Evapotranspiration (fact sheets 6,7,8,9)

Question 1

Evapotranspiration/actual evaporation (E)

Answer 1

vaporisation of water into the atmosphere

Question 2

E with a little O

Answer 2

open water evaporation

Question 3

interception loss aka

Answer 3

wet canopy evaporation

Question 4

transpiration letter

Answer 4

T

Question 5

What is evapotranspiration comprised of?

3

Answer 5

open water evaporation
wet canopy evaporation
transpiration

Question 6

how is open water evaporation measured?

2

Answer 6

evaporation pan or meteorological measurements

Question 7

evaporation pan

Answer 7

the drop in water level is measured by taking into account rainfall
eg: 4mm evaporation out and 2mm rainfall = 2mm lost from evaporation.

Question 8

why do evaporation pans normally over-estimate the evaporation rate?
2

Answer 8

1. the pan has smaller heat storage (than a lake for example)
2. higher advective transfers over the small pan area

Question 9

Penman method

Answer 9

demonstrates that open water evaporation is dependent on three factors:

1. energy (available for evaporating a water body) = net radiation
2. saturation deficit
3. wind speed

Question 10

saturation deficit

Answer 10

max. amount of water the air will hold - vapour pressure of air
vapour pressure of air = (temp. at which air saturates on cooling)

Question 11

wind speed effect on open water evaporation

Answer 11

high winds increase evaporation by 1. exchanging humid air for dry air and 2. increasing turbulance

Question 12

penman equation

Answer 12

open water evaporation = net radiation + saturation deficit

?

Question 13

measurements of the saturation deficit

Answer 13

from dry bulb and a wet bulb thermometers

Question 14

wind speed measured with

Answer 14

an anemomter

Question 15

net radiation measured with

Answer 15

a net radiometer

Question 16

what measures all the components of the penman equation?

Answer 16

weather station

Question 17

wetted-canopy evaporation

Answer 17

the vaporisation of rainfall from leaf surfaces shortly after it has fallen

Question 18

what affects wetted-canopy evaporation?

Answer 18

1. rainfall intensity

2. vegetation type

Question 19

how does rainfall intensity effect wet canopy evaporation

Answer 19

higher the intensity, the less inception opportunities there are

Question 20

how is throughfall of rain in a forest measured?

Answer 20

standard gauges on the ground, beneath the canopy

Question 21

calculation of wetted canopy evaporation

Answer 21

= precipitation reaching the canopy (P gross) - precipitation which penetrates the canopy and reaches the ground (P net)

Question 22

example of wet(ted) canopy evaporation

Answer 22

rain in the night on the canopy, sun in the morning causes evaporation, can see steam leaving the canopy.

Question 23

how is stemflow measured?

Answer 23

stemflow collars, connected to gauges

Question 24

through fall vs through flow

Answer 24

through fall = rain which penetrates the canopy and reaches the forest floor
through flow = the lateral movement of subsurface flow close to the ground surface

Question 25

Transpiration (T)

Answer 25

the water lost from leaf stomata when they open for CO2 uptake

Question 26

how does vegetation type effect wet canopy evaporation?

Answer 26

eg: wet canopy evaporation is smaller for UK grasslands than UK conifers, link to SA

Question 27

what regulates transpiration?

Answer 27

1. opening and closing of stomata (stomatal resistance)

2. the soils resistance to release water to roots (root resistance)

Question 28

measuring transpiration

Answer 28

1, porometer
2, sapflow sensors
3, lysimeter water balance
4, long-term catchment water balance
5, modified penman method

Question 29

porometer

to measure transpiration

Answer 29

an individual leaf is enclosed in a chamber and the moisture lost into the chamber is measured

Question 30

Lysimeter water balance

Answer 30

a block of soil containing one or more plants is isolated and the input (precipitation), output, (drainage) and changes in soil water storage are measured, water balance equation used: T = P - Q ±ΔS

Question 31

long-term catchment water balance

Answer 31

T = P - Q

same as for lysimeter water balance but no ±ΔS as it cancels out over long-term

Question 32

modified penman method

Answer 32

uses above canopy meteorological measurements, estimates transpiration