In Situ Soil Moisture Sensors in Undisturbed Soils

Soil water content regulates many above and below-ground processes, from crop production to weather. It has recently become a critical mission requirement for many state and federal agencies. This protocol synthesizes multi-agency efforts to measure soil moisture, using buried in-situ electromagnetic sensors.

This protocol will be useful for scientists and engineers hoping to deploy a single station or an entire network. Soil water content was recently recognized as an essential climate variable in the global climate observing system. Yet there has been little standardization to the practice of installing buried in-situ sensors.

We hope that the written protocol and video can improve data collection. There's no easy way to verify that a buried soil water content sensor is providing good data. At first, it takes confidence and assurance that the sensors are in good contact with the soil, and that the installation didn't affect the local soil hydrology.

Demonstrating the procedure will be Alex White, a physical scientist in the USDA ARS Hydrology and Remote Sensing Laboratory here in Beltsville, Maryland. To begin, separately wire each sensor to a data control platform. Use the question mark and exclamation command to query the sensor's address, and record the values along with the serial number and SDI-12 address of each sensor in a lab book.

Then label the sensor head and the cable end with the SDI-12 address using a marker. Set up the soil water content sensors and any ancillary sensors in the lab, connecting them to the data control platform and battery, leaving the soil water content sensors suspended in the air, inserted in dry place, and/or submerged in water. Verify that the data are recorded at the appropriate rates, and values are appropriate.

Query the location using the USDA SoilWeb app and dig a test hole using a post hole digger. Check that the field texture is consistent with the soil description. Lay a tarp over the excavation area to protect the soil surface from disturbance.

Uncoil a sensor and lay the sensor head at the pit face. Then position the cable end where the instrument stand will be located, verifying that the cable length is correct, adjust as needed, and mark the stand's location with a flag. Using a post hole digger, excavate an approximately 55 centimeters deep hole.

Clean up the hole using a sharp spade, and ensure that the pit face is vertical so that each sensor will have undisturbed soil above it. Keep the hole and disturbed area as small as possible so that it will recover quickly. Remove the soil in 10 centimeter increments, and place each lift on the far end of the tarp, moving closer with each increment while breaking up any clods and removing large rocks.

Next, excavate a straight, narrow trench more than 10 centimeters deep to bury the sensor cables. Uncoil all the sensors, and lay them on one side of the trench. Note the depth of any obvious changes in soil color or texture while collecting representative soil samples in a one-quart freezer bag at each sensor depth.

Check that the pit face is vertical, or slightly cut back to ensure that each sensor will have undisturbed soil above it. First, insert the 50-centimeter sensor and push the sensor horizontally into the soil, trying not to wiggle the sensor, as this can create gaps. If necessary, use leverage to gently push in the sensor, being sure the tines are fully embedded in the soil.

Orient each sensor cable to the same side of the pit, 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. For the 100-centimeter sensor, dig a hole to 100 centimeters minus half the sensor tine length, or 94 centimeters in this case.

Install the sensor vertically by pushing it into the bottom of the hole using an installation tool. Repack the auger hole with the excavated soil. Route all the sensor cables from the trench into the enclosure in a section of PVC conduit and through a bulkhead connector to enter the enclosure.

Then pull the cable lens into the bottom enclosure port, secure them with zip ties, and connect the five sensors. Verify that the voltage on the battery is sufficient using a multimeter set to DC for direct current voltage. Next, slide the terminal connector of the black negative wire over the spade terminal on the ground-negative post of the battery, and the red wire over the positive battery post.

To power up the system, plug the battery cable into the DCP. After launching the data control platform software, connect a laptop to the data control platform, and confirm that all sensors report numeric values. Once everything below the ground functions and the cables has been routed into the enclosure, fill and seal the above and below-ground enclosure openings with electrical putty.

Using the deeper excavated soil nearest to the tarp, hand pack the soil around each sensor head, starting from the bottom and working up, avoiding disturbing the sensor and ensuring it is well compacted. Backfill the pit in 10 centimeter lifts, smoothing and compacting the surface until the 20-centimeter sensor is reached. Hand pack the soil around the sensor head and continue upwards in 10 centimeter lifts until you reach the surface.

Carefully cover them with deeper soil from the tarp while compacting the soil into the bottom of the pit to secure the cables, being careful not to pull them downward with any force. Take photos of the completed station in orientations north, south, east, and west from the instrument mast. Delineate sensor installation location with flagging or other distinct items.

This field installation reports hourly air and soil temperatures, soil water content, daily precipitation, soil water storage, and its change over time. The results show a sharp increase in soil water content near the surface, with a delayed increase at greater depths following each storm. During events in early February and April of 2022, the deepest sensor at 100 centimeters reached a plateau of 0.33 cubic meters per cubic meter, indicating a period of saturation.

For a similar installation in Mississippi, soil water content reached 0.60 cubic meters per cubic meter at all depths after 40 millimeters of rainfall, while another 70 millimeters did not affect soil water storage, suggesting saturation excess runoff. For a similar installation in Montana, frozen soil and snow cover produced a dramatic decrease in soil water content in mid-March, then an increase during thawing, without any indication of precipitation. Be wary of erratic sensor behavior, such as spikes, step changes, and oscillations, which can indicate poor installation or sensor failure.

Sensor insertion can be challenging, particularly in rocky, rooty, or dry soils. Be sure the sensor head is pushed flush against the soil. This protocol will lead to more harmonized and uniformed soil water content data, for a wide range of applications, including:drought monitoring, water supply forecasting, watershed and agricultural management, and crop planning.

The National Coordinated Soil Moisture Monitoring Network is building a community of practice around soil moisture measurement, interpretation, and application. It is a network of people that links data providers, researchers, and the public. Please see the documentation for more details.