Southern Great Plains 1997 (SGP97) Hydrology Experiment Plan
Section 12 - Sampling Protocols


Goto Section in Document:
Table of Contents and Executive Summary            
1. Overview
2. Soil Moisture and Temperature
3. Vegetation and Land Cover
4. Soil Physical and Hydraulic Properties
5. Planetary Boundary Layer Studies
6. Satellite Data Acquisition
7. DOE ARM CART Program
8. Oklahoma Mesonet Program
9. Operations
10. Data Management and Availability
11. Science Investigations
12. Sampling Protocols
13. Local Information
14. References
15. List of Participants

12. SAMPLING PROTOCOLS

12.1 Gravimetric Surface Soil Moisture

The gravimetric soil moisture (GSM) sampling is intended to estimate the site average and standard deviation. Precise location within the site is not important, however, the samples should be spatially distributed to obtain meaningful statistics. For this reason a grid is used. In sites that are the subject of spatial variability studies and are marked at sampling points, it is all right to use these markings for locating GSM points. Two types of sampling designs will be employed, Full and Profile. The actual soil moisture sampling is the same but the distribution and number of samples is different.

A few important general items:

12.1.1. GSM Sampling Procedure

Preparation

* Arrive at your area operations base at assigned time. Check in with area manager and review notice board.

* Assemble GSM kit

Bucket

GSM tool

6 cm spatula

3 cm spatula

Notebook

Pens

Boxes of cans

First aid kit (per car)

Phone (if assigned)

* Check weather

Full sampling Site

The goal of this sampling is to characterize the mean of what we hope is a "homogeneous" field. A total of 14 points in each site will be sampled. Some sites are not square and the procedure will have to be adapted.

Profile Sampling Site

These are locations at which the objective is solely to correlate gravimetric surface soil moisture to the data collected by the insitu heat dissipation 5 cm sensors. Nine samples are collected on a nominal grid 10 m apart (total of 20 m by 20 m area) immediately adjacent to the sensor enclosure.

Taking a GSM Sample

1. Remove vegetation and litter.

2. Use the large spatula (6 cm) to cut a vertical face at least 5 cm deep (Figure 25 a).

3. Push the GSM tool into this vertical face. The wings of the scoop should rest on the soil surface.(Figure 25 b).

4. Use the large spatula to cut a vertical face on the front edge of the scoop (Figure 25 c).

5. Place sample in can, small spatula aids extraction (Figure 25 d).

12.1.2. GSM Sample Processing

All GSM samples are processed to obtain a wet and dry weight. It is the sampling teams responsibility to perform the wet weighing and placement of the samples in the drying ovens. A lab team will perform the removal of samples from the oven, dry weighing, and can cleaning.

The Gravimetric Soil Moisture (GSM) samples will be collected from all sites on a daily basis. After the retrieval of all samples they will be weighed in their 'wet' state and put in the oven. The next day, approximately 22 hours later, the now oven­dry samples will be weighed again. To speed up weighing and to reduce the data processing, the process has partially been automated by connecting the balances with PC's.

Wet Weight Procedure

1. The balance will be connected to a computer that will record the weight.

2. Turn on computer and get the program running. There will be an instruction sheet available on site and instruction will be provided.

3. Process your samples on a site basis and in sample numeric order.

4. Place the open cans (the lid goes on the bottom of the can) in the drying oven. Arrange them sequentially.

5. There will be sheets available for manually recording the weights if you encounter unresolvable problems with the computer operation.

Dry Weight Procedure

1. All samples should remain in the oven for approximately 20-22 hours at 105oC.

2. Try to remove samples in the order they were put in. This should result in sequential groups for sites. Only remove a few sites at a time and keep oven running.

3. These samples will be hot. Wear the gloves provided.

4. Follow the instructions provided for the software to enter the dry weight of can (with its lid).

5. Dump the soil after verifying data and clean the can with the brush provided.

6. Repack the can boxes, check that can numbers are readable and replace any damaged or lost cans with spares.

Data Processing

1. There will be a raw data file for each site on each day.

2. There will be a summary file for each day for each area that will contain the means and standard deviations.

3. All files are backed up with a floppy disk copy.

4. The summary file will be transmitted to a central collection point on a daily basis.

5. You may keep copies of raw data for any site that you actually sample at this stage. You may not take any other data until quality control has been conducted

Equipment

For the automated weighing the following equipment is required:

1 IBM compatible PC. 386 or better.

Mettler Toledo BD series portable balance (BD601)

RS­232­c/cl Interface

IBM PC/AT 15­pin 9 socket Interface cable

Mettler Toledo Balance link software

The WEIGHING program developed by the USDA­ARS­Hydrology Lab

Note: The interface with interface cable will be connected all balances prior to the SGP '97 experiment.

Installation of Equipment:

It is intended that the PC with the balance and the required software are set up and installed before the SGP '97 experiment begins. If not so, and the coordinator is not present or if the coordinator gave authorization follow the following procedure to set up the equipment. Install the PC in the laboratory. Then connect the Mettler Toldeo interface cable to serial port COM1.

Optionally the mouse can be connected to COM2. Start the computer and turn on the balance by pressing the ON button.

Insert the disk labeled "Data acquisition program, Mettler Toledo BalanceLink ME410023 Ver. 2.20" into the PC. Type A:\installe C:\balink at the prompt. If the 3.5" floppy drive is the B:\ drive type B:\ instead of A:\). Go to C:\BALINK and type LICENCSE, and follow the directions until the program quits.

Take the floppy from the 3.5" disk drive, and insert the 3.5" floppy labeled "BALINK automated soil sample weighing program" into the computer.

Go to the floppy drive and type install. The program WEIGHING is now being installed into the directory BALINK.

The WEIGHING program

Start the program by typing WEIGHING at the C:\BALINK> prompt. The program will guide you through the weighing procedure until all samples have been weighed.

All data is saved in DOS TAB delimited format, which can be imported in all major spreadsheets. Like MS­Excel, Quattro­Pro, Lotus etc. For a full description how to operate the program read the manual sheet.

Weighing procedure

1.Turn on the balance, the computer and the printer.

2.Make sure the paper in the printer is in the right position.

3.Insert the floppy labeled BACKUP into the 3.5" floppy drive.

4.Start the program WEIGHING.

5.Follow the steps as described on the WEIGHING balance program manual page.

6.Weigh the samples in ascending order.

7.When you are finished weighing you samples store the prints in the right binder (WET or DRY Binder).

8.Always finish your work cleaning up you workspace!

Example of Using the WEIGHING balance program

1. Turn the balance ON.

2. RUN the program by typing WEIGHING at the c:\prompt

Questions from the program Comments
Area you are working in (er/lw/cf) er = El Reno
lw = Little Washita
cf = Central Facility
Site this set of cans was collected from Enter Site number
Wet or Dry samples to be weighed w = wet
d = dry
Option WET
If you enter (W)ET the program
will ask you for the Letter of the
Box where the samples belong
Option DRY
If you enter (D)RY the program
will ask you for the date of the wet
samples. Enter date as MM/DD
**The WET data are loaded and
appear on the screen SKIP until
DRY section
First can in box to be weighed Enter the number of the first can to be
weighed. Always start with the can with
the lowest number! (usually can 1).
Last can in box to be weighed Enter the number of the highest
numbered can of the series to be
weighed. Normally you have 14 cans
per site.
Is this information correct? Ready to proceed? (y/n)

The input sheet appears on the screen, and you are now ready to weigh the samples.

Example of the input sheet as displayed on the screen


             Gravimetric Soil Moisture Weight Recording 
              +-------------------+------------------+ 
              |     Area: er      |      Site:  1    |
              +-------------------+------------------+ 

                                |      Sampling       |
 Sample |  Date           DOY   |   Wet (g)    Dry(g) |  Difference 
+----------+------------------------+---------+-------------+------------------+ 
  A01      05-07-1997    127         0.00
  A02

etc.

Make sure that the top cell is highlighted and put sample No. 1 on the balance!

Put the first can on the balance

Press the enter button on the balance (most right button).

Note that the balance value is exported to the screen, and that the cursor has moved one line down.

Put the next sample on the balance and repeat that procedure until you get the message:

Done collecting data. Hit any key to see results.

Hit a key. The program automatically saves your data sheet now, shows your data on the screen and prints out a paper copy. At the top of every print you will find the name of the file were the data was saved to.

At the bottom of that page you can see the message:

Done. Hit any key to continue, 'q' to quit.

Press any key to weigh the next set of sample cans, press Q to leave the program.

12.2. Soil Bulk Density

All sites involved in gravimetric soil moisture sampling will be characterized for soil bulk density. The method used is a volume extraction technique that has been employed in most of the previous experiments and is especially appropriate for the surface layer. Four replications are made for each site.

12.2.1. The Bulk Density Apparatus

The Bulk Density Apparatus itself consists of two parts. A 12" diameter Plexiglas piece with a 6" diameter hole in the center and three 3/4" holes around the perimeter. Foam is attached to the bottom of the Plexiglas. The foam is three inches high and two inches thick. The foam is attached so that it follows the circle of the Plexiglas. Figure 26 shows the basic components.

Other Materials Required for Operation

12.2.2. Selecting and Preparing an Appropriate Site

1. Select a site. An ideal site to conduct a bulk density experiment is: relatively flat, does not include any rock or roots in the actual area which will be tested and has soil which has not been disturbed.

2. Ready the site for the test. Remove all vegetation, rocks and other debris from the surface prior to beginning the test. Remove little or no soil when removing the debris.

12.2.3. Bulk Density Procedure

Securing the Apparatus to the Ground

1. Place the apparatus foam-side-down on the ground.

2. Place the three securing rods in the 3/4" holes of the apparatus.

3. Drive each dowel into the ground until they do not move easily vertically or horizontally. (Figure 26 a)

Leveling the Apparatus Horizontally to the Ground

1. Tighten each of the bolts until the apparatus appears level and the foam is compressed to 1-1/2" to 2".

2. Place the bubble level on the surface of the apparatus and tighten and loosen the bolts in order to make the surface level. (If the bubble is too far to the right, the right side is too high. Tighten the bolt(s) on the right, or loosen those on the left, until it is horizontal.)

3. Place the level in at least three directions and on three different areas of the surface of the apparatus.

Determining the Volume from the Ground to the Hook Gauge

1. Pour one liter of water into the graduated cylinder

2. Pour some of the water into a plastic storage bag.

3. Hold the plastic bag so that the water goes to one of the lower corners of the bag.

4. Place the corner of the bag into the hole. Slowly lower the bag into the hold allowing the bag and the water to snugly fill all of the crevasses.

5. Slightly raise and lower the bag in order to eliminate as many air pockets as possible.

6. Lay the remainder of the bag around the hole.

7. Place the hook-gauge on the surface of the apparatus, so that it is secure between the notches on the opposite sides of the hole.

8. Add water to the bag until the surface of the water is just touching the bottom of the hook on the hook-gauge. A turkey-baster works very well to add and subtract small volumes of water. Be sure not to leave any water remaining in the turkey-baster. (Figure 26 b)

9. Place the graduated cylinder on a flat surface. Read the cylinder from eye-level. The proper volume is at the bottom of the meniscus. Read the volume of the water remaining in the graduated cylinder. Subtract the remaining volume from the original 1000 ml to find the volume from the ground surface to the hook-gauge.

10. Carefully transfer the water from the bag to the graduated cylinder. Hold the top of the bag shut, except for two inches at either end. Then use the open end as a spout. (It is best to reuse water, especially when doing multiple tests in the field.)

Loosening the Soil and Digging the Hole

1. Label the oven-safe bag with the date and test number and other pertinent information using a permanent marker.

2. Loosen the soil. The hole should be approximately six inches deep and should have vertical sides and a flat bottom. (The hole should be a cylinder: with surface area the size of the hole of the apparatus and height of six inches.)

3. Remove the soil from the ground and very carefully place it in the oven-safe bag. (Be careful to loose as little soil as possible.) (Figure 26 c and d)

4. Continue to remove the soil until the hole fits the qualifications.

Finding the Volume of the Hole

1. Determine the volume from the bottom of the hole to the hook-gauge as described in Determining the Volume from the Ground to the Hook-Gauge. Reusing the water from the prior measurement presents no potential problems and is necessary when performing numerous experiments in the field.

2. Subtract the volume of the first measurement from the second volume measurement. The answer is the volume of the hole.

Calculating the Density of the Soil

1. Dry the soil in an oven for at least 24 hours.

2. Mass the soil.

3. Divide the mass of the soil by the volume of the hole. The answer is the density of the soil.

12.2.4. Potential Problems and Solutions

After I started digging I hit a rock. What should I do?

The best solution is to start over in another location. Also, you can remove the rock from the soil and subtract the volume of the rock from the total volume of the water. You should never include a rock in the density of the soil. Rocks have significantly higher densities than soil and will invalidate the results. Roots, corn cobs, ants and even mole holes will also invalidate the results. If you find any of these things the best thing to do is start the test again at another site.

After I began digging the hole I noticed one of the dowels wasn't the apparatus firmly in place. Do I have to start over?

Unfortunately, if you have already started digging you do have to start the experiment again. Replacing the dirt to find the volume between the ground surface and the hook-gauge will give an inaccurate volume and thus an inaccurate soil density.

I noticed that the bag holding the water has a small leak. Is there anything I can do? If the leak began after you had already found the volume, it is not necessary to start again. The volume is being measured in the graduated cylinder. If you have already removed the appropriate volume of water leaks in the bag, it will not affect the results of the test. However, if you noticed the leak before finding the volume, you will have to start again.

12.3. Surface Variability Sampling

Pre-arrival

It is expected that everyone that has purchased the Moisture Probe/Data Wizard/DGPS combination will familiarize themselves and their groups with each piece of equipment as much as possible before arriving in June. Familiarity with Microsoft Excel will also be required for data downloading purposes.

Site selection and temporal frequency of sampling

Assuming that a pair of workers constitutes one sampling team (one working the moisture probe/DGPS and the other manually and electronically recording the data); that two teams can sample one field in 2.5 hours; and considering the total number of workers available for variability sampling at each of the LW, ER and CF, daily sampling will be conducted at 3(4) quarter sections within the LW, 1(2) quarter sections at ER, and 1(2) at the CF. These sites are tentatively identified as:LW03, LW12, LW20, ER01-04, and CF03. Additional sites and measurements may be added.

Preliminary field work

June 12 and 13, LW, ER, CF. Locate sampling locations (49 points on a 100 m ( 7 X 7) grid) in selected quarter sections using the compass/tape measure/spray paint/flag approach previously described in Jackson emails. Location numbers (1-49) should be spray-painted on the ground or written on surveying flags. The numbering convention will follow the route that a sampler would follow. Location 1 will be located in the SE corner of a quarter section. Heading due north, location 2 is next, through location 7 in the NE corner of a quarter section. Moving 100 m west to the next N-S column of the sampling grid, location 8 will be located at northern end and location 14 at the southern end. The numbering scheme will proceed in this fashion until reaching location 49 in the NW corner of the quarter section.

June 12 and 13, LW, ER, CF. Enter way points for DGPS navigating into GPS receivers. Practice locating points.

June 14, LW. Instruct other researchers in the use of the surface variability equipment.

June 15 and 16, LW. Practice entire data gathering procedure (including data collection, recording, downloading and pre-processing (quality checking and calculating basic statistics).

Sampling

June 18 - July 18, LW, ER, CF. Depart for surface variability sites at 1 p.m. (two teams per vehicle, or 1 vehicle per site). At each site, one team will sample locations 1-24 and the second will sample locations 25-49. Within each team, one person will handle both the moisture probe and DGPS (the "instrument" person), and the second will record the data (the "recorder"), both manually (in a notebook) and electronically in the Data Wizard.

Entries will include sampling location number, moisture probe value, date, time, field i.d., and comments. For example, a typical entry in the Data Wizard might read:

01 .55 062197 1400 LW03 muddy

In this way, date, time and site number need only be entered at the first sampling location. After the first sample, only the location number and moisture probe value (and perhaps time) need be recorded.

Communications

A cell phone will be available at each quarter section in which surface variability studies are conducted if those sites are at a considerable distance from a home base such as the Chickasha or El Reno offices.

Data Management

Upon return from the field, the "recorder" of each team will be responsible for downloading data to PC's (Microsoft Excel) and backing up to floppy disks. This person will then "back fill" the spreadsheet entries not entered after the first sample in the field (e.g. date, site number) and quality check the data for obvious errors. El Reno and CF teams should email their quality-checked data each afternoon to Chickasha. Data will be saved in both Excel and ASCII formats.

Overall data management and quality control will be overseen by Johanna Devereaux (UT). Johanna will also provide daily updates of the moisture content statistics at each of the surface variability sites.

Equipment care and maintenance

The "instrument" member of the team will be responsible for care and maintenance of the moisture probe/DGPS/Data Wizard system on a nightly basis. This includes recharging the 12-volt DGPS battery, checking battery strength on the moisture probe and Data Wizard, packing spare batteries for the next day's field work, and cleaning all equipment as required. Problems with equipment should be reported to Jay Famiglietti in LW, Paul Houser in ER and Chip Laymon at the CF.

12.4. Profile Soil Moisture TDR

The Oklahoma Mesonet is using Moisture Point Time Domain Relectrometry (TDR) instruments as part of a program to calibrate its heat dissipation soil moisture profiling systems. Measurements are made periodically to build up a data base for calibration. Because of the manual nature of the measurement, the TDR observations are made fairly infrequently (whenever a Mesonet technician or interested researcher visits the site). A cooperative effort in SGP97 is to add additional spatial and temporal observations to this program.

The Moisture Point TDR system consists of a small portable box (Model MP-917) that is necessary to make readings and the probes. The probes are single rods 1.3 cm by 1.9 cm by 90 cm (other lengths may be present at some locations) that are inserted and left in place. Readings of volumetric water content in 5 soil layers (0-15, 15-30, 30-45, 45-60, and 60-90 cm) are made in the Mesonet configuration. Other probe designs are also in use. The supplier maintains an extensive web site that includes product descriptions as well as some very detailed technical briefs (http://www.esica.com). There are expected to be approximately 30 probes distributed over the SGP97 region and a total of 4 MP-917 instruments.

Moisture·Point probes also have integral solid state I.D. tags. The tags enable any instrument equipped with a Moisture·Point datalogger to read the probe serial number and automatically tag the data with the time/data and I.D. number.

The MP­917 interrogates the probe and reduces the segment data to a numerical data set for display, or export to a data logger. It takes approximately 15 seconds per segment for the instrument to interrogate, analyze data and log moisture. A standard five segment probe will take approximately 75 seconds to completely measure the moisture for all segments. The entire process is automatic once the MP­917's MEASURE button is pressed. Digital data displayed is an average "Volumetric Water Content" measured over the length of each probe segment. The Moisture·Point datalogger is operational when installed in the MP­917 and powered. As measurement data is displayed, it is recorded by the datalogger, along with the time and date of the measurement. Windows based software is provided to download the data into a file which can be viewed directly or exported to other applications (e.g. spreadsheets). Datalogger memory can store up to 2,500 five segment probe data sets and is expandable to over 10,000 data sets. Data acquired by the logger is numerical and consists of the probe I.D. number, time delay (in raw counts) and volumetric water content in percent to the first decimal place.

Measurement Protocol (To be expanded on through review by Mesonet)

a. Each day that soil moisture data is collected between June 18 and July 18 the probes will be read.

b. Arrive at site and inspect probe condition and check MP-917, make sure the settings are correct for the probe at that site.

c. Hook up MP-917 to probe and make measurement.

d. Replace any protective covers.

e. Record notes in field notebook.

f. At end of day record data to a computer file and turn in to the area manager

Measurements at CF by O'Neill and MARS by OSU students

Measurements at El Reno and KING by HL personnel and Houser

Measurements within the Little Washita by ARS LW personnel and Miller

12.5. Vegetation Sampling

12.5.1. Activities at Each Site

1. Park on the shoulder of public roads or stay on field roads. Do not drive through the field.

2. Enter the field through a normal entry point such as a gate. Avoid climbing fences, if possible. If you open a gate to enter a field, close it immediately.

3. If there is a meteorological station in the field, use the GPS receiver and record its location.

4. Walk ~100 meters (120 paces) to a representative area in the field. The area must be 100 m from field edges. Check the area carefully for snakes and other critters before proceeding. Insert the sampling frame horizontally at the soil surface, starting near your left foot.

5. Use GPS receiver and record location of the sampling frame (Sample #1 only) on the data form.

6. Record vegetation type and growth stage on the data form.

7. Record sky, vegetation, and soil conditions on the data form. Comment on any unusual conditions.

8. Take an oblique and vertical photograph of the vegetation in the sample frame. For the vertical photo, stand outside of the sample frame, hold the camera out over the sample frame at shoulder height with the lens facing down toward the surface and take a picture.

9. For the oblique picture, stand 3­5 m from the sampling frame and focus on the sampling area. Be sure that the entire sample area is included in the picture. Record the film roll and frame numbers on the Vegetation Data Sheet.

10. Measure leaf area index (LAI) in the sample frame with the plant canopy analyzer (LAI­2000). Use Plant Canopy Analyzer protocol.

11. Measure LAI at 4 additional locations within 3 m of the frame.

12. Measure fraction of absorbed PAR with AccuPAR in the sampling frame. Use Absorbed PAR protocol.

13. Measure fraction of absorbed PAR at 4 additional locations within 3 m of the frame.

14. Clip all of the standing vegetation at the soil surface. Use a meter stick to form the fourth side of the sampling frame. Cut all vegetation within the volume defined by the sampling frame. Plot # = A, Sample # = 1.

15. Place all clipped vegetation on a plastic sheet and sort into green and brown vegetation. Put green and brown vegetation into separate bags and staple closed. If there is no green or brown vegetation, enter 0 (zero) on the data form for the wet and dry weights. This will help remove any confusion as to whether a sample was missed. Label each bag with :

Site # e.g., LW01, ER05, CF01, etc.

Plot # A, B, or C (for the 3 locations within each field or site)

Sample # 1 or 2 (for the 2 sampling frames at each location in a field)

Plant part GREEN = green standing vegetation (G)

BROWN = brown standing vegetation (B)

RESIDUE = litter on soil surface (S)

16. Collect litter from the soil surface and place in a separate bag and staple closed. Label the bag. If there is no surface residue, enter 0 (zero) on the data form for the wet and dry weights. This will help remove any confusion as to whether a sample was missed or forgotten.

17. Move the sampling frame 5 m to the right and repeat steps #5 to 16 Plot = A, Sample = 2.

18. Walk 100 m to a new representative area within the field. Repeat steps #5 to 17. Plot = B, Sample = 1. Move 5 m for Plot = B, Sample = 2.

19. Walk 100 m to a new representative area within the field. Repeat steps #5 to 17. Plot = C, Sample = 1. Move 5 m for Plot = C, Sample = 2.

Note: This sampling protocol will produce 6 green standing biomass, 6 brown standing biomass, and 6 surface litter biomass samples per site.

20. Weigh each bag as soon as you return to the car. Weigh large samples (>300 grams) on the high capacity balance (+ 1 gram accuracy). Weight small samples on the + 0.1 g balance.

Note: For the electronic balance to operate properly, it must be leveled and protected from the wind. Use a piece of plywood and wood blocks to create a stable, level surface on the car seat. Use a aluminum tray to prevent the bag from touching the balance. Tare the balance with the appropriate bag and weigh the samples. Close the windows and doors and read the balance through the window, if necessary to eliminate the wind effects.

21. Place the bags in the dryer at Chickasha or El Reno. Allow to dry for 4 days and weigh several representative bags. Allow to dry for another day and weigh the same bags again. Continue until there is no change in weight.

22. When the samples are dry, empty each sample into an aluminum tray and weigh to nearest 0.1 gram if the weight is less than 200 grams. Record weight on the appropriate data forms.

Note: A researcher at El Reno may want us to keep the samples from range lands so that he can do nutrient analysis on them. You will be instructed on the proper procedure to dispose of the samples when you arrive in Chickasha.

12.5.2. Plant Canopy Analyzer Protocol

1. Connect sensor to X­connector (on left as viewed from keypad). Clean the lens carefully with a lens brush.

2. Setup List:

Verify X Cal data for X port. Serial No. should match sensor on X­connector. (FCT 01; 02)

Set Resolution = HIGH.

Make sure time and date are correct (FCT 05)

3. Operating Mode:

Set Op Mode= 1 sensor X (FCT 11)

Sequence= 1 above and 5 below (FCT 12)

Reps= 1

Bad Reading= BEEP AND IGNORE (FCT 16)

4. Verify that each ring (X1 thru X5) is responding to light. (BREAK)

5. View cap = 90 degrees.

Sky conditions:

Diffuse illumination is ideal, but measurements can be made on sunny days with the following precautions:

Make all Above and Below readings with your back to the sun and with the view cap blocking the sensor's view of you and the sun.

Shade the sensor with your body to prevent reflections of the sun from influencing the readings.

Shade the part of the canopy which is visible to the sensor with the umbrella. Sunlit leaves cause the sensor to underestimate LAI.

Sampling

6. Press LOG. Enter site number for SITE= prompt, e.g., LW01 or ER08. Enter plot+sample for the SAMPLE= prompt, e.g., A10, A20, or C20. Add a zero to the sample number to indicate measurement is within the sampling frame.

7. Level the sensor above the canopy, shade the sensor from direct sun, and press the button on the sensor handle.

Note: Two beeps will be heard: one when the button is pressed and the other when the reading has been completed. Between the two beeps, keep the sensor level.

8. Put the sensor beneath the vegetation and level it. The sensor should view the same direction as the Above canopy reading. Press the button on the sensor handle.

9. Move the sensor about 15 cm diagonally (relative to the field of view of the sensor) and take another beneath the canopy reading. Repeat for 5 beneath the canopy readings. After the last reading the display will show COMPUTING....

Note: The first set of LAI measurements should be within the sampling frame. The subsequent measurements should be within 3 m of the frame.

10. Move to a new area outside the sampling frame and repeat steps #6­9 four times. SITE = stays the same (press enter to retain the value). SAMPLE= the last digit increments by one, e.g, A11, A12, A13, A21, A22, etc.

Downloading LAI­2000 files to a PC (see chapters 6 and 9 of Instruction Manual)

11. Use FCT 31 to set

BAUD = 4800

DATA BITS = 7

PARITY = None

Xon/Xoff = No

12. Run communications program, PROCOMM, on personal computer. Configure the computer's RS­232 port to match the LAI­2000. Connect the computer and LAI­2000 with the appropriate cable. Specify the destination for the incoming data. c:\SGP97\LAI\yymmddn.ext

where yymmddn = year, month, day, name of team leader (c=Curry, r=Russ, w=Ward).

.ext = format of output (.std = standard, .spr = spreadsheet format)

13. Output the LAI­2000 data files in the standard (.STD) and Spreadsheet (.SPR). formats. Backup files to a floppy disk.

14. Print the spreadsheet format files.

15. Clear the files after verifying that all files have been transferred successfully.

12.5.3. Fraction of Absorbed PAR Protocol

The instruments used by each team may be different. These guidelines are generic and should apply to all instruments. Specific operating instructions will be developed for each instrument.

Background

PAR (photosynthetically­active radiation) is generally considered to be the radiation in the 400 to 700 nm waveband. It represent the portion of the spectrum which plants use for photosynthesis. In the PAR waveband, irradiances vary from full sun to almost zero over the space of a few centimeters and reliable measurements of PAR require many samples at many locations under the canopy.

Dry matter production of a plant canopy frequently is directly related to the amount of PAR intercepted by the canopy (Monteith, 1977). Dry matter production (P) is modeled as the product of three terms : P = efS, where S is the flux density of incident radiation, f is the fractional interception, and e is a conversion efficiency.

Radiation incident on a canopy (S) can be absorbed by the canopy, transmitted through the canopy (T) and absorbed or reflected at the soil surface (U), or reflected by the canopy (R).

The fraction of incident PAR transmitted by the canopy (t) is T/S,

the fraction of incident PAR reflected to a sensor above the canopy (r) is R/S,

the reflectance at the soil surface ( rs ) is U/S,

then the absorbed fraction can be calculated as: f = 1 ­ t ­ r + rs .

Sampling for fractional absorbed PAR.

1. Sky conditions: either direct sun or overcast are acceptable. Rapidly changing conditions are most troublesome. Calculations depend on illumination conditions remaining constant through the measurement sequence.

2. Measure incident PAR (S) with the instrument level and facing upward.

3. Insert the instrument under the canopy at the soil surface. Try to keep the instrument level. Measure transmitted PAR (T). Move the instrument 15 cm and take another beneath the canopy reading. Repeat for at least 5 beneath canopy readings.

Note: The first set of transmitted measurements should be within the sampling frame. The subsequent measurements should be within 3 m of the frame.

4. Measure reflected PAR (R) about 1 m above the canopy with the instrument level and facing downward.

5. Measure reflected PAR (U) from the soil by positioning the sensor about 5 cm above the surface facing downward. Alternatively the reflectance bare soil outside the canopy (Rbs) can be measured and rs calculated as t (Rbs/S).


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