5
LP-80
• PAR: radiation in the 400- to 700-nm waveband, representing the portion of the spectrum
that plants use for photosynthesis.
•
τ
(tau): ratio of below-canopy PAR measurements to the most recent above-canopy PAR
measurement, calculated automatically by the LP-80.
• LAI: the area of leaves per unit area of soil surface.
•
F
b
(fractional beam radiation): ratio of direct beam radiation coming from the sun to
radiation gathered from all ambient sources, like the atmosphere, or reflected from other
surfaces.
• χ
(leaf distribution parameter): ratio of the projected area of an average canopy element
(a leaf, for example) on a horizontal plane to its projection on a vertical plane; the default
value for
χ
is 1.0, which assumes the canopy angle distribution to be spherical.
•
z
(zenith angle): angle between the point in the sky directly overhead vertical, called the
zenith (0°), and the position of the sun. The angle is calculated based on the global position,
time of day, and location, and the correct information must be configured in the LP-80 for
this value to be accurate (
).
More in-depth explanations and examples of these measurements are in
and
the
.
The LP-80 can take measurements either manually or in unattended logging mode, as
explained in
, respectively.
2.2.1 MEASUREMENT TIPS
ABOVE-CANOPY (EXTERNAL) SENSOR
For above-canopy PAR data collection, consider connecting the external sensor to the
LP-80. This ensures an accurate measurement of intercepted PAR, especially on days where
radiation levels vary rapidly. The sensor can be mounted on a tripod and leveled with a
bubble level.
When it is not feasible to use an external sensor to obtain above-canopy measurements,
such as in timber or tropical crops, choose one of two options:
Use a separate data logger with an attached PAR sensor (such as a point quantum sensor) or
configure another LP-80 to log in the unattended mode (one reading per minute, for example)
in a clearing outside the canopy. After collecting PAR data within the canopy, download the
data from the two instruments to a computer and correlate the data.
Use the LP-80 as an above-canopy reference by regularly collecting above-canopy PAR data
in a large clearing within the canopy structure.
