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Isotopic Evolution of SnowmeltPowerPoint Presentation

Isotopic Evolution of Snowmelt

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### Feng: Theoretical model quantitatively indicating isotope exchange

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

- 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
- 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

- 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

- 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.

Varied two parameters:

Effectiveness of isotopic exchange (Ψ)

Ice-liquid ratio (γ)

Isotopic exchange 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

where a = mass of water

b = mass of ice

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

Effectiveness of exchange: 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: exchange

- 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

- Effect of varying exchangeγ (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: exchange

- 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: exchange

- Snow melted from the surface at constant rate
- Dispersion is insignificant
- 18O exchange occurs between percolating water and ice

Implications: exchange

- 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 exchanger

- 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

- Kr = Ψu*

K exchanger = 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

- Model used to calculate k exchanger as d18O is used to infer Ψ (effectiveness of exchange) so equation
Kr = Ψu* Z can be solved

Results exchange

- 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

Results exchange

- Estimate of f is uncertain
- Test 1: 0.9Tests 2-3: 0.2
- Uncertainties
- Snowpack heterogeneity
- Recrystallization

Snowpack Heterogeneity exchange

- 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 exchange

- 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

- Small ice grains melt completely

Recrystallization exchange

- 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

- Increase in f

- Taylor, S., Feng, X., Williams, M., and McNamara, J. 2002. exchange
- How isotopic fractionation of snowmelt affects hydrograph separation

Locations exchange

- 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 exchange

- 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

Results exchange

Results exchange

- 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? exchange

- Using the average d18O value of pre-melt snowpack leads to errors in the hydrograph separation

Error Equation exchange

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 Equation exchange

- 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 exchange

- 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

Error exchange

- 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 exchange

- Additional precipitation events
- Varying melt rates
- Meltwater mixing
- Spatial isotopic heterogeneity

Additional Applications exchange

- Incorporation into other models
- Mass and energy snowmelt model
- SNTHERM

- Mass and energy snowmelt model
- Glaciers
- Climate studies involving ice cores

Questions exchange

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