evapotranspiration I Flashcards
what is blue water?
saturated storage in streams, lakes, groundwater, wetlands, glaciers and snowpacks
what is green water?
evapotranspiration flux into the atmosphere
what is evaporation?
direct transfer of water from open water bodies, soil and vegetation surfaces to the atmosphere
what are the three steps to evaporation?
- a flow of energy to the evaporating surfaces
- a flow of liquid water to these surfaces
- a flow of water vapor away from these surface
how does evaporation occur?
water molecules move permanently -> some break away from the water surface into the atmosphere while some cross back to water surface -> if there are more water molecules leaving the water surface than going back, net evaporation occurs -> eventually, equilibrium will occur, when air has reached its saturation vapor pressure
what are the two basic controls on evaporation?
1.evaporation requires energy -> provided by water temperature
2. evaporation is driven (or limited) by the vapor pressure deficit
what is the energy that a water molecule needs to escape a water surface?
latent heat of evaporation: 2270 kJ/kg at 100 degrees C
where does the energy for evaporation come from?
- mostly solar radiation
- other terms of energy balance (sensible or ground heat flux)
what is the vapor pressure deficit? and its equation
the difference between the current amount of moisture in air and maximum amount of moisture air can hold when its saturated, VPD= es -ea
what is saturation vapor pressure?
maximum amount of moisture that air can hold at a given temperature (e-sat)
what are the 5 factors affecting evaporation?
- solar radiation
- vapor pressures deficit
- temperature
- wind
- quality of water
what are conditions favorable for evaporation?
strong winds + high air temperatures
what are three methods for estimating evaporation?
- water budget method
- energy budget method
- evaporation pans
what are the disadvantages of the water budget method for estimating evaporation?
- difficult to measure subsurface seepage loss and subsurface runoff
- unreliable, accuracy will increase as the time frame increases
which is the most accurate method for estimating evaporation?
energy budget method because evaporation depends on the energy state of the water system
what are the disadvantages of the energy budget method for estimating evaporation?
- difficult to evaluate all terms
- energy balance equation must be simplified
- empirical formulas are used even though radiation measurements are preferable
what is the energy budget method simplified? (estimating evaporation)
sensible heat transfer to air, heat stored in system, heat conducted to the ground, energy used in evaporation, net and total radiation
how do you calculate radiation budget?
net radiation = total radiation - reflected radiation (which is 1- albedo)
or
net radiation =latent heat x evaporation + sensible heat transfer to air + heat conducted to the ground
what are the disadvantages of using a pan measurement?
- although we use a pan, the main focus is not what evaporated from the pan -> we want to know the regional evaporation from land surface or evaporation from a lake
- pan evaporation can be a very poor indication of land surface evaporation because of the pan boundary effect, limited heat storafe and other factors
