Delta Electronics SS1-UM-1.05 - Manual (Page 41)

SunScan User Manual v 1.05 Measurement options
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41
Measurement options
Experiment design
This section discusses the factors that bear on the experimental objectives you wish
to achieve. It should help you answer questions like:
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What equipment do I need?
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What readings must be taken?
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Will I have to wait for particular times of day or weather conditions?
The type of study you propose to do will determine the time of year and duration of
the experiment, and whether you are interested, for example, in monitoring growth
by interception of solar radiation, or perhaps in the canopy structure as well.
Some canopies types (the non-uniform ones) preclude the use of the SunScan’s
direct LAI readout. You could, however, characterise the 3-dimensional light
distribution within your canopy at different heights, or along transects through it. For
brevity we will call this approach “PAR mapping” in the discussion below
Answers to the above questions are complicated, but the following tables should
give you a good appreciation of the main issues involved.
Above-canopy reference requirements
This refers to measurements of PAR incident on the canopy, made at the same time
as the below-canopy measurements. The question is whether to use a BFS.
Beam Fraction sensor
A BFS connected to the SunScan probe provides the best option, because you can
operate with fewest restrictions. However, with some canopy types this may not be
practical.
The next best option is to use the SunScan probe (without the BFS) sequentially
above and below the canopy, but you may be restricted to times when the light levels
are not changing fast.
Independent PAR sensor
If that method is not practical either, you may have to rely on independent records of
incident PAR from a separate sensor. In addition to slowly changing light levels, you
may also have to average measurements in each place over periods of a few minutes,
furthermore the direct LAI readout is not available. This is the least convenient
situation, and it will not always be considered in the analysis below.
You cannot currently use the SunData software to merge independent records of
transmitted PAR and incident PAR to get LAI.
Direct and Diffuse components
Assuming that you make your above-canopy measurements on the SunScan (with or
without a BFS), then the next table summarises whether you need to measure the
Direct and Diffuse components of the incident light. If you do not need the separate
components, you benefit because the BFS is quicker to set up (there is no need to re-
adjust its shade ring, once it has been set up correctly).
Type of study Incident PAR
Total only Incident PAR
Direct & Diffuse components
Fractional interception Yes No
LAI No Yes
PAR mapping Yes Possibly