WATER RESOURCES RESEARCH, VOL. 22, NO. 8, PAGES 1263-1272, AUGUST 1986
Storm
Runoff
1.
Generation
Where
in Humid
Does the Water
Headwater
Come
Catchments
From?
A. J. PEARCE
Forest Research Institute, Christchurch,New Zealand
M. K. STEWART
Institute of Nuclear Sciences,DSIR, New Zealand
M. G. $KLASH
Departmentof Geology,Universityof Windsor,Windsor,Ontario, Canada
Production of storm runoff in highly responsivecatchmentsis not well understood.We report in these
papersa comprehensiveset of hydrometric and natural tracer data for rainfall, soil water, and streamflow
for catchmentsin the Tawhai State Forest,Westland,New Zealand,which revealsomeof the important
runoff processes.
The catchmentsare small(< 4 ha), with short(< 300 m) steep(average34ø)slopesand
thin (< 1 m) permeablesoils.Long-term(1977-1980)weeklyobservations
of oxygen18,electricalconductivity, and chloride in the stream, groundwater,and rain in the main study catchmentindicate that
catchmentoutflowreflectsa well-mixedreservoirwith a meanresidencetime of approximately4 months.
A preliminarystormhydrographseparationusingoxygen18 (for a storm hydrographexceededby only
22% of eventssince1979)indicatesthat only 3% of storm runoff could be considered"new" (i.e.,current
storm) water. Rapid subsurfaceflow, such as macroporeflow, of new water thereforecannot explain
streamflowresponsein the study area. More detailedhydrographseparationstudieson throughflowas
well as streamfloware describedin parts 2 (M. G. Sklash et. al., this issue)and 3 (M. G. Sklash et. al.,
unpublishedmanuscript,1986).
throughflow
of stormrainfall,processes
whichdeliver"old"
INTRODUCTION
Despite decadesof increasinglymore intensivestudy, storm
runoff generationremainsa controversialtopic. Streamflowis
generated mainly by processesoperating beyond the permanent streamchannel,but the relative importanceof surface
versussubsurfaceflow and of rapid throughflow(via soil macropores)versustranslatoryflow mechanismsis still not clear.
The present state of researchis summarizedin the U.S. National Report of International Union of Geodesy and Geophysics(IUGG) for the 1979-1982quadrennium[Wood, 1983,
p. 457]:
A unifying theory for hydrologic responsesat varying basin
scaleshas not yet beendeveloped;work during the last quadrennium focusedon various issuesincluding: (1) the basin response
and flood statisticsbased upon its geomorphologicalstructure,
(2) soil variability and runoff variability and (3) role of macropore flow, interflow and contributingarea surfacerunoff in the
flow response.It is crucial that adequate and detailed data be
available so that theories concerning runoff generation can be
adequatelytested.
In. this seriesof papers we presenta comprehensiveset of
volumetric,solute chemistry,and stableisotopenatural tracer
data for rainfall, soil water, and streamflowthat permits independenttestsof runoff generationhypothesesin a responsive
hydrologic environment.These data clearly demonstratethat
current storm rainfall is a minor component not only of
streamflowbut also of throughflow resultingfrom the storms
studied. Our results indicate that in at least this hydrologic
environment, which is apparently highly suited to rapid
Copyright 1986 by the AmericanGeophysicalUnion.
Paper number 5W4214.
0043-1397/86/005W-4214505.00
water are largely responsiblefor hydrographgeneration.Currently favoredrunoff mechanisms,
which involverapid flow of
"new" water over the ground surface or through the soil
matrix, soil macropores,or other rapid transit pathways,
cannot explain the streamflowresponse.
RUNOFF GENERATING
MECHANISMS
Figure 1 broadly summarizesthe variousmechanismsand
theoriesof runoff generationin humid vegetatedareasdeveloped over the period sinceHewlett [U.S. ForestService,1961]
describedhis variable sourcearea hypothesis.It is assumed
that quasi-uniform, basin-wide generation of runoff via
Horton overland flow [Horton, 1933] cannot satisfactorilyexplain storm runoff generationin such environments[e.g.,
Freeze, 1974]. The paperslisted for each mechanismare the
major studiesproviding field evidencefor each process.An
important, but not generallyappreciated,point is that all the
mechanismsshown require a variable source area of runoff
generation.Central to all the mechanisms
is the conceptof
runoff contributingzoneswhich expand and contract seasonally and duringstorms,dependingon antecedentwetness,soil
physicalproperties,water table elevations,and storm magnitude.
The
differences
between
these variable
source area
models lie in the mechanismsand pathways of runoff production.
Partial-area Horton overland flow occurson those parts of
a catchment
where rainfall
rates exceed the soil infiltration
rate and the upper parts of the soil profile becomesaturated
from the top downward.The excessrainfall becomesavailable
for surfacedetention and flow over the ground surface.Hortonian theoryimpliesthat new water is dominantin the runoff
response.
1263
1264
PEARCEET. AL.' STORMRUNOFFGENERATION,
VARIABLE SOURCE AREA CONCEPT
SATURATI ON OVERLAND
PARTIAL-AREA HORTON
SUBSURFACE FLOW
FLOW
OVERLAND FLOW
Dotson, i 964
Amerman, 1965
Dunne and Black, 1970a, b
Dunne et al., 1975
Oeven,i g?8
9onell and 611mour,i 978
Donell et al., 1981
Ragan, 1966
Betson and Marius, 1969
Pilgrim,1966, 1976, i 985
U.S. Forest Service, i 961
Hewlett and Hlbbert, 1965
Hewlett and Nutter, 1970
Weyman,i 970, i 975
Troendle and Hofmeyer, i 97 I
Anderson and Burt, i 978
I
• RAPIDTHROU6HFLO
IDISPLACEME
OF
OLD
WATER
FNEW
WATER
EMPHASIZING
MACROPORE
FLOW
Tsukamoto, 1961
Hewlett and Hlbbert,1967
Pinder end Jones, i 969
Crouzet et al., 1970
Fritz et al., 1976
Skiash et al., 1976
Skiash and Farvolden, i 979
Whipkey, i 965, i 967
Aubertln, 197 i
Jones, 1971
Arnett, i 974
Beas!ey .i 976
•PIIgr.
lm•Tal. 1979
r,losley,197•',1982
Fig. 1. Summaryof stormrunoffmechanisms
applicable
to thevariablesourceareaconceptandstudies
providingfield
evidence for mechanisms.
Saturationoverlandflow is generatedby rain that falls di- Dunne suggestedthat subsurfaceflow dominatesthe hydrorectly onto saturatedareasnear streamchannelsor in valley graph.
floors. Water tables rise to the surfaceof theseareas (initially
There is lessobviousagreementor consensus
on the relative
fed by infiltratingrainfall,but also fed by outflowof rainfall importanceof different subsurfaceflow mechanismsin some
that has infiltrated upslopeof the runoff sourcearea) soon hydrologicenvironments.There is strongcircumstantialeviafter rainfall begins,and further rainfall generatesflow over dencefor rapid flow via preferredpathwaysin soilsin many
the surface.This mechanismalso impliesthat new water is the forestedareas (coarsetextured, highly permeablesoils with
dominantcomponentin storm runoff eventhoughsomeold obvious structural featuressuited to macropore flow; rapid
rise and recessionof hydrographs,and closedependenceof
water may appearthroughreturnflow or exfiltration.
Subsurfaceflow is generatedby rapid infiltration of rain hydrographon rainfall fluctuations),and this mechanismhas
and the associatedincreasein soil hydraulic conductivity.Intiltrated rainfall may flow rapidly through the soil mantle
more or less directly to the stream via interconnectedlarge
poresor porousstructuralfeatures(macropores),
or through
saturatedhorizonsat the baseof the soil mantle or perchedat
received considerable
renewed
attention
since about
1978
Horton and Hawkins, 1965; Hewlett and Hibbert, 1967]. The
Tracer studies,particularly thoseusingnatural unstable(tritium) or stable (deuterium, oxygen 18) isotopes,have almost
without exceptionindicated that water stored from previous
rainfalls volumetricallydominatesthe streamflowresponseto
storm rainfall. Most natural isotopetracer studieshave been
in gentle to moderate terrain, less hydrologicallyresponsive
than the areas where macropore flow and other rapid
throughflowmechanismshave been interpretedas the dominant responsemechanism.Thus there has been some scepticismabout the applicability of thesefindingsto highly responsive catchments,as well as to large volume hydrographs.The
notion that steep headwatercatchmentscan store sufficient
water to satisfythe volume of a subsequentstorm hydrograph
may also seemunlikely. However,evena shallowsoil profile 1
m thick will commonly contain 300-500 mm of water at field
capacity.
(Figure 1; alsoseeBevenand Germann[1982] for an extensive
review). Nonetheless,field evidencefor macroporeflow is
sparsein the environmentswhereit is commonlyconsidered
important [Beven and Germann,1982, Table 3]. Most macpermeabilitycontrastswithin the soil mantle. If rapid flow ropore flow studieshave been affectedto an unknown extent
through the soil occurs,current storm rainwater dominates by unnatural boundary conditions such as plot-boundary
the storm runoff. Alternatively,old water alreadystoredin the trenchesor artificial rainfall rates up to hundreds of times
soil mantle may be displacedby the infiltratingrainfall [e.g., greaterthan natural rainfall.
displacement
mechanismis poorly understood.
STORM RUNOFF GENERATION'IS THERE A CONSENSUS
9.
In humid, well-vegetatedregions, soil hydraulic conductivities,particularlynear the surface,and infiltration capacities
often greatly exceedrainfall rates. In many forestedareas,infiltration capacitiesexceedrainfall rates with return periodsof
hundreds of years by one or more orders of magnitude.
Partial-area Horton overland flow is essentiallyrestricted to
catchmentswith pooly permeablesoils, or with soil physical
propertiesso modified by land use that infiltration capacities
are exceededby rainfall rates with recurrenceintervalsof only
days or weeks.
There seemsto be a consensusthat Dunne [1973] was cor-
rect in suggestingthat in humid regionswith thin soils,gentle
terrain, concavelower slopes,and wide valley bottoms, saturation overland flow dominates the storm hydrograph. In
humid areas with steep, straight or convex slopes, incised
channels, narrow valley bottoms, and very permeable soils,
It is a reasonableassumptionthat in any catchmentthereis
somemagnitudeof runoff eventwhichexceedsthe maximum
possiblestoragecapacityof the catchment.If eventsof this
size or larger occur, rainfall from the current storm must
appearsubstantially
in and eventuallydominatethe hydro-
PEARCEET. AL.' STORMRUNOFFGENERATION,1
1265
new water, respectively.The utility of (2) and (3) for any particular storm event is controlled mainly by the magnitude of
(Co - Cn)relative to the analyticalerror and the recognitionof
areal and temporal variationsin Coand Cn.Equations(2) and
(3) can alsoprovideestimatesof old and new water percentage
contributionsto throughflowand overlandflow.
Natural chemical tracers such as electrical conductivity
conductive zones. Limited contributions
of saturation
over(EC), chloride(CI), and pH can also be insertedinto (2) and
land flow from direct rainfall on near-channel
areas and on
(3). Becauseof possiblechangesin their concentrationsrethe stream channel itself are also expected.In small runoff sulting from interactions with catchment materials [Nakaevents of only a few millimeters it seemsaccepted that the mura, 1971; Pilgrim et al., 1979; and others], interpretationsof
bulk of runoff is generated from direct rainfall on the ex- chemicaldata are often suspect.
Estimating the true isotopic composition of the rainfall
panding channelsystem.
reachingthe ground surfaceis complicatedin forestedcatchTHE USE OF DEUTERIUM AND OXYGEN 18 IN STORM RUNOFF
ments becauseof the interceptionloss (by evaporation)from
STUDIES
the forest canopy during rainfall. Evaporation from water
During the past two decades,natural isotopic tracers such storedon the forestcanopytypically occursat rates of 0.1-0.5
as deuterium(D) and oxygen18 (180) havefrequentlybeen mm/h [Stewart, 1977; Pearce et al., 1980; Pearce and Rowe,
usedto solvehydrologicalproblems.Theseisotopictracersare 1981] and can accountfor the lossof about 20% of the gross
consituent parts of natural water moleculesand, as such, are rainfall. Depending on the ambient relative humidity at the
excellenttracersof water origin and movements.Another im- canopy level, evaporation of 20% of the rainfall could subof throughfalland
portant advantage is that uniform, basin-wide tracer appli- stantiallyenrichthe D or •80 composition
cation occurs at no cost during precipitation events, since net rainfall compared with that of the gross rainfall usually
areal variationsin D and •80 in precipitationare minor in measuredand sampled.This is a potentially seriousproblem
small catchments with limited relief.
only when the grossrainfall is isotopicallylighter (more negaSince D and •80 concentrations in natural waters are much
tive in 5 notation) than the prestorm stream water.
Ambient relative humidity at canopy level during simultasmallerthan theirmorecommonlight isotopes,1H and 160,
D and •80 concentrations
are generallyexpressed
in the con- neous evaporation and rainfall is unlikely to be below 95%
ventional delta (5) notation as per rail (%o)differencesrelative relative humidity; at these RH levels,isotopic enrichment of
to the international standard,SMOW [Craig, 1961] where
the water remaining on the canopy is likely to be small and
may be negligible.To test this effectwe comparedrainfall and
or =
x 1000%o
(1) throughfallisotopiccompositionunder a densepine stand in
which interceptionlossesaverage30% of grossrainfall. Ambir$! 8 L
RSMOW d
ent relative humidity during the summer rainstorm we samand R is the D/H or •80/•60 ratio, respectively.
Analytical pled was likely to be lower than that during the winter storms
precisionfor 5D and 5•80 by massspectrometry
is better we sampled during the main part of our study. The gross
than 2%oand 0.2%o,respectively
(95% confidencelevel).
rainfall of 75 mm had 5D and 5•80 values of -38.5 and
Stableisotopessuchas D and •80 havefrequentlybeen -5.45%o, respectively,and throughfall was enrichedby 2.9%0
usedin storm and snowmeltrunoff studiesto quantify old and in 5D and 0.11%oin 5•80. These enrichments are close to the
new water contributions[Dincer et al., 1969; Martinec et al., typical analytical precisions.The above considerationsand
1974; Fritz et al., 1976; SMash and Faroolden, 1979; Rodhe, data indicate that isotopicenrichmentof D and •80 in
1981; Stichler and Hermann, 1982; and others]. Their use is throughfallcan be considerednegligibleevenin stormswhere
based on the premise(and requirement)that the new water the rain is isotopicallylighter than the prestormstreamflow.
(rain or snowmelt)and old water(groundwaterand soilwater)
have distinct isotopicsignatures.These distinctivesignatures
STUDY AREA
arise from seasonal,storm to storm, and intrastorm isotopic
variationsin precipitationin contrastto the homogenizing
E,ight smallcatchments
at the Maimaistudyareain the
effectson groundwaterof rechargeand dispersiveprocesses.
Tawhai State Forest, near Reefton, North Westland, on the
Betweenstorm events,streambaseflow reflectsthe isotopic South Island of New Zealand, have been studied since 1974
compositionof the old (stored)water. During storm runoff [Pearce et al., 1976, 1980b; Pearce and McKerchar, 1979;
events,however,the isotopiccharacterof the streammay be O'Loughlinet al., 1978, 1980, 1982]. The catchmentsare unaltered by the addition of new water from rainfall. The old derlain by firmly compacted,moderately weathered,early
and new water contributionsat any specifiedtime can be Pleistoceneconglomerate.Soils are shallow (averagemineral
calculatedby solvingthe massbalanceequationsfor the water horizons are 60 cm), podsolizedstony yellow-brown earths
and isotopicfluxesin the stream.Theseequationscan be ex- (Dystrochreptsand Humults) with a thick (average17-cm),
pressedas
well-developedupper humic horizon [Webster, 1977]. Slopes
are short (< 300 m) and steep(average34ø) and local relief is
Cs
100-150 m. Two of these catchments(M6 and M8), a subgraph. The size of suchan event,however,is not obvious,even
in highly responsivecatchmentswhich appear to have limited
opportunity for water storagein the soil mantle.
Recent published work indicatesa predominant view that
storm runoff from steepwell vegetatedheadwater catchments
in humid areas is produced by rapid flow of infiltrated rain
from the current storm through macroporesor other highly
•D [Rsample
ZRSMOW
]
Qø=Co Cn
catchment within catchment M8, and the main stream into
and
Qn= Qs- Qo
(3)
where Q is discharge,C expressestracer concentration,and
the subscriptss, o, and n refer to the stream, old water, and
which the eight monitored catchments drain were studied
during differentphasesof this research(Figure 2).
The vegetativecover is mixed evergreenforest dominated
by southernbeech (Nothofagusspp.),podocarps,and broadleaf hardwoods. The forest is multistoried, with a dense tree-
1266
PEARCEET. AL.' STORMRUNOFFGENERATION,1
171 e 41'E
I,'1
%
348
CATCHMENTS
T•HAI STATE
FOREST,
• •in .tr.amrunoff
ß t ••
82
Itructur•
13
•t we•
• Subcatc•ntofM8•tudi•d
In detail
at we•
o
M16
6oo
i
i
14•le
ef Idetr©e
I
I
I
Fig. 2. The Maimai ExperimentalCatchments,
SouthIsland,New Zealand.
fern and shrub understoreyand a ground cover of fernsand
herbs.
Mean annual grossrainfall is approximately2600 mm, producingapproximately1550 mm of runoff from 1950 mm of net
rainfall [Rowe, 1979]. The catchmentsare highly responsiveto
storm rainfall; 1000 mm (65%) of the mean annual runoff is
quick flow ((•F) as definedby Hewlett and Hibbert's [1967]
separationmethod [Pearceand McKerchar, 1979]. Quick flow
is 39% of annual total rainfall (P).
The quick flow responseratio ((•F/P) is roughly doublethat
of the most responsivebasinsdocumentedin easternUnited
States [Hewlett and Hibbert, 1967; Woodruff and Hewlett,
1970; Bryan, 1979; Olszewski,1980]. The R index ((•F/P
averaged for runoff events from rainfalls of greater than 25
mm) is 46%, comparedwith 3-35% for 11 basinsdistributed
betweenGeorgia and New Hampshire [Hewlett et al., 1977].
R indices for catchmentsin Natal, South Africa, range from
5-19% [Hope, 1983].
The average (•F/P for six catchmentsin our study area
ranged from 3 to 72% in 74 individual storms during 19741977 which yielded from 0.5-187 mm of quick flow [Pearce
and McKerchar, 1979]. Runoff coefficients(=(•F/P) from a
wide range of plot and small catchment studies listed by
Dunne [1978] show that most values from well-vegetated
catchmentsare lessthan 20%. Hart [1977] listed valuesfor a
numberof large stormson a forestedhillslopeplot in Oregon
rangingfrom 23 to 51% and averaging38%. Numerous other
comparativedata couldbe cited,but thesesufficeto showthat
the study area is extremelyresponsiveto storm rainfall. We
are unawareof any data from well-vegetated,
especiallyforested, headwater catchmentsthat exceedthe quick flow responsivenessof our study catchments.
Storms with high responseratios are frequent.On average
over a 9-year study period, 25 storms/yearhave produced
quick flow yieldsof > 5 mm. Typically,25 mm of rainfall is
neededto produce5 mm of quick flow. Quick flow yieldsin
theseeventsrangedfrom about 20 to 75% of grossrainfall
volume. Combined with the high responsiveness
of the catch-
ments,the frequentstormswith large quick flow responses
make the study area ideal for testingthe relative importance
of new and old water componentsof the storm runoff response.
Stream channelsare deeply incised and lower portions of
slopeprofilesare stronglyconvex.Areasthat could contribute
to storm responseby saturationoverlandflow are small and
limited to 4-7% of catchment area [Pearce and McKerchar,
PEARCE
ET.AL.' STORMRUNOFFGENERATION,
1
ß• .....
-,,.
0
I
1267
lOOm
scale
I
\.
M•qD-$Tt•AM
ß
o
ß wells
ß shallowl.suction
ß
/ ßdeep
/l•imeter
ß maximum rise v•sll
pit locations(see
Moslev,1979,Fi•:•)
streamtlow sample
site
'('/"J'•. WtL•.,•
•
•-•
catchment boundary
•w
subcatchment
m• $1
boundar•
Fig. 3. Instrumentation
in catchmentM8.
1979; Mosley, 1979]. The infiltrationcapacityof the soil
organichorizonand the saturatedhydraulicconductivity
of
the uppermineralhorizonare 6100and 250 mm/h [Webster,
1977]compared
with observed
1 hourmaximumrainfallrates
of 27 mm/h [O'Lou•lhlinet al., 1982]and a meanrainfallrate
of 1.6 mm/h [Pearceand Rowe,1981].Typicalrainfallrates
duringquickflowarein therange3-10 mm/h.Stormrunoffis
thusnotgenerated
byrainfallratesexceeding
infiltration
rates.
Only eventswith QF/P < 10% couldbe fully accounted
for
by saturationoverlandflow.Subsurface
flow dominates
the
production
of stormrunoffin all eventsyieldingmorethana
few millimetersof quick flow [Pearceand McKerchar,1979,
low-flow) samplesof streamflowfrom catchmentM8 (3.8 ha),
groundwater samplesfrom two wells in catchment M8, and
bulk rainfall samplescollectedat the meteorologicalstation
(Figure 2). We analyzed samplesfor electrical conductivity
(EC), chlorideconcentration,
and 6x80. During the early
study period, 1977-1980 we intensivelysampledtwo storms,
but only one sampleset was suitablefor discriminationof new
and old water componentsin the resultingrunoff.
During the August-September1983intensivestudy,streamflow was sampledfrom catchmentM8, from the undisturbed
control catchment M6 (1.6 ha), and from the main stream
(2.5-km2 drainagearea)into whichthe monitoredcatchments
drain (Figure 2). Rainfall was sampledat two locations(Figure
2). Groundwaterwas sampledat two wellsin catchmentM8,
and soil water was collectedat two depths at theselocations
cablesystem.
A narrow(3-10 m) zoneof riparianvegetation, usingsuctionlysimeters(Figure 3). Sevenof the measurement
includingthe full widthof the incisedvalleybottom,wasleft sitesusedby Mosley [1979] for his studyof streamflowgener-
Figure4; Mosley,1979].
In 1979,midwaythroughthe periodof study,the foreston
catchment M8 was clearfelledand harvestedusing a skyline
undisturbed. Soil disturbance was limited to the removal of
ation in a first-order
the organichorizonalongthe main routesoverwhichlogs
wereextracted
excepton somesmallridgetopareas( <<1% of
the catchmentarea)wherethe mineralhorizonwas also re-
Data werecollectedin a long-termseriesof weeklysamples
covering
muchof theperiod1977-1980andduringa 2-month
(pits 1, 2, 3, 5, sitesA and D, and the seep).Suctionlysimeters
were usedto samplesoil water immediatelyalongsidesite A
and pit 5. Water table elevations were monitored with
maximum-rise,narrow diameter piezometersinserted to the
baseof the soil mantle at site A, pits 1, 3, and 5, at well B, and
in the streamchannelupstreamof well B and upstreamof the
M8 weir (Figure 3). Suctionlysimetersand piezometersat the
pits were strategicallylocated to minimize the influenceof
hydraulic drawdown. All samplescollectedduring this study
period were analyzedfor electricalconductivityand pH, and
mostwereanalyzedfor chlorideand deuterium(D).
Rainfall during both study periods was measuredusing a
Lambrechtnatural siphonrecordingrain gauge,correctedto a
standard 127 mm gauge,300 mm above ground. Streamflow
intensivestudyperiod(August-September,
1983).
from catchments
moved. In 1980 the residual slash was broadcast burned.
Comparison
with the unlogged
controlcatchment
M6 indicates that storm runoff volumes in M8 have increased after
loggingby amounts
andproportions
closely
equivalent
to the
interception
losses
thatwouldhaveoccurred
in thepresence
of
the forest[Pearceet al., 1980,alsounpublished
data].
DATA COLLECTED
M8
subcatchment
and M6
of M8
were reactivated
was measured at 90 ø V-notch
weirswith stagerecordedto + 1 mm. Outflow
The long-termsampleseriesconsisted
of weekly(usually sharp-crested
1268
-I0
PEARCE
ET.AL.'.STORMRUNOFF
GENERATION,
1
-
I I I I I I I•//I////I
•
N•wZ•ola•dm•taoric
water
Ii•//
_/ •ram
-
Maimai
r•r•$$io• Ii•
-20
-5.4 and -7.4%oduring1978,1979,and 1980.Samplesfrom
well A (nearthe M8 weir)closelyfollowthe M8 6•80 values,
showingthat the well samplessimilarwater.Well B (nearsite
D) hassimilar6•80 valuesbut is out of phasewith M8; i.e.,it
tendsto haveheavier6•80 in winterandlighterin summer.
The dampedtemporalvariationsin streamand groundwater
-30
•/• -ram
-
main
j
7•8.
storm
peak
S/te
•_ I _•_Sr
5S
-40
6•80 suggest
thatmostof themixingof old andnewwaters
occurson the hillslopeand that subsurface
waterdischarge
to
the streamis an isotopicallyuniformmixtureof storedwater.
Of the 55 weekly samples,11 were collectedat flow rates
•/ Seep
which were exceededfor only 20% of the long-run(19741983)flow duration(0.5-2.2L s- • ha-•) and threewereat
flowratesexceeded
for only 10% of theflowduration(> 1.0L
-60
.
I
-9
I
-8
I
-7
I
-6
I
-5
I
-4
I
-$
I
-2
I
-I
s"o (%ø)
Fig. 4.
Local meteoric water line.
s- • ha-•). Flowratesat thesesampletimesrangedfrom1 to
4.5 timesthe mean annual dischargeand exceededthe flow
rate at whichquickflowusuallyends(0.1-0.5 L s-• ha-i).
These11 samplepointsare markedby opencirclesor squares
(> 1.0L s- • ha) in Figure5 andshowno greatdepartures
ratesand volumesfor the pitsand naturalseepweremeasured from the seasonal
isotopictrendsor from preceding
samples,
volumetrically
by timingthe fillingof 1-L graduatedcylinders despitelarge associatedfluctuationsin rainfall 6D or 6•80.
and by measuringthe depth of water storedin 210-L tanks. Theseresultsindicateonly small contributionsof new water at
Overflowsduring high runoff eventspreventedus from ob- higherflow ratesand duringquickflow.
taininga completevolumerecordfor all but threeof the pits.
An estimate of the mean residence time of water in the
Streamflowin the main streamwasmeasuredat a rectangular catchmentcan be obtainedfrom the smoothingof the seasonconcretecontrol sectionusingstagerecords(+ 1 mm) and a
al isotopicvariation betweenthe input (rain) and output
rating equationdeterminedfrom currentmeterand dye dilu- (streamflow)by the methodof Maloszewskiet al. [1983]. The
tion gaugings.
rainfall t5 valuesare approximatedby a sinusoidalfunction
Electrical conductivitywas measuredand correctedto a
with a periodof 1 year(w = 2• per year):
25øC standard temperatureusing portable Metrohm and
6in(t) = A sin wt + M
(4)
pHox conductivitymeterscheckedagainsta laboratory-grade
conductivitymeter.Chloridewas analyzedusingan automawhere A is the amplitudeof the •80 variation and M its
ted mereuric thiocyanate-ferricnitrate method [American
mean value. Assuminga steady state (i.e., the flow rate and
Public Health Association,
1976] on a Pye-Unicamdiscrete
volumeof waterin the systemare constant),
we can applya
analyzer.
convolutionintegralto producethe outputfunction,whichis
The isotopicanalyseswereprovidedby the Instituteof Nualso sinusoidalbut with smalleramplitude(B) and a phase
clearSciences,
LowerHutt, New Zealand.Deuteriumsamples shift (•b):
(August-September
1983 data) were preparedby the zinc reductionmethod[Colemanet al., 1982; Stanleyet al., 1984]
5o•t(t)= B sin (wt + •b)+ M
(5)
and analyses
wererun on a V. G. Micromass602 massspectrometer.Oxygen18 samples(all samples)werepreparedby The systemis nonsteadyin the short term, so assuminga
resultsrelevantto the meanhydrologic
the carbondioxidegasequilibrationtechnique.
The local"me- steadystateproduces
conditions.
The
convolution
integral dependson the distriteoric water line" (Figure4) to relate the 6D and 6•80 data
bution of flow ratesin the system.We assumethe catchment
wascalculatedfrom the watersamplescollectedin September
system,in whichincoming
1983.The Maimai regression
equation,6D - 7.36•80 + 7.9,is to be a well-mixed(exponential)
rain mixeswith all the waterin storage.The convolutioninteslightlydifferentfrom the equationusuallyfound for New
Zealandrainwaters,
6D = 8.06•80+ 13 [StewartandTaylor,
gral is then
1981], but only one sampledeviatessignificantlyfrom the
latter equation(Figure4).
t$out(t
) = T-1
t$in(t
-- t') exp(-- t'/T) dt'
©
(6)
RESULTS
whereT is the meanresidence
time of waterin the system.
The predominance
of old waterin streamflowduringstorm
Long-Term SampleSeries
events(shownby short-termexperimentsdescribedbelow)
The 6•80 valuesof the long-term
seriesof weeklystream suggests
that the exponentialmodel shouldbe a good ap(M8), rain, and groundwatersamplesare summarizedin proximationto the storageof water in the catchment.SubstiFigure 5. The data from the longestuninterrupted
sampling tution of (4) for 6i, and solvinggives
period(February-November,
1977)showthat theweeklyrainfall samplesrangedfrom lessthan -3 to morethan -12%o in
6•80, withtheheavier
values
(less
negative)
occurring
during
the summermonthsand the lighter(morenegative)valuesin
winter.Both the streamand groundwatersamplesfollow the
rain 6•80 trends;however,the seasonal
and short-termvariationsare much smaller.The samepattern is also shownfor
samplingperiodsin 1978,1979,and 1980.The stream(M8)
T = w- 1 [(A/B)2_ 1]1/2
(7)
COS
tp= (w2T2 -I- 1)- 1/2
(8)
and
The rainfallcollections
at Maimai havetoo manygapsto
be satisfactoryfor the aboveanalysis,but a continuousseries
of monthly sampleswere collectedat Takaka during the
6•80 varied between -6.0 and -7.4%o in 1977 and between period1976-1979[Stewartand Williams,1981].Takakais 160
PEARCEET. AL.' STORMRUNOFFGENERATION,1
1269
•-20•
-40
•
-60
-80
,•
L e½OSL/$echa
• 0•
I I
I I I
I I,
ß
••oo•
F •M • A • M
IA
S ' 0 •N ]A IS •0 ] M I a I
1977
197 8
1979
is j
!
1980
Fig. 5. Long-termrain,stream,andgroundwater
5xsO,Maimaicatchment
M8, andrainfall5xsOat Takaka.
km north of Reefton and occupiesa similar position with rangingfrom 15 min to severalhours.The rainfall, streamflow,
regardto altitude,climate,and topographyof the surrounding electricalconductivity,
and 180 data are presented
in Figure
country. The amplitude of variation of rainfall 5180 at 6. A total of 16.2 mm of rain fell between 1500 and 2200 h on
Takaka due to seasonal effects should be similar to that at
April 11. A further 2.7 mm fell between 0000 and 0900 h on
Maimai. Since we are concernedwith seasonaleffects,we have April 12, and the streamthen beganto rise again in response
screened out short term variations in the Takaka 5180 data to 7 mm of rain in the secondstorm on April 12. The recesby computingthe three monthsrunningaverageweightedby sion had not crossedthe hydrograph separation line before
the amount of precipitation;this is plotted in Figure 5. The streamflow increased again on April 12; thus storm runoff
averagesummer-winter
5180 difference
(A in equations(4) calculated as the quick flow before the second rise of the
and (7)) in the Takaka rainfall is 4.9 + 0.6%0for the period hydrograph is somewhat smaller than it would have been in
1976-1979.The summer-winter
5x80 difference
for M8 (B in
the absence of the second storm. Storm runoff before 0900 h
equations(5) and (7)) is about 2.0%0.With thesevaluesin (7),
on April 12 was 4.1 mm (22% of rainfall). For the two storms
combined the quickflow was 9.5 mm (37% of the 25.9-mm
rainfall).
the mean residence time obtained is four + 1 month and the
phaselag expectedis 2 months.(The data in Figure 5 are not
inconsistentwith a phaselag of 2 months.The phaselag is
Antecedent
conditions
were somewhat
wetter
than
usual.
mathematically limited to 3 months,becausethe contributions
of youngand old water havecancellingeffects,and it is not an
accurateindicator of meanresidencetime.)
We concludethat the old componentin catchmentM8 has
An 8.4-mm rainfall on April 5 had produced0.3 mm quick
flow (4% of P), and a 60.2-mm rainfall from April 6 to 8 had
produced 37.7 mm quick flow (63% of P). Antecedentflow
rate beforethe two earlier eventshad been0.9 mm/d, but was
a mean residencetime of about 4 months,implying dynamic 2.9 mm/d beforethe storm of April 11. The higherantecedent
storageof about one third of the annual runoff (• 500 mm of flow reflectsthe continuationof quick flow from the storm of
water) on average.The forest floor (L, F, and H horizons) April 6-8 until 0030 h on April 10. Only 22% of all storm
averages 19 crn in thicknessin catchment MS, with a satu- hydrographsrecordedafter logging of the forest cover have
ration water contentof 50% by volume[Webster,1977], and antecedentflows greaterthan that for the April 11 storm.
the mineralhorizonsaverage60 crnin thickness,indicatinga
Figure 6 showsthat the oxygenisotope content of storm
total water storagecapacityof at least 350 mm. This is some- runoff fluctuated only slightly from the antecedent conwhat smaller than our estimatedstoragebasedon residence centration. If shifts in 180 concentration of less than 0.15%o
time, but is of similar order, suggesting
that the conglomerate are discountedas nonsignificant(<2 standard deviations of
bedrockdoesnot providemuch water to streamflow.
repeat measurements),the contribution of new rainwater is
The greatersmoothingand longertime lag of 5180 values lessthan 3% to the storm runoff. The only significantchanges
for well B suggestslonger storageand that a differentflow in 180 concentration
areduringthelaterpart of therecession
model is appropriate(suchas a pipe flow model).This implies after rainfall ceasedat 1900 h. Thesechangesare away from
the compositionof the rainwater (i.e., the streamflowbecomes
that the water is isolatedby poor permeability.
slightlylessnegativeor heavier),thus even they do not reflect
An Exampleof StormflowResponse
inputs of new water. As is demonstratedin part 2 [Sklashet
In April 1979,shortlyafter loggingof MS, two smallstorms al., this issue],the soil water storeis not completelymixed so
were sampledintensivelyto assess
the relativecontributionsof that the assumptionof a simpletwo-componentmixing model
new (storm)and old (prestorm)water.The first stormproved of old and new water is not entirely satisfied.As the observed
suitablefor this purposeas the rainfallaveraged• 5• lighter 5180 shiftis barelysignificant,
theassumption
is reasonable.
in x80 than the streamflow
immediatelybeforethe storm.
Electrical conductivity(EC) and chloride data confirm the
Insufficient difference in 180 content between rainfall and
antecedent streamflow was found in the second storm.
Rainfall and streamflowsampleswere collectedat intervals
negligibleinput of new rain into the storm runoff. Rainfall EC
fluctuatedbetween 17 and 29 pS/cm exceptfor the first 2 mm
of rain on April 11 and 2 mm of rain during the recession
1270
PEARCEET. AL.' STORMRUNOFFGENERATION,1
'• 120
E
160
• 80
"'40
-5
-6
•
-7'-
rain
•
rain
o
-I0
rain
-II
-12
0.7
• 0.6
4.8
E 0.5
,go.4
3.2E
o 0.3
2.4•
02
1.6
0.1
0.8
0
1200
1600
2000
II/4 / 79
2400
I
0400
0800
1200
12/4/79
Fig. 6. Storm responsein catchmentM8, April 11-12, 1979.
between0000 and 0800 h on April 12, which had EC of 50 and
51 #S/cm, respectively.The streamEC was initially 47 #S/cm
and roseto 80-100 + #S/crnin severalpulsesduring April 11.
It did not fall below the antecedentEC at any time. During
the rain on April 12, conductivityrose to over 140 #S/cm,
laggingthe hydrographpeak by about 1 hour. Changesin EC
consistentlyindicated increasesin total solutesrather than
dilution, which would be expectedif water from the storm
1982] and in suggestionsfor integration of field and model
studies[Dunne, 1983].
Our study refutestheseearlier conclusionsand castsserious
doubt on the relevanceof macroporeflow and other forms of
rapid throughflow of new water in steep,forestedcatchments
that have shallow,highly permeablesoils,and are highly responsiveto storm rainfall. The appropriatenessof field methods which have demonstratedrapid throughflowvelocitiesis
rainfall had dominated
the storm runoff. Chloride conalsoplacedin seriousdoubt.
centrationsin streamflowremained relatively steadybetween
The April 1979 storm results and persistentlags between
6.9 and 8.0 mg/L comparedwith concentrations
in rainfall in rainfall and streamflowcompositionfound in the long-term
the range0.4-1.0 mg/L, exceptin the initial 2 mm of eachfall, sample seriesled to an intensive study undertaken in 1983.
when concentrationswere 2.5 and 5.1 mg/L. The observed The subsequentparts of this seriesof papersdescribein detail
increases in EC and the lack of dilution of chloride indicate
the hillslopeand low-orderstreamresponses
to rainfall during
that the storm runoff response was predominantly water our intensive 2-month study and the relationship between
catchmentsizeand runoff response.
which has had a substantialperiod of contactwith the soil.
The solutechemistry
data are lessconclusive
thanthe •80
data becauseconcentrationcontacttime relationshipsfor EC
and chloride are not known for the soil water of the study
area. The combination of isotope concentration and solute
chemistrydata, however,providesstrongevidencethat at least
in small eventsunder moderately wet antecedentconditions,
rapid throughflowof infiltrated rainwateris not the mechanism that producesstorm runoff. The contributionof new
water (< 3% of hydrographvolume) to this particular storm
hydrographcan be accountedfor entirely by direct precipitation on the stream channel.
Acknowledgments.The Forest ResearchInstitute (New Zealand),
the Institute of Nuclear Sciences(New Zealand), the Weizmann Institute of Science(Israel),the GeographyDepartment of the University
of Bristol (England),and the Natural Sciencesand EngineeringResearchCouncil of Canada providedsupportfor M. G. Sklashduring
this study. B. J. O'Brien, Director of the Institute of Nuclear Sciences,
was instrumentalin arrangingfunding for M. G. Sklash'swork at the
Institute.J. Geany, J. Gray, P. Hinchey,C. Pruder,C. Woolmore,and
K. Sklashassistedin the collectionand analysisof water samples.K.
R. Lasseyof the Institute of Nuclear Scienceshelpedus in the mathematical treatment of our long-term data set. We are grateful to M.
Anderson,K. Beven,F. Hall, R. Jackson,J. Orwin, C. Rogers,and R.
Ward for their commentson variousdraftsof the manuscript.
SIGNIFICANCE OF THIS STUDY
Our study is the first comprehensivetest of the importance
of stored water in generatingstorm runoff in a highly responsive area. Previous studiesof the same area [Mosley, 1979]
had concludedthat rapid subsurfaceflow via macroporeswas
the predominant mechanismof storm runoff generation,that
new water was dominant in this flow, and that there was no
evidencefor displacementor translatory flow [Hewlett and
Hibbert, 1967]. These conclusionshave been considered in
recent modeling of rapid subsurfaceflow [e.g., Beven, 1981,
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