WATER RESOURCES RESEARCH,
VOL. 35, NO. 1, PAGES 319-328, JANUARY
1999
Seasonal and spatial variations in infiltration rates in badland surfaces
under
Mediterranean
climatic
conditions
A. Cerdh 1 Departamentde Geografia,Universitatde Valencia,Valencia,Spain
Abstract. This paper investigates the spatialand temporalvariationsof infiltrationrates on badlandlandscapes under Mediterraneanclimaticconditionsof the easternIberian Peninsula.Soil infiltration was measuredunder simulatedrainfall and by cylinder infiltrometerfor typicalwinter, spring,summer,and autumnconditions.The spatial variabilityof infiltrationwithin the badlandlandscapes is determinedby the two main geomorphological units:slopesand pediments.Slopesshowgreatersteadystateinfiltration
ratesthanpediments (averaging 6.6and11.9mmh-•, respectively), buttheygenerate faster runoff and the runoff curvesare steeperthan on the slopes.For short thunderstorms, pedimentsgeneratemore runoff becausesoilson the slopeshave a wider and deepercracknetwork,whichfavorshigherinfiltrationratesat the beginningof the rainfall. However, for simulatedstormsof 40 min duration, the runoff rate on the slopes
(74.3%) is slightlygreaterthanon the pediments(72.5%) due to their lowersteadystate infiltrationrate. For both slopesand pediments,greatersteadystateinfiltrationrateswere
measured undersimulated rainfall(fc) for summer (18.9and11.6mmh- •) thanfor winter(11.8and6.8mmh-•), spring (3.8and8.7mmh-•), andautumn (4.1and8.3mm h-•) for slopeandpediment positions, respectively. Measurements bymeansof cylinder infiltrometer(if c) confirmtheseseasonaltrends.On the pedimentplots,ifc variesfrom
43 mmh-• in winterto 26 mmh-• in spring, risesto 62 mmh-• in summer, andfinally dropsto 26 mmh-• in autumn. Fortheslopepositions, ifc hasa similarseasonal trend: 18mmh-• in winter,9 mmh-• in spring, 52 mmh-• in summer, and10mmh-• in autumn.Measurementsby meansof cylinderinfiltrometerresultsin 3 timesgreatersteady state infiltrationrates due to the effect of water depth pressureand crustdevelopment under simulatedrainfall. In summerthe developmentof wide and deep cracksresultsin macroporeflow, and theseare importantfactorsin pipe initiation. 1.
Againstthisbackgroundthe main objectiveof thiswork wasto investigatespatialand temporalvariationsof infiltrationrates
Introduction
Badlandsare widely developedunder semiaridconditions, and their landformprocesses have been extensivelystudied [e.g.,Campbell,1974;Bryanand Yair, 1982].In the southeastof the Iberian Peninsula,where they affectlarge areasof unconsolidatedmaterial,the development of badlandareashasalso been well documentedlimesonand Verstraten,1988; Harvey and Calvo,1991;Cerdaand Garcia-Fayos,1996].Nevertheless, little is known about the seasonalvariability of the rates of infiltration and runoff in badlandslandforms[Schummand Lusby,1963],where Hortonian overlandflow occurs[Horton, 19331. In spite of the importanceof the infiltration processon badlandslopeslimeson,1983;Bryanet al., 1984],the dearthof
information abouttheseasonal changes of infiltration ratesis
in badlands under Mediterranean
climatic conditions in order
to explaintheir effectson other geomorphological processes and to showhow seasonalchangesinfluencethe annual evolution of badland morphology.Measurementsof infiltration were madein the field usingsimulatedrainfall experimentson permanentmicroplotsand cylinderinfiltrometermeasurementswhich were installedevery seasoncloseto the microplots.Both measurements were donein order to comparethe two techniquesand to identify the differencesbetween the
infiltration process undersimulated rainfallat 55mmh-• and underpondedconditions,whichcanbe consideredthe potential infiltration
2.
Materials
rate.
and Methods
probablydue to the complexbehaviorof the infiltrationpro-
A typicalbadlandareawasselectedin southeastSpain,near cessand the detailedmeasurements that are required.In Medthe village of Petrer in the provinceof Alicante (30ø30'N; iterraneanecosystems the few experimentalfield studiesof the 0ø45'W)(Figure1). The sitesfor the rainfallsimulationexpervariabilityof infiltration rates focusedon spatialvariability imentsand soil samplingwere selectedfrom the most repre[Scoging,1982;Imesonand Verstraten,1988; Cerda,1995]. sentativesurfacemorphologies on the slopes.In total, 17 experimentalmicroplotswere selected,12 on slopesand five on •Also at Centrode Investigaciones sobreDesertificaci6n-CIDE, pediments.Becausethe pedimentsare more homogeneous Universitatde Valencia, Generalitat Valenciana,Valencia, Spain. surfacesand are more limited in spatialextent,only five plots Copyright1999by the American GeophysicalUnion. were selected.The Petrer badlandsdevelopedon Cretaceous
(Senonian)marls as a resultof Pleistocenevalley dissection and recentland usechanges.On the badlandslopes,the dom-
Paper number98WR01659. 0043-1397/99/98WR-01659509.00 319
320
CERD3,,: VARIATIONS
x
IN INFILTRATION
summer,and autumn(1990). Distilledwaterwasusedin order to avoid effectsof changingchemicalcompositionof the rain on the soil hydrologicalresponse[Agassiet al., 1994]. The showersperformed by this rainfall simulator are characterizedby a drop size (D so) of 2.5 mm, which is found in very intense thunderstormsin the western Mediterranean ba-
Bene•xarn, a e •_ ....
[Caude•te , •,•r
sin[Cerda,1997a].The measured dropvelocitywas3.4m s-•, andthe meankineticenergywas7.1 J m-• mm-•, whichare
-,,
I •x•Vfllena• Itl I ß ', e Biar Saltworks "-,• - -,. % _/ Study
I!1•!
Endorhe•c
•L I.... •L• Sax .,, area e•'•-'tl//r
area
Wetlands
•'Alluvial fan
•'•" Elda Monbver
• Novelda ß
-
RATES IN BADLAND SURFACES
F EIx
Portugal
Santa Pola
Mediterranean sea
Figure 1. Locationof the studyarea near the Vinalop6 River arid Petrer.
inant processesare rilling, swelling,and crackingand the developmentof shallowbridge piping. The infiltration rates were measuredby means of rainfall simulationexperimentsand ponding(cylinderinfiltrometer) experiments.The sitesfor rainfall simulationexperimentsand samplingwere selectedat representativesoil surfaces.Seventeen permanent experimentalplots were constructedfor the simulatedrainfall experimentsusingaluminumsheetsfor the borders(0.8 mm thick) and zinc sheetsfor the collector(0.35 mm thick) (Figure 2). The cylinderinfiltrometerconsists of a 15 cm high cylinderwith a diameter of 7 cm. This smallapparatuswas selecteddue to the high slopeanglesfound on badlands. The cylinder was installed every experimentalseason, and infiltration measurementswere done simultaneouslywith the simulatedrainfall experimentat time stepsof 1 min, 2 min, and every 5 min up to 60 min. Steadystate infiltration rates were alwaysreached.The simulatedrainfall plotswere set up in the field during September-October1989, and 136 measurementswere doneduringthe year 1990(4 seasonsx 17 plotsx 2 methods). The focusof this studyis on processesat the interrill scale.
lower than the natural values due to the low height of the nozzle (2 m). Althoughthe kinetic energyof the simulated thunderstorms is lowerthan the naturalones,the experimental designis similar to other rainfall simulatorsdevelopedsince the 1960s [Meyer, 1994]. The simulatedrainfall applications have a return period of 4-5 yearsin the studiedregion [Elias and Ruiz, 1979]. The time to ponding(tp) wastakenfrom the startof rainfall to when40% of the surfacewasponded[Imeson,1983].The tp value gives information about the time and the amount of rainfall necessaryto saturateat the top soil layer, which is a cruston the badlandsurfaces.Surfacerunoff starts(tsr) when ponding contributesto runoff, and the time to plot runoff (tro) startswhen runoff is measuredin the collector(runoff outlet). The time to crackclosure(tcc) is reachedwhen the cracksare completelyclosed;this is very importantin termsof understandingthe surfacerunoff behaviorand the infiltration process.The runoff was measuredevery30 or 60 s in order to draw the infiltration curveby subtractingthe runoff rate from the rainfall intensity.The absenceof vegetationinterception permitshighly accuratemeasurementsof both runoff and infiltration
under
simulated
rainfall.
Field
infiltration
values
were fitted to the Horton infiltration equation[Horton,1940] both for the cylinderinfiltrometer and for the simulatedrainfall measurements[Cerda,1995]:
f = fc + (fo - fc)e -"t
F = fct - (fo - fc)/a(e -"t-•)
where
f
instantaneousinfiltration rate;
F
cumulative infiltration rate;
fc fo a
steadystate infiltration rate; initial infiltrationrate (t = 0); empiricalconstant(infiltrationcurveform).
Among the above parameters,the most important is fc, which is equivalentto the saturatedhydraulicconductivityof the soilsurface[Dunin, 1976].Another hydrologicalparameter is the runoff coefficient(Rc), whichis the percentageof rain transformedinto runoff. Substractingtotal runoff from rainfall givesthe amount of infiltration during the 40 min of rainfall. The
infiltrated
volume
of rainfall
has been measured
for
15
min rain duration(/v15), which is related to more frequent thunderstormsand can explainthe geomorphologicaldynamics of badlandsunder high-frequency(every year), mediummagnitude(13.75 mm/15 min) events.The parameterRc is Forthispurpose a special circular microplot of 0.24m2 (55cm more related to medium-frequency (4-5 years), highdiameter)was installedto measurerunoff and infiltrationun- magnitude(36.7 mm/40 min) events.The parameterfc is reder simulated rainfallperformedon 1 m2. Figures2a and2b lated to the steady state conditions, which reflect lowshowa slopeplot and Figures2c and 2d showa pedimentplot. frequency,very high magnitudeevents. Rainfall simulationexperimentswere performedby meansof a Two samplesto determine the soil moisture content were portable rainfall simulator, which has been describedelse- taken before the experimentsat the surface(0-2 cm) and at where [Cerda, 1996; Cerda et al., 1997]. The showershad a 4-6 cm depth.The soilwater contentwasmeasuredgravimetdurationof 40minat a design intensity of 55mmh-1, andthey rically.The organicmatter content(Walkley-Blackmethod), were done at eachof the 17 permanentplotsin winter, spring, grainsizedistribution(U.S. Departmentof Agricultureclassi-
CERD•: VARIATIONS IN INFILTRATION RATES IN BADLAND SURFACES
321
Figure 2. View of a typicalbadlandslopeand pedimentsurfacebefore and after the simulatedrainfall. (a) Slopesurfacebefore the simulatedrainfall. (b) Slopesurfaceafter 20 min of simulatedrainfall when open crackscanstillbe found.(c) Pedimentsurfacebeforethe simulatedrainfall.(d) Pedimentsurfaceafter 20 min of simulatedrainfall when cracksalreadywere completelyclosed.
fication),and calciumcarbonatecontent(Bernardcalcimetry) were alsodeterminedat eachof the soilplots.Bulk densitywas measuredby the ring method for the 0-6 cm top layer. For all plotsthe sampleswere taken duringthe summerof 1990.It is expectedthat changesin organic matter, calcium carbonate, and texturethroughoutthe year shouldbe limited due to weak soil developmentand the absenceof vegetation.However,the seasonalvariabilityof soilbulk densityshouldbe very high due to the presenceof the swellingclays.
Samplesfrom only the upper 6 cm were taken becauseunder semiaridconditionsthe first soil layer determinesthe infiltration processand overlandflow generation[Dunneet al., 1991] and becausesoil depth on badland slopesis very shallow.In fact, the 4-6 cm depth sampleis related to recentlyweathered parent matedhal,and the 0-2 cm surfacelayer is the crusted surfaceregolith. Profile and surfacedescriptionswere done during 4 years (1989-1993) in order to identify the seasonal changesof the cracksand the surfacecrust.
322
3.
CERD3,: VARIATIONS
IN INFILTRATION
RATES IN BADLAND SURFACES
Climatic Conditions During the Experiment
Period
An accuratestudy of the climatologicalconditionsduring the experimentperiod and a comparisonto the averagevalues for 23 previousyearsare necessary to understandthe behavior of the infiltration processunder Mediterranean climatic conditions because of the climate influence on soil conditions,
August, 1989- December, 1990 Average, 1968-1990
150
Field measurements
Rainfall
(mm)
A
100
spnng
mainly on crack formation. The mean annual precipitationat
thenearestmetereological stationis316mmyr-•. Thewettest month is October (43 mm), and the driestis July (9 mm). The seasonaldistributionof th• rainfall is characterizedby a very dry summer,low rainfall in winter, wet spring,and very we• autumn.The meannumberof rainydaysis 35 per year, and the mean annual temperature is 16øC, ranging from the mean monthlymaximumtemperatureof 26øCin Augustto the mean monthlyminimumtemperatureof 11øCin January(Figure 3). The experimentyear of 1990wasquite dry (254.8mm), but 37% of the recordedyearsreceivedlower rainfall volumesthan 1990 (Figure 3a). Thus,while 1990wasnot an averageyear, it is representativeof the region'ssemiaridenvironment.In addition to the large spatialand temporalvariabilityof the rainfall, another characteristic of the climatic conditions at the
studysite is the high intensitiesof the thunderstorms.During September1989,178.2mm fell, whichis more than 5 timesthe monthlyaverage.On September4, 102 mm fell, and on September 7 a further 68 mm fell (Figures3b and 3c). In total, Septemberrecorded54% of the meanannualprecipitationfor the period 1968-1990, and more than someof the driestyears. Although hourly data are not availablefrom the nearestmeteorologicalstation, data from other nearby metereological stationsindicatethat probably90% of the daily rainfall fell in 1 or 2 hours and reachedvery high intensities. The mean, maximum, and minimum daily temperature showsnormalpatternsunder semiaridconditionsfor Mediterranean marine locations,a warm winter with negligiblefrost events,hot weather in summer(up to 30øCfor the maximum daily temperature),and two transitionseasons:autumn and spring(Figure 3b). The seasonalchangesof temperatureand precipitationresult in greaterpotential evaporationrates for the summerseason(331 mm) thanin spring(272 mm), autumn (260 mm) and winter (213 mm) (Figure 3d).
autumn
5O
A SO N DIJ F MAM JJI AS O N D 1989
In the badland areas, soils are bare, crusted, and cracked
(Figures2a and 2b). The pediment crustshave fewer cracks than the slopes(Figures 2c and 2d), and the depth of the cracking(2-4 mm) is much less than on the slopes(20-60 mm), asis the crackdensity(Figure4). The meansoildepthon the badlandslopesisverythin (2.8 cm (1-5 cm)) and it reaches 12.4cm (7-18 cm) on the pediments.Cracksaffectonlythe top surfacecrustin the pediment,whereasthey are connectedto crackswithin the parent material on the slopes.The regolith thicknesson the slopeschangesfrom seasonto seasondue to the weatheringof the parent material and from erosionof the top layer by splashand surfacewash. Changesin soil moisturecontentresult in large soil surface morphologychangesas is seenin Figures2a and 2b for slope badlandsurfacesbefore and after the simulatedrainfall experiments.As the soil becomesdrier, crack formation produces wider crackswhich can rapidly amalgamatewith cracksin the
Months
•
1990
i
100
a Maximum
spnng
summer
4 50
1
winter •
• autumn
pp•[]I•J , I•,• ail, B 40 cM,n,mum Il•,l,II }[tI•v[',f Ta
60•{l•)l,,rll Dally temperature •eC) '•l•[JJ]•'•'•i '•_•?•;•j I 30 0,AS
,,,..
,,,,,,,,., JJIA SON
•
•
IOND I ! •J IFMAM
1989
Months
D
,0
1990
I
(ram) 0
,
ttJ
, i,
•0
8
• .......
i
1
,
Daily
i 1077mm
--O•
Tendayaverage
I
--• •
Season average
Month average sum_merll /
D
!•wlnter,i •sprlng ' iiTi tautur• n
Evp 6
I II
(mm)
4. Soil, Surface Morphology, and Moisture Content Changes
---
4
, I
I
!
Ill
I
II
2
0
j
D
Months, 1990
Figure 3. (a) Monthly precipitation(millimeters)from August 1989 to December 1990 at the Monbver metereological station (38ø28'N;0ø53'W) in comparisonto the averagefor 1968-1990. (b) Daily precipitation(Pp, millimeters)and temperature (T a, øC) (curve a, maximum;curveb, medium;and curvec, minimum)from August1989to December1990at the Monbver metereologicalstation(38ø28'N;0ø53'W). (c) Daily precipitation (Pp, millimeters) from January to December 1990 at the metereologicalstation of Ciutat Jard• (Alicante city, 38ø21'N;0ø30'W). (d) Potential evaporation(EVP, millimeters)for the sametime period and locationas in Figure 3c.
CERDA: VARIATIONS
Slope
10
cm 20
Profiles
IN INFILTRATION
RATES IN BADLAND
SURFACES
323
parent material. A 4-year detailed studyof the soils (19891993)showedthat the widestcracksoccurredin summer,when the most developedcracknetworkformed. The amount of organic matter is very low in the badland surfaces,both on the pediments(0.24% and 0.18% at 0-2 and 4-6 cm depth,respectively)and on the slopes(0.30% at 0-2 cm and 0.22% at 4-6 cm depth) (Table 1). The bulk densityat
Pediment
'.'• .., '. ß :..', .. --'--'•-•'• "-:-: ... surface morphology
0
10
the top layer(0-6 cm depth)is veryhigh:1.47g cm-3 and 1.36g cm-3 for slopesandpedimentsurfaces, respectively.
2O
Cm
Calcium carbonatecontentis extremelyhigh: 69% and 66% for slopesand 64% and 62% for pedimentsat 0-2 and 4-6 cm 0-1 mm -5-10 mm • depth, respectively(Table 1). The grain sizesof the badland materialsshowtwo distinctdistributions(Figure 4). The texture is loam/siltyloam for the pediment and silty clay/silty 10• OPediment clay-loamfor the slopes.The slopesare an erosivesurfacewith a texture similar to the many parent material. The pediments, 9yx-X "$'øPe however,are sedimentationor transportsurfaces,with a higher sand content and lower clay content due to the preferential sedimentationof the coarseparticles.The range of slopeanglesclearlydifferentiatesthe two units. On the pedimentsthe mean slopeangleis very low (1ø-3ø);on the badlandslopesit rangesfrom 10ø to 40ø (31ø average).Vegetationon all sampled surfaceswasnegligible,and rock fragmentcoverwaslow, rangingform 1 to 25% (8% on average)on the slopesandfrom 0 to 2% on the pediment. Figure 5 showsthe soil moisturecontent at individualplots and their averagevalues.Soil moisturevaries spatiallyfrom • 00 90 80 70 60 50 40 30 20 •0 0 pedimentto slopesand from the top surfacelayer (0-2 cm) to ( Sand (%) 4-6 cm depth. Seasonally,there are clear differences,espeFigure 4. Soil profile and crackne•ork on badlandslopes cially during a very dry summerwhen plot variabilityis very and pediments,and soil grain sizefor pedimentsand slopes. low. During the summer,moisturedifferencesbetween pediData collectedduring summer1990 at each of the 17 experi- mentsand slopeswere negligible,but for the wet seasonsthe mental studyplots. average soil moisture content was higher on the pediments 30
Table 1. SlopeAngle, Soil Depth, Rock Fragment,OrganicMatter, and Calcium Carbonate on the Slope and Pediment Plots Organic Matter, % Slope Angle,
Soil Depth,
ø
cm
35 37 30 34 30 30 10 10 25 40 40 27
3 1 3 2 2 3 5 5 2 2 3 5
30.25
2.92
10.05
1.35
Rock Fragment,
Calcium Carbonate, %
0-2
4-6
0-2
4-6
%
cm
cm
cm
cm
4 18 10 5 25 1 10 10 2 2 5 1 7.17 7.41
0.26 0.20 0.23 0.38 0.42 0.23 0.31 0.31 0.21 0.10 0.11 0.21 0.24 0.10
0.16 0.22 0.24 0.26 0.23 0.15 0.10 0.10 0.15 0.12 0.15 0.20 0.17 0.06
76.30 72.00 73.29 68.40 66.00 68.32 63.26 63.26 70.23 71.34 69.80 61.30 69.62 4.49
70.60 71.34 74.21 67.50 65.30 66.95 59.36 59.36 66.32 68.39 65.31 55.30 65.85 5.47
2 0 1 1 2 1.20 0.84
0.21 0.35 0.26 0.38 0.30 0.30 0.07
0.18 0.24 0.21 0.25 0.24 0.22 0.03
64.22 64.12 60.31 66.88 64.39 63.98 2.35
62.15 63.18 60.98 63.25 62.17 82.35 0.93
Slope 1 2 3 4 5 6 7 8 9 10 11 12
Average Standard Pediment 1 2 3 4 5
deviation
Average Standard
deviation
2 1 2 3 2
10 12 15 18 7
2.00
12.40
0.71
4.28
324
CERD•: VARIATIONS IN INFILTRATION RATES IN BADLAND SURFACES
0.2
Soil moisture
'
1'•1•
lit
_
••
1• •
delayedresponse,and in summerrunoff startsmuchlater (4 min 11 s) after a delayedponding(2 min 49 s) (Table 2). In
[]Pediment (0-2 cm) ß Pediment (4-6 cm)
summerit is necessaryfor at least4.45 mm of rainfall at 55 mm
x.x O Slope (0-2cm)/
h-• rainfallintensity to generate plotrunoff;meanwhile, dur-
_ •.•'••'•ø•iem (m4•y,•
ing the other seasons, 2.5 mm of rainfallis enoughto generate runoff.The tcc valueswere slightlylongerthantro (an average of 1 m 2 s for winter, spring,and autumnwas recorded)but 0.05 muchlater in summer(7 min 21 s). The crackspersistedduring winter 0 the experimenton the slopes,whichfavoredinfiltration(see 0 40 80 120 160 200 240 280 320 Figure 2b). Normally, the surfacerunoff infiltrated at the beDays (year 1990) ginning of the rainfall into the crack networks.Later, cracks were closedby the flow generatedin the areasbetweencracks. Figure 5. Seasonalchangesof the soil moisture content for pedimentsand slopesat 0-2 cm and 4-6 cm depth. Symbols The cracksbecamesaturated(bywater),closed(by swellingof indicate averagevaluesand error bars indicate standarddevi- clays),or plugged(by sediment).This resultedin sheetoverations. landflowinto the collector.Also, incipientrilling (concentrated flow) was found at the interrill measurementsite (55 cm diameter),whichwas mainlydevelopedalongcracks.During particularly,therewere open than on the slopes:2.02 timesin winter, 1.53in spring,and 1.34 the summerseasonexperiments in autumnfor the 0-2 cm surfacelayer.For the 4-6 cm depth cracksor smallmacroporeswhere cracksexistedwhichfavored layer the soil moisturecontentwas also higher on the pedi- macroporeflow. This allows larger infiltration rates during mentsthan on the slopes:2.24 timesin winter, 1.42 in spring, summerthan during the other seasons.The surfacecrackson and 1.21 in autumn. badlandslopesare very differentfrom thoseon pedimentsin both morphologyand functionbecausethoseon the slopes allow deep and concentratedsubsurfaceflow, which favors 5. Seasonal Hydrological Response pipe and rill pipe development.
{g.g-1)
F spring •••./'/
to Simulated
autumn
Rainfall
The badland surfaceundergoesquick morphologicaland hydrologicalchangesinduced by raindrop impacts and the runoff surfacewash,with a seasonaltrend in the soil hydrological responseto the rainfall due to the previoussoil water contentand surfacemorphology(Table 2). On the pediments, pondingand surfacerunoff were rapid during springand autumn, slightly delayed in winter, and much later in summer (Table 2). This seasonaltrend is repeated for the average valuesof tro: 1 min 12 s for springand autumn,1 rain 34 s, for winter and finally 3 min 1 s for summer.This means that in
6.
SeasonalVariability of Infiltration and Runoff
Badlandsare well knownfor their highrunoff,whichresults in someof the greatestrecordederosionratesin the world and in their fascinatinglandforms[Howard,1994]. This is due to the low infiltrationratesof badlandsurfaces[Campbell,1987], evenfor the firstminutesof rainfallwhensoilsorptivityis still very large, and it resultsin fast pondingand runoff contribution to the overlandflow.Althoughinfiltrationratesare always very low, a seasonaltrend of the infiltration and runoff rates summer at least 2.93 mm of rainfall at 55 mm h -• rainfall throughoutthe year (1990) is shownfor the pedimentsand intensityis neededto generaterunoff; meanwhile,in the other slopepositions(Table 2). seasons,1.5 mm of rainfall was enoughto contributesurface For the first 15 min of rainfall (/v15), infiltrationratesare runoff. The tcc occurredslightlylater than tro for winter, greaterduringthe summerthan in winter, autumn,and spring spring, and autumn seasons(1 min 4 s) but much later in (Figure 6). On average,the slopeplotsshowlargervaluesof summer (6 min 32 s). Cracks were ponded from the first lv15 thanfc for everyseason.Seasonally, lv15 evolvesfrom minute, and they did not contributeto deep infiltration due to 4.4 and 5.6 mm in winter to 2.7 and 3.4 mm in spring;it later the shallowdepth of the cracks. risesto 7.4 and9.5 mm in summerandfinallydropsto 3.3 and On the slopes,pondingand surfacerunoff also appeared 4.1 mm in autumn,for slopeand pediment,respectively.On very quicklyin springand autumn. Winter showeda slightly average,the slopesinfiltrated 1 mm (27%) of rainfall more
Table2. SoilResponse totheSimulated Rainfall fortheSlopes andPediment Throughout theYear Average
Standard Deviation
tp
tsr
tcc
tro
tp
tsr
Winter
1 min 44 s
2 min 18 s
4 min 47 s
Spring
1 min 18 s
1 min 51 s
Summer Autumn
2 min 49 s 1 min 18 s
Winter
tcc
tro
3 min 11 s
0 min 58 s
0 min 57 s
1 min 14 s
1 min 21 s
3 min 25 s
2 min 32 s
1 min 11 s
1 min 10 s
2 min 17 s
1 min 15 s
4 min 11 s 1 min 46 s
12 min 12 s 3 min 13 s
4 min 51 s 2 min 34 s
1 min 16 s 0 min 58 s
1 min 3 s 1 min 1 s
4 min 28 s 1 min 57 s
1 min 25 s 1 min 16 s
0 min 49 s
1 min 14 s
3 min 5 s
1 min 34 s
0 min 6 s
0 min 10 s
0 min 49 s
0 min 12 s
Spring
0 min 34 s
0 min 55 s
2 min 13 s
1 min 12 s
0 min 6 s
0 min 27 s
0 min 44 s
0 min 7 s
Summer Autumn
1 min 41 s 0 min 39 s
21 min 22 s 0 min 56 s
9 min 33 s 1 min 54 s
3 min 1 s 1 min 12 s
0 min 31 s 0 min 10 s
0 min 11 s 0 min 29 s
1 min 43 s 0 min 35 s
0 min 35 s 0 min 11 s
Slope
Pediment
Here, tp denotestime to ponding;trs denotestime to surfacerunoff; tcc denotestime to crackclosed;and tro denotestime to runoff outlet.
CERD•: VARIATIONS IN INFILTRATION RATES IN BADLAND SURFACES
325
15Pediment ISlopesummer summer
Iv
wß
(mm) 5 0
spring
autumn /,
spri,ng ,
aut, umn
spring
spring
autumn
autumn
80nter•• •
Rc
60
wi
(%)
summer
winter
40 Pediment Pediment
S,I ISlope
summer/
fC20 Wl
summer
I winter•umn
(mmh-1) 10
spring
autumn
Pediment
summer
80 ifc
SIo• te•'ned by m w•
(mm h-1) 40
20
0
40
spring 120
200
autumn 280
Days (year 1990)
40
120
200
280
Days (year 1990)
Figure 6. Seasonalchangesin hydrologicalparameterson badlandlandscapes coveringpedimentsand slopes(seeTable3 for meanvalues).Here lv15 denotesinfiltratedvolumeof rainfallafter 15min of rain;Rc denotes runoffcoefficient (percentof rainfall);fc denotes steadystateinfiltrationratemeasured bymeansof
simulated rainfallintensity of55mmh-•; andifc denotes steady stateinfiltration ratemeasured bymeans of thecylinderinfiltrometer. Fiveplotsmeasured duringsummerhadmeasured ifc valuesgreaterthan100mm
dueto themacopore flow(plot1,106mmh-•; plot7, 185mmh-•; plot9, 115mmh-•; plot11,540mmh-J; andplot 12,690mmh-•).
than the pedimentsduring the 0-15 m interval due to the strongeffect of crackson the slopes.The differencesbetween
atedmorerunoff(Rc) thanthe slopesin winter(2%), autumn (3%), spring(7%), and summer(5%).
slopesandpedimentsfor lv15 are largerduringsummer(2.1 Pedimentshad larger infiltration rates for the first 15 min, mm) due to the greatersizeof the crackson the slopes. but after40minof rainfall,infiltrationislargerin thepediment However, for the whole duration of the experiment,the (27.5%) thanon the slope(25.75%).Thisis due to the differaveragerunoffcoefficient(Rc) wasslightlylargerfor the slope encesin the steadystateinfiltrationrates(fc). In yearlymean
surfaces (74.25%)thanfor thepediments (72.5%).The larger . values, fc is 11.9mm h-1 for pediments and6.6 mm h-1 for runoff rateswere foundduringthe spring(79 and 80%) and the autumnseason(86 and 83% for slopeandpedimentposition, respectively).Winter showedslightlylower runoff rates (70 and 72%), while during summersurfacerunoff is much lower than during the other three seasons(61 and 56% for slopeand pedimentposition,respectively). Pedimentsgener-
slopes,but seasonally infiltrationratesmovefrom very low in
spring (3.8and8.7mmh-l) andautumn (4 and8.3mmh-1) to slightlyhigherin winter(11.8and6.8 mm h-1) andmuch higherin summer(18.9and 11.6mm h-1 for pedimentand slopesurfaces, respectively). In averagevalues,pedimentshave
valuesoffc of 4.3mmh-1 (autumn), 4.9mmh-1 (spring), 5
326
CERD•: VARIATIONS IN INFILTRATION
Table 3.
Main Soil HydrologicalParameters Iv15,
Rc,
fc,
ifc,
mm
%
mm h- 1
mm h- 1
Winter
5.6
71.8
6.8
Spring
3.4
86.5
3.8
Summer
9.5
56.2
11.6
51.6'
Autumn
4.1
82.6
4.1
10.2
Average
5.7
74.3
6.6
22.3
Winter
4.4
Pediment 70.2
Spring
2.7
79.4
Summer Autumn
7.4 3.3
61.1 79.6
Slope 18.1
9.2
11.8
meansof simulatedrainfall (fc) is confirmedby the measurements of the steadystate infiltration rate using the cylinder infiltrometer(if c), althoughslopesurfacetypeshave a very complexbehaviorunder very dry conditionsdue to the deep and wide cracks.Pedimentplotshave a mean annualifc of 39 mm h-•, but variedfrom 42 mm h- • in winter to 26 mm h-•
in spring; theyroseto 62 mmh- • in summer andfinallyfell to 26 mmh-• in autumn.For the slopeposition, ifc alsohasa similar seasonaltrend: 18 mm h-• in winter, 9 mm h-• in
spring,and18mmh- z in autumn.The average valuesfor the 12slopetestsin summerwas166mmh-•, whichis dueto the
42.5
8.7
RATES IN BADLAND SURFACES
26.4
influenceof cracksand macroporeflow during the measurements.Five measurementswere clearly affectedby the crack inflow (see Table 3 and Figure 8). If thesefive testsare not Average 4.4 72.5 11.9 39.1 taken into account,the mean ifc value for the slopesunder Here, Iv15 denotes infiltrated volume of rainfall after 15 min of summerconditionsis 52 mm h-•, which is lessthan for the rain; Rc denotesrunoff coefficient(percent)of runoff from the rainfall; fc denotessteadystate infiltration rate measuredby means of pediment positionsduring similar seasonalconditions. Figure 8 illustratesthe instrumentinstallationfor cylinder simulated rainfallintensity of 55 mmh-1; andifc denotes steadystate infiltration rate measuredby meansof cylinderinfiltrometer.Average infiltrometermeasurements, which explainthe very high spavaluesare for 12 plots on slopesand five plots on pediments. tial variability of the infiltration rates at interrill scaleon bad*Five valuesaffectedbymacroporeflow (cracks)were not takeninto land slopesduringsummerdeterminedby the cracks.Placing account. the cylinderon the cracks(positionb in Figure 8) had to be avoidedbecausesteadystate infiltration rateswere controlled mm h-• (winter),and7.2 mm h-• (summer)largerthanthe by a preferentialflow throughthe regolith cracksand into the parent material cracks.As a result, the infiltration rates rise slopeplots (Table 3). The differencesin the hydrologicalcharacteristicsbetween with time, due to the growth of the pipes, which assistin slopeand pedimentsurfacetypesare summarizedin Figure 7. developingbadlandpiping.Althoughthe cylinderinfiltrometer Runoff curvesunder simulated rainfall show that pediment wasalwayslocatedbetweencracks(seepositiona in Figure8), surfacesgeneratedfasterrunoff and steeperrunoff curves,but the existenceof largevoidsor subsurfacecrackson the badland lower steadystate infiltration rates. This explainshow pedi- slopes,which may not be visibleon the soil surface,favorsthe ments can work as erosive, transport, or sedimentationsur- connectionof the surface pending to the parent material facesdependingon the durationof the thunderstorms [Hedges, cracksduringthe test due to the pendingpressureas shownat 1982].The differencesbetweenpedimentsand slopespartially positiona'. This resultsin valuesof ifc greater than 100 mm explain the very high spatialvariability of the infiltration and h- • in crusted soils and can sometimes result in values of more runoff rates within the badland landscapes[Scoging,1982]. than 1000mm h-• for ifc [Cer&i,1993].The fivetestswith Moreover, the large seasonalvariabilityof the infiltration rates macroporeflow to the parent material reachedvery large inrates:plot1, 106mmh-•; plot7, 185mmh-•; plot9, resultsin changeson the runoff and erosionrate throughthe filtration 115mmh-•; plot 11,540mmh-•, andplot 12,690mmh-• year on badlandsenvironmentsand under different Mediterranean environments[Cerdd,1996, 1997b].Experimentsper- (Figure 6) and alwaysoccurredduringthe summerseason. formedwith mobileplots,which allow accessto the soilprofile after the simulatedrainfall, showvery shallowwetting fronts due to the low infiltration in badlands[Cerdd,1995]. The seasonalevolutionof the infiltration rates measuredby 18.9 8.3
61.6 25.9
(0-2 cm)
/
v I '• ' .1' '1.. $...!
60
}--
Precip•tabon
50
0
•
t --I
/ •
t-- Preferential flow I:_':'?•.:"-' ß
• b I I '/
',
/ a' / toparent•. I - / material cracks I: L' ß•'' , ß., ,, '' ß
'- ß ' ,'
40
Discharge
Runoff
(mmh'• ) 20
20
Cm 0 10
10
20
30
40
50
60
70
•, Slope surfacetype [] Pediment surfacetype
o
o
6
12
18
24
30
36
42
Time (minutes)
Figure 7. Comparison between the typical runoff curves measuredon the slope and pediment soil surfacetype. Pedimentsgeneraterunoff earlier than the slopes,and the runoff curvesare steeper. However, pedimentshave greater steady state infiltration rates than slopes.
Regolith ':.•
Parent material •
Internal cracks
Figure 8. The influenceof cylinder infi]trometer location on the infiltration measurements.Positions a, b, and a' are the
three different position of the cylinder.Positionb (on the crack)wasavoided.Positiona wasalwaysselected,but during summerwhen internal cracks(largevoids)are ve• abundant, preferentialflowwasfoundbe•een cracksafter someminutes due to the flow of the pondedwater to the internal cracksand later to cracksin the parent material, as shownat positiona'.
CERD•: VARIATIONSIN INFILTRATIONRATESIN BADLANDSURFACES winter, spring and autumn
tSlope
[]
winter,springand autumn ß
Pediment
summer
summer
(• Affected bymacropore flow 600
ifc 400
scrubland
on limestones
the mean annual val-
ues of infiltration are also much greater than under badland conditions[Cerdg,1997b].
2OO
8.
0
8O
plainedby Hills [1970].Also, the crustingprocessis not simulated when usingthe cylinderinfiltrometer method,which resultsin larger steadystateinfiltrationrates.Other publications have shownvaluesof ifc 8 times greater thanfc on soilswith greater infiltration rates [Cerdg, 1996]. For example, in the Anna researchstudyarea on vegetatedKeuper clayssoils,fc was 4.8 times (11.5 times for ifc) greater than in the Petrer study area under identical experimental conditions. Under Mediterranean
(mm h'" )
327
Seasonal variations
y = -0.24+ 3.17(X) Without plots affected by macropore flow r 2= 0.73
60
ifc 40
(mm h ) 2O
0
4
8
12
16
fc (mmh )
2o
Conclusions
24
in infiltration
rates on badlands
of the
southeasternIberian Peninsulaare determinedby the variation in soil conditionsfor swelling-claysoilsunder the seasonally contrastedcharacteristicsof the Mediterranean climate. Spatially,infiltration and runoff are determinedby the soil surfacemorphology,which is different on the two main geomorphologicalunits of the badlands:slopesand pediments. Runoff volumesare very large due to the quick runoff generation, the steep runoff curves,and the low steadystate infiltration rates. The researchresultsfrom this study show that pediments have greater steady state infiltration rates than slopes,but they generatefasterrunoff, and the runoff, and the runoff curvesare even steeperthan on the slopes.This means that for short thunderstormspedimentsgeneratemore runoff becausesoilson the slopeshave wide and deep crackswhich favor deep infiltration at the beginningof the rainfall. How-
Figure 9. Relationshipbetweenifc (steadystateinfiltration rate measuredby means of pondingwith cylinder infiltrometer) andfc (steadystateinfiltrationrate measuredby means ever,for stormslongerthan40 min durationat 55 mm h-•, as of simulated rainfallat 55 mm h-•). On the lowergraphthe were simulatedhere, the runoff rates on the slopesare larger measurementsclearly affected by macropore flow were re- than on pedimentsbecauseof their lower steadystate infiltramoved.
tion rates.
For both pedimentsand slopes,seasonalchangesresult in greater infiltration rates in summer (the driest and hottest Cylin.derinfiltrometermeasurements, apart from indicating season),followedby the dry and coldwinter. The lowestinfilthat in summer cracksare deep enough to connectwith the tration rateswere reachedduringthe wet andwarm springand parent material, are usefulto comparedifferent typesof soils autumn. These trends were found under both simulated rainand the some soil through the year. However, the absolute fall and ponding(cylinderinfiltrometer)methods. values are not significantfor quantitativelyexaminingrunoff Infiltration ratesmeasuredby cylinderinfiltrometerswere 3 generation. timeshigherthan thosemeasuredby the rainfall simulatordue to the effect of pondingpressureand becausethe raindrop impact favorscrustingand thus reducesinfiltration. Neverthe7. Relationship Between Simulated Rainfall (fc) less,both methods are suitablefor studyingthe infiltration and Ponding (ifc) Measurements process.It is necessaryto avoid installing the cylinder over A positiverelationshipexistsbetweenthe steadystateinfil- cracks, because even during the driest periods subsurface trationratesmeasuredby meansof simulatedrainfall (fc) and cracksand large voidscan result in preferentialflowsthrough ponding(if c), althoughto understandthis relationshipit is macropores.Sucha processcan explainthe formationof pipes necessaryto ignore the five measurementsdone in summer and later formation of pipe-relatedrills. with their dear preferentialflow throughthe cracks.Figure 9 Finally, the seasonalchangesof fc demonstratethat soil showsthe relationshipincludingand excludingthesetests.For hydraulic conductivitychangesthroughout the year, even in pediments,the ifc is 3.6 (winter),3 (spring),3.3 (summer),and soilswith about 20 cm of intenselyactiveand seasonallyvary3.1 (autumn) times greaterthanfc. For the slopes,ifc is 2.7 ing clay soil as in the badlands.Such dynamicpatterns of (winter),2.4 (spring),4.4 (summer,withoutthe fivemacropore change,operatingwithin quite narrow seasonalgeomorphic flowtests),and 2.5 (autumn)timesgreaterthanfc. In termsof thresholdconditions,go someway to explainingthe complex mean yearly values, for both slopesand pedimentsifc is 3 nature of badlandstopography. times greater thanfc, but for the pedimentsis it very regular throughoutthe year, while for the slopesthe valuesof ifc are Acknowledgments. Metereorologicaldata were suppliedby the Inmuch greater in summerthan in winter, spring,and autumn, stitutoNacionalde Metereologia(INM). I thank Mario Payhand his due to the crack'sinfluenceon soil hydrology. family for help during the field campaigns,project CLI95-1890 for The larger infiltration rates measuredby meansof cylinder financialsupport,and Ian A. Campbell,T. Dunne, and an anonymous infiltrometer are due to the water ponding pressure,as ex- refereefor suggestions, commentsandcriticalrevisionof the manuscript.
328
CERD3.: VARIATIONS
IN INFILTRATION
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(ReceivedFebruary26, 1998;revisedMay 1, 1998; acceptedMay 13, 1998.)
