Optional Gypsum Block Soil Moisture Protocol
Optional Gypsum Block Soil Moisture Protocol
PurposeTo measure the water content of the soil based on the electrical resistance of gypsum blocks. OverviewThe Gypsum Block Protocol consists of:
Time10 minutes per day; initial calibration requires following gravimetric protocol for six to eight weeks. FrequencyMonitor gypsum blocks daily. Re-installation and calibration of gypsum blocks should be done annually. LevelAdvanced ConceptsA Gypsum block's wetness is related to soil moisture and is a function of their electrical resistance (AC).
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The forces that hold water in the soil change with soil texture and this affects the saturation of gypsum blocks and requires us to determine what this relationship is by calibration. Soil moisture increases after precipitation. The amount of increase in soil moisture after precipitation depends on many factors. Soil moisture decreases on dry, sunny days. The rate of soil drying also depends on many factors. Skills
PreparationLocate the soil moisture site, enter the requested soil moisture site metadata and collect the tools and materials. PrerequisitesIt is useful to have established a precipitation gauging station nearby and to have started a soil characterization and soil profile study of your soil moisture site. |
1. Place the gypsum blocks into a container of water and soak for 5 minutes.
2. Auger a hole to the appropriate depth for each gypsum block sensor (10, 30, 60 and 90cm). A soil auger works like a cork screw - simply lean on the handle as you turn it. It is best to remove the auger bucket from the hole after several turns and clean the soil out of the bucket. If you fill it too full, it will be very difficult to remove the soil. Place the extracted soil in a large pail to keep the site clean. The four holes should be placed next to one another in sequence to reduce potential confusion while taking readings and recording data.
3. Put two large handfuls of the soil extracted from the hole into a small bucket or similar container. Add a small amount of water and stir to create a mudball. The mudball should stick together. Remove any rocks.
4.Drop the mudball to the bottom of the hole. Make sure it reaches the bottom.
5. Place the wire lead from one of the sensors through the PVC guide tube.
6. Grab the end of the lead and pull the sensor up tight against the end of the pipe. Lower the sensor into the hole while holding it against the end of the pipe. Holding the wire lead tightly at the top of the pipe, gently push the pipe down to seat the sensor in the mud at the bottom of the hole. Note: Since it is difficult to pack soil tightly around the sensor, the purpose of the mud is to establish good contact between the sensor and the soil particles.
7. Hold the sensor in place with the pipe while you begin to backfill the hole. Add just a few handfuls of soil and gently tamp with a broom stick or similar pole. Then add a little more soil and remove the pipe as you tamp. Continue adding soil a few handfuls at a time and tamping firmly as you backfill the hole. Hold on to the wire lead as you backfill so that it will come straight to the surface.
8. Place a short piece (about 10 to 20 cm long) of PVC pipe, tin can, or coffee can (with the top and bottom removed) around the wire lead at the surface to protect it and make it more visible to anyone walking in the vicinity.
9. Repeat the above steps for each sensor.
Do not report measurements for a week after installation. The sensors require at least one week to equilibrate to natural conditions. The wire leads are fragile, especially where they connect to the meter. If the end of the wire leads to the gypsum blocks break, peel back the wire insulation and make new leads. It is important to leave enough wire above the ground for this.
Congratulations! Your gypsum blocks are installed. Wait at least one week before beginning to take data which you report to the GLOBE Student Data Archive. After this, monitor your gypsum blocks daily for soil moisture variations. This is the fun and easy part of this investigation. Do not monitor the blocks when the ground is frozen.
Test the soil-moisture meter to ensure it is functioning properly according to the manufacturer's instructions. Do this before each use.
1. Obtain the reading for each gypsum block.
a. Connect the soil-moisture meter to the wire leads of the gypsum block located at the 10 cm depth.
b. Push READ button. Wait for the meter to reach a constant value - it should not be negative.
c. Record the date, time, current soil conditions (CC's), and soil moisture meter reading on the "Daily Gypsum Block Data Work Sheet" in the appropriate depth column.
d. Disconnect the meter and store the wire leads.
e. Replace the cover over the PVC pipe.
f. Repeat a - e for each of the remaining gypsum blocks (30, 60, 90 cm).
2. Report all four meter readings to the GLOBE Student Data Server.
3. Convert each meter reading to soil water content as in part D below.
How to use the "Daily Gypsum Block data worksheet": There are numbers 1 to 0 in the far left column. Please keep a running count of your measurements by adding a tens (or more!) digit as you accumulate more data. This allows someone reviewing your data sheets to ascertain if any pages are missing. There is also space to plot your data in the field as you collect it. You would normally expect gradual transitions except for the rapid increase in soil moisture after a rain.
The gypsum blocks must be calibrated so that the meter reading you make can be related to soil water content (SWC). This process can take 6-8 weeks, depending upon how quickly your soil moves through its full drying cycle. Rather than calibrate your gypsum blocks at every depth, we have adopted a policy of basing each calibration on observations made from the 30 cm sensor. Technically, this assumes your soil profile is uniform and your gypsum blocks are identical. It takes about 30 minutes to complete the steps below.
The materials for the Soil Moisture Protocol are also needed
1. Make a soil meter reading from the 30 cm gypsum block sensor.
2. Select a random location within 5 m of the gypsum block hole .
3. Clear surface debris.
4. Auger to 30 cm and collect a 100 g sample centered at this depth. Place the soil sample in a soil can and number the can.
5. Backfill the hole and replace the surface cover.
6. Record the date, time, depth and can number.
7. Follow the "Gravimetric Drying Technique" found in Task B of the Soil Moisture Protocol and make note of your drying method and average drying time.
8. Record on the "Annual Gypsum Block Calibration Work Sheet" the date and time of your measurement, the three weights and the soil moisture meter reading that you obtained. There is also space to calculate soil water content (SWC).
9. Repeat steps 1 - 8 about twenty times as the soil moves through one or two complete drying cycles. Wait until your meter reading changes 5% before collecting another gravimetric sample. Re-install and recalibrate your gypsum blocks once a year.
SWC = (wet weight - dry weight)/(dry weight - can weight) x 100
Remember:
wet weight = wet soil + can + lid
dry weight = dry soil + can + lid
If you have any questions about creating your calibration curve or if you need any assistance with the curve, Dr. Washburne, the principal investigator for the Soil Moisture Investigation, is glad to provide answers and assistance.
You can contact him by phone, fax, e-mail or conventional mail at:
When you have finished determining your calibration curve, please mail or fax a copy of your curve and of your "Annual Gypsum Block Calibration Data Work Sheet" to Dr. Washburne at the above address.
During the year, if you get readings either higher or lower than any of the readings on your Data Work Sheet, take a gravimetric sample following the steps in Part C of this protocol and use the values you measure for this sample to extend your calibration curve. Again, please send a copy of your revised calibration curve and extended "Annual Gypsum Block Calibration Data Work Sheet" to Dr. Washburne.