Lec 6: ET I Flashcards
What does evapotranspiration involve?
plants, waterbodies, and soils
Define evaporation.
What is required for evaporation to occur?
Direct transfer of water from open water bodies, soil and vegetation surfaces to
the atmosphere
- A flow of energy to the evaporating surfaces;
- A flow of liquid water to these surfaces; and
- A flow of water vapor away from these surfac
What two conditions control phase change (evaporation)
- enough energy is provided (high water temperature)
latent heat of evaporation: The energy that a water molecule needs to escape a water surface (2270 kJ/kg (= 540 cal/g) at 100°C) - evaporation is limited or driven by the vapor pressure deficit
VPD = es - ea = saturation vapour pressure - actual vapour pressure
How does evaporation occur?
- water molecules move permanently
- some break away from water surface while some cross back into the water
- net evaporation occurs if more water molecules leave the water surface than molecules cross back into it
Where does the energy required for evaporation come from?
‒ Mostly from solar radiation (insolation)
‒ Or from other terms of the energy balance (e.g., from sensible or ground
heat flux)
‒ Air temperature is a good proxy for average energy availability
define VPD
VPD: the difference between the current amount of moisture in air and the
maximum amount of moisture air can hold when it is saturated
What is saturation vapor pressure?
What is it a function of?
Maximum amount of moisture that air can hold at a given air temperature
it’s only a function of temperature
What is actual vapor pressure?
How can you derive actual vapor pressure
Actual amount of moisture that air holds at a given air temperature
can derive by:
- knowing air temperature and relative humidity
OR
- knowing the dew point temperature
What is dew point temperature?
Dew point temperature is the temperature at which air starts to condensate (i.e., the temperature at which the ea is equal to es)
What does the magnitude of evaporation depend on?
(a) Solar radiation
(b) Vapor pressure deficit
(c) Temperature (higher)
(d) Wind (strong)
(e) Quality of water
Name 3 methods to estimate evaporation. Name the pros and cons for each of them.
- Water budget method: measure all other inputs and outputs, difference is evaporation
pros: simple
cons:
– Difficult to estimate Qd and Qs (subsurface seepage loss and runoff)
– Unreliable, accuracy will increase as Δt increases - Energy budget methods
pro: Most accurate method
cons:
‒ Difficult to evaluate all terms
‒ Energy balance equation must be simplified
‒ Empirical formulas are used (although radiation measurements are
preferable) - Evaporation pans
pros:
‒ Easy method, inexpensive
cons:
‒Pan evaporation can be a very poor indicator of land surface evaporation or
lake evaporation (because of pan boundary effect, limited heat storage, and
other factors)
What are the input (2) and outputs (4) of the energy budget method?
energy in:
- net energy advected (from incoming and outgoing water)
- solar radiation
energy out:
- reflected solar radiation
- latent heat of evaporation
- sensible heat loss from the water body to the atmosphere
- net long wave radiation
What percentage of earth’s energy budget is carried to clouds and the atmosphere through latent heat of evaporation?
23%
What is the formula for the simplified energy budget method?
R(net) = lambdaE + H + G = R(total)(1 - albedo)
lambda = latent heat of vaporization
E = evaporation
H = sensible heat transfer to air
G = heat conducted to the ground
What is the mass balance equation for estimating evaporation using the pan method? What is the other formula necessary for estimation?
E(pan) = precip - (H2 - H1)
E(true) = C(p) - E(pan)
