The amount of foliage in a vegetative canopy can be deduced from measurements of radiation attenuation as it passes through the canopy at several angles from the zenith. Foliage orientation information can also be obtained.
The Indirect Leaf Area Obtained from the UNL Light Wand Data Set contains indirect leaf area index (LAI), and mean tilt angle and diffuse transmittance data through the vegetative canopy. Measurements were made during late July and early August in 1988 with prototype LAI-2000 plant canopy analyzers provided by LI-COR in areas adjacent to 4 plots at site 811 (4439-PAM) with similar foliage cover. Measurements were made at solar zenith angles greater than 75 degrees or during overcast sky conditions. The LAI-2000 was used to measure the attenuation of diffuse sky radiation at five zenith angles simultaneously. Measurements were also made in June, July and August of 1989 at three sites, 906 (2133-ECA), 916 (4439-ECV) and 966 (2437-BBS). Measurements were made in the plots as well as in areas adjacent to the plots with similar foliage cover.
FIFE LAI (Indirect): Light Wand - UNL.
(Indirect Leaf Area Obtained from the UNL Light Wand).
The Indirect Leaf Area Obtained from the UNL Light Wand Data Set contains indirect leaf area index (LAI), and mean tilt angle and diffuse transmittance data through the vegetative canopy.
Indirect leaf area index (LAI) (1988 & 1989), and mean tilt angle and diffuse transmittance through Canopy (1989).
Measurements were made during late July and early August with a prototype LAI-2000 plant canopy analyzers provided by LI-COR in areas adjacent to the 4 plots at site 811 (4439-PAM) with similar foliage cover. Measurements were made at solar zenith angles greater than 75 degrees or during overcast sky conditions with the instrument held in a horizontal position.
Measurements were made in June, July and August at three sites, 906 (2133-ECA), 916 (4439-ECV) and 966 (2437-BBS). Measurements were made in the plots as well as in areas adjacent to the plots with similar foliage cover. Measurements were made at solar zenith angles greater than 75 degrees with the instrument held in a horizontal position.
Blaine L. Blad, Professor and Head
Elizabeth A. Walter-Shea, Asst. Professor
University of Nebraska
Measuring and Modeling Near-Surface Reflected and Emitted Radiation Fluxes at the FIFE Site.
Mark A. Mesarch
Cynthia J. Hays
Elizabeth A. Walter-Shea
The Indirect Leaf Area Obtained from the UNL Light Wand data were collected under the direction of B.L. Blad and E.A. Walter-Shea at the University of Nebraska. The dedicated efforts of C.J. Hays and M.A. Mesarch in the collection and preparation of these data is particularly appreciated.
There is a certain probability that a beam of radiation passing through some distance of a vegetative canopy will be intercepted by foliage. The probability of interception is proportional to the path length, foliage density and foliage orientation. Therefore, if the beam transmittance is known, then it is possible to invert foliage information (Welles and Norman 1991).
Non-interception (i.e., transmittance) is described by Beer's law:
Since the LAI-2000's optical sensor averages over azimuth, the azimuth angle (azi) is omitted from the following equations with the understanding that the various quantities are azimuthal averages.
Rewriting equation 1 yields:
K(zen) is the contact frequency, and is equivalent to the average number of contacts per unit length of travel that a beam would make passing through a canopy at zenith angle (zen).
The foliage density u is defined as:
Foliage density is related to leaf area index (LAI) by canopy height (z):
Path length S is related to canopy height (z) by zenith angle (zen):
Using equations 2, 3, 4 and 5 we can define the leaf area index in terms of canopy transmittance:
Because canopy height cancels out of Equation 6 it is numerically identical to Equation 3 when S(zen) = 1 / cos(zen). Thus Equation 3 can be used for either LAI or foliage density: if the distances are 1 / cos(zen), then the results should be interpreted as LAI; otherwise, foliage density is computed.
Once the LAI is estimated from transmittance measurements, 5 values of G(zen) are determined using Equation 2. A straight line is fit to the data and the slope of that line (dG(zen) / dzen) is used in the following equation to predict mean tilt angle:
The LAI-2000 plant canopy analyzer is composed of a LAI-2070 control unit and a LAI-2050 sensor head. The control unit is 21 cm x 11.4 cm by 6.9 cm and has connectors for 2 sensor heads, 2 BNC connectors for other LI-COR sensors, and a connector for RS-232 communication. The sensor head projects the image of its nearly hemispheric view onto five detectors arranged in concentric rings (approximately 0-13, 16-28, 32-43, 47-58, 61-74 degrees). Radiation above 490 nm is rejected. Lenses are coated with MgF2 to improve transmission at high oblique angles. For further information consult the LAI-2000 plant canopy analyzer instruction manual. Several different prototype LAI-2000's were used in 1988. Serial number PCH23, PCA122 was used in 1989.
Hand-held in a horizontal position.
Indirect LAI (1988 & 1989), mean tilt angle and diffuse non-interceptance (1989) transmittance through canopy.
The amount of foliage in a vegetative canopy can be deduced from measurements of radiation attenuation as it passes through the canopy at several angles from the zenith. Foliage orientation information can also be obtained. The LAI-2000 measures the attenuation of diffuse sky radiation at five zenith angles simultaneously. For further information consult the LAI-2000 plant canopy analyzer instruction manual.
The LAI-2000 was hand-held in a horizontal position. The LAI-2050 (sensor head) has a near hemispherical field-of-view. The effective view area is:
The potential view area is larger than the effective since the effective range of view is reduced by foliage. The potential area viewed is:
4421 Superior Street
P.O. Box 4425
Lincoln, NE 68504
Calibration data for the sensor head is not needed to compute LAI when only one LAI-2050 sensor head is used. Transmittances are calculated for each ring independently of the other rings so the calibration factors cancel out.
The front lens of the LAI-2050 sensor head should be kept clean and dry for comparable readings. Recalibration of the sensor head may never be necessary, as long as the optics within the sensor remain in place. The detectors may have long-term electrical drift, but this would not affect LAI determinations.
It is very difficult (if not impossible) to determine the precision and accuracy of leaf area index measurements. Leaf area index is estimated by the LAI-2000 from all light not intercepted by any object in the sensor's field-of-view, so foliage area index is a more appropriate term.
Some assumptions must be met for accurate estimates of foliage amount and orientation when using the LAI-2000.
The degree to which these assumptions are violated will affect the degree to which the calculations can be trusted. The major assumptions are:
No canopy conforms exactly to the first 4 assumptions. Foliage is never random, but is clumped along stems and branches, and is certainly not black. Many species exhibit some degree of heliotropism, which violates the azimuthal randomness assumption. However, the practical compromises that must be made are often not serious. Many canopies can be considered to be random, and living foliage does have relatively low transmittance and reflectance below 490 nm. Offsetting errors may be common, such as when leaves are grouped along stems (increasing light transmittance), but arranged to minimize overlap (decreasing transmittance). View restrictors on the LAI-2050 can reduce errors associated with assumptions 5 and 6 but at the potential cost of reduction of sample size.
Measurement procedures varied because of the experimental nature in using the prototype LAI-2000 plant canopy analyzers provided by LI-COR. The number of incoming and transmitted readings and the number of replications varied. The following conditions were consistently met: 1) the LAI-2050 (sensor head) held in a horizontal position, 2) measurements at large solar zenith angles (greater than 75 degrees) or under completely overcast sky conditions, 3) measurements in areas adjacent to the 4 plots identified at site 811(4439-LWN) with similar foliage cover.
Measurements were made at three sites, 906 (2133-LWN), 916 (4439-LWN) and 966 (2437-LWN). Measurements were made in the plots as well as in areas adjacent to the plots with similar foliage cover. Measurements were made at solar zenith angles greater than 75 degrees with the instrument held in a horizontal position. Measurements were made in the following manner: first an incoming then a transmitted measurement were made. On June 2 three replications of these pair of measurements were made in each plot. The remaining days measurements were made, 5 replications were made in each plot.
Measurements were always made in areas adjacent to the four plots identified at site 811 (4439-PAM) in areas of similar foliage cover.
The FIFE study area, with areal extent of 15 km by 15 km, is located south of the Tuttle Reservoir and Kansas River, and about 10 km from Manhattan, Kansas, USA. The northwest corner of the area has UTM coordinates of 4,334,000 Northing and 705,000 Easting in UTM Zone 14.
The LAI-2000 has a near hemispherical field-of-view. The plot size was approximately 3 m x 3 m. This area was larger than the potential view area of the LAI-2000.
In 1988 measurement plot encircled the AMS station located at site 811 (4439-LWN). In 1989 measurement plots were located northeast of the Wind Aligned Blob (WAB) at sites 906 (2133-LWN) and 916 (4439-LWN) (Sellers et al., 1989). Topography files containing the northing and easting of the plots at each site, except for site 966 (SITEGRID_ID = 2437-LWN) in 1989, are available in the GRAB-BAG section of FIFE CD-ROM Volume 1 in the UNL directory, in files UNL_PLOT.T88 and UNL_PLOT.T89. These files also include the slope, aspect, soil depth, species and vegetative height of the plots.
Detailed description of the location of each site follows:
SITEGRID STN NORTHING EASTING LATITUDE LONGITUDE ELEV -------- --- -------- ------- -------- --------- ----- 2133-LWN 906 4329726 711604 39 05 34 -96 33 12 443 2437-LWN 966 4329150 712375 39 05 15 -96 32 41 4439-LWN 916 4325193 712773 39 03 06 -96 32 28 443 4439-LWN 811 4325219 712795 39 03 07 -96 32 27 445 SITEGRID SLOPE ASPECT -------- ------ ------ 2133-LWN 1 TOP 2437-LWN 4439-LWN 2 N 4439-LWN 2 N
Measurements were made at large solar zenith angles (greater than 75 degrees) or under overcast sky conditions. Measurements at a plot typically took 1 to 5 minutes depending on the number of replications. Measurements at a site required 10 to 60 minutes depending on the terrain and distance between plots.
The measurement time ranged from 1100 to 2030 GMT.
Measurements were not continuous over this time period. Measurements were made during 2 periods: July 29 through August 12, 1989, and June 13 through August 11, 1989. During these periods data were collected on the following twelve days:
OBS_DATE OBS_DATE --------- --------- 29-JUN-88 25-JUL-89 30-JUN-88 27-JUL-89 14-JUL-88 05-AUG-89 12-AUG-88 08-AUG-89 13-JUN-89 10-AUG-89 11-JUL-89 11-AUG-89
The optimal time interval between plot measurements was approximately 2 minutes. The time interval depended on the distance between the plots and the terrain.
The SQL definition for this table is found in the LIGHTWND.TDF file located on FIFE CD-ROM Volume 1.
Parameter/Variable Description Range Units Source
SITEGRID_ID This is a FIS grid location code. Site grid codes (SSEE-III) give the south (SS) and the east (EE) cell number in a 100 x 100 array of 200 m square cells. The last 3 characters (III) are an instrument identifier.
STATION_ID This column contains the station ID designating the location of the observations.
OBS_DATE This column contains the date of the observations, in the format (DD-MMM-YY).
OBS_TIME This column contains the time that the observation was taken in GMT. The format is (HHMM).
PI_NAME $ This column contains the name of the Principal Investigator who oversaw the collection of the data.
PLOT_NUM This column contains the plot number at the site where the observations were made.
SAMPLE_TYPE # This column contains information about the treatment of the plot where the data were collected, such as if the plot was watered or destructively sampled.
LAI_2000_LAI This column contains the leaf area index as calculated by the LAI-2000.
STD_ERR_LAI This column contains the standard error of the leaf area index measurement.
MEAN_LEAF_INCL_ANG This column contains the mean inclination angle of the leaves in the canopy as determined by the LAI-2000.
STD_ERR_LEAF_INCL_ANG This column contains the standard error of the mean inclination angle.
DIFFUSE_TRNSMTNC This column contains the diffuse transmittance of the canopy. This is the probability of diffuse radiation from above penetrating the canopy to a particular location.
NUM_OBS This column contains the number of pairs of above and below canopy observations used in the calculations.
FIFE_DATA_CRTFCN_CODE * This column contains the FIFE Certification Code for the data, in the following format: CPI (Certified by PI), CPI-??? (CPI - questionable data).
LAST_REVISION_DATE This column contains the last revision date for the data, in the format (DD-MMM-YY).
$ For the data described here the PI_NAME = B. BLAD.
# Decode the SAMPLE_TYPE field as follows:
* Decode the FIFE_DATA_CRTFCN_CODE field as follows:
The primary certification codes are:
The certification code modifiers are:
SITEGRID_ID STATION_ID OBS_DATE OBS_TIME PI_NAME PLOT_NUM SAMPLE_TYPE ----------- ---------- --------- -------- -------- -------- ----------- 2133-LWN 906 05-AUG-89 1443 B. BLAD 1 2133-LWN 906 05-AUG-89 1452 B. BLAD 101 DS 2133-LWN 906 05-AUG-89 1457 B. BLAD 2 2133-LWN 906 05-AUG-89 1505 B. BLAD 102 DS LAI_2000_LAI LAI_ST_ERR MEAN_LEAF_INCL_ANG LEAF_INCL_ANG_ST_ERR ------------ ---------- ------------------ -------------------- 1.590 .120 61 4 1.990 .060 58 4 1.510 .100 58 10 1.940 .090 63 8 DIFFUSE_TRNSMTNC NUM_OBS FIFE_DATA_CRTFCN_CODE LAST_REVISION_DATE ---------------- ------- --------------------- ------------------ .300 5 CPI 17-APR-91 .225 5 CPI 17-APR-91 .308 5 CPI 17-APR-91 .246 5 CPI 17-APR-91
The plot size was approximately 3 x 3 m. Topography files containing northing and easting, slope, aspect, soil depth, species, and vegetative height are available of the plots for each site.
A general description of data granularity as it applies to the IMS appears in the EOSDIS Glossary.
The CD-ROM file format consists of numerical and character fields of varying length separated by commas. The character fields are enclosed with a single apostrophe. There are no spaces between the fields. Each file begins with five header records. Header records contain the following information:
Each field represents one of the attributes listed in the chart in the Data Characteristics Sectionand described in detail in the TDF file. These fields are in the same order as in the chart.
Leaf area index is estimated with the LAI-2000 from all light not intercepted by any object in the sensor's field-of-view so foliage area index is a more appropriate term.
Some assumptions must be met for accurate estimates of foliage amount and orientation when using the LAI-2000. The degree to which these assumptions are violated will affect the degree to which the calculations can be trusted. The major assumptions are:
No canopy conforms exactly to the first 4 assumptions. Foliage is never random, but is clumped along stems and branches, and is certainly not black. Many species exhibit some degree of heliotropism, which violates the azimuthal randomness assumption. However, the practical compromises that must be made are often not serious. Many canopies can be considered to be random, and living foliage does have relatively low transmittance and reflectance below 490 nm. Offsetting errors may be common, such as when leaves are grouped along stems (increasing light transmittance), but arranged to minimize overlap (decreasing transmittance). View restrictors on the LAI-2050 (sensor head) can reduce errors associated with assumptions 5 and 6 but at the potential cost of reduction of sample size.
Leaf area index values were compared with values obtained from different methods and values from other investigators (Kim et al., 1989).
Generally green leaf area index was over-estimated since the LAI-2000 leaf area index estimate includes the contribution of leaves and stems and does not distinguish between green and dead foliage.
Comparison with destructive green leaf area index measurements showed good agreement in 1988 except for plot 2 on July 14th (a significant number of forbs were in the general location of this plot that would have been seen by the LAI-2000 that were not included in the destructive sample) and for August 12th (considerable number of senescenced leaves due to moisture stress that were "seen" by the LAI-2000 but were not included in the destructive method)(Kim et al., 1989).
Comparison with destructive foliage area index (green and dead leaves and stems) measurements yielded a mean bias error and mean absolute bias error of -0.12 and 0.37, respectively. The most likely reason for the disagreement was the length of time spent preparing and measuring the destructive sample components.
Welles and Norman (1991) contains information on other verification tests.
The number of transmitted readings needed can be determined to give a leaf area index within +/- 10% at a 95% confidence level. For further information see the LAI-2000 plant canopy analyzer instruction manual.
Not available at this revision.
Not available at this revision.
The data verification performed by the ORNL DAAC deals with the quality of the data format, media, and readability. The ORNL DAAC does not make an assessment of the quality of the data itself except during the course of performing other QA procedures as described below.
The FIFE data were transferred to the ORNL DAAC via CD-ROM. These CD-ROMs are distributed by the ORNL DAAC unmodified as a set or in individual volumes, as requested. In addition, the DAAC has incorporated each of the 98 FIFE tabular datasets from the CD-ROMs into its online data holdings. Incorporation of these data involved the following steps:
Each distinct type of data (i.e. "data set" on the CD-ROM), is accompanied by a documentation file (i.e., .doc file) and a data format/structure definition file (i.e., .tdf file). The data format files on the CD-ROM are Oracle SQL commands (e.g., "create table") that can be used to set up a relational database table structure. This file provides column/variable names, character/numeric type, length, and format, and labels/comments. These SQL commands were converted to SAS code and were used to create SAS data sets and subsequently to input data files directly from the CD-ROM into a SAS dataset. During this process, file names and directory paths were captured and metadata was extracted to the extent possible electronically. No files were found to be corrupted or unreadable during the conversion process.
Additional Quality Assurance procedures were performed as follows:
As errors are discovered in the online tabular data by investigators, users, or DAAC staff, corrections are made in cooperation with the principal investigators. These corrections are then distributed to users. CD-ROM data are corrected when re-mastering occurs for replenishment of CD-ROM stock.
1989: Data taken at site 966 (2437-LWN) were taken with the sensor held in a horizontal position rather than parallel to the ground. LI-COR (LAI-2000 manual 1991) now suggests that the sensor should be parallel to the ground. No possible error is associated with holding the sensor in any other position.
There can be considerable variability in leaf area index values, especially in natural ecosystems. Grazing can enhance the natural variability. Extreme water stress causing leaf rolling could change the effective LAI.
The LAI-2000 is definitely a field proven instrument since we were allowed to test the prototype in 1988. Our field experiments seem to be governed by Murphy's law (what can go wrong will go wrong). When using the prototype sensor we managed to find a few quirks including negative numbers that only seemed to happened when we used the sensor. LI-COR representatives were finally able to replicate the negative numbers but for a while they were baffled. We have used the LAI-2000 since 1988 with no problems, which rarely happens with any instrument we use.
This data set can be utilized to characterize bi-directional reflectance factor distributions in the solar principal plane for a tall grass prairie; estimate surface albedo from bi-directional reflectance factor and radiance data; determine the variability of reflected and emitted fluxes in selected spectral wavebands as a function of topography, vegetative community and management practice; determine the influence of plant water status on surface reflectance factors; and determine sun angle affects on radiation fluxes.
The FIFE field campaigns were held in 1987 and 1989 and there are no plans for new data collection. Field work continues near the FIFE site at the Long-Term Ecological Research (LTER) Network Konza research site (i.e., LTER continues to monitor the site). The FIFE investigators are continuing to analyze and model the data from the field campaigns to produce new data products.
Software to access the data set is available on the all volumes of the FIFE CD-ROM set. For a detailed description of the available software see the Software Description Document.
ORNL DAAC User Services
Oak Ridge National Laboratory
Telephone: (865) 241-3952
FAX: (865) 574-4665
ORNL Distributed Active Archive Center
Oak Ridge National Laboratory
Telephone: (865) 241-3952
FAX: (865) 574-4665
Users may place requests by telephone, electronic mail, or FAX. Data is also available via the World Wide Web at http://daac.ornl.gov.
FIFE data are available from the ORNL DAAC. Please contact the ORNL DAAC User Services Office for the most current information about these data.
The Indirect Leaf Area Index Obtained from UNL Light Wand data are available on FIFE CD-ROM Volume 1. The CD-ROM filename is as follows:
Where xxxx is the four digit code for the location within the FIFE site grid. Note: capital letters indicate fixed values that appear on the CD-ROM exactly as shown here, lower case indicates characters (values) that change for each path and file.
The format used for the filenames is: ydddgrid.sfx, where grid is the four-number code for the location within the FIFE site grid, y is the last digit of the year (e.g., 7 = 1987, and 9 = 1989), and ddd is the day of the year (e.g., 061 = sixty-first day in the year). The filename extension (.sfx), identifies the data set content for the file (see the Data Characteristics Section) and is equal to .LWN for this data set.
Anonymous. 1989. LI-COR LAI-2000 plant canopy analyzer instruction manual. LI-COR. Inc. Lincoln, NE (1991).
Anonymous. 1991. LI-COR LAI-2000 plant canopy analyzer technical information. LI-COR. Inc. Lincoln, NE (1989).
Kim, J., C. Hays, S. Verma, B. Blad. 1989. A preliminary report on LAI values obtained during FIFE by various methods. Department of Agricultural Meteorology. University of Nebraska-Lincoln. Lincoln, NE 68583-0728.
Welles, J.M., J.M. Norman. 1991. Instrument for indirect measurement of canopy architecture. Agronomy Journal. 83:818-825.
Contact the EOS Distributed Active Archive Center (DAAC) at Oak Ridge National Laboratory (ORNL), Oak Ridge, Tennessee (see the Data Center Identification Section). Documentation about using the archive and/or online access to the data at the ORNL DAAC is not available at this revision.
A general glossary for the DAAC is located at Glossary.
A general list of acronyms for the DAAC is available at Acronyms.
May 6, 1994.
Warning: This document has not been checked for technical or editorial accuracy by the FIFE Information Scientist. There may be inconsistencies with other documents, technical or editorial errors that were inadvertently introduced when the document was compiled or references to preliminary data that were not included on the final CD-ROM.
Previous versions of this document have been reviewed by the Principal Investigator, the person who transmitted the data to FIS, a FIS staff member, or a FIFE scientist generally familiar with the data.
July 29, 1996.
Blad, B. L., and E. A. Walter-Shea. 1994. First ISLSCP Field Experiment Indirect Leaf Area Obtained from the UNL Light Wand Data Set, In D. E. Strebel, D. R. Landis, K. F. Huemmrich, and B. W. Meeson (1994), Collected Data of The First ISLSCP Field Experiment, Vol. 1: Surface Observations and Non-Image Data Sets. Published on CD-ROM by National Aeronautics and Space Administration. Available on CD-ROM and on-line [http://daac.ornl.gov] from Oak Ridge National Laboratory Distributed Active Archive Center, Oak Ridge, Tennessee, U.S.A.