EVAPORATION GEOG 405 Tom Giambelluca 1
Evaporation The change of phase of water from liquid to gas; the net vertical transport of water vapor from the surface to the atmosphere. 2
Definitions Evaporation: (specific) flux of water vapor from open water surface, wet vegetation, or wet soil; (general) total flux of water vapor from the surface to the atmosphere. Transpiration: water vapor transfer to the atmosphere occurring primarily through the stomata of living plants. Evapotranspiration (ET): total flux of water vapor from the surface to the atmosphere (same as the general use of evaporation ). Potential Evapotranspiration (PE): the environmental demand for evaporation; the evaporation rate if moisture is not limiting. 3
ET and Hydrological Cycle ET is one of the primary links in the hydrological cycle. 4
What Controls Evaporation? For a water surface, the net rate of liquid-to-gas transition depends on: Humidity of the air (vapor pressure) Temperature of the water surface (saturation vapor pressure) 5
Evaporation Aerodynamic issues The humidity gradient (vapor pressure gradient) near a moisture source controls the rate of evaporation. The mean vapor pressure gradient over a complex vegetated surface is impossible to measure. Turbulent exchange influences the vapor pressure gradient by transporting humid air away from the evaporating surface and replacing it with drier air. Wind speed, the vertical temperature gradient, and the roughness of the surface influence the vapor pressure gradient by controlling the amount of turbulence. Temperature influences the vapor pressure gradient by controlling the saturation value. 6
Evaporation Energy balance issues Evaporation requires energy. If no energy is added to the evaporating surface, via radiation or advection, the surface will cool, causing evaporation to be reduced or stopped. To maintain evaporation at a certain rate, the net energy input must be sufficient. By accounting for all the other inputs and outputs of energy to the surface, the amount of energy used for evaporation (latent energy) can be estimated. 7
NET RADIATION R where : R net net = K K + A L = net radiation A = downward longwave radiation absorbed by the surface L = upward longwave radiation emitted by the surface GEOG 402: Radiation Balance 8
NET RADIATION R R R = K K + A L = (1 α) K where α = albedo ε T s s net net net = (1 α) K + A ε σt + ε ( L = surfaceemissivity = surface temperature s s L = downward longwave radiation from atmosphere 4 s σt 4 s ) GEOG 402: Radiation Balance 9
Latent Heat of Vaporization λ = 2.454 x 10 6 J kg -1 at 20ºC The specific heat of water (C w ) = 4186 J kg -1 K -1. Thus, it takes about 586 times as much energy to evaporate a kg of water as it does to raise its temperature by 1º. Example: Typical summer evaporation rate: 5 mm day -1 (mm per day) Water density: 1000 kg m -3 5 mm = 5 kg per square meter 5 kg m 2 day 1 day 86400 s 2454000 J kg =142 W Convert days to Multiply by latent heat of seconds vaporization of water of water m 2 Energy used to evaporate 5 mm
Latent Heat of Vaporization λ = 2.454 x 10 6 J kg -1 at 20ºC Another way of stating the latent heat of vaporization: The amount of latent heat flux per mm/day of evaporation: Examples: (a) E = 5 mm per day: λe = 5 mm day -1 x 28.4 W m -2 per mm day -1 = 142 W m -2 (b) λe = 110 W m -2 : E = 110 W m -2 / 28.4 W m -2 per mm day -1 = 3.88 mm day -1
Evaporation Estimation approaches Direct water loss measurements Evaporation pans Lysimeters Soil water balance Soil water balance modeling Potential evaporation: Penman, Priestley-Taylor, Penman-Monteith Meteorological Methods Profile method Penman-Montieth equation Bowen ratio-energy balance Resistance-energy balance Temperature variance-energy balance Scintillometer-energy balance Eddy covariance 12
Evaporation Pan 13
Weighing Lysimeter 14
Soil Water Balance Measure: RF Irr ET RF RO Irr ΔSM ΔSM Estimate: RO GWR GWR Get ET by difference 15
Soil Water Balance Modeling Measure: RF Irr ET RF, Irr Rn, T, RH, U RO Estimate: PE ΔSM RO Use water balance to get GWR ET GWR ΔSM 16
Meteorological Approaches The evaporation rate can also be understood in terms of an Ohm s Law analogue. R In this case, the evaporation rate is driven by the vapor pressure gradient and limited by resistance to transfer of water vapor molecules away from the surface: λe = ρc p γ (e s [T surface ] e) r W ρ = air density (kg m -3 ) γ = psychrometric "constant" = C p P ε λ C p = specific heat of air at constant pressure = 1005 J kg -1 K -1 λ = latent heat of vaporization (MJ kg 1 ) r W = resistance to transfer of water vapor from the surface; depends on wind speed and surface aerodynamic characteristics 17
Meteorological Approaches e s [T surface ] e Note that the vapor pressure gradient is sometimes approximated (using the saturation vapor pressure of the air rather than that of the water surface) as the vapor pressure deficit: VPD = e s - e 18
Potential Evapotranspiration Penman Equation: E 0 = ΔH +γe a Δ +γ where: E 0 = open water evaporation (pan evaporation; potential evaporation) Δ = slope of the saturation vapor pressure vs. temperature curve at T a H = net radiation in evaporation units (Rnet/28.4) γ = psychrometric constant E a = aerodynamic term 19 aerodynamic term is an empirical function of wind speed and vapor pressure deficit
Parameter Estimation Δ = 4096*(0.6108*exp(17.27*T/(T+237.3)))/((T+237.3)^2) γ = Cp*P/(λ*0.622) λ = (2500.8-2.36*T+0.0016*T^2-0.00006*T^3)/1000 Constant: C p = 0.001013 specific heat of air at constant press. (MJ kg -1 K -1 ) Variables: T = air temperature ( C) P = air pressure (kpa) 20
Potential Evapotranspiration Priestley-Taylor Equation: 21
Penman-Monteith Equation 22
Bowen Ratio H Bowen Ratio : β = or H = β LE LE Energy Balance: R G = H + LE Combining, we get : LE = Need independent c p ΔT β = λ Δq where : β = c p the Bowen ratio = specificheat of λ = latent heat of ΔT Δq net estimate of Rnet G β + 1 vaporization = vertical temperature gradient = vertical humidity gradient β : air at constant pressure 23
Bowen Ratio 24
Resistance Method 25
Other Methods of Estimating H Temperature Variance Method: based on rapid fluctuations in air temperature H = hσρc p z -d g T σ 1.5 T Scintillation Method: based on optical disturbance of air by thermally-induced density differences 26
Scintillometer 27
Eddy Covariance Method based on correlation of a scalar (e.g. temperature, humidity, carbon dioxide concentration) fluctuations with vertical wind speed variations. LE = L v ρ' v w' H = C ρt' p w' 28
Eddy Covariance 29
Eddy Covariance 30
Eddy Covariance 5 0-5 0 100 200 300 400 500 600 700 800 900 31