Evapotranspiration (fact sheets 6,7,8,9) Flashcards
Evapotranspiration/actual evaporation (E)
vaporisation of water into the atmosphere
E with a little O
open water evaporation
interception loss aka
wet canopy evaporation
transpiration letter
T
What is evapotranspiration comprised of?
3
open water evaporation
wet canopy evaporation
transpiration
how is open water evaporation measured?
2
evaporation pan or meteorological measurements
evaporation pan
the drop in water level is measured by taking into account rainfall
eg: 4mm evaporation out and 2mm rainfall = 2mm lost from evaporation.
why do evaporation pans normally over-estimate the evaporation rate?
2
- the pan has smaller heat storage (than a lake for example)
- higher advective transfers over the small pan area
Penman method
demonstrates that open water evaporation is dependent on three factors:
- energy (available for evaporating a water body) = net radiation
- saturation deficit
- wind speed
saturation deficit
max. amount of water the air will hold - vapour pressure of air
vapour pressure of air = (temp. at which air saturates on cooling)
wind speed effect on open water evaporation
high winds increase evaporation by 1. exchanging humid air for dry air and 2. increasing turbulance
penman equation
open water evaporation = net radiation + saturation deficit
?
measurements of the saturation deficit
from dry bulb and a wet bulb thermometers
wind speed measured with
an anemomter
net radiation measured with
a net radiometer
what measures all the components of the penman equation?
weather station
wetted-canopy evaporation
the vaporisation of rainfall from leaf surfaces shortly after it has fallen
what affects wetted-canopy evaporation?
- rainfall intensity
2. vegetation type
how does rainfall intensity effect wet canopy evaporation
higher the intensity, the less inception opportunities there are
how is throughfall of rain in a forest measured?
standard gauges on the ground, beneath the canopy
calculation of wetted canopy evaporation
= precipitation reaching the canopy (P gross) - precipitation which penetrates the canopy and reaches the ground (P net)
example of wet(ted) canopy evaporation
rain in the night on the canopy, sun in the morning causes evaporation, can see steam leaving the canopy.
how is stemflow measured?
stemflow collars, connected to gauges
through fall vs through flow
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
Transpiration (T)
the water lost from leaf stomata when they open for CO2 uptake
how does vegetation type effect wet canopy evaporation?
eg: wet canopy evaporation is smaller for UK grasslands than UK conifers, link to SA
what regulates transpiration?
- opening and closing of stomata (stomatal resistance)
2. the soils resistance to release water to roots (root resistance)
measuring transpiration
1, porometer 2, sapflow sensors 3, lysimeter water balance 4, long-term catchment water balance 5, modified penman method
porometer
to measure transpiration
an individual leaf is enclosed in a chamber and the moisture lost into the chamber is measured
Lysimeter water balance
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
long-term catchment water balance
T = P - Q
same as for lysimeter water balance but no ±ΔS as it cancels out over long-term
modified penman method
uses above canopy meteorological measurements, estimates transpiration
