evapotranspiration

Question 1

evaporation

Answer 1

direct transfer of water from open water body/soil/ vegetation surfaces to atmo

Question 2

how evaporation occurs

Answer 2

-water molecule move constantly
-some break away from surface VS other come back
-net evaporation = more molecule leaving surface than returning

Question 3

2 controls of evaporation

Answer 3

-vapor pressure deficit
-energy

Question 4

latent heat of vaporization

Answer 4

energy a water molecule needs to escape water surface
-source of energy -> mostly solar radiation

Question 5

Vapor pressure deficit

Answer 5

Diff between how much moisture air CAN hold when saturated VS actual amount of moisture in air
VPD = es-ea

Question 6

Relative humidity equation

Answer 6

RH = (ea/es)x100%

Question 7

Dew point temperature

Answer 7

temp at which air starts to condensate (100% relative humidity)

Question 8

Saturation vapor pressure (es)

Answer 8

max amount of moisture air can hold at given temp

Question 9

Actual vapor pressure (ea)

Answer 9

actual amount of moisture air holds at air temp

Question 10

equation for ea

Answer 10

ea = es x (RH / 100)

Question 11

factors affecting evaporation

Answer 11

Vapor pressure diff between water surface & air
Temperature
Wind
Atmo pressure
Water Quality

Question 12

Methods used for Estimating evaporation

Answer 12

-water budget equation
-energy budget method -> simplified version
-evapo pan measurements

Question 13

Water budget method for evapo

Answer 13

Isolate Evapo from equation of water budget
Advantage: simple
Disadvantage: Difficult to estimate seepage loss & subsurface runoff AND unreliable

Question 14

Energy budget method (full one)

Answer 14

Isolate energy used for evapo from equation
Advantage: most accurate
Disadvantage: difficult to estimate all terms/ equation must be simplified/ empirical formula used

Question 15

Energy budget simplified equation

Answer 15

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

Question 16

Rnet equation

Answer 16

Rnet = Rtotal x (1 - albedo)

Question 17

assumptions of simplified energy method

Answer 17

Energy is limiting factor
Energy balance without water input

Question 18

Evapo pan measurements issues

Answer 18

Overestimate evapo
-less heat storage capacity (small volume)
-heat transfer
-wind effects

Question 19

Solution to overestimation of evapo pan

Answer 19

Etrue = Cp x Epan -> use of pan coefficient

Question 20

+ and - of evapo pan measure

Answer 20

+ : simple
-: overestimate evapo
poor indicator of land/ lake evapor

Question 21

Transpiration

Answer 21

indirect transfer of water from root-stomatal system to atmo

Question 22

what drives water movement in plants

Answer 22

Energy difference -> from less negative moisture tension in soil to more negative tension in atmo

Question 23

Stomata

Answer 23

air opening allowing plant to exchange gas with atmo
open/close with diurnal effect & water tension

Question 24

Stomatal conductance

Answer 24

rate of gase exchange with air via stomata
-highest with crops
-many variables affect it
-strong relationship with transpiration

Question 25

Importance of transpiration

Answer 25

Plant-mediated diffusion of soil water to atmo Primary leaf cooling mechanism under high radiation pathway for nutrient uptake & matrix for chem reaction

Question 26

Evapotranspiration

Answer 26

summarize all process returning water to atmo in vapor forms

Question 27

Factor affecting transpiration

Answer 27

Temperature solar radiation wind plant type soil moisture

Question 28

phytometer

Answer 28

completely sealed contained with soil & plant growing in it -> measure transpi

Question 29

potometer

Answer 29

System connecting tube to plant stem to see speed & distance travel of air bubble in water tube -> transpi measurements

Question 30

Transpi VS Evapo VS interception with land cover type

Answer 30

At plant lvl: transpi > evapor Forest: + interception, - evapo, + transpi Agri field: - interception, + evapo, - transpi

Question 31

Ways to measure evapotranspiration

Answer 31

Lysimeter measurements inflow-outflow measure (water-balance principle) micrometeorological measures (Flux tower) ET equation Study of GW fluctuation

Question 32

Lysimeter conditions, + and -

Answer 32

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

Question 33

Eddy flux tower functioning, + & -

Answer 33

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)

Question 34

PET

Answer 34

ET that would occur with no water limitation -> determined by weather and energy

Question 35

AET

Answer 35

actual evapo rate from any surface under prevailing conditions of moisture availability and radiative input -> determined by weather, energy AND water availability

Question 36

Comparing AET and PET

Answer 36

Always AET <= PET -dry soil & other natural conditions: AET < PET -open water body & over-saturated bare soil: AET = PET

Question 37

Reference crop evapotranspi (ETrc) conditions

Answer 37

well-watered grass specific canopy resistance specific albedo completely shades ground

Question 38

Blaney-Criddle method

Answer 38

Only focus on temperature -> simplest

Question 39

Thornthwaite method

Answer 39

Applicable to arid areas only need temperature data But indirect reference to radiative balance

Question 40

Penman model

Answer 40

Requires great amount of data Strong physical knowledge originally designed for free water surface BUT ignores soil heat conduction

Question 41

Penman-Monteith model:

Answer 41

requires even more data consider surface and atmo resistance BUT consider vegetation canopy as one leaf

Question 42

Hargreaves method

Answer 42

Only requires air temperature and extraterrestrial radiation Can get daily estimates BUT for ETrc and underpredicts arid & windy conditions

Question 43

Hamon

Answer 43

Requires Temp, daylight hours and considers saturated vapor pressure