1. Isotopic Evolution of Snowmelt
Vicky Roberts
Paul Abood
Watershed Biogeochemistry
2/20/06

2. 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

3. 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

4. Snowmelt Isotopes Snowmelt Why?
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

5. 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 )

6. Papers Theory Lab Field Feng, X., Taylor, S., and Renshaw, C.E. 2002.
Taylor, S., Feng, X., and Renshaw, C.E
Field
Taylor, S., Feng, X., Williams, M., and McNamara, J

7. Feng: Theoretical model quantitatively indicating isotope exchange
Varied two parameters:
Effectiveness of isotopic exchange (Ψ)
Ice-liquid ratio (γ)

8. 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
where a = mass of water
b = mass of ice
per unit volume of snow i.e. ratio of liquid to ice

9. 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

10. 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

11. 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

12. 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

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

14. Implications:
Variation in d18O between snowmelt and bulk snow causes errors in hydrograph separation if bulk snow values are used

15. 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

16. 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

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

18. 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

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

20. 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

21. 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

22. 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

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

24. 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

25. Methods Sample collection Determination of d18O for meltwater samples
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

26. Results

27. 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

28. Why is this important?
Using the average d18O value of pre-melt snowpack leads to errors in the hydrograph separation
Timing
early
late
d18O
lower
higher
New water estimation
overestimated
underestimated

29. 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

30. 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)

31. 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

32. 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

33. Other Factors Additional precipitation events Varying melt rates
Meltwater mixing
Spatial isotopic heterogeneity

34. Additional Applications
Incorporation into other models
Mass and energy snowmelt model
SNTHERM
Glaciers
Climate studies involving ice cores

35. Questions