Delta Electronics SS1-UM-1.05 user manual LAI theory, Ingredients of the LAI computation method

Page 54

LAI theory

In this section we shall explain as fully as we can how the SunScan computes its readings of leaf area index, and what the main limitations and provisos are in interpreting these for real canopies.

Ingredients of the LAI computation method

There are three broad areas contributing to the final result.

Geometric analysis

The first is the analysis of what happens to a ray of light passing through the canopy. In order to do this, we have to make some general assumptions about the canopy, i.e. uniformity, randomness and total absorption by canopy elements. This was done by Campbell (1986) for a beam of light from a single direction (the Direct solar beam) passing through a canopy with a generalised ellipsoidal leaf angle distribution function. This function allows a wide range of different canopy types to be described by the value of a single parameter ELADP.

Wood then integrated Campbell’s result over the whole sky to give a description of the transmission of Diffuse light through the same canopy. This is important because the transmission of Diffuse light is different, and in reality there is usually a combination of both Direct and Diffuse illumination. In particular, the analysis shows that Diffuse transmission is strongly dependent on the leaf angle distribution, a point which has not generally been recognised.

These functions are integrals which do not have direct analytical solutions, so have to be solved numerically, and computable functions fitted to the results. This has been done to a high degree of accuracy, improving on Campbell’s original approximation.

Incomplete absorption - more elaborate analysis

The above analysis based on black leaves is relatively straightforward. However, real leaves also reflect or scatter some of the light that falls on them. Typically, only about 85% of the incident light is absorbed. This means that in reality, every leaf element in the canopy is re-emitting light, as well as absorbing it, which makes the situation much more complicated.

Because the direction of any particular light ray can be changed by reflection or scattering, it means the spatial distribution of the light changes through the canopy. Therefore it is no longer adequate to consider just the vertical component of the light (as measured by a cosine corrected sensor), the horizontal component must also be included. This is why Wood’s analysis also considers a hemispherical response sensor (which measures both horizontal and vertical components).

The relentless advance of computing power has made it possible to model the situation in ways that were not feasible in the past. By integrating the "black leaf" analysis into a computer model Wood has calculated the light levels in the canopy across the whole range of canopy and incident light parameters.

54 • LAI theory

Document code: SS1-UM-1.05

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Contents SunScan SS1-UM-1.05Trademarks CopyrightAcknowledgements CE conformityContents LAI theory Menus and Screens More Psion and file handling notesMeasurement options Technical Reference sectionAppendices IndexSunScan Canopy Analysis System How to use the manualsIntroduction Organisation of this manualBeam Fraction sensor Field accessoriesSunScan probe Data Collection TerminalGetting Started Preliminary checksWorkabout and SunScan probe Checking the Workabout hardware SunData s/w Diskette Installing the SunData s/w in your PCInstalling the s/w What the s/w doesSetting up your PCs COM port Running SunDataCommunication checks SunScan probe to PCWorkabout to PC Running SunData in Windows SunScan Tutorial About this tutorialWorking with the Workabout What to do if you get lost Starting SunData in the WorkaboutUsing a PC instead of the Workabout Starting the PC softwareThis page is intentionally blank Setting up a measurement session Working through the menu optionsFile Saving Hot KeysUsing the Emulator mode Taking readingsConnecting the SunScan probe Without a Beam Fraction SensorConnecting the Beam Fraction Sensor Measuring Leaf Area Index without a Beam Fraction Sensor Reviewing your data file AveragesTransferring the data file to your PC From the WorkaboutTo the PC Initiating the file transfer from the Workabout Meanwhile, on the PC RS232 communication problems Conclusion of the TutorialSunData Screens on the Workabout Settings Menus and ScreensFile Contd UtilsQuit Workabout User Guide More Psion and file handling notesNavigating Psion directories and screens Navigating the Psion directories and screensPsion subdirectory usage in file select dialogs Re-installing the SunData application icon Deleting unwanted Workabout filesFlashcards reformatting Configuration and data file handling SunData Configuration filesData memory management Restoring a configuration Default .cfgCreating a configuration file Data filesDisplaying data files on your PC .PRN file.CSV file Data file layouts and data groups Page Measurement options Experiment designAbove-canopy reference requirements Canopy Sampling volume Canopy type and BFS practicalitiesCanopy type and LAI estimates Preferred light and weather conditions Planning for the sun’s positionAdvice on Absorption and Eladp values Setting EladpAbsorption Relationship between Mean Leaf Angle and Eladp Estimating Eladp in the fieldWorkabout setup SunScan System Measurement modesLAI, PAR and All displays Autolog function Probe GO button Measurement procedures in the fieldProbe handling in the field Levelling the probeUse of the tripod Using the tripodBFS handling in the field WorkaboutFinding North, and setting the shade ring Levelling the BFSExtension cables, and the location of the BFS Checking the probe/BFS matching PAR calibrationsFactory light calibration Recalibrate optionEffect of the shade ring on the BFS Routine maintenance and cleaningRestoring the factory calibration Comparing the calibration with other PAR sensorsEnvironmental and moisture protection SunScan probe and Beam Fraction SensorLAI theory Ingredients of the LAI computation methodDerivation of Wood’s SunScan canopy analysis equations Major assumptionsTheory versus reality Beers law for canopy absorption Campbells Ellipsoidal LAD equationsTransmission of Diffuse Light Transmission fraction τ Is given by I/I0 Modelling the canopy transmission Diffuse light transmission cosine corrected sensorDetail Diffuse light cosine response sensor Functions used to model canopy transmissionAccuracy of LAI calculations Diffuse light hemispherical response sensorModelling incomplete PAR absorption and scattering Exp 0.1 . x . atan 0.9 Atan L Q SpherScientific references Calculating zenith anglesSummary Jones, Hamlyn G Plants and Microclimate second edition. CUP Checking the batteries Maintenance and repairTechnical Reference section SunScan probe batteriesChecking the desiccant Re-setting the factory calibration Checking the PAR calibrationFactory calibration method Psion Workabout TroubleshootingProblems running the SunData application While running SunDataInsufficient power to write data reported Technical Support Beam Fraction Sensor type BF1 SpecificationsSunScan Probe Type SS1 Data Collection Terminal type DCT1 Psion WorkaboutSpares Kit type SPS1 Carrying Case type SCC1Telescopic Tripod type BFT1 Logging CablesPAR Performance Spectral responseCosine responses of probe and BFS SunScan system cosine responseAppendices Logging the probe as a Linear Quantum SensorLogging the Beam Fraction sensor Workabout SunData s/w Upgrading the SunScan systemSunData PC s/w Revision HistoryPsiWin File transfer between Workabout and PCChoice of different programs Workabout Remote LinkExample using Slink Slink and RcomDrives on the PC are referred to as REMA, Remc REM = remote Example using Rcom Documentation of Rcom and SlinkSending a file to a Communication program Using the Workabout Comms applicationAlternative file transfer mechanisms Sending a file to a serial printerGlossary Page Index AccuracyIndex Field use 39, 46, 48, 50, 51, 58

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