Leaf area index (LAI), a measure of the foliage in a canopy
How can crop coefficients be computed from data collected with the LP-80?
Crop coefficients are used to estimate crop water needs. The procedure for computing a crop coefficient for a vineyard using the AccuPAR LP-80 and Williams (2001) correlation is as follows:
Make measurements on a clear day within a couple of hours of noon.
Make one PAR measurement above the canopy and several equally spaced measurements below the canopy from row center to row center following the instructions in the AccuPAR manual.
Don’t sample preferentially in sun or shade areas, and take enough samples to give a good average for the area.
The AccuPAR automatically computes t*. Subtract this value from 1.0 to get the interception. Williams’ (2001) correlation multiplies this value by 1.7 to get Kc (crop coefficient). For example, if t were 0.60, interception would be 1 – 0.60 = 0.40, and then Kc would be 1.7 x 0.40 = 0.68.
*Interception is computed as 1 – t, where t, the fractional transmission, is the ratio of one or more measurements below the canopy to one or more measurements above.
Is there a lower limit of "above PAR" to collect reliable data with the LP-80?
There isn’t a lower limit of “above PAR” to collect reliable LAI data; however, very large zenith angles around dawn and dusk can cause more variability. As long as sampling does not occur within an hour of dawn and dusk, the data collected are reliable (even 300 µmol m−2 s −1 on a diffuse day). Most peer-reviewed research would prefer sampling with the LP-80 between 10 am and 2 pm.
I took my readings, but I'm still getting LAI values of 0.0. What's going on?
1. Make sure that the location (latitude and longitude), date, and time are entered correctly in the AccuPAR LP-80 handheld.
2. Perform the user calibration on a clear and sunny day with the light bar level and green cap removed from the external PAR sensor. Ensure that the external PAR sensor is connected and level. The user calibration should be performed in an open area, where there are no shadows on the sensors.
3. Read the measurement tips in the LP-80 user manual.
4. Contact METER if you have additional problems.
Can we add more locations (cities) to the hand-held options for setting the latitude/longitude?
The cities are there to at least provide an approximate location to start from, and then you make adjustments from there. Select the closest city, then just use the up and down arrows to move to the Latitude and Longitude lines. Press “ENTER” to make adjustments to these values to better reflect the actual location. Once it is set correctly, they just press the MENU button again to get out of this menu and the setting will be saved.