Wetlands ET
-
Upload
oumaima-guerouh -
Category
Documents
-
view
230 -
download
3
description
Transcript of Wetlands ET
-
1
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, [-]
-
2
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
-
3
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
-
4
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
-
5
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
-
6
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
-
7
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
-
8
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
-
9
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[ ]
-
10
Measuring/Estimating Evaporation
Measuring/Estimating Evaporation
-
11
Crop Coefficients
We introduce a crop coefficient, Kc, toaccount for the differences between thereference crop and any other vegetation
E = KcErc
Crop Coefficients
-
12
Crop Coefficients
Artichoke Pinto bean Broccoli
Lettuce Melon Onion
Crop Coefficients
-
13
Fakahatchee Strand