In Situ Soil Moisture Sensors in Undisturbed Soils

1. Preinstallation preparation of sensors

1. Check the SDI-12 address. Sensors are set to a default address by the manufacturer. Separately wire each sensor to a data control platform (DCP) using the ?! command to query the sensor's address.
   NOTE: Each sensor on a common data line must have a unique address (e.g., 0-9). Refer to the sensor manual for SDI-12 addressing and changing a sensor's value, if needed.
2. Take a measurement (e.g., "aM!", where a is the address) in air, dry sand, and submerged in water.
   NOTE: Air measurements should read 0.00 m³m^-3 (relative permittivity [ε] ~1), play sand < 0.02 m³m³ (ε < 4), and water ~1.00 m³m^-3 (ε ~80).
3. Record these values along with the serial number and SDI-12 address of each sensor in a lab book. Using a marker, label the sensor head and the cable end with the address number.
4. Check the DCP program. Some DCPs are plug-and-play, but most require a program to make measurements and record data. Set up the SWC sensors and any ancillary sensors in the lab, connecting all of them to the DCP and battery. Leave the SWC sensors suspended in the air, inserted in dry play sand, or submerged in water, ensuring the tines are not touching.
   NOTE: Air measurements should read 0.00 m³m^-3 (relative permittivity [ε] ~1), play sand < 0.02 m³m³ (ε < 4), and water ~1.00 m³m^-3 (ε ~80).
5. Let the system run overnight or longer. Verify that the data are recording at the appropriate rates and the values (e.g., right number of columns, significant digits) are appropriate.
6. Check any ancillary SWC sensor outputs (e.g., temperature and BEC) as well. Allow the system to run for at least 1 day. Verify that the data tables are correct.
   NOTE: Some DCPs are plug-and-play but most require a program to make measurements and record data.

2. Determine the field layout

1. Before beginning any digging, call 811 (US and Canada) at least 2 days before excavation to verify the presence of any underground infrastructure (e.g., electrical wires, water supply, gas pipes). Failure to assure such clearances can lead to substantial penalties and liability.
2. Verify the soil map unit at the pit location. Use the USDA SoilWeb app, available for iOS and Android smartphones, to query the location. Dig a test hole using a 5-10 cm diameter hand auger to check that the field texture is consistent with the map unit description. Check for any issues such as hard layers (e.g., plow pans, caliche, or argillic horizons) or layers with high rock fragments; either case can make probe insertion difficult, or even impossible.
3. Determine the best location for the sensors. Each sensor will be installed into the vertical face of an undisturbed soil.
   NOTE: If any slope exists, the face should be upslope to minimize preferential flow occurring through disturbed soil and along cable trenches.
4. Use a small (1 m²) sheet of plywood or a tarp to protect the soil surface and keep field workers from milling around on the undisturbed soil.
5. Determine the location of the instrument mast. Ensure that the sensors are suitably far away from the mast to minimize foot traffic and any effects from the tower.
   NOTE: Stock 5 m cables are generally sufficient for most installations.
   1. Use the shortest possible cable length to minimize surface disturbance and the potential for breakage.
      NOTE: If the instrument mast is already in place at an existing site, a longer lead may be required to reach a representative soil; alternatively, wireless technologies could be considered (see "Additional thoughts on site selection").
   2. Ensure that the total distance to the instrument stand is 80%-90% of the cable length to account for extra cable needed to route from the installation depth, through conduit, and up into the enclosure.
      NOTE: Wire management can be awkward when many SWC sensors come to a central point.
   3. Thinner cables require burial in PVC conduits, while more rigid, thick cables can be buried directly. For both, dig a trench that is >10 cm deep and 10-15 cm wide.
   4. Ensure that the enclosure has an entry point for any aboveground sensors and a conduit port for below-ground sensors (Figure 5). Mount the enclosure at a comfortable height (1 m) for wiring.
   5. Recommendation: Uncoil a sensor. Lay the sensor head at the pit face and position to the cable end at the instrument stand. Verify that the cable length is correct and adjust as needed.

3. Excavation of the soil pit

NOTE: The soil pit can be excavated manually or mechanically. The goal is to minimize overall site disturbance.

1. For the hand-dug pit, lay another larger tarp (2 m²) adjacent to the excavation area. Use a narrow spade (e.g., sharpshooter) to excavate a rectangular hole to a depth of ~55 cm. Ensure that the pit face, currently protected by plywood or tarp (step 2.4), is vertical (or slightly cut back) so that each sensor will have undisturbed soil above it. Also ensure that the pit is 20-40 cm wide and ~25% longer than the total sensor length. Begin removing soil in 10 cm increments and placing each lift on the far end of the tarp, moving closer with each increment; break up any clods and remove large rocks.
   NOTE: Ensure that the excavation area is as small as possible and allows enough room to insert the deepest horizontal probe.
2. For the hydraulic post hole auger, use a wide diameter (>30 cm) and a 1 m long, trailer-mounted auger.
   ​NOTE: Two- or one-person fence-post augers can be dangerous.
   1. Set up the auger ~5 cm back from the intended pit face.
   2. Drill down until >50 cm, raising the auger occasionally to expel soil.
   3. Use a narrow spade to create a flat and vertical pit face.
   4. Use a spade or a hand trowel to move soil from the pit to the tarp.
      NOTE: The excavated soil will be well-mixed; there is no way to avoid this.
3. Make a mechanically dug trench using heavy equipment.
   NOTE: Unless horizontal installation below 100 cm is necessary, large digging equipment is not encouraged. Dealing with the spoil pile (i.e., excavated soil) can be challenging, and the tracks and stabilizers of the backhoe cause significant disturbance.
   1. Use a lightweight backhoe with a narrow bucket, ideally less than 50 cm, to dig a similar narrow trench to a 100 or 200 cm depth.
      NOTE: Avoid moving the backhoe to minimize surface impact.
   2. Begin removing the soil in 10 cm increments and placing each lift on the far end of the tarp, moving closer with each increment. Ensure that the excavation area is as small as possible and at a depth of ~55 cm, allowing enough room to insert the deepest horizontal probe.
4. For the sensor cable trench, dig a trench from the back of the soil pit to the instrument tower. Use a trenching shovel assisted by a pick mattock or Pulaski in hard sections. Excavate a straight, narrow (~10 cm), >10 cm deep trench, laying the soil on one side of the trench.

4. Assembling/erecting the instrument stand and enclosure

NOTE: The instrument stand has three options: a simple pole, a tripod, or a tower. For a basic soil moisture station with a PPT gage, a galvanized steel pole or stainless-steel instrument stand (120 cm tall) with legs is sufficient. For basic meteorological measurements, a taller mast is needed to install sensors at 2 m. Most mesonets prefer 10 m tall towers; however, such towers are beyond the scope of this protocol.

1. Use a galvanized steel pole.
   NOTE: A 4 cm diameter, galvanized steel water pipe, ~3 m long is the most economical method.
   1. Hand auger a small hole to a minimum of 60 cm depth. Position the pole in the hole. Ensure that the pole height is sufficiently aboveground to hold the enclosure, solar panel, and any antennas needed.
      NOTE: A height of <2 m is recommended.
   2. Mix quick-set concrete or fence-post foam, according to instructions.
      NOTE: Concrete is not allowed on some federal lands, and some private landowners may object. Foam alternatives for fence-post installations are a good alternative and require no water.
   3. Pour either material around the pole and ensure it is level using a torpedo level. Let the concrete cure for several hours (ideally overnight) and secure the pole with braces to ensure it remains level. Although foam cures in 30 minutes, be sure to hold the pipe in place for at least 2 minutes, ensuring it remains vertical.
2. Instrument stand or tripod (see manufacturer's instructions)
   1. Loosen or unbolt each of the three stand legs.
   2. Rotate or extend each leg and position over the end of the excavated trench.
   3. Insert the instrument mast into the legs and tighten.
   4. Adjust the length of each leg to assure the mast is vertical.
   5. Stake each leg into the soil and check the mast again with a torpedo level.
3. Using U-bolts, mount the enclosure on the instrument stand at 1-1.5 m. Hand-tighten the bolts to secure it; its final height and tightening will occur later.
   NOTE: It is recommended to mount on the northern side of the pole to avoid hitting one's head on the solar panel later.

5. Soil characterization and sample collection

NOTE: Visually characterizing the soil is critical for interpreting soil moisture dynamics after installation. Sample collection can aid the interpretation with quantitative data. Collect samples even if funding is unavailable or in-house facilities are unable to process them. Air-dry and archive them, in case soil characterization is needed in the future.

1. For basic soil description, note the depth of any obvious changes in soil color or texture (horizons).
   NOTE: The National Soil Survey Center provides an excellent overview of soil profile descriptions and interpretations²⁴. If the location is not ideal, now is the time to move.
2. For basic soil characterization, collect representative soil samples in a 1 quart (1 L) freezer bag at each sensor depth, following the procedure of Lawrence et al.²⁵.
   1. On return to the office or lab, lay all the 1 quart bags on the counter, open, and allow them to air-dry for at least 48 hours.
      NOTE: Air-drying removes most of the soil moisture while preserving the organic and chemical properties for future analyses.
   2. Submit the samples either to a university extension laboratory (e.g., https://agsci.colostate.edu/soiltestinglab/) or a commercial laboratory (e.g., http://www.al-labs-west.com/) for further analyses. Alternatively, run the sample in-house by trained technicians, using accepted methods noted below.
   3. Perform basic laboratory analysis, including physical soil parameters such as particle size distribution²⁶, rock fraction (RF; weight percentage greater than 2 mm), soil fraction (SF; percentage less than 2 mm), and texture (sand, silt, and clay percentages). Check the basic chemical parameters, including saturated paste electrical conductivity (dS m^-1)²⁷ and organic matter²⁸.
   4. Recommended: Perform volumetric soil core sampling at 5, 10, 20, and 50 cm depths using a coring device to collect an undisturbed, volumetric sample. Determine the soil bulk density (BD; g cm^-3) from the total dry soil weight and the core volume²⁹. Soil porosity (φ; [-]) is the physical upper boundary of SWC. For mineral soils, estimate φ as 1 – BD/PD, where the particle density (PD) for predominantly quartz mineral soils is 2.65 g cm^-3.
      NOTE: Samples for BD are either collected in a core of known volume or using soil peds²⁹.

6. Horizontal insertion of the 5, 10, 20, and 50 cm probes

NOTE: The goal is to ensure complete soil contact around the sensor tines, avoiding any air gaps.

1. Carefully snip any zip ties and unroll each sensor, removing any coil in the cables. Position the sensor head near the soil pit and the cable in the trench.
2. Installation depth is defined as the center of the sensor when installed horizontally, regardless of whether the face of the sensor is round or rectangular. Install the sensor at the precise depth below the land surface and as horizontally into the soil as possible. Use a zero-depth reference and a measuring device (tape measure or ruler) for accurate sensor depth (Figure 2) and a spacer to maintain tine spacing during insertion (Figure 2C).
3. First, insert the 50 cm sensor. Push the sensor horizontally into the soil, trying not to wiggle the sensor as this can create gaps. As the 50 cm probe is often the most difficult, use a grounding rod to provide more leverage to push that sensor in, being careful not to break the epoxy head or separate the tines. Repeat the insertion process, working upward to the 20, 10, and 5 cm sensor depths. Stagger (Figure 2D) or stack (Figure 2B) the sensors.
   NOTE: The measurement timing in SDI-12 protocols generally prevents sensors from reading simultaneously and generating interference between adjacent sensors (e.g., the 5 and 10 cm depths).
4. Orient each sensor cable to the same side of the pit face, allowing them to hang to the bottom of the excavation pit. Take a photo of the excavated hole and sensors with a tape measure for scale (Figure 6A). Use a GPS to determine the latitude and longitude within a few meters of the pit. If excavating multiple sites in a day, use a placard with a unique identifier to differentiate between the pits.

Figure 6: Example photographs for metadata. (A) The instrumented soil pit with tape measure for scale, (B) the cable trench excavated back to the instrument mast, and final site photos facing (C) north and (D) south. Please click here to view a larger version of this figure.

7. Vertical insertion for the 100 cm probe

1. For sensor installations at depths greater than 50 cm, auger a separate hole for each sensor in or near the cable trench (Figure 4A).
2. Using a hand auger (5-10 cm diameter), dig to the appropriate installation depth(s). The depth is defined as the center of measurement (e.g., 50 cm) minus half of the tine length relative to the zero-depth reference (Figure 4B).
3. Arrange the excavated soil onto a tarp in the order in which it was removed.
4. Install the sensor vertically by pushing it into the bottom of the hole using an installation tool (Figure 4C,4D). Repack the auger hole with the excavated soil from the deepest to shallowest. Replace the soil in small lifts, packing it sufficiently to prevent soil bridging in the hole and the creation of voids.
   NOTE: Packing tools may be a capped piece of PVC or wooden dowels. Avoid damage to the sensor head or cable.

8. Completing the sensor installation and wiring to DCP

1. If the sensor cables are directly buried, ensure that the aboveground ends running into the enclosure are in PVC conduit using a bulkhead connector to enter the enclosure (Figure 5).
   NOTE: If using a separated rain gage (step 9.1), be sure to include this cable when routing into the enclosure.
2. If using conduit, lay it out in the cable trench and cut to the desired length. Feed the cable through the conduit-this may require a pull string or fish tape to pull the cables through. Use flexible conduit or a 90° sweep elbow plus a length of vertical conduit to route the cables from into a conduit port onto the bottom of the enclosure.
3. Lay the cable or cable/conduit in the bottom of the cable trench. Pull the cable ends through the bottom enclosure port and secure with zip ties. If there is excess cable in the enclosure, pull it back through the conduit and coil in the bottom of the excavation trench.
4. Take a photo of the installation pit and trench with cables leading back to the enclosure (Figure 6B).
5. For soil moisture sensor wiring, use a common power (5-12 volt) and ground wire for each SDI-12 sensor. Use lever connectors, splice connectors, or terminal blocks (Figure 5) to make these connections easier and more secure. If using more than one type of sensor, use a different communications port on the DCP, if available.
   ​NOTE: One faulty SDI-12 sensor can interrupt other sensors in a series.

9. Ancillary sensors and hardware setup

1. Precipitation (PPT) gage
   NOTE: To improve catch, rain gages should be installed on a separate vertical mast as close to ground level as possible. Installing the gage higher on the cross-arm mount can reduce catch due to greater wind speeds.
   1. Determine the location. Install the rain gage as low as possible above groundcover (~1 m) and at a distance twice the height of any nearby obstruction³⁰. The ideal location is close to the cable trench.
      NOTE: The rain gage cable will be buried alongside the sensor cables before entering the bottom of the enclosure.
   2. Install a vertical mast. Using a hand auger, excavate a hole to ~50 cm depth. Set a section of galvanized steel pipe of sufficient length in cement or foam (see step 4.1).
   3. After curing, install the gage using hose clamps or a flat mount base, as per the sensor instructions. Ensure that the gage is perfectly level.
      NOTE: Most gages have a built-in bubble level.
   4. Run cables between the rain gage and enclosure in the underground conduit with the soil moisture cables.
   5. For a tipping gage, wire the two leads into a pulse count channel on the DCP.
      NOTE: The wires can go in either side.
   6. Be sure to remove the top and check that the tipping mechanism is free moving. The buckets are often secured during shipment with rubber bands.
      NOTE: Rain gages require routine cleaning and calibration. If mounting the rain gage directly on the instrument stand or cross-arm, follow step 9.2.
2. Other sensors
   1. Install ancillary measurements and any antenna to the vertical mast or cross-arm mounts at the appropriate height above ground^10,30.
   2. Route leads into the aboveground enclosure entry, and secure where needed with cable ties. Wire into the appropriate measurement channels on the DCP.
3. Grounding rod
   1. Install a >1 m long copper grounding rod 0.5 m from the instrument mast. Use a fence-post driver to insert the rod into ground, leaving ~20 cm exposed.
   2. Secure heavy (8-10) gage copper wire to the rod using a ground clamp. Secure the other end to the enclosure or tripod.
      NOTE: Grounding may not be advisable in all situations.
4. Connect the battery.
   NOTE: Most DCPs need 5-24 volts (V), although 12 V is most common and 7 or 12 Ampere hours (AH) is sufficient to power most soil moisture stations. A 12V 12AH battery pack and voltage regulator are used here.
   1. Ensure the charge regulator is in the off position. Using a multi-meter set to DC for direct current voltage, verify the voltage on the battery is sufficient (>10V for a 12V battery) and identify the + and – terminals, if not marked.
   2. Slide the terminal connector of the black (–) wire over the spade terminal on the ground (–) post of the battery, and the red wire over the + battery post. Plug the other end of the red/black wires into the BAT port on the voltage regulator.
5. Solar panel
   NOTE: A 10 or 20 watt panel is typically sufficient. Increased wattage is needed at higher latitudes, more shaded areas, or on systems with a high power draw (e.g., cellular modems, cameras). The panel should be oriented to receive maximum incident solar radiation over the course of 1 year.
   1. Wrap electrical tape separately around each lead on the solar panel.
      NOTE: These wires will carry current if the panel is exposed to sunlight.
   2. Using U-bolts, mount the solar panel above the enclosure and on the side of the instrument stand facing the equator (e.g., south in the USA). Use the appropriate angle for the site latitude, typically 25° to 35° in the conterminous USA.
   3. Route the cable into the aboveground enclosure entry point. Remove the tape from the panel leads. Using a multi-meter set to A for amperage, verify the output of the solar panel is >0.1 A.
   4. Using a multi-meter set to DC for direct current voltage, verify the output of the solar panel is >10V and identify the + (usually red) and – (usually black) leads, if not marked. Connect the – lead from the solar panel to the G (ground) port, then the + lead from the SOLAR port on the charge regulator.
      NOTE: Cover the solar panel with a tarp or something opaque to minimize sparking.
   5. Check that the CHG or charging light is now illuminated.
6. Remote data communications
   NOTE: Cellular data telemetry provides the ability to transmit and send data from the DCP. Smartphone apps, such as OpenSignal, can measure  signal strength and the heading to the nearest cell tower. Omni-directional, multi-band antennas are preferred; however, a directional (Yagi) type antenna can improve signal in more remote areas.
   1. Attach the antenna to the top of the instrument mast using supplied U-bolts.
   2. Connect the coaxial cable to the antenna and route other end into the enclosure through the aboveground sensor conduit. Secure the cable with zip ties.
   3. Connect the other end to the cellular modem in the enclosure.
7. Powering up the system
   NOTE: At this point, the assumption is that the DCP program is written, and all sensors are wired in an appropriate manner. The solar panel and rechargeable battery are wired to a voltage regulator with a red/black power wire connected to the DCP power ports.
   1. Switch the toggle on the voltage regulator to on. Launch the DCP software and connect a laptop to the DCP. Confirm that all sensors are reporting values and not not-a-number (NaN) or an error value.
   2. Check each soil sensor for SWC, BEC, and T values. Ensure that the SWC values are >0.05 m³/m³ and <0.60 m³/m³. Check any out-of-range sensor; reinsert or replace any sensor that is questionable. Pour some water through the rain gage and verify that the DCP is recording counts.
      NOTE: Low BEC values (<0.001) can indicate poor sensor contact (or very dry soils). When installing in warmer seasons, T is generally warmest at the top and coolest at the bottom.
   3. Check cellular communication strength. Follow the manufacturer's documentation to determine signal strength.
      ​NOTE: Signal strength should be > -100 dBm to ensure decent signal quality. Direction antennas can be rotated to possibly improve signal. Many other communication options exist beyond cellular (e.g., satellite).

10.Site completion

1. Once certain everything below ground is functioning, and the cables or cables in conduit are all in the trench and routed into the enclosure, fill and seal the openings of the above- and belowground enclosure entries with electrical putty to protect from moisture and keep insects out of the enclosure.
2. Delineate the outside perimeter of sensor locations on the surface with permanent stakes with bright flagging.
3. Backfill the excavated area using the soil on the tarp and in the reverse order of  removal (Step 3.1) (deepest to shallowest). Start by hand-packing the soil against the trench face and around the sensor head at 50 cm, taking care to avoid disturbing the sensor. Support the sensor head while packing soil around it so that the sensor tines do not move.
4. Ensure that all the remaining sensor cables are still positioned near the bottom of the trench; then, carefully cover them with deeper soil from the tarp. Compact the soil into the bottom of the pit to secure the cables, being careful not to pull them downward with any force. Use sufficient force during compaction to assure similar bulk density of the removed material.
   NOTE: Wetter soils during installation can easily be over-compacted, while drier soils may stay loose regardless of force.
5. Backfill the pit in 10 cm lifts, smoothing and compacting the surface until the 20 cm sensor is reached. Again, carefully hand-pack the soil under and around the sensor, before returning to backfilling another 10 cm lift of soil.
6. Lastly, hand-pack the soil around the 10 cm sensor, then the 5 cm sensor, assuring both remain horizontal and in place. Fill the remainder of the soil pit with upper soils from the tarp.
   NOTE: All the soil removed should go back into the pit. Leftover soil indicates that the soil was not packed to the original bulk density.
7. Using the trenching shovel, push the excavated soil beside the trench over the conduit. Ensure everything is buried completely and below 5 cm.
8. Use a steel rake to level the repacked soil in the pit and trench flush with the original surface. Compact soil in the conduit trench sufficiently to minimize any preferential flow into the installation site.
9. Optional: Sprinkle some diatomaceous earth around any subsurface opening and on the surface to discourage ants, slugs, and other insects.
10. Recommended: use a portable SWC sensor to take readings of the surface soil around the in situ sensors to aid data verification over time and any scaling needs. Take readings in cardinal directions (north, south, east, and west) at consistent distances away (e.g., 5, 10, 25, and 50 m).

11. Record station metadata, the data behind the data²³

NOTE: Document metadata at installation and each site visit (see Table 1). Consistent metadata reporting supports the growing community of practice and is critical to ensure data and network integrity. 

1. Document the installation details, including a unique site identifier, installation date, sensor serial numbers, corresponding SDI-12 addresses, insertion orientations (horizonal or vertical), and depths.
2. Describe the soil profile and take associated photos. Record sample identifiers for any soil samples collected.
3. For the site location, record the latitude and longitude, elevation, slope, aspect, land use, and land cover.
4. Make a note of landowner and contact information, as well as site accessibility, including gate or lock codes.
5. Using the Compass App on a smartphone (or an actual compass) and a measuring tape, measure the angle and distance to the sensor pit (and any sensor auger holes) from two reference points (e.g., grounding rod or a tripod leg).
   NOTE: This will help triangulate their positions later.
6. Take photos of the completed station and orientations north (Figure 6C), south (Figure 6D), east, and west from the instrument mast. Delineate sensor installation location with flagging or other distinct items.

Table 1: Station metadata for soil moisture data collection. Abbreviations: Dec. = decreasing; GPS = Global Positioning System; 3DEP = 3D Elevation Program; O&M = operation and maintenance; SSURGO = Soil Survey Geographic Database; Mukey = map unit key. Please click here to download this Table.

12. Operations and maintenance

NOTE: A detailed maintenance log should be added to the metadata record, including sensor replacement, vegetation health or changes, or any site disturbance.

1. Perform routine site inspections annually at a minimum (Table 2). Record any sensor calibrations or replacements.
2. Ensure regular vegetation management, especially for permanent stations so that the site does not become overgrown or anomalous to the surrounding area.
3. Adapt animal management to local wildlife, possibly including fencing.
4. In case of sensor failure, do an emergency site visit and install a replacement (Table 2).

Table 2: Example maintenance schedule. Abbreviation: DCP = data control platform. Please click here to download this Table.