Evaporation and Evapotranspiration

Question 1

What drives evaporation?

Answer 1

The Sun

Question 2

What is evaporation?

Answer 2

Vaporisation of liquid water

Question 3

What is the equation and unit for flux?

Answer 3

flow rate / area and W/m2

Question 4

What is a black body?

Answer 4

Idealised object that absorbs all electromagnetic radiation that falls on it
(i.e. no reflection).
It emits temperature dependent on spectrum of light
(i.e. higher T = lower wavelength = higher frequency)

Question 5

What is radiation emission linked with?

Answer 5

Radiation is emitted from all bodies at rates linked with their surface temperatures.
(i.e. higher T = lower wavelength = higher frequency)

Question 6

Explain the terms and units found in the radiation emission equation

Answer 6

Re = emitted energy flux (W/m2)
E = emissivity of surface to correct assumption of black body
sigma = stefan boltzmann constant
T = surface temp. in Celsius

Question 7

What is emissivity of a black body?

Answer 7

1

Question 8

How can we calculate the power emitted from a body?

Answer 8

P = A x Remission

Question 9

What happens when radiation strikes a surface?

Answer 9

Partially reflected and partially absorbed

Question 10

What is the albedo?

Answer 10

Reflected fraction of radiation

Question 11

What is the net radiation flux?

Answer 11

Difference between radiation absorbed and emitted

Question 12

What does temperature govern in air?

Answer 12

Moisture content air can hold - warmer air carries more moisture

Question 13

What is water vapor pressure and its symbol?

Answer 13

Partial pressure contributed by water vapor, e

Question 14

What is saturated vapor pressure and its symbol?

Answer 14

Vapor pressure when pressure is in equilibrium, es

Question 15

If saturation vapor is constant what occurs?

Answer 15

NO precipitation occurs

Question 16

What unit is vapor pressure in?

Answer 16

Pa or N/m2

Question 17

What is symbol of relative humidity?

Answer 17

Rh

Question 18

What does the triangle represent and what does a higher value mean? Also give its units

Answer 18

Gradient of saturated vapor pressure curve at temperature T. Higher gradient = higher T
Units = Pa/C

Question 19

What is sensible heat?

Answer 19

Energy per kg responsible for liquid water temperature change.

Question 20

What is the constant in sensible heat equation?

Answer 20

Specific heat

Question 21

What is latent heat?

Answer 21

Represents energy/kg required to vaporise water into water vapor.

Question 22

What is evaporation from open water surface influenced by?

Answer 22

Energy Input - provides latent heat
Vapor Transport - moves vapor away from water surface

Question 23

Define the symbols and units in the evaporation rate equation

Answer 23

Er = evaporation rate (m/s)
lv = latent heat of vaporisation (J/kg)
pw = water density (kg/m3)
Rn = net radiation flux (W/m2)
Hs = sensible heat flux (W/m2)
G = ground heat flux (W/m2)

Question 24

What is transport rate governed by?

Answer 24

Humidity gradient in the air near the surface and the wind speed across the surface

Question 25

Define the symbols in the evaporation aerodynamics equation

Answer 25

D = coefficient linked with air and water vapor densities u2 = wind speed at 2m height e_os = saturated vapor pressure at water surface T e_2a = actual vapor pressure of air at 2m height

Question 26

What is the vapor pressure deficit equation and when does it decrease?

Answer 26

(e_os - e_2a) - decreases when air pressure increases

Question 27

When can energy balance or aerodynamic method be used?

Answer 27

Energy balance - vapor transport is not limiting Aerodynamic - energy supply is not limiting

Question 28

What does the psychometric constant represent?

Answer 28

A balance between the sensible heat gained from air flowing past a wet bulb thermometer and the sensible heat converted to latent heat

Question 29

What does the p from the psychometric constant represent?

Answer 29

ATMOSPHERIC pressure given by equation p

Question 30

What is evapotranspiration?

Answer 30

Combination of evaporation from soil surface and transpiration from vegetation.

Question 31

What is transpiration and what is it dependent on?

Answer 31

Evaporation of water from aerial parts of plants. Dependent on type of crop and soil moisture

Question 32

What are important factors for ET on land?

Answer 32

Energy Input Water Transport Availability of Soil Water

Question 33

When does ET reach its full potential?

Answer 33

When water availability is not a limiting factor

Question 34

What is reference ET and what can it be used for?

Answer 34

Potential ET calculated on a reference surface at local climate station. Can be converted to potential ET by multiplying with a surface (crop) coefficient.

Question 35

What happens to rate of ET when soil dries out?

Answer 35

Drops below potential ET rate

Question 36

What is the reference ET given in the equation sheet for and what must be remembered?

Answer 36

Hypothetical grass reference. Rn = net radiation and units must be in MJ/m2 day

Question 37

What does ETc stand for and when can it be used?

Answer 37

Actual ET for a vegetation type. Is only applicable if soil moisture is not a limiting factor.

Question 38

What are 4 ways of measuring evaporation in the field?

Answer 38

Pan Lysimeter Eddy covariance Catchment/reservoir water balance

Question 39

Describe how an evaporation pan is used. (include when, why and the drawbacks of the usage)

Answer 39

Used to hold water to determine evaporation at a given location, automated with water sensors. Requires a rain measurement to subtract from values. Estimates evaporation from lakes and land Usually overestimates evaporation since has metal sides which heat up with the sun.

Question 40

Describe how a lysimeter is used. (include when, why and the drawbacks of the usage)

Answer 40

Is a measuring device used to measure the amount of ET released by plants. Records amount of precipitation in area and the amount lost through soil. ET is found by isolating vegetation root zone. Requires vegetation both inside and immediately outside meter zone perfectly match which is rarely adhered to leading to unrepresentative data. They are also difficult to use, expensive and require maintanence.

Question 41

Describe how eddy covariance is used. (include when, why and the drawbacks of the usage)

Answer 41

Prime atmospheric flux measurement technique used to measure and calculate vertical turbulent fluxes within atmospheric boundary layers. Analyses high frequency wind and scalar atmospheric data series to yield values for evaporation and ET. Technique is mathematically complex.

Question 42

Describe how catchment/water balance is used. (include when, why and the drawbacks of the usage)

Answer 42

ET can be estimated by creating an equation of water balance of a catchment.

Question 43

When can storage be ignored in water balance equation?

Answer 43

When an annual time step is used.

Question 44

What is important to note when using the Heffner empirical relationship for evaporation of an open water body?

Answer 44

Although using standard SI units with wind speed in m/s and area in m2, the output is automatically in mm/day

Question 45

Explain how to use the combined method to find evaporation - note down units for each output

Answer 45

Find lv using T_air (J/kg) Find E_r using lv_air (m/s) Find gradient using es_2 and T_air (Pa/C) Find p using elevation with output in kPa Find E_a using equation (m/s) Find Y (Pa/C) Use E combined equation (m/s)

Question 46

When calculating the reference ET using the FAO-Penrith equation, what must be remembered and how is it achieved?

Answer 46

Rn needs to be in MJ/day To convert Rn from W/m2 to MJ/day you multiply by seconds in a day (E=Pt)