Delta Electronics SS1-UM-1.05 - Manual (Page 43)

SunScan User Manual v 1.05 Measurement options
••••
43
This means that in strong sun (high Beam Fraction) the canopy volume sampled is
fairly small and well defined. As the Beam Fraction decreases, the volume sampled
increases, and has less well defined limits.
Preferred light and weather conditions
These also will significantly influence your field operations.
Limitation Comments
Time of day Preferably within 3 hours either side of solar noon depending on
the location and season, to meet the next two conditions.
Solar zenith angle Measurements are easier when the sun is high. Probe and BFS
levelling errors become larger beyond zenith angles of greater
than 60
°
, especially for LAI
Incident light level -
absolute Preferably above about 200
µ
mol.m-2.s-1. Accuracy degrades
below this figure.
Light level - rate of
change With the BFS, avoid only the very fastest changes between bright
sun and cloud.
With no BFS: slow-changing conditions needed.. For LAI, with no
BFS, slow change of direct and diffuse components.
Full overcast, or full
sun in blue sky SunScan LAI model copes with both, but full sun will usually give
the best results. Broken cloud is also satisfactory.
Planning for the sun’s position
The SunData software on the PC contains a useful calculator for solar zenith angles
on any date, to help you plan appropriate times to make your measurements. This
can be accessed from the Utilities menu
Alt+Utilities, Zenith calculator. The default
values are taken from the Site Settings you are currently using, but can be changed
within the solar predictor without affecting any settings elsewhere in the program.