataix-Solera and Doerr, 2004). However, these meth-ods do not provide direct information on the impact ofwater repellency on surface runoff generation, whereasthe spraying experiment used in this study providedsuch direct evidence. As shown in Figure 4, the spray-ing experiment clearly showed that the surface runoffcoefficient decreased when using the ethanol solution,which is less affected by soil water repellency, than whenusing pure water. However, we still observed surfacerunoff when we sprayed the 36% ethanol solution withintensities (182 to 336 mm h1) lower than the saturatedhydraulic conductivity of the surface soil (Figure 2(a)). Itwas presumed that the surface tension of the 36% ethanolsolution was not low enough, i.e. the ethanol concen-tration not high enough, to fully eliminate the effect ofsoil water repellency, so that not all of the applied solu-tion did infiltrate the soil. In fact, when we sprayed pureethanol on the same location and at a similar intensity,no generation of surface runoff was observed.

Some earlier studies examined the impact of waterrepellency on surface runoff generation by conduct-ing in situ artificial rainfall experiments. These stud-ies attempted to clarify the effects of water repel-lency by comparing infiltration rates at hydrophobic andhydrophilic sites. Conclusions were inconsistent; Cerda`et al. (1998) and Benito et al. (2003) strongly assertedthat water repellency enhanced surface runoff generation,while Coelho et al. (2005) reported that water repellencyhad less impact on surface runoff. This inconsistencymay be partly attributed to a problem in the experimen-tal method, namely that the paired rainfall experimentsconducted at hydrophobic and hydrophilic sites could notnecessarily eliminate the effects of factors other than soilwater repellency that may have affected surface runoffgeneration, such as the hydraulic conductivity of thesurface soil, initial soil moisture condition, soil surfaceroughness, and topography. In contrast, our experimentused the different surface tension of spraying liquids,which allowed us to isolate the effects of soil water repel-lency. Also, the effects of small variations among theplots were successfully eliminated by repeating the exper-iment twice for the same plots while interchanging thespraying liquid used. Thus, the spraying experiment pro-posed here clearly demonstrated that the soil water repel-lency caused the surface runoff in the Japanese cypressplantation forest.

Relationship between surface runoff and antecedent soilmoisture condition

In the infiltration experiment, the surface runoff coeffi-cient was higher on 10 May than 24 February (Figure 4),suggesting that the soil was more water repellent on 10May. It is thought that the difference in water repellencybetween these 2 days was due to a difference in soilmoisture conditions. As indicated by the thick arrowsin Figure 5(b), the soil water content at 5 cm depth on10 May (19%) was lower than on 24 February (26%).



This is consistent with the known tendency for waterrepellency to be exhibited more strongly when the soilis drier (Witter et al., 1991; Ritsema and Dekker, 1994).To examine the effect of soil dryness, which could bestrongly associated with the severity of water repellency,on surface runoff generation at plots A-1 through A-3,Figure 6 shows the amount of surface runoff for eachstorm event in comparison with the total event rainfall forfour different conditions of the initial soil water content at5 cm depth, i. The figure clearly shows that the surfacerunoff coefficient increased for lower initial soil moisture.

Under dry initial conditions, hydrophilic soils usuallyattain a high infiltration rate because of a high waterpotential. As the soil becomes wet, the infiltration ratedecreases to a constant value controlled by the saturatedhydraulic conductivity (Burch et al., 1989; Gimenezet al., 1992). Thus, for hydrophilic soils, the surfacerunoff coefficient tends to decrease with initial soilmoisture. The trend shown in Figure 6 is opposite thatexpected for hydrophilic soils and seems typical forhydrophobic soils, which have low infiltration rates underdry conditions and gradually increasing rates as the soilbecomes wetter (Burch et al., 1989).Changes of surface runoff affected by changes in soilwater repellency

Figure 6 shows that the surface runoff coefficient var-ied a little, even when the initial soil moisture was simi-lar. This suggests that the runoff coefficient is not entirely

determined by the initial soil moisture. Figures 5(c)through 5(e) plot the coefficient of surface runoff to totalprecipitation for each storm event. Storm event 18, whichoccurred after the end of Period 2, produced a largesurface runoff coefficient for every plot (Figure 5(ce))because of strong water repellency associated with ini-tially dry soil conditions (Figure 5(b)). During Period3, small storm events with total precipitation rangingfrom 47 to 20 mm occurred eight times at relativelyshort intervals from 1 to 6 days. During this period, therunoff coefficient for each storm event decreased grad-ually (Figures 5(ce)) although the initial soil moistureconditions were similar (Figure 5(b)). This suggests thatthe initial soil water repellency for each event graduallydecreased as storms repeatedly occurred at short intervals.Thus, the water repellency of field soil did not dependonly on soil moisture content but also on the history ofrainfall at the site.

Doerr and Thomas (2000) demonstrated that once thewater repellent soils collected at pine and eucalypt plan-tation forest wetted up, they were no longer water repel-lent even when they were dried again. On the basisof this result, they deduced that the reestablishment ofwater repellency requires new input and/or redistribu-tion of hydrophobic substances. In addition, Crockfordet al. (1991) observed soil water repellency periodicallyin an eucalypt forest and found that the water repel-lency broke down after some weeks of consistently wet

(b) dry (0.20 < qi < 0.24)

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Figure 6. Relationships between total surface runoff and total precipitation for each storm under (a) very dry, (b) dry, (c) wet, and (d) very wet initialsoil moisture conditions. The parameter i represents the soil water content at 5 cm depth just before the storm event. Symbols represent average

values, and error bars represent minimal and maximal values for the three plots



weather and that when more frequent than usual rain-fall continued, the water repellency disappeared. Ourresults obtained during Period 3 suggested that successivestorm events with short intervals can gradually decreasesoil water repellency, resulting in a gradual decrease inthe coefficient of surface runoff from a forested hills-lope.

On the other hand, the runoff coefficient graduallyincreased during Period 4 for every plot (Figures 5(ce)).In view of the explanation suggested by Doerr andThomas (2000), the relatively longer intervals betweenstorm events during Period 4 (115 days with an averageof 82 days) than those during Period 3 (16 dayswith an average of 45 days) probably allowed forthe reestablishment of water repellency in the surfacesoil. Crockford et al. (1991) also observed that waterrepellency was reestablished after one week of hot dryweather. During Period 4, the soil was becoming drier(Figure 5(b)) because of the longer intervals betweenstorm events and the increased evapotranspiration ratesfrom increases in air temperature (Figure 5(a)). Theincreased air temperature might also directly enhancethe water repellency. This climatic condition probablyallowed new inputs and redistributions of hydrophobicsubstances (Doerr and Thomas, 2000) during Period 4,which likely caused the gradual increases in the surfacerunoff coefficient.

Change of soil water repellency during a storm eventAs shown in the previous subsection, a series of storm

events with short intervals during Period 3 graduallydecreased the soil water repellency. A similar decreasemay happen during a single storm event. In Figure 7,hydrological data observed during a storm event from 4to 5 December (i.e. Event 10 in Figure 5(a)) are shown.Total precipitation and storm duration for this event were123 mm and 207 h respectively, the third greatest forthe monitoring period (Figure 5(a)), and the maximumrainfall intensity was 26 mm h1. The initial soil watercontent was relatively low (22%; Figure 5(b)).

Generally, when soil is hydrophilic, the surface runoffcoefficient is expected to gradually increase during stormsas the soil becomes wet. As shown in Figure 7(a), therainfall intensity gradually increased during Event 10,causing the expectation of a large surface runoff coef-ficient during the latter half of the event. However, theobserved runoff coefficient exhibited a gradual decline inevery plot (Figures 7(ce)). This result can be explainedby increases in the infiltration rate during the storm eventas the water repellency declined. Similar declines inthe surface runoff coefficient were observed for Event4, which was the heaviest event during the monitoringperiod, with total precipitation of 244 mm and stormduration of 559 h (Figure 5(a)). The initial soil watercontent at 5 cm depth was 25% for Event 4; this value

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Figure 7. Time series of (a) rainfall, (b) soil water potential and water content at 5 cm depth, and (ce) surface runoff and the surface runoffcoefficient measured at Plot A-1, 2, and 3, respectively, during Event 10



was slightly higher than that for Event 10 but still rel-atively low (Figure 5(b)). On the other hand, no cleardecreases in the runoff coefficient occurred during Event3, which was the second greatest storm event (total pre-cipitation of 198 mm and storm duration of 353 h) witha wet initial soil moisture condition (29%; Figure 5(b)).

In hydrophilic soils, capillary forces generally causesuction. In contrast, when water infiltrates hydrophobicsoils, capillary forces in small soil pores exclude waterbecause the contact angle between the surface of thesoil particles and the water is greater than 90. As aresult, water excluded from the small pores infiltratespreferentially into larger pores and causes a rapid increasein soil water potential with little increase in water content(Bauters et al., 2000). In Figure 7(b), a rapid increasein potential at 5 cm depth was observed without anysignificant increase in water content at the beginning ofEvent 10. After the potential reached its maximal value of33 kPa, it decreased slightly, although rainfall intensityvaried greatly. After the end of the storm, the potentialimmediately started to recede. On the other hand, thesoil water content at the same depth increased graduallyduring the storm event and changed to recession at theend of the rainfall (Figure 7(b)).

In Figure 8, the correlations between soil water poten-tial ( ) and soil water content () from Figure 7(b) (i.e.the field water retention curve) are compared with thewater retention curve measured in the laboratory for thedrainage process from complete saturation (the curveshown in Figure 2(b)). The field water retention curveshowed different relationships between wetting anddrying processes. During the wetting process, roseto nearly 0 kPa with little increase in (Phase 1 inFigure 8); this was followed by slight decreases in and gradual increases (Phase 2). During the drying pro-cess after the rainfall, both and decreased gradually,making the field water retention curve very similar tothat measured in the laboratory (Phase 3). In each phase,an interaction between the soil water repellency and theinfiltration and drainage processes can be summarized asfollows:

Phase 1. Infiltration water at the surface soil horizon,which was excluded from the small pores, constitutedbypass flow (i.e. vertical preferential flow) due towater repellency, resulting in a rapid increase in .Because of the small increase in , the hydraulicconductivity of the surface soil did not increasesubstantially, which resulted in the generation ofsurface runoff (Figures 7(ce)).

Phase 2. Infiltration water gradually attenuated soil waterrepellency, resulting in increases in as well as inthe infiltration rate. As a result, the surface runoffcoefficient gradually decreased (Figures 7(ce)). Inthe last stages of Phase 2, the field-measured relationship agreed well with the water retention curvemeasured in the laboratory (Figure 8), suggesting thatwater repellency was almost broken down.

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Figure 8. Relationships between soil water potential and water contentat 5 cm depth during Event 10 and shown in Figure 7(b) comparedwith the water retention curve measured in the laboratory and shown

in Figure 2(b). Arrows indicate directions of change with time

Phase 3. After water repellency was broken down, waterdrained following the retention curve measured in thelaboratory (Figure 8).

By conducting a laboratory infiltration and drainageexperiment, Kobayashi et al. (1996) observed stronghysteresis in the water retention curve for hydrophobicsoil samples; the wetting branch of the curve wascharacterized by much lower water content than thedrying branch. They reported that during the waterinfiltration process the potential increased to almost0 kPa with little increase in water content. This findingcorresponds well with the phenomenon we observed inPhase 1; our study showed that the sudden increasein soil water potential along with little water contentincrease actually occurred in the field soil and that thisphenomenon was closely related to the occurrence ofsurface runoff on the forested hillslope.

Also, Phase 2 in Figure 8, in which the field retentioncurve did not corresponded the laboratory observedretention curve, may correspond to the transition zoneproposed by Dekker et al. (2001) in which the watercontent lies between a lower limit below which soilsare completely water repellent and an upper limit abovewhich soils are completely hydrophilic. In spite ofsuccessive decrease in the water repellency, the waterrepellency might remain before the last stage of Phase 2.

SUMMARY AND CONCLUSIONS

Measurements, observations, and experiments were con-ducted to determine soil hydrological properties, soil



water repellency, and surface runoff processes on a hills-lope covered with a dense plantation of Japanese cypress.The main conclusions are as follows:

1. The CST test revealed that surface soils in the Japanesecypress forest exhibited water repellency as severe asthat for soils at pine and eucalypt forests reported byprevious studies.

2. The spraying experiment proposed in this study pro-vided direct evidence of soil water repellency causingHortonian overland flow despite the high inherent infil-tration capacity of the surface soils.

3. The surface runoff coefficient was negatively corre-lated with the initial soil moisture condition. This isconsidered to result from severe water repellency underdry conditions, which was reduced as the soil becamewetter.

4. At the same time, successive storm events with shortinter-rainfall intervals reduced water repellency in thesoil, resulting in a gradual decrease in the surfacerunoff coefficient despite similar initial soil moistureconditions. Thus, the water repellency did not dependonly on soil moisture content but also on the historyof rainfall at the site.

5. Longer rainfall intervals, high air temperature, anddrier surface soils due to evapotranspiration, allowedstrengthening of repellency in soils. This was asso-ciated with a gradual increase in the surface runoffcoefficient.

6. Decreases in soil water repellency were also detectedduring an individual storm event. On the basis of mea-surements of the surface runoff coefficient, water con-tent, and soil water potential in surface soils, interac-tions between the soil water repellency and rainwater-infiltration/redistribution processes were clarified. Inthe beginning of a rainfall event, the soil water poten-tial rapidly increased with small increase in watercontent, resulting in low hydraulic conductivity in thesurface soil and surface runoff generation. As rainfallaccumulating, infiltration water gradually reduced soilwater repellency, resulting in increases in the soil watercontent. Thus, the surface runoff coefficient graduallydecreased with increases in the infiltration rate. Finally,the effect of the soil water repellency became neg-ligible. After the end of the event, water drained orredistributed following the laboratory measured reten-tion curve.

While some studies have modelled preferential infil-tration in water repellent soils (e.g. Nguyen et al., 1999;Kramers et al., 2005; Ritsema et al., 2005), field-scalerainwater runoff models do not adequately account for theeffects of soil water repellency. This study demonstratedthat the temporal change of water repellency affects thesurface runoff coefficient at a forested hillslope. Thisstudy also suggested possible mechanisms of infiltrationprocesses and surface runoff generation in water repellentsoil. This is potentially a key phenomenon for modelling

hydrological processes at a forested watershed with waterrepellent soils.

ACKNOWLEDGEMENTS

This work was supported by a CREST project of JapanScience and Technology Agency. We appreciate Prof.Roy C. Sidle, Kyoto University and two anonymousreviewers for their useful comments. We also thankthe members of the Laboratory of Erosion Control andLaboratory of Forest Hydrology, Graduate School ofAgriculture, Kyoto University for their assistance withfield work.

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surface runoff as affected by soil water repellency in a japanese cypress forest

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