alluvial fan indicate that alluvial fans can be significant hydrologic buffers. .... 3. Methods. The study fan was instrumented to continuously monitor weather ...
WATER RESOURCES RESEARCH, VOL. 37, NO. 2, PAGES 341-351, FEBRUARY,
2001
A water balance approach to assessingthe hydrologic buffering potential of an alluvial fan Natasha
Herron
•
CooperativeResearchCentrefor CatchmentHydrology,CSIRO Land andWater, Canberra,ACT, Australia Departmentof Geography,Universityof Melbourne,Parkville,Victoria,Australia
Cathy Wilson2 CSIRO Land and Water, Canberra, ACT, Australia
Abstract. The hydrologicconnectivityof a catchmentdeterminesthe efficiencywith whichrunoff is movedthroughthe catchmentand henceopportunitiesfor runoff storage. Potentialstoragesincludevalleyfloorsand alluvialfans.Monitoringresultsfrom a small alluvialfan indicatethat alluvialfanscanbe significanthydrologicbuffers.Between20% and 100% of surfacerunoff deliveredto the studyfan from a 26-ha catchmentwas slowed and/orstoredduringthe monitoredevents.Intereventdifferencesin bufferingresponse are attributed to antecedent moisture conditions. In one event, when catchment conditions
were verywet, only 20% of incomingrunoff was absorbedby the fan. Under dry conditions,smallerproportionsof total rainfallwere deliveredto the fan headas surface runoff, of which >60% was absorbedby the fan. Becausesedimentstoragescan modify surfacerunoff delivery,becauseof their distinctivemorphologicand sedimentologic properties,it is suggested that their distributionthrougha catchmentwill influence catchmenthydrologicconnectivityhencerunoff and sedimentdeliverythroughcatchments. their soilproperties,particularlysoildepth,permeability,and potentialwater storagecapacity,can mean greaterpotential The efficiencywith whichrunoff movesfrom sourceareasto for surfacerunoff absorptionthan on hillslopes.Assessing the streamsand then throughthe streamnetworkdefinesa catch- bufferingpotentialof near-streamareas,particularlywith rement'shydrologicconnectivity.Runoff deliveryefficiencyre- spectto sedimentand nutrient transfersto streams,has beflectsthe proportionof rainfall that runsoff to a streamand comea significant researchareain recentyears[Peterjohn and the rate of delivery.As such,it has implicationsfor both the Correll, 1984;Lowranceet al., 1985;Jacobsand GillJam, 1985a, quantityandqualityof water deliveredto the streamnetwork. 1985b;Cooperet al., 1987; Cooke,1988; Cookeand Cooper, In orderto predictfloodresponses in ungaugedcatchments or 1988;Pinay and Decamps,1988;Pionkeet al., 1988; Cooper, to minimizethe lossof sedimentsand nutrientsfrom hillslopes 1990;Boschet al., 1994, 1996;Hairsine, 1996;Herron and Hairthrough,for example,the strategicsitingof vegetativefilter sine,1998]. strips,a better understanding of the controlson hydrologic The extentto whicha valleyfill depositinhibitsthe efficient deliveryof runoffto the next-orderstreamis influencedby the connectivityis needed. on the fan.Herronand The geomorphicunitswithin a catchmentinfluencerunoff dominantrunoffgenerationmechanism of a riparianbuffer processes in differentways.A geomorphic featurethat diverts Hairsine[1998]foundthat the effectiveness surfacerunoff to subsurface pathwaysand/orsubstantially dis- is stronglyinfluencedby its availablesoilwater storagecapacsipatesthe energyof surfacerunoff (e.g.,by flow dispersion, ity. In humid environments,near-streamareascan be satuflow attenuation due to storagedelay, or increasingsurface rated or nearly saturatedfor significantlengthsof the year. roughness)is definedhere as a hydrologicbuffer becauseit Rather than absorbingsurfacerunoff, thesenear-streamareas reducesthe hydrologicconnectivityof the runoff pathway. are dominantsourceareasof streamstormflow [Betson,1964; Maximumbufferingoccurswhen a "buffer" infiltratesall sur- Dunne and Black, 1970a,1970b].As a result,hydrologicconface runoff delivered to it. Less effectivebuffers may slow nectivityis high becausethe sourceof runoff is immediately surfacerunoff rates,leadingto dispersionof the flow hydro- adjacentto the streamnetwork.In more arid environments, where conditionstend to favor the generationof patchyHorgraph,but not affect actualrunoff volumes. Valley fill areas are potential hydrologicbuffers because tonianrunoffon the hillslopes,toe slopeandvalleyfill deposits theirrelativelylowgradientscausehillsloperunoffto slow,and havebeenfoundto functionassinksfor runoff [Puigdefabregas et al., 1998;Yair et al., 1980;Yair, 1983].Hydrologicconnectivity between the hillslopesand streamsis broken by the •Now at Land and Water Sciences Division,Bureauof Rural Sci- infiltrationof runoff into the interveningcolluvialand alluvial ences, Canberra, ACT, Australia. deposits.In theseenvironments, alluvialfansappearto havea 2Now at Earth and EnvironmentalScienceDivision,Los Alamos considerable influenceon the hydrologicconnectivitybetween National Laboratory,Los Alamos,New Mexico. 1.
Introduction
source areas and streams.
Publishedin 2001 by the American GeophysicalUnion.
Alluvial fansare characterizedby semiconicalmorphologies whichcausewater to spreadout overincreasingly large areas,
Paper number2000WR900253. 341
342
HERRON
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HYDROLOGIC
BUFFERING
BY AN ALLUVIAL
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FAN
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Contour interval 0.5m
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Lower Weir KEY
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Overland Flow Indicators
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therebyreducingflow rates and specificrunoff yieldsto the higher-orderstream.Moreover,theydevelopbelowthe outlets of catchments, or convergent sourceareas,identifiedbyHerron and Hairsine[1998] as areas most in need of bufferingto minimizerunoff and pollutantdeliveryto streams.While the role of large fans in arid systemshas receivedconsiderable attention,very little is known about the role of smallfans in other environments, yet they are a ubiquitouslandscapefeature in all environments.In this paper, the role of a small alluvialfan in bufferinga fifth-orderstreamfrom runoff generatedon a second-order tributarycatchmentis exploredfor three eventsof varyingmagnitudeand antecedentconditions. We use a water balanceapproachto quantifythe impactof fans in temperateclimateson modifyingrunoff deliveryto
fan selected for instrumentation
extends below a second-order
catchmentof approximately 26 ha. The fan andvalleyfloorare partially dissectedby two discontinuousgullies. A grasscoveredshallowwaterwayconnectsthe head of the fan with the gullies.For smallrunoffevents,flow arrivingat the headof the fan is generallyconfinedto the preferredpathway,but duringlargereventsit can disperseonto the wider fan as the capacityof the preferred pathwayis exceedednear the fan head.Runoff to the fan is ephemeral. The area is underlainby Ordovicianmetasediments, predominantlyslates,shales,siltstones,and quartzite.Fan soils vary in texture from fine sandyclay loams to medium and heavyclaysbut with gravellensesoccurringat variabledepths through the profile[Herron,1999;Butterworth et al., 2000]. streams. Tussockgrasscoversmuchof the upperfan surfaceand the lowerpart of thefan-valleyfloorarea,withnativegrasses filling 2. Site Description the gapsbetweenclumps.The grassis replacedby opendry The studyarea is locatedin the BrooksCreekvalleyon the sclerophyllforeston the valley. The BrooksCreekareahasa temperate,subhumidclimate, SouthernTablelands of New SouthWales(149ø21'E,35ø12'S) (Figure 1). Small alluvialfans are commonalongthis fifth- with an averageannualrainfall of about 630 mm. There is no order streamvalley,occurringbelow discontinuous tributaries pronouncedseasonaldistribution to the rainfall. Moderateon thevalleyfloorandevenon somehillslopes. The 4380-m 2 intensitythunderstormsoccurin the summerwhenthe amount
HERRON
AND
WILSON:
HYDROLOGIC
of convectionalrainfall is greater.In winter a westerlyflow of low-pressuretroughsand depressions bring rain.
BUFFERING
BY AN
FAN
343
recommendedrecessionparameter value of 0.9. The API is usuallycalculatedfor the 30 daysprecedingan eventto negate the effectof initial conditions.The largerthe APT is,the wetter the catchment
3.
ALLUVIAL
is.
Methods
Rainfallwasrecordedusinga tippingbucketrain gauge(0.2 The studyfan was instrumentedto continuouslymonitor mm bucket) and loggedby the climate station. Total daily weather, surfacerunoff inputs and outputsto the fan, and evaporationwasautomatically calculatedby the climatestation water table fluctuations on the fan. Weirs were constructed at usingthe Penman equation.Results are reported usingthese the head of the fan, to measurethe stageheight of incoming valuessincepotentialevaporationrepresentsan upper limit to catchment runoff, and at the mouth of the lower discontinuous fan losses.
gully,to measurethe main outflowpathway(Figure 1). Stage heights(h) were convertedto discharge(q) accordingto Bos 4. Results [1989] and then to total dischargefor each event (Quw). A standard error of 0.0024 m was determined for h, which as a 4.1. Rainfall-Runoff Events During the Study Period
percentageof h variesfrom 1.2% at h = 0.2 m to 24% for h = 0.01 m. Since averagestage height tended to be less than 0.01 m during many events,large errors are associatedwith many of the individualreadings.However,when the errors arisingfrom the measurement of h are propagatedthroughto the calculationof Quw, they are found to have a relatively minor impacton the estimationof total discharge(
