Soil, Water and Atmospheric Processes 2h
Tutorial – Evaporation
Simulation of Evaporation.
This tutorial will help reinforce some of the lecture material on water vapour, evaporation and
turbulent transfer processes. You will use a simulation model of the evaporation process (essentially the
Penman-Monteith equation we covered in Lecture #8) to answer a small number of questions. By
doing so, you should begin to appreciate how sensitive the evaporation process is to its driving
parameters e.g. solar radiation, vapour pressure gradient, atmospheric turbulence and vegetation-related
factors.
Your answers to the questions form part of the continuous assessment for this course. You will be
given a worksheet to record your answers and you should hand in the completed worksheet by
next Wednesday (7 March) to Helen McKeating.
Activities
Log on to the university network and use the FireFox browser to navigate to one of the pages on the
micrometeorology site:
http://www.geos.ed.ac.uk/abs/research/micromet/java/ and select the first
Applet for the SWAP course (click on the Web Start (gold) icon)
When you enter this web page you will see text explaining what you have to do plus the image below.
You can control the simulation
by altering the scrollbars labeled
‘Solar
radiation’,
vapour pressure’ etc. this will change the driving
variables for evaporation. The
model was demonstrated to you
briefly at the end of Lecture #8
in JBM’s lecture series. It is
important you are confident with
the model before you attempt
the worksheet- you should ask
the demonstrators to help if you
are unsure of anything before
you start.
The model has a default set of
weather conditions which are
restored
whenever
the
’reset’ button is pressed.
You can alter the sliders to
explore the sensitivity of the model to changes in solar radiation, vapour pressure, wind speed etc. You
can also choose different types of vegetation and explore how sensitive they are to changes in the
weather conditions. The different surface types will differ in their albedo, roughness length and canopy
resistance.
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Soil, Water and Atmospheric Processes 2h
Tutorial – Evaporation
About the Penman-Monteith model
Surface
In lecture #8, we discussed
roughness
the advance that Monteith
term
made to the Penman
equation when he introduced
n
p
z
o
a
resistances to water vapor
transport
that
were
c
associated
with
the
vegetation. You can think of
resistances in an analogous
a
Surface
form to Ohm’s Law – for a
wetness
given potential difference
term
(voltage), current will flow at
a rate determined by the electrical resistance of the circuit (I=V/R). The aerodynamic roughness of the
vegetation will influence the ease with which water vapour can be removed from leaves, hence the
‘surface roughness term’, introduces the aerodynamic resistance (ra) which is a function of wind speed
and roughness length (see the tutorial web page for the formula). The taller and rougher the vegetation,
the smaller will be ra (all else being equal). See Oke (1987) BoundaryLayer Climates, page 70 for more
details.
LE =
'R + *c (e ( e ) / r
& r
' + ) $$1 +
% r
#
!!
"
It was recognized in the 1960’s that plants were not merely ‘passive wicks’ in the evaporation process
but were able to control their loss of water to some extent by opening and closing their stoma. The
degree of closure of the stomatal aperture can be a function of solar radiation, internal CO2
concentration, soil water availability and other factors as discussed in the lecture. The P-M equation
treats canopies comprised of individual leaves as one big leaf with canopy (or surface) resistance (rc)
being a measure of stomatal movement. Liquid water sitting on the surface of leaves, of course, is
readily available for evaporation and wet leaves in the model should be given a surface resistance of 0.
Some things to note about the model.
SVP
1. You can determine the relationship
(mbar)
between air temperature and saturation
vapour pressure (svp) and plot it here.
Simply record the svp at any
temperature (take 5 ºC steps from 0 ºC
to 40 ºC) and plot it. You should
recognize this curve as describing
equilibrium evaporation.
2. Note the relative Humidity value
whenever you change T or VP – it can
never be allowed to exceed 100% (this
will be indicated by the text turning red
10
20
30
40
in the RH textbox.
3. The questions on the worksheet are just
Air temperature ( ºC)
a start - you should make use of such
simulation models to perform your own
‘what if?’ experiments. You can gain greater understanding of the subject by just ‘playing
around’ with such models.
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Soil, Water and Atmospheric Processes 2h
Tutorial – Evaporation
WORKSHEET for TUTORIAL #1 (Evaporation)
Mark (out
of 50)
NAME (CAPITAL LETTERS) …………………… Matriculation No. ……………………..
Q1.
Use the model to determine the evapotranspiration rate for both dry and wet (i.e. just after rain)
surfaces using the default meteorological conditions. Wet surfaces have an r s of zero. Fill in
the gaps in this Table. Some values have been pre-calculated - you should be able to re-create
these results to convince yourself that you are using the model correctly.
Surface
Bare soil
Grass
Cereal
Coniferous forest
Upland
Water
Evapotranspiration rate (Wm-2)
Dry (use the default
Wet
values)
...
268
213
...
...
...
465
157
...
...
...
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Q2.
It has been hypothesized that the conversion of a catchment from grassland to coniferous
plantation would reduce the discharge rate of the river which drained it and that the mean
annual number of days of rain would be an important factor to take into account. What
comments do you have on this, bearing in mind the results in the Table above?
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Q3.
Studies have shown that for surfaces like grass which are poorly coupled to the atmosphere, the
humidity immediately above the canopy is higher than above a rougher canopy such as a
coniferous plantation. What effect would this have on evapotranspiration from the two
contrasting surfaces?
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Soil, Water and Atmospheric Processes 2h
Q4.
Tutorial – Evaporation
The model can be used to examine the sensitivity of wet and dry grassland and wet and dry
coniferous forest to solar radiation, humidity, wind speed and temperature. Do this by filling in
the blanks in the Table below. You should record the difference in latent heat flux between the
maximum and minimum values for each factor in turn while keeping the other factors constant.
(again, some example values are included – make sure you can re-create them. You need to
keep an eye on the RH value – it can never exceed 100% so you may find when you alter T or
vpd that the slider need not go all the way to the minimum position!)
Difference in latent heat flux (W m-2)
Surface
solar radiation
wet grassland
Dry grassland
wet coniferous forest
Dry coniferous forest
(508-44)
temperature
vpd
(keep
air
wind speed
temperature fixed
at default value)
464
(270-167) 103
(784-190) 594
(227-179) 48
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Q5. What can you conclude from these estimates?
-----------End of Problems-------------------
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