Isotopic evolution of snowmelt
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Isotopic Evolution of Snowmelt. Vicky Roberts Paul Abood Watershed Biogeochemistry 2/20/06. Isotopes in Hydrograph Separation. Used to separate stream discharge into a small number of sources

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Isotopic Evolution of Snowmelt

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Isotopic evolution of snowmelt

Isotopic Evolution of Snowmelt

Vicky Roberts

Paul Abood

Watershed Biogeochemistry

2/20/06


Isotopes in hydrograph separation

Isotopes 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


Problem

Problem

  • For hydrograph separations involving snowmelt runoff

    • Some studies assume snowmelt to have a constant d18O value equal to the average d18O of the snowpack

    • d18O in snowmelt ≠ d18O snowpack


Snowmelt isotopes

Snowmelt Isotopes

  • Snowmelt

    • Depleted in d18O early in melting season

    • Enriched in d18O later in melting season

  • Why?

    • Isotopic exchange between liquid water and solid ice as water percolates down the snow column


Physical process

Physical Process

  • At equilibrium, the d18O of water is less than the d18O of ice; initial snowmelt has lower d18O than the snowpack

  • Snowpack becomes enriched in d18O ; melt from the enriched pack is itself enriched (d18O )


Papers

Papers

  • 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 quantitatively indicating isotope exchange

Feng: Theoretical model quantitatively indicating isotope exchange

Varied two parameters:

Effectiveness of isotopic exchange (Ψ)

Ice-liquid ratio (γ)


Isotopic exchange

Isotopic 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 surface

      Ice-liquid ratio quantified by: γ = bf

      a + bf

      wherea = 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 (d18O=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


Isotopic evolution of snowmelt

  • 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 recrystallization

      Therefore 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 d18O between snowmelt and bulk snow causes errors in hydrograph separation if bulk snow values are used


Taylor laboratory experiment to determine k r

Taylor: Laboratory experiment to determine kr

  • 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


K r u z

Kr = Ψu*Z

  • 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


Isotopic evolution of snowmelt

  • Model used to calculate kr as d18O is used to infer Ψ (effectiveness of exchange) so equation

    Kr = Ψu* Z can be solved


Results1

Results

  • 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


Results2

Results

  • Estimate of f is uncertain

    • Test 1:0.9Tests 2-3: 0.2

    • Uncertainties

      • Snowpack heterogeneity

      • Recrystallization


Snowpack heterogeneity

Snowpack 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


Recrystallization

Recrystallization

  • 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


Recrystallization1

Recrystallization

  • 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


Isotopic evolution of snowmelt

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

  • How isotopic fractionation of snowmelt affects hydrograph separation


Locations

Locations

  • 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


Methods

Methods

  • 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 d18O for meltwater samples


Results3

Results


Results4

Results

  • At all locations, meltwater had lower d18O 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 d18O value of pre-melt snowpack leads to errors in the hydrograph separation


Error equation

Error Equation

Dx = estimated error in x

x = fraction of new water

d18ONew - d18OOld = isotopic difference between new and old water

Dd18ONew = difference between d18O in average

snowpack and meltwater samples


Error equation1

Error Equation

  • Error is proportional to:

    • Fraction of new water in discharge (x)

    • Difference in d18O between snowpack and meltwater (Dd18ONew)

  • Error is inversely proportional to:

    • Isotopic difference between new and old water (d18ONew - d18OOld)


Error

Error

  • 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


Error1

Error

  • 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

      • d18OMelt ≈ d18OPack during middle of melt season

      • Negative error and positive error cancel out


Other factors

Other Factors

  • Additional precipitation events

  • Varying melt rates

  • Meltwater mixing

  • Spatial isotopic heterogeneity


Additional applications

Additional Applications

  • Incorporation into other models

    • Mass and energy snowmelt model

      • SNTHERM

  • Glaciers

    • Climate studies involving ice cores


Questions

Questions


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