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 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 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

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 Lab Field Feng, X., Taylor, S., and Renshaw, C.E. 2002. 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 Varied two parameters: Effectiveness of isotopic exchange (Ψ) Ice-liquid ratio (γ)

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

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

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 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

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

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

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

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

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 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

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

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

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 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

Results

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? 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

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 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 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 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 Additional precipitation events Varying melt rates Meltwater mixing Spatial isotopic heterogeneity

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

Questions