Soil Moisture Tensiometer Pilot Testing
Draft v.0.4
www.hwr.arizona.edu/globe/pilot/2000smsum.html
Overview
Soil moisture varies as a function of soil, weather and vegetation cover. Most places experience relatively slow changes (seasonal) in soil moisture below 20 cm but the top 10 cm might change rapidly, particularly in hot, dry climates. As the soil dries out, soil tension (the pressure force or suction plants must use to extract soil moisture) increases. Gypsum blocks are also sensitive to changes in soil tension, but most gypsum block meters measure resistivity.
A Soil Moisture model 2725 Jet Fill tensiometer will be used to monitor DAILY changes in soil tension (or suction - a negative pressure). This should be done at your Soil Moisture study site, within 100 m of a raingauge. We recommend that you use three tensiometers to observe suction at 10, 30 and 60 cm depths.
Protocol Checklist
Pre-protocol
Complete the Initial Equipment Check
Complete a Wet-Dry lab test of tensiometer in at least one typical soil
Email me the results
Protocol
Establish an operation and site plan
Make daily observations
Post-protocol
Data analysis
Soil Moisture modeling (coming soon)
Equipment List
3 Soil Moisture model 2725 Jet Fill Tensiometers (6,18,24"; 15,45,60cm)
Body tube
Ceramic Tip and O-ring
0-100 cm pressure gauge
Jet-Fill reservoir
Laboratory squeeze bottle with narrow tip
Initial Equipment Check
- Completely read the manufacture’s "Operating Instructions – 2725A".
- Carefully follow the assembly instructions and screw together (FINGER TIGHT) the ceramic tip and pressure gauge to the plastic body tube.
- Fill a laboratory squeeze bottle with distilled water, place the tip into the top of the body tube and fill to the top (optional: add one drop of blue food coloring).
- Screw on the Jet Fill reservoir and fill 3/4 full.
- Pump the Jet Fill plunger many times (~20) until no more bubbles rise to the top.
- Let it sit for several hours. You should observe water "sweating" out of ceramic very slowly.
Lab Tests
Wet-Dry lab test
Container preparation
Clean half gallon (2 L) milk box.
Use duct tape to seal pour spout.
Cut out one side panel (see figure).
Procedure
- Using a 2.5 L bucket, collect ~2.2 L of soil from your field site.
- Spread and sun dry, if necessary.
- Sieve with 2 mm screen back into bucket.
- Compact the soil slightly with palm of hand.
- Using a 100-200 mL soil can (diced chili type), collect two full cans of soil.
- One is for a bulk density sample. If time allows, also complete the other soil lab protocols of PSD, pH and TDS (using pH mixture).
- One can be used to estimate porosity: Add a known volume of soil to a 500 mL graduated cylinder with 250 mL of water. Measure the displacement. Porosity is the ratio of displaced water divided by the sample or can volume. f
= Vdisplacement / Vcan .
- Starting with a known volume of soil, Vstart, add 1 cm depth to container. Stand the tensiometer upright near the middle of the container. Support it somehow (I taped mine to the wall). Add soil to within 1-2 cm of container top, fully covering the ceramic tip. Slightly compact the soil as before. Find Vsoil = Vstart – Vend .
- Estimate the amount of water needed to just saturate the soil: Vwater = Vsoil x f
.
- Add this amount of water and take daily readings while the soil dries.
Field Installation
Extra Equipment
- Meter stick
- Auger or shovel
- 2 mm sieve
- Tie/Mounting rack (such as 2 x 12-24" spikes and a metal or plastic cross bar)
Place the tensiometers about 10 cm apart, in order from deepest to shallowest. Do not place the tensiometers together so they are touching (see figure).
Site Considerations: Install your tensiometer "nest" at your soil moisture study site, within 100 m of a raingauge. A flat slope is best – precipitation must not be inhibited from wetting the soil by surrounding trees, structures or ground coverings.
Steps
- Plan the installation carefully. If the soil is damp, allow time for the backfill soil to dry before completing the installation. Do NOT hammer the tensiometers into the ground. Are the holes lined up so you can tie-off and support the tensiometer tubes?
- Auger or shovel a hole or holes to the appropriate depths (10, 30 and 60 cm).
- Complete a soil characterization on the soil column.
- Starting with the deepest hole, place the water-filled tensiometer in the hole and backfill around the ceramic tip with dry, sieved soil removed from that depth. Lightly tamp the soil firm. Then complete backfilling that hole.
- Follow the same procedure for each tensiometer that you install.
- Secure the tensiometers to a tie-off rack and make sure that this "nest" does not poise a tripping hazard.
Operation
Normal Operation
It is particularly useful to have a daily record of soil moisture values when the ground is drying out and the pressure gauge is recording daily changes. Readings during wet or cold periods are less valuable.
- Observe your observations daily, within one hour of local solar noon.
- Read the pressure meter and record your observation to the nearest cm.
- Maintain the water level in the tube above ground level. This will require "pumping" water into the tensiometer tube from the Jet-Fill reservoir, which will be more often for a fine soil than for a coarse soil. Do not let the Jet-Fill reservoir go dry.
- Report your pilot observations to me on a weekly-monthly basis using the following column format. When this protocol becomes official, your pilot data will be incorporated into the GLOBE data archive.
- If you have gypsum blocks, please continue to report this data.
- Make near-surface gravimetric measurements weekly during soil dry-down periods and report this data to the GLOBE server.
Pilot Data Format: Send to jwash@hwr.arizona.edu
School Name:
Location:
Site Lat/Lon:
QC check: must be between. 0-100
[UT] Tension (cm)
Date Time 10 cm 30 cm 60 cm Notes
11/24/98 19:30 45 30 18 holiday next 4 days
11/30/98 19:30 50 30 19
….
Special Conditions
- Tensiometers cannot stand extended cold temperatures and should be removed from the ground each winter at locations where temperature remain below freezing for more than 3 hours. This means reinstalling the tensiometer "nest" every spring after the ground becomes workable. As the temperature gets cold, let the water level in the tube fall below the pressure gauge in case of a sudden cold spell.
Critical issues to investigate during the pilot study
Here are some questions I am trying to answer. I appreciate both teacher and student observations. Please let me know what other questions or issues you have – pilot studies like this require regular communication between you and I. Please Email me your comments or observations on the following issues:
- What problems did you encounter during installation or assembly?
- What problems did you have selecting a measuring site?
- How are you keeping the tensiometers safe from vandalism and a tripping hazard?
- How often do you have to add water to Jet-Fill reservoir?
- Can your students see patterns emerging from this data?
- If a teacher could only spend two months on this protocol, what two months would you choose and why?
- What is confusing about this protocol or the instructions by SoilMoisture Corp.?
Data Quality – Correlative Measurements
One purpose of the pilot test phase of data collection is to pay particular close attention to other, related GLOBE observations to help evaluate and understand their value and problems associated with the technique. So it is important to maintain regular (not heroic) observation and reporting schedules of the standard protocols, with particular emphasis on the following measurements:
- Daily Max/Min/Current Air temperature – to help understand drying regime.
- Daily precipitation – to help model soil moisture using API method.
- Weekly near-surface gravimetric soil moisture (during dry-downs)
- Soil characterization – of top 60 cm in the area near the tensiometer nest.
- Land Cover – what is the MUC for your measurement site?
Data Analysis
Spreadsheet Graphing
see (Soil Temperature Pilot Protocol)
Understanding Soil Tensiometer Data
(under development)
Modeling Soil Moisture using the Antecedent Precipitation Index (API) Method
see (www.hwr.arizona.edu/globe/pilot/api.html)