Wetlands ET

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1 CEE 261 Watershed and Wetlands Hydrology May 20, 2002 • Evapotranspiration in Wetlands • Florida Marshes and Cypress Strands Evaporation • Net flux to vapor phase Evaporative flux, E, [mm day -1 ] Latent heat of vaporization, λ, J kg -1 ] Vapor pressure, e Saturation vapor pressure e s = e s (T ° , solutes), [kPa] Saturation vapor pressure gradient = de s / dT, [kPa °C -1 ] Relative humidity RH = e/ e s , [-]

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Transcript of Wetlands ET

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    CEE 261Watershed and Wetlands Hydrology

    May 20, 2002

    Evapotranspiration in Wetlands

    Florida Marshes and Cypress Strands

    Evaporation Net flux to vapor phase

    Evaporative flux, E, [mm day-1]Latent heat of vaporization,

    , [J kg-1]

    Vapor pressure, e

    Saturation vapor pressure e s = e s(T , solutes), [kPa]Saturation vapor pressure gradient = de s/ dT, [kPa C-1]Relative humidity RH = e/ e s, [-]

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    Evaporation Requirements

    WaterEnergy

    F Solar radiation (net), R n, [MJ m-2 day-1]

    F Sensible heat transfer from air, H, [MJ m-2 day-1]

    Vapor pressure gradient in the atmosphereF Molecular diffusion

    F Turbulent mixing (diffusion)

    Evaporative surfacesFree water (puddles, lakes, reservoirs)SnowSoilPlants (transpiration)

    Transpiration

    Surface resistance, rs [s m-1]

    Stomatal resistance of the entire canopy

    E =

    k es e[ ]rs

    Stomatalresistance

    Vapor flux

    Units coefficient

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    Transpiration

    Water flux through a plant is controlled by:

    Atmospheric conditions (energy flux, vapor pressuregradient)

    Surface resistancePlant and root structureSoil water movement/availability

    Evapotranspiration

    Because in practice it is extremely difficult tomeasure separately the evaporation from plantsurfaces and the evaporation from the soilsurface, puddles, ditches, etc. surrounding theplant, we commonly lump all of the evaporativesurfaces and adopt the terminologyevapotranspiration or ET

    Sometimes called consumptive use

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    Standard Evaporation Rates

    Potential Evaporation, E0, [mm day-1]The quantity of water evaporated per unit area per unit timefrom an idealized, extensive free water surface under existingatmospheric conditions

    Reference Crop Evaporation, Erc (ET0), [mm day-1]The rate of evaporation from an idealized grass crop with afixed crop height of 0.12 m, an albedo of 0.23, and a surfaceresistance of 69 sec/m

    Measuring/Estimating Evaporation

    Liquid water loss

    Vapor/energy flow in the atmosphere

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    Measuring/Estimating Evaporation

    Liquid water lossEvaporation pans

    F Pan coefficients Anemometer

    Stilling well

    Rain gage

    Measuring/Estimating Evaporation Liquid water loss

    Lake and watershed water balancesSoil moisture depletionLysimeters

    Plant physiological techniques

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    Measuring/Estimating Evaporation Vapor/energy flow in the atmosphere

    Aerodynamic methodsEddy correlation measurementsEnergy balance methods

    CIMIS ET station

    Energy Budgets and Evaporation

    Rn = net incoming radiant energy, [MJ m-2 day-1]

    H = outgoing sensible heat flux, [MJ m-2 day-1]wE = outgoing energy as evaporation, [MJ m-2 day-1] = latent heat of vaporization, [MJ kg-1]w = density of water, [kg m-3]

    E = evaporative flux, [mm day-1]

    G = outgoing heat conduction into soil, [MJ m-2 day-1]Ad = Ad

    o-Adi = energy advected by horizontal air

    flow , [MJ m-2 day-1]

    S = energy stored , [MJ m-2 day-1]P = energy used biochemically in plants , [MJ m-2 day-1]

    Control Volume

    wE

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    Energy Budgets and Evaporation

    E =1

    wRn H G Ad P S[ ]

    wE

    Energy Budgets and Evaporation

    E =1

    wRn H G Ad P S[ ]

    Rn = Sn + Ln = St 1 [ ] + Li LoSn = net short-wave radiationLn = net long-wave radiationSt = total incoming short-wave radiation = albedoLi = incoming long-wave radiationLo = outgoing long-wave radiation

    [Rn] = MJ m-2 day-1

    [Rn/w] = mm day-1

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    Energy Budgets and Evaporation

    E =1

    wRn H G Ad P S[ ]

    Hs =

    ks es e[ ]rs

    Ha =

    ka es e[ ]ra

    Stomatal control

    Aerodynamic control

    es = saturation vapor pressuree = vapor pressurers = surface resistancera = aerodynamic resistanceks, ka = units coefficients

    Energy Budgets and Evaporation

    Big Leaf Model

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    Energy Budgets and Evaporation

    E =1

    w Rn G S P Ad[ ] + ac p es e[ ] / ra

    + 1 + rs / ra[ ]

    = des/dT, [kPa C-1]a = air density, [kg m-3]cp = specific heat of moist air, [kJ kg-1 C-1] = psychrometric constant, [kPa C-1]

    [rs, ra] = s m-1

    [Rn] = MJ m-2 day-1

    [es, e] = kPa

    Penman-Monteith Equation

    Reference Crop Evaporation

    * = [1+0.33 U2]U2 = wind speed at 2 m, [m s-1]T = temperature, [C]

    [] = kPa C-1[*, ] = kPa C-1[Rn] = mm day-1

    [es, e] = kPa

    [Erc] = mm day-1

    Erc =

    + *Rn +

    + *

    900T + 275

    U2 es e[ ]

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    Measuring/Estimating Evaporation

    Measuring/Estimating Evaporation

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    Crop Coefficients

    We introduce a crop coefficient, Kc, toaccount for the differences between thereference crop and any other vegetation

    E = KcErc

    Crop Coefficients

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    Crop Coefficients

    Artichoke Pinto bean Broccoli

    Lettuce Melon Onion

    Crop Coefficients

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    Fakahatchee Strand