Comparison of modeled vs. measured
below-canopy PAR
at the Wind River Canopy Crane Facility
Scott N. Martens
Measurements of below-canopy PAR were made by
Jiquan Chen on October 31, 1997 at seven points. The locations are
plotted below relative to mapped point 398 (coordinates 275 east, 75 north
in the coordinates used in this study). The vertical
transect of fisheye views is located at point 398.
The time-course of transmitted PAR
at the seven points is shown below along with that measured above-canopy
(in the open) for the same time period on October 31, 1997.
Simulations were made using the measured
above-canopy PAR, a leaf area density of 0.25, k=0.5, and spherical leaf
angle distribution. Diffuse PAR was estimated at 1 degree intervals
in both azimuth and zenith.
There are two difficulties in using the measured
dataset for model comparisons:
1) The fraction of direct vs diffuse in the
total measured above-canopy PAR was unknown because there was no shadowband
on the above-canopy PAR sensor. Thus, for the simulations the direct
fraction of PAR was varied from 0.0 to 1.0 to determine the effect on the
transmitted PAR.
2) The difference in measurement frequency
of above- (30 min) and below-canopy (5 min) PAR required comparisons be
made in two ways. The first involved comparing simulated and measured
results at 30 min intervals, i.e., when above-canopy PAR was known (measured).
The second involved interpolating above-canopy PAR to 5 min intervals to
allow comparisons at finer temporal scales (< 30 min) because the measured
data (see figure above) show features ("sunflecks") at this scale.
The figure below shows the time-course of simulated
PAR for all seven locations, at three values of direct fraction (0.0, 0.5
and 1.0) shown in a rotating animation.
The simulated time-course of PAR for each of the
seven points is shown individually below along with the measured above-
and below-canopy PAR to enable a comparison. Only simulations at
30 min intervals are shown below.
View the graph for one location: 1
2 3
4 5
6 7
Last Updated on March 22, 1998
By Scott N. Martens
Email: scott@cstars.ucdavis.edu
