Isotopic Evolution of Isotopic Evolution of SnowmeltSnowmelt

Vicky RobertsVicky RobertsPaul AboodPaul AboodWatershed Watershed BiogeochemistryBiogeochemistry2/20/062/20/06

Isotopes in Hydrograph SeparationIsotopes in Hydrograph Separation

• Used to separate stream discharge into a small number of sources

• Oxygen and hydrogen isotopes are widely used because they are components of water and are conservative over short time scales

ProblemProblem

• For hydrograph separations involving snowmelt runoff– Some studies assume snowmelt to have a

constant 18O value equal to the average 18O of the snowpack

– 18O in snowmelt ≠ 18O snowpack

Snowmelt IsotopesSnowmelt Isotopes

• Snowmelt– Depleted in 18O early in melting season– Enriched in 18O later in melting season

• Why? – Isotopic exchange between liquid water and

solid ice as water percolates down the snow column

Physical ProcessPhysical Process

• At equilibrium, the 18O of water is less than the 18O of ice; initial snowmelt has lower 18O than the snowpack

• Snowpack becomes enriched in 18O ; melt from the enriched pack is itself enriched (18O )

PapersPapers

• Theory– Feng, X., Taylor, S., and Renshaw, C.E.

2002.

• Lab– Taylor, S., Feng, X., and Renshaw, C.E.

2002.

• Field– Taylor, S., Feng, X., Williams, M., and

McNamara, J. 2002.

Feng: Theoretical model Feng: Theoretical model quantitatively indicating quantitatively indicating

isotope exchangeisotope exchange•Varied two parameters:Varied two parameters:

–Effectiveness of isotopic exchange (Ψ)–Ice-liquid ratio (γ)

Isotopic exchangeIsotopic exchange

• Rliq controlled by advection, dispersion and ice-water isotopic exchange

• Rice controlled by ice-water exchange• Rate of isotopic exchange dependent on:

Fraction of ice involved in exchange, f– Dependent on size and surface roughness of ice grains– Accessibility of ice surface to infiltrating water– Extent of diffusion within ice– Amount of melting and refreezing at ice surfaceIce-liquid ratio quantified by: γ = bf

a + bf where a = mass of water

b = mass of ice per unit volume of snow i.e. ratio of liquid to ice

Effectiveness of exchange:Effectiveness of exchange:

Ψ= krZ

u*

• Kr is a constant

• Z = snow depth

• U* = flow velocity

Ψ and γ dependent on melt rate and snow properties e.g. grain size, permeability

Results: Results:

• Effect of varying ψ (effectiveness of isotope exchange)

• Relative to original bulk snow (18O=0)

• Where Ψ is large = curved trend (a) – Base of snowpack is 18O

depleted as substantial exchange occurs

– Low melt rate so slower percolation velocity

• Where Ψ is small = linear trend (e)– Constant 3‰ difference

between liquid and ice

• Effect of varying γ (and therefore f):

• Relative to original bulk snow (δ18O=0)

• Low γ = curved trend (e)– Slow melt rate– Lower liquid: ice ratio as

lower water content

• High γ = linear trend (a)– Fast melt rate– Higher water content so

more recrystallizationTherefore constant difference

in 18O of snowmelt and bulk snow

Conclusions:Conclusions:

• High melt rate = effective exchange and high liquid: ice ratio. Higher percolation velocity so constant difference in 18O. Increased water content triggers recrystallisation, a mechanism of isotope exchange.– linear trend

• Low melt rate = Large difference in 18O initially due to substantial exchange– Only a small proportion of ice is involved in isotopic exchange

therefore insignificant change in 18O of bulk ice– 18O of liquid and ice reach steady state resulting in curved trend

as equilibrium is reached

Assumptions:Assumptions:

• Snow melted from the surface at constant rate

• Dispersion is insignificant

• 18O exchange occurs between percolating water and ice

Implications:Implications:

• Variation in 18O between snowmelt and bulk snow causes errors in hydrograph separation if bulk snow values are used

Taylor: Laboratory experiment to Taylor: Laboratory experiment to determine kdetermine krr

• Determination of kr to allow implementation of model in the field

• Controlled melting experiments:– Melted 3 snow columns of different heights at

different rates– 18O content of snowmelt relative to snow

column substituted into model equation to obtain kr

• Kr = Ψu*

Z

KKrr = = ΨΨuu**

ZZ• Range of ψ (effectiveness of isotopic

exchange) values obtained by melting a short column rapidly (low ψ) and long column slowly (high ψ)

• Z = initial snow depth

• U* = percolation velocity

• Model used to calculate kr

as 18O is used to infer Ψ (effectiveness of exchange) so equation

Kr = Ψu*

Z can be solved

ResultsResults

• kr = 0.16 0.02 hr-1

• Small range (0.14 – 0.17 hr-1)

• Small standard deviation (15%)

• Successful parameterization of kr indicates that the model captures the physical processes that control the isotopic composition of meltwater

ResultsResults

• Estimate of f is uncertain– Test 1: 0.9

Tests 2-3: 0.2– Uncertainties

• Snowpack heterogeneity• Recrystallization

Snowpack HeterogeneitySnowpack Heterogeneity

• Real snowpacks are not homogeneous in terms of pore size

• If water content is low, water may only percolate in small pores

• Reduces surface area where isotopic exchange can occur

RecrystallizationRecrystallization

• Snow metamorphism due to wetting of snow– Small ice grains melt completely

• No isotopic fractionation

– Water refreezes onto larger ice crystals• 18O preferentially enters ice• Liquid becomes depleted

RecrystallizationRecrystallization

• Change to fraction of ice participating in isotope exchange (f) depends on two processes– Increase in f

• High mass of snow involved in melt – freeze

– Decrease in f• Larger mean particle size reduces surface area

available for ice – liquid interaction

• Taylor, S., Feng, X., Williams, M., and McNamara, J. 2002.

• How isotopic fractionation of snowmelt affects hydrograph separation

LocationsLocations

• Central Sierra Snow Laboratory (CA)– Warm, maritime snowpack

• Sleeper River Research Watershed (VT)– Temperate, continental snowpack

• Niwot Ridge (CO)– Cold, continental snowpack

• Imnavit Creek (AK)– Arctic snowpack

MethodsMethods

• Sample collection– Meltwater collected from a pipe draining a

meltpan (CA, VT, CO)– Plastic tray inserted into the snowpack at the

base of a snow pit (AK)

• Determination of 18O for meltwater samples

ResultsResults

ResultsResults

• At all locations, meltwater had lower 18O values at the beginning of the melt event and increasingly higher values throughout the event (3.5% to 5.6%)

• Trend holds despite widely different climate conditions

Why is this important?Why is this important?

• Using the average 18O value of pre-melt snowpack leads to errors in the hydrograph separation

Timing early late

18O lower higher

New water estimation

overestimated underestimated

Error EquationError Equation

NewOldNew

OOO

xx 18

1818

x = estimated error in x

x = fraction of new water

18ONew - 18OOld = isotopic difference between new and old water

18ONew = difference between 18O in average snowpack and meltwater samples

Error EquationError Equation

• Error is proportional to:– Fraction of new water in discharge (x)– Difference in 18O between snowpack and

meltwater (18ONew)

• Error is inversely proportional to:– Isotopic difference between new and old

water (18ONew - 18OOld)

ErrorError

• Large error if meltwater dominates the hydrograph

• Expected in areas of low infiltration– Permafrost– Cities

• Underestimate new water – Assume more enriched water is a mixture of

new and old water

ErrorError

• Error magnitude depends on time frame of interest– Maximum error at a given instant in time– Error is lower if entire melt event is considered

• 18OMelt ≈ 18OPack during middle of melt season

• Negative error and positive error cancel out

Other FactorsOther Factors

• Additional precipitation events

• Varying melt rates

• Meltwater mixing

• Spatial isotopic heterogeneity

Additional ApplicationsAdditional Applications

• Incorporation into other models– Mass and energy snowmelt model

• SNTHERM

• Glaciers– Climate studies involving ice cores

QuestionsQuestions