Source waters and flow paths in an alpine catchment, Colorado, Front Range, United States Fengjing Liu, Mark W. Williams, and Nel Caine 2004

Overview Source waters and flow paths of stream flow draining high- elevation catchments of the Colorado Rocky Mountains were determined using isotopic and geochemical tracers during the 1996 snowmelt runoff season at two subcatchments of the Green Lakes Valley, Colorado Front Range. δ O-18 used to determine new vs. old water Geochemical tracers used to analyze flow paths

End Member Mixing Analysis (EMMA) EMMA was used for three member mixing analysis to answer these questions –What is the magnitude of δ O-18 fractionation in snowmelt runoff? –How do we extrapolate information on δ O-18 fractionation measured in snow lysimeters to the catchment scale? –How does the amount of event water change as the catchment size increases from 8 ha to 225 ha? –What role does groundwater play in the discharge of streams? –What if any is the role of talus fields in stream flow quality and quantity?

Green Lakes Valley

Site Description Green Lake 4 (GL4) –225-ha catchment –45% coarse poorly sorted debris – “talus” Martinelli –8-ha catchment –Poor soil development –little vegetation

Methods Sample Collection –Snow Snow Lysimeters sampled daily for isotopes and solutes 13 snow pits were dug on Apr. maximum accumulation –Rain National Atmospheric Deposition Program (NADP) Analyzed for isotopes and solutes

Methods (cont.) Sample Collection –Surface Waters Stream flow sample weekly during melt and monthly during non- melt Talus sampled as a time series for 8 sites; once available in August –Soil Zero tension soil lysimeters both sites –4 site in GL4 –25 sites on 5x5 grid in Martinelli –Sampled weekly to biweekly once site were snow free

Methods (cont.) Analysis –All water and snow samples were tested for pH, ANC, conductance, major ions, dissolved Si Hydrograph Separation δ O-18 and geochemical tracers balanced using these equations

Hydrograph Separation Assumptions –Tracer values of each component must be significantly different –Only 2 components contributing to stream flow –Tracer composition of each component must be constant or changing at a known rate

Results Solute concentrations follow similar patterns at both catchments –Ionic Pulse evident w/ high concentrations in 1 st 20 days of snowmelt –Solute concentrations decrease as discharge increases on day 155 –Solute concentration reach annual low after maximum discharge on about day 190 –Concentrations begin to increase on recession limb of hydrograph

Solute Concentrations

δ O-18 Ratios Snowmelt depleted becoming enriched w/ time Summer rain enriched Soil water enriched compared to snowmelt or stream flow ~ 5 ppm more than stream flow for the same day

Source Waters Old Water –Stored in basin prior to initiation of snowmelt Soil lysimeter measurements varied over time Base flow measurements were temporally constant –Last stream flow sample of 1996 New water –Current year’s runoff Bulk value from snow pits Mean value from snow pits Volume-weighted mean from snow lysimeters Time series from snow lysimeters

Source Waters (cont.) Martinelli dominated by new waters –~82% new water when old water parameterized by base flow GL4 new water contribution varied with time –~40% during rising limb –Near 0% during base flow

Flow Paths Martinelli –37% surface flow –9% soil water –54% base flow GL4 –36% surface flow –36% talus –28% base flow

Source Waters and Flow Paths

Possible End Members

Possible End Member Sources

Discussion Source Waters –Ignoring the temporal variation in δ O-18 introduced error –Error is proportional to the fraction of new water and to the difference between the average snow pack value and melt water –The δ O-18 is significantly correlated to the cumulative amount of melt water (R2=0.87) –So, δ O-18 values in snowmelt measured at the point cannot be directly used for the entire catchment –Old water GL4 is 64% Other studies may underestimate based on use of VWM δ O-18 value in melt water and a constant δ O-18 value in snow pack This study may overestimate based on ignoring rainfall

Discussion Flow Paths –Old water in Martinelli is much less than subsurface water Subsurface event water is substantial contributor Same δ O-18 as snowmelt but chemical composition between new and old End Member with δ O-18 of snowmelt but Si of base flow bounds stream flow well. –Talus water has δ O-18 signature of old water and chemical composition distinct from base flow Subsurface event water is GL4 compared to Martinelli Two different reacted water components are needed –Unreacted water is overestimated due to difference

Flow Generation

Flow Contributions

Martinelli Stage 1 –Ionic Pulse and low discharge explain high solute concentrations Stage 2 –Saturation-excess overland flow occurs near stream channels –Depression of solute contents results from dilution by surface flow –Subsurface event water primarily occur during this time –Subsurface event water may be routed laterally through thin saturated layer above saturated zone Stage 3 –New water dominates until base flow conditions return when old water dominates

Flow Generation

Flow Contributions

Green Lake 4 Stage 1 –Melt water infiltrates ground but soil/talus waters do not contribute to stream flow Stage 2 –Surface flow makes up 40% of stream flow –Solute concentration dilute by surface flow –Subsurface flow matches old water in magnitude and temporal pattern Old water displaced by new water in subsurface –Talus water major contributor to stream flow Stage 3 –Talus flow invariant making increasing proportion –Ends with base flow dominating

Conclusions Ignorance of temporal variation of δ O-18 in snow melt may result in underestimation old water contribution δ O-18 values measured at a point should be adapted to the snowmelt regime at the catchment scale Soil water does not seem to be a significant contributor of stream flow in high elevation catchments Surface and groundwater interactions are much more important to the quality and quantity of stream flow in high elevation catchments