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What can be done with echosounder bubble fluxes in a lake or reservoir : Spatiotemporal variability and atmospheric emissions. Tonya DelSontro Eawag & ETH Zurich , Switzerland. The Bible. Helge Balk and Alex Rynskiy. Simrad EY60/EK60 split - beam echosounder

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Tonya DelSontro Eawag & ETH Zurich , Switzerland

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Tonya delsontro eawag eth zurich switzerland

Whatcanbedonewithechosounderbubblefluxes in a lakeorreservoir: Spatiotemporalvariabilityandatmosphericemissions

Tonya DelSontro

Eawag & ETH Zurich, Switzerland


Tonya delsontro eawag eth zurich switzerland

The Bible

Helge Balk

and

Alex Rynskiy


Tonya delsontro eawag eth zurich switzerland

Simrad EY60/EK60 split-beam echosounder

with 120 kHz, 7° beam angle transducer

Woh

Wohlensee,

Switzerland

Lake Kariba,

Zambia/

Zimbabwe


Bubble size calibration

Bubble Size Calibration


Tonya delsontro eawag eth zurich switzerland

  • How an echosoundercanhelpillustratethespatiotemporalvaribilityofebullition

  • Howatmosphericechosounderfluxes, whicharedependent on bubblesizeand a dissolutionmodel, comparewithothermethods


Tonya delsontro eawag eth zurich switzerland

Lake Kariba (Zambia/Zimbabwe)

  • Subtropical

  • Oligotrophic

  • Monomictic

    • Anoxichypolimnion

  • 3 yearres. time

  • Level ± 4 m

  • Oneofworld‘slargestreservoirs

    • 5250 km2, 280 km long

    • 100 m maxdepth

  • Dendritic

    • 2100 km ofshoreline

1) Howimportantisebullition

in a large reservoir?

2) Large-scalespatialvariability

relatedtoriverinflows?

DelSontro et al. (2011) ES&T 45: 9866-9873


Methodology

Methodology

  • Comparingebullitiondynamics in bayswithriverinflowswithbayswithout

  • Echosounderandsurfacechambersurveys

Hydroacousticprocessingnote:

- Erased all non-bubbletargets

- Estimatedflux in 5 m segments


Tonya delsontro eawag eth zurich switzerland

Ebullitiondynamics in riverbays

Frequency

Magnitude

  • Data richness & newaspects traditional methodscannotreveal


Small scale spatial variability in a hotspot

Small-scalespatialvariability in a hotspot

  • Can makegeneralizationsthatitisrelatedtobathymetry, sedimentationdynamics, or ??


Tonya delsontro eawag eth zurich switzerland

Large-scalevariability in surfaceemissionsandmethodcomparison

  • River deltas do emitmore CH4, mostly via ebullition

  • Ebullitionhotspots (riverdeltas) moreobvious via acoustics


Tonya delsontro eawag eth zurich switzerland

Large-scalevariability in surfaceemissionsandmethodcomparison

Surface Chambers

EchosounderSurfaceFlux

  • Echosoundercoverage an orderofmagnitudemorethanchambercoverage

  • Chambers integrateover all fluxes, eventhelowones –

  • But methodsarewithinerror


Wohlensee lake wohlen switzerland

Wohlensee (Lake Wohlen), Switzerland

  • Run-of-river reservoir

  • 2.5 km2

  • Meandepth, 10 m

  • 2 dayres. time

  • Waterlevel ± 10 cm


Methodology1

Methodology

  • Reverseditforbubbledensity in thepresenceoffish (all non-bubbletargets)

  • Segment divisionbased on bubble-to-non-bubbleratios (nostandard bin length)


Spatiotemporal emission variability in a localized hotspot

Spatiotemporalemissionvariabilityin a localizedhotspot

  • Obviousdifferenceseven on consecutivedays


Bubble size variability per segment

Bubble sizevariability per segment

  • Meanbubblediameterillustratesonepossiblecauseforfluxvariability


Bubble size distribution

Bubble sizedistribution

  • Larger bubblescontributemoreto total gas volume

  • Variability in meanbubblesizedistribution per segment, but preferredsizerangeis 4-6 mm diameter


Surface emission method comparison

Surfaceemissionmethodcomparison

  • Surfacefluxcalculatedfromhydroacoustics – high variabilitywith median between 100 and 1000 mg m-2 d-1

  • Surfacefluxesfromhydroacoustics (H) andchambers (C) on same day

  • Chamberfluxeshigher – integratingoverlowfluxes


Temporal variability comparison

Eddy covariance

Hydroacoustics

Temporal variabilitycomparison

  • Ebullitioncorrelatedwith time ofday,

    mostlikely due towaterlevelfluctuations

Chambers


Spatial limitations of all methods

Spatiallimitationsof all methods

AdaptedfromEugster et al. (2011) Biogeosciences 8: 2815-2831


Spatiotemporal variability of surface ebullition flux via hydroacoustics

Spatiotemporalvariabilityofsurfaceebullitionfluxvia hydroacoustics

  • Surfacefluxesaveragedfrom all surveysandcontouredoverbathymetry

  • Standard deviationshowswheremost variable fluxareascanbefound


Conclusions

Conclusions

  • Echosoundingcanhelpefficientlysurvey larger areas

    • Expandingthespatialcoverageof traditional methodsandhelpingtoidentify large-scalevariabilty

  • Per segmentanalysisthenhelpsidentifysmall-scalespatialvariabilitywithinobservedebullitionhotspots

    • Thus aiding in futureresearchforcausesof such variability

  • Bubble sizemaybeimportantcauseforobservedvariability

    • Echosoundercalibrationsallowforbubblesizedetermination

  • Surfacefluxcalculationsaredependent on bubblesizeandthedissolutionmodel

    • Thenhydroacousticmethodscanbeappliedandcomparedwithotheratmosphericemissiontechniques


Thanks to

Thanksto

I. Ostrovsky, D. McGinnis, W. Eugster, D. Senn, M. Kunz, T. Diem, A. Zwyssig, M. Schurter, C. Dinkel, H. Balk, A. Rynskiy, J. Wüest, B. Wehrli

Zimbabweancrew, ZRA, BKW

Swiss National Science Foundation


Ch 4 emissions from basin iv

CH4 emissions from Basin IV

Surface CH4

concentrations

3 other deltas = avg 3 measured

Vertical CH4

profile near dam

Ebullition emits 100x more CH4 than other pathways


L kariba relative to other tropical reservoirs

L. Kariba relative to other tropical reservoirs

  • Rough upscaling shows ebullition remains most important

    • Higher than that in other tropical reservoirs

      • Dam emissions highest in other reservoirs

  • Total CH4 emission comparatively low

    • Due to dam release of CH4-poor epilimnion water

  • Total CH4 emissions from tropical reservoirs

    1Abril et al., 2005; 2Kemenes et al., 2007; 3Bambace et al., 2007; 4dos Santos et al., 2006


    Tonya delsontro eawag eth zurich switzerland

    Ebullition areas

    < 11% of lake area

    CH4 emission from Lake Kariba

    • Dam: 170 t y-1

    • Diffusion: 540 t y-1

    • Ebullition: 60,000 t y-1

    • Total: 10 t km-2 y-1

    • Less than all recorded tropical reservoirs


    Tonya delsontro eawag eth zurich switzerland

    Chap. 6 – KARIBA Acoustics


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