evapotranspiration Flashcards
evaporation
direct transfer of water from open water body/soil/ vegetation surfaces to atmo
how evaporation occurs
-water molecule move constantly
-some break away from surface VS other come back
-net evaporation = more molecule leaving surface than returning
2 controls of evaporation
-vapor pressure deficit
-energy
latent heat of vaporization
energy a water molecule needs to escape water surface
-source of energy -> mostly solar radiation
Vapor pressure deficit
Diff between how much moisture air CAN hold when saturated VS actual amount of moisture in air
VPD = es-ea
Relative humidity equation
RH = (ea/es)x100%
Dew point temperature
temp at which air starts to condensate (100% relative humidity)
Saturation vapor pressure (es)
max amount of moisture air can hold at given temp
Actual vapor pressure (ea)
actual amount of moisture air holds at air temp
equation for ea
ea = es x (RH / 100)
factors affecting evaporation
Vapor pressure diff between water surface & air
Temperature
Wind
Atmo pressure
Water Quality
Methods used for Estimating evaporation
-water budget equation
-energy budget method -> simplified version
-evapo pan measurements
Water budget method for evapo
Isolate Evapo from equation of water budget
Advantage: simple
Disadvantage: Difficult to estimate seepage loss & subsurface runoff AND unreliable
Energy budget method (full one)
Isolate energy used for evapo from equation
Advantage: most accurate
Disadvantage: difficult to estimate all terms/ equation must be simplified/ empirical formula used
Energy budget simplified equation
Rnet = lambda x E + H + G
Where, H is sensible heat transfer to air, G is heat conducted to ground, lambda is latent heat of vaporization
Rnet equation
Rnet = Rtotal x (1 - albedo)
assumptions of simplified energy method
Energy is limiting factor
Energy balance without water input
Evapo pan measurements issues
Overestimate evapo
-less heat storage capacity (small volume)
-heat transfer
-wind effects
Solution to overestimation of evapo pan
Etrue = Cp x Epan -> use of pan coefficient
+ and - of evapo pan measure
+ : simple
-: overestimate evapo
poor indicator of land/ lake evapor
Transpiration
indirect transfer of water from root-stomatal system to atmo
what drives water movement in plants
Energy difference -> from less negative moisture tension in soil to more negative tension in atmo
Stomata
air opening allowing plant to exchange gas with atmo
open/close with diurnal effect & water tension
Stomatal conductance
rate of gase exchange with air via stomata
-highest with crops
-many variables affect it
-strong relationship with transpiration
Importance of transpiration
Plant-mediated diffusion of soil water to atmo
Primary leaf cooling mechanism under high radiation
pathway for nutrient uptake & matrix for chem reaction
Evapotranspiration
summarize all process returning water to atmo in vapor forms
Factor affecting transpiration
Temperature
solar radiation
wind
plant type
soil moisture
phytometer
completely sealed contained with soil & plant growing in it -> measure transpi
potometer
System connecting tube to plant stem to see speed & distance travel of air bubble in water tube -> transpi measurements
Transpi VS Evapo VS interception with land cover type
At plant lvl: transpi > evapor
Forest: + interception, - evapo, + transpi
Agri field: - interception, + evapo, - transpi
Ways to measure evapotranspiration
Lysimeter measurements
inflow-outflow measure (water-balance principle)
micrometeorological measures (Flux tower)
ET equation
Study of GW fluctuation
Lysimeter conditions, + and -
condition: same soil & vegetation inside and outside tank
+: direct ET measure that are very precise
primary tool for evaluating weather effect on ET
-: difficult & expensive to construct
require careful operation and maintenance
primarily for research
Eddy flux tower functioning, + & -
measure vertical transfer of water vapor & CO2 driven by convection moisture
+ : continuous measurement
dont disturb surface monitored
direct ecosystem lvl measures
-: expensive
require air turbulence
require flat terrain & homogenous vegetation
Gap-filling dataset is difficult
difficult to operate in remote location (e.g. trees breaking)
PET
ET that would occur with no water limitation -> determined by weather and energy
AET
actual evapo rate from any surface under prevailing conditions of moisture availability and radiative input -> determined by weather, energy AND water availability
Comparing AET and PET
Always AET <= PET
-dry soil & other natural conditions: AET < PET
-open water body & over-saturated bare soil: AET = PET
Reference crop evapotranspi (ETrc) conditions
well-watered grass
specific canopy resistance
specific albedo
completely shades ground
Blaney-Criddle method
Only focus on temperature -> simplest
Thornthwaite method
Applicable to arid areas
only need temperature data But indirect reference to radiative balance
Penman model
Requires great amount of data
Strong physical knowledge
originally designed for free water surface
BUT ignores soil heat conduction
Penman-Monteith model:
requires even more data
consider surface and atmo resistance
BUT consider vegetation canopy as one leaf
Hargreaves method
Only requires air temperature and extraterrestrial radiation
Can get daily estimates
BUT for ETrc and underpredicts arid & windy conditions
Hamon
Requires Temp, daylight hours and considers saturated vapor pressure
