Isotopic Evolution of Snowmelt

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 • 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 • 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.

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: 0.9Tests 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 • 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

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