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GLOBAL BIOGEOCHEMICAL
CYCLES, VOL. 10, NO. 3, PAGES 473-482, SEPTEMBER
1996
Diffusional flux of CO: through snow:
Spatial and temporal variability amongalpine-subalpinesites
RichardA. Sommerfeld,William J. Massman,andRobertC. Musselman
U.S. Department
of AgricultureForestService,RockyMountainForestandRangeExperiment
Station,Fort
Collins,Colorado
Arvin R. Mosier
U.S. Departmentof AgricultureAgriculturalResearchService,FortCollins,Colorado
Abstract. Threealpineandthreesubalpine
sitesweremonitored
for upto 4 yearsto acquire
dataonthetemporalandspatialvariabilityof CO2flux throughsnowpacks.
We conclude
that
the snowformeda passivecapwhichcontrolledthe concentration
of CO2at the snow-soil
interface,whiletheflux of CO2intotheatmosphere
wascontrolled
by CO2production
in the
soil. Seasonal
variabilityin theflux at all siteswascharacterized
by earlywinterminima
followedby a risein flux thataveraged
70% abovetheminimaoverabouta 1-monthperiod.
The seasonal
variabilitywasnotrelatedto soiltemperatures
whichremainedrelatively
constant.Interannual
variabilitywassmall,andspatialvariabilitywassmallerthanpreviously
reported.Spatialvariabilityon a scaleof 1 to 10 m waslessthan30% of theaveragefluxes
andnotsignificantly
greaterthanestimated
errorin mosteases.Spatialvariabilityon a scaleof
10- to 100-m was about a factor of 2 and on a scale of 100 to 1000 m was about a factor of 4.
The 100-to 1000-mvariabilitywascomplicated
by thefactthatthesiteswerein different
ecosystems,
alpineandsubalpine,
andat differentelevations.We attributethesmallvariability
at the 1- to 10-mscaleto thedeepsnowcover,from 1.4 to 5 m. We hypothesize
that
horizontaldiffusionunderthesnowcoverreducedsmall-scale
horizontalgradients,
whilethe
insulatingeffectof thedeepsnowcoverkeptthesoiltemperature
andmoisturerelatively
constant.
Equivalent
annual
wintertime
fluxaveraged
about
95g C m'2yr'l inthealpine
and
about
232g Cm'2yr'l inthesubalpine
sites.Measurements
ofCO2concentrations
at0.2and
0.5 m in thesoilof oneof thesubalpine
sitesindicatedthatproduction
earlyin thesnowseason
occurredat or below0.5 m whileproduction
between0.5 m, andthe surfacebecameimportant
afar the start of the melt season.
Introduction
The flux of CO2 throughsnow coversmay constitutean
importantpart of the carbonbalancein alpine and subalpine
ecosystems
[SommerfeM
et al., 1993]. Snowmaycoverbetween
44 and 53% of the land areaof the northernhemisphere
during
the winter [Barry, 1992]. In high elevationand high latitude
areasthe soil may be snowcoveredfor the greaterpart of the
year. The insulating
effectof deepersnowcoverscanallowthe
underlyingsoilto maintaintemperatures
above -6.5'C, whichis
thoughtto be the lowesttemperatureat which CO2 production
occurs[Coxsonand Parkinson,1987]. CO2productionin soils
varies in time and space,and this variationis reflectedin
variationsin the fluxes throughsnow. It is importantto
characterize
this spatialandtemporalvariabilitybothto evaluate
theimportance
of subnivian
CO• production
onthe globalcarbon
Thispaperis notsubject
to U.S. copyright.Published
in 1996by the
.american
Geophysical
Union.
Papernumber96GB01610.
473
budgetandto help understand
the controlson CO2production
under snow. We have measuredthe fluxes of CO2 through
snowpacksat alpine and subalpinesites in the Glacier Lakes
Ecosystem
Experiments
Site(GLEES)in southcentralWyoming
for four seasons.The resultsshouldprovidea basisagainst
which other measurements
can be compared. Musselman
[1994] gives a comprehensivedescriptionof the GLEES.
Sitesfor SampleLocations
To investigatethe spatialvariabilityof CO2flux, six sitesat
separationsbetweenabout 50 and 2000 m were established.
Three of the sites were representative
of the local alpine
ecosystem,
andthreewere representative
of the local subalpine
ecosystem.In selectingthe sites,we attemptedto maximizethe
differencesin CO2 productionby choosingsiteswith different
vegetation, soil development,and wetness. To evaluate
temporalvariability,one alpine and one subalpinesite were
maintainedfor more than 3 years. Four additionalsiteswere
sampledfor onesnowseasoneach;two in 1992-1993andtwo in
1993-1994.At five of the siteswe established
pairsof sampling
474
SOMMERFELD ET AL.: DIFFUSIONAL FLUX OF CO• THROUGH SNOW
locationsseparated
by 2 to 4 m. At the sixth(subalpine)sitewe
usedfour of the samplinglocationsdescribedby Mosier et al.
[1994],whichwere separated
by 5 m eachandincludedsample
tubesat 0.2 and 0.5 m in the soil. This site was sampledin
1992-1993
Alpine Sites
The alpine sites are locatedat the alpine and subalpine
ecotoneat the GLEES. The main alpine site at about3290 m
abovesealevel (HILL) was maintainedfor 3 years,April 1991
throughJune 1993. The site is a dry, wind-exposedridge
between East and West Glacier Iatkes and the environment here
isharsh.Thewindaverages
11m s4 inthewinterand5 m s4 in
the summer.The vegetationis primarily grassesand cushion
plants; although there are a few small, scattered,Picea
engelmannii,Abies lasiocarpa,Juniperuscommunisvariety
depressa,and Salix brachycarpanear the site. Groundcover
speciesfound here include Vacciniumscoparium,Festuca
brachyphylla,
andSelaginella
densa.Vegetative
coveris sparse.
Soilsare thin and rockywith very little soil development.
The
maximummowpackhere averages1.8 m, about 90% of the
averagefor GLEES [Musselman,1994], and the well-drained
soil dries soonaRer mowmelt. This site is the highestin
elevationandthereforehasa loweraverageair temperature
with
the snowmeltbeginninglater in the seasonthan averagefor
GLEES. This sitewaschosenbecauseit wasexpectedto havea
relativelylow CO2 flux. The two samplelocationsat this site
wereseparated
by about4 m.
The secondalpine site, West Glacier Lake (WGL), at the
elevation of the HILL site and about 400 m to the southwest was
sampledfrom late December1993 throughApril 1994. It is a
largealpinemeadow,fed by snowmeltearly in the season,but
typicallybecomingdry in Julyor August.It is protectedfromthe
prevailingwind directionby topography
andnearbytrees.There
is a considerable
amountof exposed
rock. Primarycoverspecies
are Geum rossii and Sibbaldia procurebens. Artenna•qa
umb•qnella,Potentilia diversifolia, Danthonia intermedi,
Juncusdrummondii, Vacciniumscopariumand Carex rossii
alsooccuron the site. The two samplelocationsat this sitewere
separated
by about3 m and were sampledfor one snowseason
in 1993-1994.
The third alpine site, a wet alpinemeadow(WAM) located
about200 m northeastof the HILL site in a smallgully,was
sampled from November 1992 through June 1993. An
ephemeral
streamrunsthroughthe gullyin the springandearly
sununer,fed by a large, uphill snowpack. The gully also
accumulates
the deepestsnowpack
of all the sites;5 m during
the samplingyear. The area is vegetatedprimarily with
Deschampsiacespitosa, Carex nigricans, and Caltha
leptosepala,
with smalleramounts
of CarexaquatilisandJuncats
drummondii
present. The site is somewhat
shelteredfromthe
prevailingwinds and is surroundedby sparseforest near
timberline. Soil is thicker than the nearby HILL site.
Vegetationis alsodenser,becau• of the protection
andwetter
conditionsthan the HILL site. This site was sampledfor one
snowseasonin 1992-1993at two samplelocationsseparated
by
about 4 m.
SubalpineSites
The main subalpinesite (BRK) was maintainedfor 4 years,
April 1991 throughMay 1994. It was chosento contrastwith
the main alpine (HILL) site. The BRK site is a smallmeadow,
about25 m in diameter,surrounded
by forestexceptfor a small
openingleading to a larger open area about 20 m to the
southwest.The surrounding
forestprotectsthe locationfrom
prevailingwinds. Wind belowthe canopytop averages
about3
m s4 in winter and about2 m s'• in the summer. The common
species
areDeschampsia
cepitosaandCalthaleptosepala.Soils
are shallow,rocky,and dry after snowmelt.The environment
is
muchlessharshthanthealpinesite. The soilappears
to holdits
moisturelonger,and beingat lower elevation(3180 m), it is
warmer. This sitewaschosen
with the expectation
thatthe CO:
flux wouldbe higherthan at the alpinesites. The two sample
locations
at this sitewereabout10 m eastof the forestedgeand
wereseparated
by about3 m.
The secondsubalpinesite(FOR) is about100m westof BRK
in a densePicea engelmanniiforest stand,with almost no
understoryvegetationpresent at the site, but Vaccinium
scopa•umoccursin the immediatearea. Thereis a thick layer
of freewoodydebrisandduff fromtwigsandneedlesfromthe
overstory
trees. The snowpack
hereis thethinnestandspatially
the most variablein depth. This site was sampledfor one
season,September 1993 through April 1994 at locations
separated
by about2 m.
Locations
M3, M4, M5, andM6 (3 to 6 in the workof Mosier
et al. [1993])were 130 (M6) to 145m (M3) westof BRK. They
are in a wet meadowabout20 m wide by 70 m long,alonga
relativelyflat drainagebasin,with a ephemeralstreamchannel
which flows during snowmeltbut, the stream dries after
snowmeltis complete,generallyin early Julyto early August.
M3 and M4 grade from the drier Picea engelmanniiforest
towardthe wettercenterof the meadow.Commonspecies
here
are Deschampsiacepitosa and Caltha leptosepala. Salix
planifolia occursfrequentlyon the east edge (M3) of the
meadow.The centerof the meadow(M5 andM6) is wet, with
characteristicspeciesbeing E•geron ursinusand Plantago
tweedyi. Also presentare Trisetumspicatumand Achillea
millefolium.The four samplelocationswere eachseparated
by
about5 m each, startingabout3 m east of the forestedge.
Samplingwas conductedfrom Decem• 1992 throughMay
1993. This sitewas chosenwith the expectation
that it would
havethehighestCO• fluxes.
Methods
At each site, gas collectors[Soreinertialet al., 1991] were
installed at the soil surface before accumulation of the •nal
snowpack.Accesstubingwastapedto a 3-m-longpieceof 1/2
inch(13 nun)metaltubingsetverticallymorethan1 m fromthe
samplelocationto allow sampling
withoutdisturbing
the snow
directlyoverthe samplelocation.After thewinter'ssnowcover
beganto accumulate,sampleswere drawnfrom the collectors
andfrom the ambientatmosphere
on a variableschedule.The
snowdepthat thetimeof sampling
wasmeasured
by theheight
of themetaltubingabovethe snowsurfaceat eachlocation.The
samples
wereanalyzed
by gaschromatography.
Thediffusional
SOMMERFELD ET AL.: DIFFUSIONAL FLUX OF CO• THROUGH SNOW
fluxesthroughthe snowcoverswere estimatedby two different
methods. When snowstratigraphy
datawere availablefrom
snowpitsdugwithin 1 dayof the sample,themodelof Massman
et al. [1995] was used. This was the moreaccuratetechnique
becausethe model accountsfor layer porosity,tortuosity,and
thickness.At othertimesa voltune-weighted
meanporosityand
total snowthicknesswere used. The volume-weighted
mean
porosities
wereestimated
frominterpolations
of snowpit data.
475
calibrationaccuracy.Whenthe 4000 ppmvtankwasexhausted,
we concentrated
our calibrationeffortsin the range0 to 2000
ppmv where the gas chromatograph
exhibited nonlinearities
largerthan2%. This wasjustifiedby the manycomparisons
betweenthe 2000 and 4000 ppmv tanks which were linear
within 2%. We assumed from the calibration data between 2000
and4000 ppmvthat extrapolation
from 4000 to 10,000ppmv
couldbe donewith sufficientaccuracy
for our purposes.This
assumption
is requiredto recoverdatagreaterthan4000 ppmv.
Gas Analysis
Data in this range (about 14% of the total) were initially
of the
Gascollectorinstallations
at the baseof the snowpacks
were unexpected.Becauseof the extrapolation,the accuracy
concentrations
above
about
2500
ppmv
is
conservatively
as described
by Sommerfeld
et al. [1991]. Soil gascollectorsat
to be 5%, andfor simplicityin the followingwe have
M3 throughM6 were sectionsof 1/8 inch (3 mm) stainless estimated
robingembedded
in epoxyin 1 inch(25 ram)polyvinylchloride taken a conservativeestimate of the accuracyof all
measurements
as 5%. Thereare threemeasurements
(about1%
(PVC) robing. Eachsectionof stainless
robingpiercedthe wall
of
the
total)
above
6000
ppmv,
but
since
the
nonlinearfly
of the
of the PVC tubingat a samplinglevel from 0.1 to 0.5 m. The
ends of the stainless sections were finished
flush with
the
outsidesurfaceof the PVC tubingat eachlevel. The otherends
protruded
abovethe soil surfacewhen theseprobeswere forced
intoclosefittingholescoredin the soil.The levels0.2 and0.5 m
in the soilweresampledfor oneseason
in 1992-1993.
Twenty-mLnylonsyringes
wereusedto drawthe gassamples
and transport them to a Hewlett Packard 9730 gas
chromatograph
with a 3 M I-IaysepD columnand a thermal
conductivity
detector.Sampleswereanalyzedwithin4 hoursof
collection.Testsshowedthat samplesof 2000 partsper million
by volume (ppmv) stored in the syringes decreasedin
concentration
by about5% in 24 hours.
In 1991,standardgasesof approximately
350 and2000 ppmv
fromAir Products,Inc. were usedas workingstandards
for the
analyses.Startingin 1992,the 350 ppmvworkingstandard
was
calibratedagainsta primarystandardtank of 343.2 ñ 0.4 ppmv
obtained
from
the
National
Institute
of
Standards
and
Technology.
In addition, higher-concentration
working
standards
of approximately
1000, 2000, and 4000 ppmvwere
calibratedagainsta mixtureof 99.99% CO• (Air ProductsInc.)
anddry artificialair. Mixing was doneusingmassflow meters
calibratedagainsta Hastingsmodel HBM-1A bubble flow
meter,whichhasan accuracyof betterthan 1%. The precision
measurementsdecreasedwith concentration,we believe their
accuracyis also within our estimate. As discussed
below, the
errorsinvolvedin estimating
the porosities
of the snowcovers
for the flux calculations
are greaterthan 5%, so the errorsof
theseanalyses
are smallcompared
to othererrors.
SnowpackProperties
The mostimportantsnowpropertiesfor the estimationof
diffusionalflux are the porosityprofilesand the layer
thicknesses
[Massman
et al., 1995]. Snowpitsweredugat
timesthroughout
eachwinterto determiae
theporosity
profiles
frommeasurements
of densities
andstratigraphy
usingstandard
snowpit techniques.
Pitsweredugascloseaspractical
to the
sampling
sitesbut not socloseas to affectthe gasflux at the
sampling
sites.All sitesexceptFORhadsnowpacks
whichwere
relativelyfiat anduniform. Therefore
the stratigraphy
of the
samplingsiteswas assumedto be the sameas that determined
fromthe snowpits,with individual
layerslinearlyscaledfor
differences
in snowdepth.Thisassumption
maynothavebeen
accuratefor the FOR site. Snow pits were backfilledafter
stratigraphy
determinations.
For samplingtimesthat did not
correspond
closelyto the snowpit times, we used a time-
weighted
average
of thevolume-weighted
meanporosities
of the
snowpitsbeforeandal•erthesampling
dates.
was better than 1%. On occasion,the precisionof the gas
E•ence
has shownthat if the layeringis properly
chomatograph
was no betterthan2% as estimatedfrom scatter identified,scatterin density(porosity)measurements
within
in the calibrationsusingthe workingstandards.Thereforewe each layer is about 10%. Since this scatteris random,
of thesecalibrations,as estimatedfrom the scatterin the results,
conservativelyestimate the accuracy of the calibration inaccuracies
in theaverages
areexpected
to decrease
by na,
measurements
as ñ 2% in the range300 to 4000 ppmv.
wheren is thenumberof layers,usuallyabout10. However,the
The calibrations
allowed
determination
of the calibration
constant
(ppmv/signal
curvearea)for the range0 to 4000 ppmv.
This calibrationand subsequent
operationshowedthat the gas
chromatograph
hada small,nonlinearresponse
in the range0 to
2000 ppmv, which varied from run to run. A linear
extrapolation
from0 through350 ppmvcouldresultin an error
at and above 4000 ppmv of up to 5%. This result was
unexpected
sincethis typeof instrument
is usuallyassumed
to
be linearwithin about2% overits entirerange. To compensate
for this behaviorbetween0 and 2500 ppmv, we fitted a
polynomial
to theworkinggascalibrations
for eachdayandused
thatcalibration
curveto correctthegasanalyses.Fortherange0
to 2500 ppmv, accuracyis about 2%, the estimateof the
snowpitswereduga fewmetersfromthegasmeasurement
sites
andoftenporosities
wereestimatedfromthe porosities
of the
two snowpits closestin time. We thereforeconcludedthat
unless
thelayersweremorethan20 cmthick,takingmorethan
onesampleperlayerwouldnotmateriallyincrease
theaccuracy
of the porosityestimates.We makea conservative
estimateof
the accuracy
of the volume-weighted
meanporosityestimates
as
10%. The error estimatemustremainsomewhatsubjective
basedon ext•rience. An objectiveestimateof the errorwould
haveinvolveddestroying
the samplesite.
Icelenses
wererare. Onlysinglelayerswereobserved
in pits
dugafterthe middleof April, andtheydisappeared
in about2
weeks.Theirporosity
is difficultto characterize.
However,they
476
SOMMERFELD ET AL.' DIFFUSIONAL FLUX OF CO• THROUGH SNOW
Table 1. Comparisonof Fluxes Estimatedby a Layer Model and a
Volume-Weighted-Mean
PorosityModel for Days on Which Snow Pit
Information Was Available
Site
Layer Model
VWM Model
Mean
Average,
mgm'2s'•
Average
DifferencesofDifferences
StandardDeviation
0.0149
0.0123
0.0188
0.0176
0.0266
0.0261
0.0005
0.0008
0.0009
0.0008
0.0011
0.0011
cch
HILL-A
HILL-B
WAM-A
WAM-B
BRK-A
BRK-B
0.0143
0.0114
0.0178
0.0168
0.0255
0.0250
areverythincompared
to thetotalsnowthickness,
sothatlarge
errorsin estimating
theirporosities
addonlya smallerrorto the
volume-weighted
porosities.The influenceof an ice layerwould
onlybe dramaticif its porositywere 0, but suchice layersare
neverobservedin this region. That suchlayersdo not exist is
shownby the concentration
profilesreportedby SommerfeM
et
al. [1993], Mast et al. (1994), Massman,et al. (1995), and
unpublished
profilesobtainedduringthe courseof this study.
These profiles were basically linear and thus showed no
evidencefor anysignificantimpermeable
layer.
Flux Calculations
Assumingone dimensionalflux, the diffusionalflux follows
Fick's law
0.0004
0.0012
0.0001
0.0002
0.0002
0.0004
Results
StratigraphyModel Comparisons
Beforeflux datafromindividualsiteswereevaluated,
it was
necessary
to det•e
and correctany inaccuracy
involvedin
usingvolume-weighted
meanporosities
whensnowpit data
werenot available.A comparison
betweenthe fluxescalculated
usingthe layer modeland fluxescalculated
usingvolumeweightedmeanporositiesfrom 28 setsof concentration
datais
shown
in Table1. Errorscreated
byusingthevolume-weighted
meanporosities
areup to about+5% andaveraged
+4.4% of the
flux for all the measurements.
On the basisof our results,
diffusional
fluxescalculated
for timeperiods
withoutsnowpit
datawerereduced
by4.4%. Thiscorrection
wasalsoappliedin
determining
averagefluxespresented
in Table2.
Alpine Sites
J=-•xD
dz
I-HLL
Data for the I-IILL site include measurementsfrom
April1991through
June1993.Theaverage
fluxesfortheperiod
studied
areshown
in Table2. Thetwolocations
differbyabout
where• is the porosityof the layer,x is the tormosity
of the 6% of the averageflux, not a significant
difference
givenan
layer,D is the diffusioncoefficientfor CO2 in air and dc/dz is
errorestimateof 11%. The standard
deviationof the daily
the concentration
gradientacrossa uniformlayer. We assumed differences
is about30% of the flux indicatingshort-time
that x = 1. Tortuosityis a difficultquantityto measureand variations
in the fluxes. The seasonal
temporal
variabilityis
proved impossible for the number of samples taken.
Nevertheless,limited measurementsof the tormositiesof snow
consistent
from year to year. Eachyear exhibitsa minimtun
earlyin thecalendar
year,a riseto a maximum
whichbeginsat
aboutthetimeof onsetsnowmelt,anda dropbeforethe snow
(W. Massman
et al., manuscript
in preparation,
1996)suggesting disappears
(Figurela). The riseaveraged
about95%overthe
minima with a duration of about 1 month.
thatwe mayhaveoverestimated
ourfluxby 10to 25%.
Windpumping
mayalsoaffectthe flux [AlbertandHardy,
WAM. Forthisstudy,the lowestdatapoint(WAM A, May
1995;Massmanet al., 1995;W. Massmanet al. manuscript
in
25, 1993) was removedfrom the data set. At the time that
preparation,
1996] However,the magnitude
of the effectis still
measurement
was made,water was observed
to be running
unclear.
beneath
thesnowcover.In a few daysa largeholeopened
in
The snowdepthandthe diffusioncoefficientof CO2in air are thissnowcoverandtheconcentrations
at thissampling
location
knownto betterthan 1%. Thus,apart from tortuosityand droppedto about2000 ppmv. Becauseof the waterflow, we
possiblewind effects,mostof the errorresidesin estimatesof
think this point is more representative
of the concentration
concentration
(5%)and porosity(10%). A conservative
estimate upstream
of this samplinglocation. Temporalvariabilityis
of the accuracy
of the productof porosityandconcentration
is similarat the two samplelocationsbothof whichexhibitthe
then11%[Goodman,
1960]whichwetakeastheaccuracy
of the earlywinterminimaanda riseto a maximumaveraging
70%
fluxestimates.
Again,theestimates
at theFORsitemaynotbe higherovera 1 monthperiod,corresponding
approximately
to
thisaccurate
because
of variablesnowdepth.If ourestimate
of theonsetof melting.Thissitehasanaverage
fluxintermediate
the accuracyof the of the concentration
measurements
is
between
theotheralpinesitesandthe subalpine
sites(below).
degraded
to 10%, the overallerrorwouldbe 14% [Goodman, The averages
differby about10%, andthe standard
deviationof
•960].
thedailydifferences
is about15%of theaverage
flux. Bothare
layersnear sitesM3-M6 indicatea value between0.75 and 0.9
SOMMERFELD ET AL.' DIFFUSIONAL FLUX OF CO2 THROUGH SNOW
477
Table 2. Summaryof CO2Fluxes
Location
Numberof
Hill A
Hill B
WGL 2
WGL 4
WAMA
WAMB
BRK A
BRK B
FOR A
FOR B
M3
M4
M5
M6
25
25
7
5
12
10
30
30
9
7
11
g
11
11
Coefficientof
Standard
Deviation
Average,mg
Samples m':s4 CO•
Variation
0.44
0.0043
0.0097
0.0103
0.0088
0.0071
0.0136
0.0167
0.01 g9
0.01 g4
0.0140
0.0229
O.O372
0.0369
0.0320
0.0359
0.35
0.30
0.37
0.37
0.37
0.27
0.19
0.65
0.37
0.17
0.21
0.25
0.15
0.0046
0.0024
0.0026
0.0051
0.0062
0.0050
0.0035
0.0090
0.0084
0.0062
0.0076
0.0080
0.0055
Maximum
0.0224
0.023g
0.0112
0.0096
0.0265
0.0304
0.0330
0.0253
0.0363
0.0394
0.0494
0.04g0
0.0476
0.0456
M'mimum
0.0040
0.0042
0.0054
0.0044
0.003g
0.0115
0.0093
0.0071
0.0064
0.0156
0.0293
0.0233
0.0215
0.0300
Range
0.0184
0.0196
0.0057
0.0052
0.022g
0.01 g9
0.0247
0.01 g2
0.0300
0.022g
0.0200
0.0247
0.0261
0.0156
$. D. of Daily
Differences
0.0032
0.0012
0.005g
0.0040
0.0151
0.0043
0.0034
closeto the estimatederror of 11%, indicatingno seasonalor
short-time
0.04
--
HillA
¸
Hill B
+
0.00
i
i i i [ i i i
1991
1992
i
i
11993
difference
between
locations.
This
site had the
thickestsnowpack,
whichshouldhavebeenthe mosteffectivein
decreasing
horizontalgradients.
WGL This sitehad averagefluxesabout20% smallerthan
the HILL siteandwith a smallerrange. Temporalvariabilityis
similarbetweenthe two samplelocationsand to the HILL site,
with the earlywinter•m
risingto an average80% higher
overa 1-monthperiod. In this casethe rise startedwell before
the melt period(Figurel c). The averages
differ by about15%,
andthe standarddeviationof the daily differencesis about16%,
onlyslightlyhigherthanthe estimatederror. Thusthe seasonal
andshort-termdifferences
maynothavebeensignificant.
SubalpineSites
q- WAM A
0.04
--
¸
WAM B
BRK.
The most extensive data are available from the BRK
site (Figure ld). The 1991 data collectionbegan in the
beginningof April and continuedto the end of March 1994. In
1992-1994, minima in the fluxes were observed in the first
b
0.00
;19'92' 963'
+ WGL 2
0.04--
¸
WGL 4
-
c
0.00
I1993
i i I • 994
i i
Figure 1. Timeseriesof CO• at thedifferentsites:(a) HILL, (b)
WAM, (c) WGL, (d) BRK, (e)FOR, and(f) M3 - M6.
quarterof eachyear, and maximawere observedin April at
approximatelythe time that melt starts. In 1991-1993 the
maximaare followedby a distinctdropin flux just beforethe
snowdisappears.The maximaaveragedabout25% higherthan
the minimaandroseoverabouta 1-monthperiod. Rangesand
averagesof the fluxesare in closeagreementfor the 4 years.
The averages
differby about10%, nothigherthanthe estimated
error. The standarddeviationof the daily differencesbetween
the locationsat this site is about 20% of the averageflux,
indicatingsomeshort-time
variabilitybetweenthefluxes.
FOR. This site showedthe greatestrangeof fluxesbut also
had the early winter minima rising to maxima averaging60%
higherin about2 weeks. The rise startedbeforethe onsetof
meltingandpersistedabout1 month. Driftingandto a smaller
extentinterception
causedunevensnowdepositionwhichmade
it difficultto estimatethe effectivesnowdepthat eachlocation.
The averageflux was aboutthe sameas BRK. The averages
differedby about20%, and the standarddeviationof the daily
differences
wasabout80% of the averageflux, bothsignificantly
largerthanthe estimatederror. However,because
the estimated
errorin the porosityand depthmay not be reliableat this site,
the20% differencebetweenaverages
maynotbe significant.
478
SOMMERFELD ET AL.: DIFFUSIONAL FLUX OF CCh THROUGH SNOW
0.04
+ BRK A
--
0.00
i
i
¸
i
i
BRK B
i
i
i
i
i
i
11992 11993
1991
i
i
1994
d
FOR A
0.04
--
¸
¸ FOR B
+
+
0.00
-I- M3
-I- M5
¸
¸
M4
M6
0.04
0.00
I
1992
1993
I
1992
I
I
1993
Figure 1. (continued)
M3 through M6. There is a greatertemporalvariation
amongthe measurements
at thesefour samplelocationsthanat
any but the FOR site. However,they all exhibitminimaand
maximasimilarto thoseof theothersites.An average70% rise
overthe minima startedbeforethe onsetof meltingand lasted
about6 weeks. The averagesdifferedby almost50% across
locationsprobably,reflecting different soil regimes at the
locations.The standarddeviationof the daily differenceswas
about10% of the averageflux, whichis not significantly
larger
than the estimated error.
the slope of the linear regressionto the 0.5-m data is
substantially
lowerthantheslopes
of0.2and0 m. Also,R2for
0.5 m is lower for all thesedata, and the concentrations
at 0.5 m
arehigherfor mostof theperiod.However,at thebeginning
of
snowmelt
theconcentrations
amongthedepthstendto converge.
A constant
concentration
throughout
the depthof soil, combined
withthemeasured
flux throughtheoverlying
snow,impliesthat
there is a distributed source in the soil between 0.2 and 0.5 m.
Discussion
Soil concentrations. Figures 2a-2d show the temporal
TemporalVariability
variationof the concentrations
at the soil surface,0.2 and 0.5 m
belowthe soil surface.All locationsat this site showa general
Longer-termtemporalvariability amongall the locations
increasingtrend in concentration
duringeach sampleseason. show distinct similarities. Concentrations at the snow-soil
Table3 givesthe parameters
of linearfits to data. For all cases interface increase from the start of the snow season until the
SOMMERFELD ET AL.: DIFFUSIONAL FLUX OF CO• THROUGH SNOW
DEPTH
10000
--
I
•
479
IN SOIL
ocm
20
cm
5000
•ANUARY
I
I
JANUS.'
APRIL
1993
1993
b. M4
a. M3
10000
5000
JANUARY
APRIl.
1993
c. M5
1993
d. M6
Figure 2. Temporalvariationof CChconcentrations
at the soilsurface,andat depthsof 0.2 and0.5 m in
the soil,at samplelocations
(a) M3, (b) M4, (c) M5, and(d) M6.
startof themelt season.Figure3 showsa representative
plot of
concentration
andsnowwaterequivalentwith the axesscaledfor
correspondence
betweenthe two curves.Fick's law showsthat
if the flux is constant,
the concentration
is linearlyrelatedto the
snowdepthand inverselyrelatedto the porosity. If the snow
depthis constant,
the concentration
will increasewith increasing
snowwaterequivalent.Figure3 showsthatthe concentration
at
thebaseof the snowpack
tracksthe snowwaterequivalent.
Figure2 showsthatthe concentration
at the baseof the
snowpack
at M3-M6 increasessteadily. In contrast,the
diffusio• fluxes(Figure l f) showdistinctminima. The most
extreme case of this behavior was at WAM A, where the
concentration
increasedmonotonically
up to a maxim.urnof
10,464ppmvwhilethe flux (Figurelb) wentthrougha distinct
minimumand then a maximum. The changebetweenthe
minimumandmaximumwasaboutaverageforthisstudy.Thus,
to a first order, control of the concentrationat the snow-soil
Table 3. Linear TemporalTrendsof C02
Concentrations in Soil
interface
isthesnow
depth,
whilethefluxismainly
controlled
by production
in the soil. In somecases,concentration
increased
by more than an order of magnitudefrom the start of snow
accumulationto the time of maximum accumulation,while the
Level, m
_
CO• Concentration,
ppmv/day
M3
0.93
flux went througha minimumand then a maximum. Secondordercontrolof the concentration
is by meansof the flux from
the soil. This is indicatedby the factthatthe concentration
and
snow water equivalent(SWE) plots do not track perfectly
(Figure3). If the SWEwereconstant,
Fick'slaw showsthatthe
concentration
would changeproportionalto the flux. Data
0.0
38
-0.2
38
0.95
-0.5
26
0.80
0.0
38
0.94
-0.2
38
0.79
-0.5
16
0.70
of 3 overthe minimumbecauseof flux changes
but that the
concentration
was observedto changeby morethan a factorof
10. It is alsointeresting
thatthe veryhighconcentration
nearthe
M4
M5
0.0
27
0.86
-0.2
-0.5
29
14
0.90
0.65
0.0
31
0.83
-0.2
38
0.93
-0.5
22
0.88
M6
presented
hereshowthatthechange
couldbeashighasa factor
time of maximum
flux before snowmelt at WAM
A indicates
that the productionof CCh is not limited by high CCh
concentrations
up to at least1%.
It is possiblethatproduction
or consumption
of CChoccursin
the snowpack.Suchprocesses
within the snowthat affectthe
flux wouldcauseflux divergence
whichwouldbe evidencedby
480
SOMMERFELD ET AL.: DWFUSIONAL FLUX OF CO• THROUGH SNOW
-- 1oo
4000
--
1992 I
•
BRK-A
•
BRK-B
- 5o
1993
Figure3. Concentrations
of CO•at themow-soilinterface
andthesnowwaterequivalent
at BRK,the
main subalpinesite.
nonlinearities
in concentration
profilesnot relatedto porosityor
tortuositydifferences. Sommerfeldet al. [1993] show such
evidencefor the BRK (their "Subalpine")site in 1991. The
production
of CO: in the snowat that siteaddedat most10% to
the total flux. Subsequentprofiles, includingprofilestaken
duringthis study,indicatethat the 1991production
in the snow
wasan extremeevent. Mast et al. [1994] fittedtheir CO:-snow
profiledatawithlinearregressions
R2 between
0.82and0.99,
similarlyindicatinglittle if anyflux divergence
in the snow.
Likewise,storageprocesses
withinthe snoware not likely to
be importantontimescales
of morethana few hours. In the dry
snowpack,
surfaceadsorption
ontheiceis insignificant
[Ocampo
and Kilnget, 1983]. In a wet snowpack
at 10% liquid water
content, a cubic meter of snow with a CO2 concentration
increasing
from0 to 1000PPMV, the storagein solutioncould
absorb
2 to3 hours
ofCO:ata fluxrateofabout
0.02mgm'2s4.
Approximate calculationsshoxv that flushing of CO: by
downward
movingmeltwateris about2-ordersof magnitude
less
than the upwardfluxes we measured. Winstonet al. [1992]
measuredlower fluxesfrom the top of the snowthan from the
soil. However,they hypothesized
that the CO: was channeled
alongtree trunk and stem macrochannels
at their site which
couldaccountfor the discrepancy.This processwas possible
onlyat ourFOR site,butbecause
of driring, thetreewellswere
not as well developedas thosedescribedby Winstonet al.
[1995]. Theseresultsleadto the conclusion
thatthe maincause
of temporalvariabilityin the concentrations
at the soil surfaceis
the snow layer which forms a passive,porouscap over the
samplinglocationbut that snowprocesses
do not affectthe flux
significantly
ontimescales
greaterthana fewhours.In all cases
the flux reachesa maximumat approximately
the time the snow
water equivalentreachesa maximum:oppositethe behavior
expectedif the snowpackwere controllingthe flux. The
diffusionmodelwe usedandthe concentration
profilesshowthat
ice lensesarenota significant
factorin ourflux estimates.
Thus it is clear that there is a secondarycause of
concentration
variabilitywhichis relatedto temporalvariability
in the fluxesinto the baseof the snowpack.Withoutsignificant
sources
or sinksof CO• in the snow,processes
in the soilcontrol
theflux. Theseprocesses
weresignificantly
differentamongour
differentsites, as would be expected. Differencesbetween
locationsat eachsiteweremorevariable. Sometimes
theywere
greaterthanthe estimatederrorandsometimes
not.
All sitesexhibita minimumin flux earlyin the winter. This
is in agreement
with the measurements
of Winston
et al. [1995],
who attributed the minimum in their measurements to low soil
temperature. A flux minimum early in the winter is also
apparentin the measurements
of Brookset al. [1995]on Niwot
Ridge, Colorado. Three candidatesfor causingtemporal
variationsin CO2fluxesfrom the soil are temperature
changes,
soilmoisturechanges,andpopulationchanges
in the soilbiota.
Figure4 showsthe 5- and 20-cmsoil temperatures
at two sites,
onenearthe alpineandonenearthe subalpinesites. ARer the
establishment
of a significantsnowcoverin late November,
thereis no significantchangein the soil temperatures.These
remainaround-1ø to IøC, well abovethe approximately
-6.5øC
thatCoxsonandParlanson[1987]giveas a cutofftemperature.
for biologicalactivity. The constant
temperature
lastsuntil the
snowmeltsawaywhenthereis a steeprise in soiltemperature.
Notethattherisein CO• production
beginsbeforetherisein soil
temperature.The temperaturechangesat 0.5 m, whichappears
to be nearthe sourceof CO• production,
wouldbe evensmaller.
Thussoiltemperature
changes
cannotbe a factorin thetemporal
variabilityof the fluxesat thesesites. It is possiblethat air
temperaturechangesaffected root respirationat those sites
wherevegetation
protrudedabovethe snowpack,
mainlyin the
subalpine.However,this couldnot be the caseat WAM, where
the snowpack
was 3 m deepon January28 and 4 m deepon
February27. The othertwo alpine siteswere alsofar enough
from prong
vegetationthat air temperaturerelatedeffects
areunlikely.
No soil moisture measurements are available for these time
periods.However,watervaporis transported
fromthe soil into
the snow cover during the courseof the winter, driven by
temperate gradients[SmithandBoyne,1982]. Thesesitesare
characterizedby a clefmiteonset of melting each year, as
contrasted
with otherclimateswheremeltingmayoccurseveral
timesduringthe winter. Moistureflowsbackintothe soilfrom
thesnowpack
whensignificant
meltingoccurs.Finally,highsoil
moisture leads to anaerobic conditions, which limits CO•
production,
causinga decreasein flux. Brookset al. [1996]
observedsuchdecreases
in soil moistureat their alpine site.
However,theminimumsoilmoisturewhichtheyobserved
seems
too high to limit microbialactivity. Furthermore,someof the
risesto maximaappearedto start beforethere was significant
melting. While the variationin CCh may correlatewith the
SOMMERFELD ET AL.: DIFFUSIONAL FLUX OF CO: THROUGH SNOW
481
15--
10--
5--
-5
a Alpine
b Subalpine
Figure4. Temperatures
at 0.05 and0.2 m in thesoilat (a) GlacierLakesEcosystem
Experiments
Site
towerand(b) BrooklynLake Tower.
expected
changes
in soil moisture,this remainsa Slx•ulation chosenin an attemptto maximizethe differences.Thesediffer
bylessthana factorof 4 between
thealpineandsubalpine.
The
Thepopulation
dynamics
of thesoilbiotaresponsible
forthe spatial variability betweenthese two types of sites is
production
of CO•arenotknown.Brookset al. [1995]speculate complicated
bythefactthattheyrepresent
different
ecosystems.
thatthevariations
in CO• flux .whichtheyobserved
weredueto TheWGL siteis, in fact,about420 m fromtheI-BLLsite,but
a combination
of factorsaffecting
microbialactivity.We think the difference in flux is less than a factor of 2.
thismaybethemostpromising
explanation.
which should be tested.
Errors
SpatialVariability
It is apparent
fromFiguresl a-lf thatthe horizontal
spatial
variability
ona scaleof about1 to 10m is notverygreat.Table
2 liststhestandard
deviations
of thepaireddifferences
for each
sampling
siteforeachsampling
day. M3 is pairedwithM4, and
M5 is pairedwithM6 for thisanalysis.The largestdifferences
are for the FOR site.Herethe averagefluxesdifferby about
The measurement
of gasflux acrossthe Earth'ssurfacehas
problems
whichhavenotbeenentirelyresolved.Most studies
use sometype of chamberembeddedin the surface. Here we
haveuseda method
whosephysics
is basically
simple,Fick's
law diffusionthrougha porousmedium.Because
thismethodis
notdirectlycomparable
to othermethods,
we havegivena more
50%, and the standarddeviationof the differencesis about 80%
detailed discussionof errors than is usual. We have concluded
that most of the error resides in the estimates of the snow
oftheaverage
fluxforthelocation.Thishighvariability
maybe
theresultof treewellsasdescribed
by Winston
et al. [1995]or
porosity
andthattheseerrorscannotbe evaluated
objectively
withoutgreat,andprobablyunwarranted,
effort. Thereforewe
the variablesnowdepthobserved.For the othersitesthe within
have made what we considera' fair and conservativeestimate of
sitevariations
on a scaleof a few metersrangebetween32% our errors. Our error analysisindicatesthat estimatesof CO•
(HILL) and 8% (M5-M6). Thus they are of marg'mal fluxesthat differ by lessthan 15% may not be significant.
significance
comparedto the overall accuracyof these However,flux differences
greaterthan25% aresignificant.
measurements.
TheHILL sitehasa largepercentage
difference
becauseits averageflux is low, but the standarddeviationis
consistent with those at the other sites.
Zimovet al. [1993]and Winstonet al. [1995]reportmuch
higher small-scalespatial variability. We attribute the
differencein our resultsto the fact that our snowpacks
were
muchdeeperand of lowerporosity.By impedingthe vertical
flux, the deepersnowpackwould allow horizontalfluxes to
decrease
the spatialdifferences
betweenthe sampling
locations.
Our snowpacks
werealsoflat with no macrochannels
causedby
treethinksor stems,exceptat the FOR site. It is alsopossible
that someof the measurements
of Zimov et al. [1993] and
Winstonet al. [1995] were influencedby wind pumpingwhich
wouldhavea stronger
influencewith thinner,moreporoussnow
covers[AlbertandHardy, 1995;Massmanet al. 1995].
Analysisof differences
betweensiteswithineitherthealpine
or subalpine
indicates
thatspatialvariabilityof averageflux ona
scaleof 10 to 100m is withina factorof 2 (Table2). Largescalespatialvariabilityover100-to 1000-mspacing
is alsolow,
eSlXXiallyconsideringthe fact that the different sites were
Conclusions
CO• flux valuesestimated
in this study.
yieldedequivalent
annual
wintertime
fluxesthataveraged
about
95 g C m'2yr4 in
thealpine
andabout
232gCma yr4 inthesubalpine
sites.This
compares
withanannual
fluxrange
of29 to 95g C m'2yr4 for
tundraand 120 to 550 g C m'2 yr4 for borealforestand
woodland(Raich and Schlesinger,
1992). Sincethe snowcovered
periodcanbe morethan70% of the year,the winter
CO• flux shouldbe significantin the carbonbalancein the
ecosystems
we studied[Sommerfeld
etal., 1993].
Temporalvariabilityin CO• concentrations
at the soil-snow
interface
wascaused
primarilyby the variabilityin snowwater
equivalent
of the overlying
snow. Secondary
variations
in CO•
concentrations
were causedby variabilityin the flux from the
soil.
Spatialvariationin flux on a scaleof about1-10 m was small
in thisstudy.Averagefluxdifferences
betweenlocations
at each
of the samplesitesweregenerally
not significantly
largerthan
482
SOMMERFELD ET AL.: DWFUSIONAL FLUX OF CO2 THROUGH SNOW
the estimatederror. This was probablythe resultof the deep Massman,W., R. Sommerfeld,K Zeller,, T. Hehn, L. Hudnell, and S.
Rochelle,, CO2 flux througha Wyoming seasonalsnowpack:
snowpack,whichwouldhave facilitatedhorizontaldiffusionto
Diffusional effects, pressurepumping effects, and eddy
decrease
horizontalgradients.Differencesbetweenlocationsat
covariance measurements,
in Biogeochemistry
of Seasonally
eachsitewere sometimes
differentfor samplingdaysindicating
Snow-CoveredCatchements,
editedby K. A. Tonnessen,
M. W.
someshorter-term
flux variations.Spatialvariabilityon a scale
Williams, and M. Tranter,IAHS Publ.-228, 71-80, 1995.
of 10 to 100m wasabouta factorof 2. Spatialvariabilityonthe Mash M. A., D. W. Clow, and R. G. Striegl,Flux of carbondioxide
scaleof 100 to 1000 m was about a factorof 4, but this seemed
correlated with elevation.
Almostall of the cch originates
in the soilbelowthe snowso
that temporaland spatialvariationsin the fluxesthroughthe
snowmustbe causedby variationsin soilproduction.An early
winterminirotanin CO2production
was observed
at all sites.
The minima are similar to those observedat Niwot Ridge
Colorado[Brookset al., 1996] about150 km fromthe GLEES
andin Manitoba[Winstonet aL, 1995]approximately
1500km
from the GLEES. However, the cause of the early winter
minirotanin production
is obscure.Oar datadidnotsupport
air
or soiltemperature
variations
asa cause.Whilethevariationin
flux probablycorrelates
with soilmoisturedepletioncausedby
thermalgradients,
themount of depletion
doesnotappearto be
great enoughto limit CO2 production[Brookset al., 1995;
Mosieret aL, 1993]. Theroleof population
dynamics
of thesoil
microbiota
remainsan openquestion.
Acknowledgments.We wouldlike to thankthe EditorJorgeL.
Sarmiento,and two anonymous
refereesfor suggestions
which
materiallyimprovedthispaper.
and methane through a seasonalsnowpackin a subalpine
watershed,
(abstract)Eos Trans.AGU, 75(44) Fall Meet. Suppl.
205, 1993.
Mosier, A. R., L. K. Klemedtsson,R.A.
Sommerfeld,and R. C.
Musselman,Methane and nitrous oxide flux in a Wyoming
subalpinemeadow,Global BiogeochemCycles, 7(4) 771-784,
1993.
Musselman,R. C., The glacierlakes ecosystemexperimentssite,
U.S.Dep. of Agric. For. Serv., Gen. Tech.Rep. RM-249, Fort
Collins, Colo., 1994.
•po,
J., andJ. Klinger,Modificationof the surfacestructureof ice
duringaging,J. Phys.Chent,87, 4167-4170, 1983.
Raich,J.W., andW. H. Schlesinger,
The globalcarbondioxideflux in
soilrespiration
andits relationship
to vegetation
andclimate,Ser.
B. 44, 81-99, 1992.
Smith,F. W., and H. S. Boyne,Snow pillow systembehaviorfor
Snotelapplication,final reportfor the periodSept. 1980 to Dec.
1981, SnotelRes. Team,U.S. Dep. ofAgfic. Soil Conserv.Serv.,
Fort Collins, Colo., 1982.
Sommerfeld,R. A., R. C. Musselman,J. O. Reuss,and A. R. Mosier,
Preliminarymeasurements
of CO2 in meltingsnow,Geophys.
Res.Lett., 18(7), 1225-1228, 1991.
Sommerfeld,R. A., A, R Mosier, and R. C. Musselman,CO2, CH4
andN20 fluxthrougha Wyomingsnowpack
and implications
for
globalbudgets,Nature, 361, 140-142, 1993.
Winston,G. C., E. T. Sundquish
and A. B. Shortlidge,CO• fluxes
throughthe snowpackat SleepersRiver,Vermont(abstract),Eos
References
Trans.AGU, Fall Meet.Suppl.,73(43), 174, 1992.
E. T. Sundquish
J.P. Hardy,andR.
Albert,M. R., and J.P. Hardy,Ventilation experiments
in a seasonal Winston,G. C., B. B. Stephens,
E. Davis,Seasonal
variabilityin CO2 transportthroughsnowin a
snow cover, in Biogeochemistryof SeasonallySnow-Covered
borealforest,in Biogeochemistry
of SeasonallySnow-Covered
Catchements,
editedby K. A. Tonnessen,
M. W. Williams,and M.
Catchements,
editedby K. P• Tonnessen,
M. W. Williams,andM.
Tranter, IAHS Publ.-228, 41-50, 1995.
Tranter,IAHS Publ.-228, 61-70, 1995.
Barry,R. G., Climate-IceInteractions,Encyclopediaof Earth System
Zimov, S. A., G. M. Zimova, S. P. Daviodov,A. I. Daviodova,Y. V.
Science,vol. 1, pp. 517-524,Academic,SanDiego,Calit•1992.
Voropaev, Z. V. Voropaeva,S. F. Prosiannikov,and O. V.
Brooks, P. D., M. W. Williams, D. A. Walker, and D. A. Schmidt,
Prosiannikova,I. V. Semiletova,and I. P. Swmiletov,Winter
The Niwot Ridge snow fence experiment: biogeochemical
bioticactivityand production
of CO2 in Siberian soils:A factorin
responses to changes in the seasonal snowpack, in
thegreenhouse
effect,J. Geophys.
Res.98(3) 5017-5023,1993.
Biogeochernistryof Seasonally Snow-CoveredCatchements,
editedby K. A. Tonnessen,
M. W. Williams,andM. Tranter,/AHS
Publ.-228, 293-302, 1995.
Brooks,P. D., M. W. Williams,and S. K Schmidt,Microbialactivity
under alpine snowpacks:Implicationsfor immobilizationof
atmospheric
N inputs,Biogeochemistry,
in press,1996.
Coxson,D. S., and D. Parkinson,Winter respiratoryactivityin aspen
woodlandforest floor litter and soils,Soil Biol. Biochem, 19, 4959, 1987.
Goodman,L. A., On the exact varianceof products,J. Am Stat.
Assoc., 55, 708-713, 1960.
R. A. Sommerfeld,W. J. Massman,and Robert C. Musselman,
U.S. Departmentof AgricultureForest Service,Rocky Mountain
Forest and Range ExperimentStation,240 W. ProspectRd., Fort
Collins, CO 80526.
A. R. Mosier, U.S. Departmentof Agriculture, Agricultural
ResearchService,P.O. Box E, Fort Collins,CO 80522.
(Received•mber
5, 1995; revisedMay 22, 1995;
acceptedMay 23, 1996.)