Delta SS1-UM-1.05 User Manual download pdf (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 µmol.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.

1 2 ... 38 39 40 41 42 43 44 45 46 47 48 ... 85 86

No comments

Delta SS1-UM-1.05 User Manual Page 43