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. 23-24 @ 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 installed @ 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 1st 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/ timeSummer rain enrichedSoil 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

0

10

20

30

40

50

60

-24 -20 -16 -12 -8

d18O(‰)

Si (m m

ol L

-1)

Stream FlowIndex SnowpitSnowmeltTalus EN1-LTalus EN1-MTalus EN1-UTalus EN2-LTalus EN2-UTalus EN4-VTalus EN4-LTalus EN4-USoil WaterBase Flow

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 component @ 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 insignificant @ 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