12. SAMPLING PROTOCOLS (refinements added as available)
12.1 Surface Soil Moisture
Two types of sampling designs will be employed, Field and Profile. The actual soil moisture sampling is the same but the distribution and number of samples is different.
Before getting started each person should establish their average pace in meters. This can be done beforehand (strongly suggested), however, a measured distance will be available at each area for this purpose. Obviously the actual pace in a field will vary with obstacles encountered. The pace is used to aid in distributing samples, not for precise location so do your best to adjust.
12.1.1. Field Sites.
The goal of this sampling is to characterize the mean of what we hope is a "homogeneous" field. A total of 12 points in each field will be sampled. For each field, an orientation and starting point will be determined based upon individual field characteristics and flightline orientation. The general layout is shown in Figure 17 (in some cases
this orientation may be rotated 90o.
All sampling will be done by teams of two for safety and in some cases efficiency. This doesn't mean the team has to sample together, it means that they are in close proximity in case there is problem. Sampling will be conducted every day except when it is raining. The procedure follows:
Sample tins come in boxes of 36. Use cans sequentially, this always helps resolve discrepancies. A box can be used for more than one site but never use two different boxes for a site. Be careful not to spill the cans and try to keep them in a shaded location after collection. Note that the field notebooks will be turned in to the experiment, if you want to keep your own records or make copies of your own sites either make a duplicate set of notes or zerox.
The procedure for surface gravimetric soil moisture sampling follows.
1. Check in at designated coordination location (i.e. Chickasha Office) at standard time that will be established.
2. Pick up sampling kit (use the same kit each day)
Bucket
Sampling tool (0-5 cm scoop)
Large spatula
Small spatula
Notebook and pen
Cans
3. Conduct sampling for each field and point
Remove vegetation and litter
Use the large spatula to cut a vertical face at least 5 cm deep (Figure18a)
Use the scoop to take the sample, small spatula aids sample removal (Figure 18b)
4. Record the following information in the notebook, always use a new page for each Day-Site
Date
Site
Names of samplers
Start time-Finish time
Drawing indicating the nominal location of the samples by can number and any relevant landmarks
5. When you return to the lab check in and weigh your samples, recording the field, date, can number and weight on the Gravimetric Soil Moisture sheets
6. Place samples in oven
7. Leave the data sheets in lab
12.1.2. Profile Sites
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.
12.2. Soil Bulk Density (USDA ARS Hydrology Lab)
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 plexiglass 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 plexiglass. The foam is three inches high and two inches thick. The foam is attached so that it follows the circle of the plexiglass. Figure 19 shows the basic components.
Other Materials Required for Operation
* Three 12" threaded dowel rods and nuts are used to secure the apparatus to the ground.
* A hammer or mallet is used to drive the securing rods into the ground.
* A bubble level is used to insure the surface of the apparatus is horizontal to the ground.
* A trowel is used to break up the soil and to remove the soil from the hole.
* Oven-safe kitchen bags are used to hold the soil as it is removed from the ground. The soil is left in the bag when it is dried in the oven.
* Water is used to determine the volume of the hole.
* A plastic gasoline can is used to carry the water to the site.
* One gallon plastic storage bags are used as liners for the hole and to hold the water.
* A 1000 ml graduated cylinder is used to determine the volume of the water. Plastic is best because glass can be easily broken in the field.
* A hook-gauge is used to insure water fills the apparatus to the same level each time.
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 19a)
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. (Figure 19b)
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 19c)
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 19d)
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. Vegetation Sampling
1. Identify site (field) that exceeds minimum size requirements.
2. Determine vegetation type and phenological stage measure plant height
3. Take vertical photograph and oblique photographs of sample area.
4. Clip standing vegetation in 0.5 m2 sample area, bag, and weigh. (Standing Wet Biomass).
5. Gather liter on the soil surface, bag, and weigh. (Litter Wet Biomass)
6. Measure fraction absorbed PAR in a 10 m radius surrounding the sample area.
7. Measure LAI with Plant Canopy Analyzer in a 10 m radius surrounding the sample area.
8. Dry vegetation and litter samples in drying ovens and weigh. Standing Dry Biomass; Litter, Dry Biomass.
9. Separate green and brown leaves, weigh separately. Green Standing Dry Biomass; Brown, Standing Dry Biomass
