Name: in situ soil moisture sensors in undisturbed soils
Uploaded: 2022-11-18T14:10:00
Duration: 8 min 20 s
Description: Watch this Scientific Journal Video about In Situ Soil Moisture Sensors in Undisturbed Soils at JoVE.com

The authors acknowledge financial support from NOAA-NIDIS, the National Coordinated Soil Moisture Monitoring Network (NCSMMN), and the USGS Next Generation Water Observing Systems (NGWOS) program. We thank members of the NCSMMN Executive Committee, including B. Baker, J. Bolten, S. Connelly, P. Goble, T. Ochsner, S. Quiring, M. Svoboda, and M. Woloszyn for input on this protocol. We thank M. Weaver (USGS) for their initial review of the draft protocol.

Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. This article is a U.S. Government work and is in the public domain in the USA. USDA is an equal opportunity employer and provider.

Soil moisture state is a result of many different environmental factors, including precipitation, vegetation, solar irradiance, and relative humidity, along with soil hydraulic and physical properties. These interact over space and time at differing spatial and temporal scales. To model and forecast the water, energy, and carbon cycles, it is necessary to understand the SWC state. One of the most common types of automated measurement technologies is an electromagnetic SWC sensor with tines meant to be inserted in situ into undisturbed soil. This protocol is designed to provide guidance for the process of installing these common types of buriable sensors. Accuracy, performance, and cost are typically proportional to the operating frequency of the sensors; lower-frequency sensors cost less but are more confounded by soil and environmental factors45. Soil- or site-specific calibration can improve the accuracy of lower-frequency sensors. The measurement method also affects sensor performance due to the underlying physics of the electromagnetic field (EMF).
Two major electromagnetic physical laws govern electromagnetic sensing. One is Gauss&#39; law, which describes how the sensor&#39;s propagated EMF is dependent on both the &#949; and BEC of the medium. Permittivity increases with SWC, however, so too does the BEC. Therefore, sensors dependent on Gauss&#39; law are affected by SWC, BEC, and the temperature effect on BEC, as well as any interference from salinity. Capacitance sensing methods obey Gauss&#39; law and thus are more prone to these effects46. Additionally, Gauss&#39; law describes the dependence of the capacitance on a geometric factor, which changes with the shape of the EMF in the soil. Research has demonstrated that the EMF shape changes with soil structure and the small-scale spatial variability of water content around the sensor tines. Small-scale spatial variability of water content and soil structure is large in most soils, resulting in geometric factor changes and consequent capacitance changes that have little to do with bulk mean soil water content changes. These factors decrease capacitance sensor accuracy and increase data variability46,47,48. The impedance and transmission line oscillation methods also depend on Gauss&#39; law, while time domain reflectometry and time domain transmissometry methods depend on Maxwell&#39;s equations, which do not include a geometric factor and are not dependent on BEC. While no sensor is problem-free, the time domain methods tend to be appreciably more accurate and less biased than either capacitance- or impedance-based methods.
There are several critical steps in the procedure. For a sparse network, proper site selection and sensor location are needed to have the most appropriate spatial representation of SWC. Site selection may be more influenced by external factors, such as land access, or other atmospheric monitoring requirements where soil moisture is the ancillary measurement. Meso-scale meteorological sites are located on broad and open, well-manicured grassy surfaces to minimize any microscale influences. Such locations may be less ideal for SWC monitoring. If applicable, wireless sensor technologies should be considered49,50,51,52,53 to allow SWC monitoring to occur away from the existing environmental monitoring station and in representative soil. Working around active farm operations and irrigation equipment is challenging. Most networks (e.g., SCAN and USDA-ARS) stay on the fringes of fields to avoid tillage activities such as plows or harvesters that can cut the cables and unearth sensors. Any in situ sensor and cable needs to be sufficiently buried and have a low-enough surface profile to avoid inferring with on-farm operations. Wireless systems53 and removable borehole sensors47 may be more appropriate. Groundwater conservation using large-scale, soil moisture-based irrigation54 is a growing field for SWC sensors; this protocol pertains to spatially representative long-term SWC data in undisturbed soils.
Some soils are more difficult to measure than others. In rocky, gravelly, or very dry soils, it may be impossible to insert the tines without any damage. One option is to excavate the soil pit and lay the sensors in place while backfilling, trying to compact to the original BD. Rocky soils tend to have little structure, which will likely heal after several wetting and drying cycles; however, such disturbance may never be truly representative of the soil hydrology of the site. Alternatively, if sensors are installed in the bottom of auger holes, the soil removed can be sieved to remove stones and repacked in the hole just deeply enough to accommodate the sensor tines. The sensor can then be installed vertically, and the auger hole refilled with the remaining non-sieved soil, with frequent compaction as soil is added.
Roots in forest soil pose similar challenges to probe insertion, however roots can be cut in some situations. Forest soils often have organic (O) horizons on top of the mineral soil, which can have very low BD and high specific surface area, with large amounts of bound water resulting in very nonlinear sensor responses at higher SWCs55. Furthermore, the practitioner sets the zero-datum as either the top of the O-horizon or the mineral soil-noting which in the metadata. Clay-rich soils and expansive clays with high shrink/swell potential can be extremely conductive to electromagnetic signals when wet and may crack when dry. Such soils may need additional corrections to get reasonable SWC from the raw measurements56,57. In shallow soils, bedrock or a restrictive soil horizon (e.g., caliche or hardpan) may be encountered before reaching the ideal maximum depth. It may be necessary to change location or simply not install the deeper sensor(s). Excessively dry or wet soils can be challenging, and it is also preferable to choose installation dates outside of seasonal extremes. Dry soil can be very strong, and it may prove impossible to insert a sensor without damage. If necessary, preaugered holes can be filled with water to soften the pit face, although it may take some time before the soils return to a natural state. Wet soils can be too weak to support pit faces or the trench can fill with water. It is also easier to over-compact a wet soil.
The sensor output should include permittivity, not just SWC, so that corrections or soil-specific calibrations can be made later. Higher-frequency sensors are more appropriate in high BEC soils, while shorter tines may be easier to install in more compact soils. Perhaps the most critical step, however, is soil contact; poor contact degrades the signal from any electromagnetic sensor. Finally, backfilling the excavation sounds trivial, but it is key to minimizing preferential flow into the area of the sensors, keeping cables protected, and discouraging animals from disturbing the area. A soil- or site-specific calibration can improve sensor accuracy but requires more detail than is possible in this protocol. Field soils adjusted or repacked to varying SWC levels are ideal to check the linearity of response and can serve as a site-specific calibration for some sensor types21. Dielectric liquids can also be effective media to check sensor response58. Temperature-controlled water baths can be used to improve soil temperature calibrations59. This protocol is the first step toward the establishment of a standard operating procedure for in situ SWC sensor installation, as there is no existing method, nor any accepted method of calibration for SWC sensors60,61.
While SWC monitoring has been the focus of this protocol, the method has limitations, and SWC alone cannot give a complete picture of soil water status. Many ecosystem processes are also regulated by soil water potential, which is less commonly measured in situ62. Soil water potential, recently reviewed by S. Luo, N. Lu, C. Zhang, and W. Likos 63, is the energy state of water; such sensors may be less affected by soil properties and provide quality control for SWC sensors64. Furthermore, the bulk field SWC includes gravels, rocks, roots, and void space (e.g., preferential flow paths). In situ SWC sensors are generally repositioned around rocks and roots, and the limited measurement volume, concentrated around the tines, may miss discrete yet important aspects of the bulk field SWC.
This protocol will hopefully lead to a more harmonized and uniform SWC data for a wide range of applications, including drought monitoring, water supply forecasting, watershed management, agricultural management, and crop planning. The advent of remote sensing platforms4 has greatly enhanced the ability to estimate SWC globally, but these products need ground validation, which is still only reasonably collected by in situ networks65. Computer advancements have allowed hyper-resolution SWC modeling66 to be developed, producing high-resolution and sub-daily SWC status, but these products also need&#160;in situ estimates of SWC to provide some basis for calculating uncertainty. Often, the first question asked when a new product is introduced is &#34;what is the uncertainty?&#34; For SWC products, the primary comparison for validation is in situ network data67.
There have been recent network expansions associated with the National Coordinated Soil Moisture Monitoring Network (NCSMMN), including the U.S. Army Corps of Engineers Upper Missouri River Basin soil moisture project and the NOAA-supported Southeast U.S. network buildup, all designed to improve water hazard prediction, monitoring, and provide resource management decision support. Certainty and accuracy of SWC estimates for such applications can only be accomplished with thorough protocols and procedures to provide confidence in data integrity. The NCSMMN is a federally led, multi-institution effort that aims to provide assistance, guidance, and support by building a community of practice around soil moisture measurement, interpretation, and application-a &#34;network of people&#34; that links data providers, researchers, and the public68. This protocol is a product of NCSMMN efforts. A data quality control workflow is forthcoming.

Soil moisture was recently recognized as an Essential Climate Variable in the Global Observing Climate System1. Soil moisture, or quantitatively volumetric soil water content (SWC), plays a major role in partitioning the flux of incoming radiation into latent and sensible heat between the earth&#39;s surface and the atmosphere, and partitioning precipitation between runoff and infiltration2. However, the spatiotemporal variability of soil moisture at the point, field, and watershed scales complicates our ability to measure SWC at the appropriate scale needed to meet research or management goals3. New methods to quantify SWC, including ground-based networks of in situ sensors, proximal detectors, and remote sensing, provide unique opportunities to map the variation of SWC at an unprecedented resolution4. In situ SWC sensors provide the most temporally continuous and depth-specific data records, but are also subject to small sensing volumes and local-scale variability inherent in soil properties, topography, and vegetation cover5.
Furthermore, there is a lack of standards or widely accepted methods for the installation, calibration, validation, maintenance, and quality control of in situ SWC sensors. Soil moisture is inherently a challenging parameter to measure and may be the most difficult variable to quality-assure6. While general protocols for SWC data collection have been produced by the International Atomic Energy Agency7, the Committee on Earth Observation Satellites8, federal agency reports9, and the American Association of State Climatologists10, there is limited specific guidance on the installation, maintenance, quality control, and verification of SWC data from buried in situ probes. This has made the adoption of such technologies challenging for operational monitoring networks, such as state Mesonets, to add SWC measurements. Similarly, it is also challenging for operational hydrologists, for example, at river forecast centers, to incorporate these data into their workflow. The objective of this videography and accompanying paper is to provide such guidance and document a cohesive installation protocol for buried in situ SWC probes.
Selecting a location for in situ soil moisture monitoring 
The soils within any area of interest (AOI) form through a unique and coupled feedback over time between the topography, ecology, geology, and climate11,12. The variability of SWC across landscapes makes site selection a critical aspect to any soil moisture study. For some research goals, a site may be chosen to represent a particular feature or microsite on the landscape or ecosystem. For the purposes of monitoring networks, the site should be spatially representative of a larger landscape component. The goal is to find a location that provides the best spatial representation of the AOI. In the field, more pragmatic considerations must be reached, such as the requirements of other meteorological instrumentation, accessibility, or permittance. However, the dominant soil map unit within the AOI is usually a good spatial representation of the environmental conditions of a larger area13. The dominant soil map unit can be determined using the Web Soil Survey (https://websoilsurvey.sc.egov.usda.gov/); this soil map unit should also be verified with a shallow pit or test hole.
A typical monitoring station can occupy 5-50 m2, depending on the sensor needs and number of ancillary measurements. Figure 1 depicts a typical monitoring station with a 3 m tower that holds an anemometer for wind speed and direction, an air temperature and relative humidity sensor, a pyranometer for solar radiation, and a National Electrical Manufacturers Association (NEMA) weather-resistant and watertight enclosure (NEMA rating 4). The NEMA enclosure houses the data control platform (DCP), cellular modem, solar panel charge regulator, battery, and other related hardware (see Table of Materials; System components). The tower also provides a platform for the communication antenna, solar panel, and lightning rod. A liquid precipitation (PPT) gage is also typically included, which should be placed away from the tower and at the lowest elevation possible to reduce wind effects on PPT capture. The SWC sensors should be installed a sufficient distance away (3-4 m) and upslope so that there is no potential interference from the tower on either rainfall or overland flow. Any related cables should be buried in conduit at least 5 cm below the surface.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/64498/64498fig01.jpg" /> 
Figure 1: A typical monitoring station. The USDA SCAN collects hourly information on soil water content and temperature at standard depths (5, 10, 20, 50, and 100 cm), air temperature, relative humidity, solar radiation, wind speed and direction, precipitation, and barometric pressure. There are over 200 SCAN sites across the USA. Abbreviations: SCAN = Soil Climate Analysis Network; NEMA = National Electrical Manufacturers&#39; Association. <a href="https://www.jove.com/files/ftp_upload/64498/64498fig01large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Measurement depth, orientation, and number of sensors 
In situ SWC sensors are typically installed horizontally to represent specific depths in the soil (Figure 2). Federally funded, national networks such as the Soil Climate Network (SCAN)14, Snow Telemetry Network (SNOTEL)15, and U.S. Climate Reference Network (USCRN)16 measure SWC at 5, 10, 20, 50, and 100 cm. These depths were reached by consensus during SCAN development for a variety of reasons. The 5 cm depth corresponds to remote sensing capabilities17; 10 and 20 cm depths are historical measurements for soil temperature18; 50 and 100 cm depths complete root zone soil water storage.
Probes can be oriented vertically, horizontally, or inclined/angled (Figure 3). Horizontal installation is most common to achieve a uniform soil temperature measurement at a discrete depth. While the sensor may be centered at a discrete depth, SWC measurement is a volume around the tines (i.e., electrodes), which can vary with moisture levels, measurement frequency, and the geometry of the installation (horizontal, vertical, or angled). For horizontal installation, the sensing volume integrates moisture above and below the depth, and 95% of the sensing volume is typically within 3 cm of the tines19. Vertical or angled installations integrate SWC along the tines, so vertical installation can represent the storage along the entire length of the sensor depths20. Some sensors do not evenly measure along their tines. For example, transmission line oscillators are more sensitive to moisture near the probe head where the electromagnetic pulses are generated21. Vertical installations are more suitable for deeper probes where temperature and moisture gradients tend to be reduced.
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/64498/64498fig02.jpg" /> 
Figure 2: Installation of in situ SWC sensors. Horizontal sensor placement at chosen depths using (A,B) a zero-depth reference jig and (C) a zero-depth board or (D) a zero-depth shovel handle for reference. <a href="https://www.jove.com/files/ftp_upload/64498/64498fig02large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/64498/64498fig03.jpg" /> 
Figure 3: Orientation of probes vertically, horizontally, or at an inclination. (A) Slanted and vertical insertion and (B) horizontal-vertical insertion and horizontal-horizontal insertion center depth of a three-tined SWC sensor. Abbreviation: SWC = soil water content. <a href="https://www.jove.com/files/ftp_upload/64498/64498fig03large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Installation to depths less than 50 cm is relatively intuitive, while deeper sensors take slightly more effort. Root zone SWC or profile soil water storage typically requires measurements to 1 or 2 m. As illustrated in this protocol, 0-50 cm installations are completed in an excavated pit or auger-hole with probes installed horizontally into the undisturbed soil, minimizing the surface disturbance. For deeper sensors (e.g., 100 cm), both SCAN and USCRN install the sensor vertically in separate, hand-augered holes using an extension pole (Figure 4).
Given the heterogeneity of SWC, particularly near the surface, and the small measurement volumes of sensors, triplicate measurements allow for a better statistical representation of SWC. However, one profile of in situ sensors is typical for most networks (e.g., SCAN and SNOTEL). The USCRN uses three profiles spaced 3-4 m apart to make triplicate measurements at each depth16. Furthermore, redundancy in measurement adds resiliency and continuity to the station record if financial resources are available.
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/64498/64498fig04.jpg" /> 
Figure 4: Installation of sensors. (A) Shallow sensors are typically installed horizontally into the side wall of an excavated soil pit. For deeper sensors, (B) a hand auger is used to dig a hole to depth using a zero-depth reference (e.g., wood spanning the trench) and the sensors are pushed vertically into the bottom of holes using (C) a section of PVC pipe modified to secure the sensor and cable during installation or (D) an installation tool. Soil layers are noted as topsoil (A horizon) and subsoil horizons with translocated clays (Bt) and carbonate accumulation (Bk). <a href="https://www.jove.com/files/ftp_upload/64498/64498fig04large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
In situ SWC sensor type 
Commercially available sensors infer SWC from the measured response to an electromagnetic signal propagated along tines&#160;in direct contact with the soil22. Buried sensors fall into five classes depending on the type of electromagnetic signal propagated and the method of measuring the response: capacitance, impedance, time-domain reflectometry, time-domain transmissometry, and transmission line oscillation (Supplemental&#160;Table S1, with links to each manufacturer&#39;s information). These technologies tend to group by operating frequency and manufacturer. Longer tines integrate a larger volume of soil; however, they can be more difficult to insert and are more subject to signal loss in soils with clay and higher bulk electrical conductivity (BEC). Manufacturers report SWC measurement errors of 0.02-0.03 m3m&#8722;3, while users typically find these to be significantly larger23. Proper calibration and standardization of electromagnetic sensors improves performance22; however, these soil-specific calibrations are beyond the scope of this protocol, which focuses on installation.
Sensor selection should consider the desired output, measurement method, operating frequency, and compatibility with other measurements. Prior to 2010, most SWC sensors were analog and&#160; required the DCP to make measurements of differential voltages, resistances or pulse counts, which required more expensive components and individual channels (or multiplexers) for each sensor. Now, serial data interface at 1,200 baud (SDI-12) communication protocols (http://www.sdi-12.org/) allow smart sensors to implement internal measurement algorithms and then transmit digital data along a single communication cable. Each sensor can be wired together in sequence (i.e., a daisy chain) using a common wire connected by lever nut or terminal block connectors (Figure 5) with each sensor having a unique SDI-12 address (0-9, a-z, and A-Z).&#160;The common communication wire of SDI-12 sensors forms a single circuit along with a power and ground wire. Multiplexers or any measurements at the DCP are not required; instead, the DCP simply sends and receives digital commands and lines of text. Many SDI-12 SWC sensors also include soil temperature, relative permittivity (&#949;), and BEC measurements. Such measurements are useful for sensor diagnostics and soil-specific calibration. At this point, the user has selected a site, determined the sensor type, number, and depths, and obtained all the necessary hardware and field tools (Table of Materials). Thus, they can proceed to the installation protocol.
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/64498/64498fig05.jpg" /> 
Figure 5: Wire splice connectors and terminal blocks used to join common power, ground, and communication wires to a single input on the data collection platform. <a href="https://www.jove.com/files/ftp_upload/64498/64498fig05large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>System components, essential</td>
			<td></td>
			<td></td>
			<td>This system is the typcial micro-station used in the TxSON soil moisture network. The TxSON meteorlogical station is listed under optional components. https://acsess.onlinelibrary.wiley.com/doi/full/10.2136/vzj2019.04.0034&#160;</td>
		</tr>
		<tr>
			<td>Battery, sealed rechargable 12 V 12 AH&#160;</td>
			<td>Campbell Scientific&#160;</td>
			<td>BP12</td>
			<td>7 amp-hour (AH) minimum</td>
		</tr>
		<tr>
			<td>Charging regulator</td>
			<td>Campbell Scientific&#160;</td>
			<td>CH200</td>
			<td>Charge regulator, needed for any unregulated solar panel</td>
		</tr>
		<tr>
			<td>Conduit, schedule 40 PVC, 1 to 2&#34; diameter&#160;&#160;</td>
			<td>Any home supply store</td>
			<td></td>
			<td>Diameter sized appropriate to number of sensors and cable thickness. Length dependent on height of enclosure</td>
		</tr>
		<tr>
			<td>Data aquistion software</td>
			<td>Campbell Scientific&#160;</td>
			<td>PC400</td>
			<td>Free versions with limited programability, for more basic applications, manual downloads and simple sensor configurations</td>
		</tr>
		<tr>
			<td>Data control platform</td>
			<td>Campbell Scientific&#160;</td>
			<td>CR300</td>
			<td>Any SDI-12 compatible DCP is sufficint. Many also have integrated cellular modems available&#160;</td>
		</tr>
		<tr>
			<td>Enclosure (NEMA), 10 x 12 inch, -DC 2 conduits for cables, -MM tripod mast mount</td>
			<td>Campbell Scientific&#160;</td>
			<td>ENC10/12-DC-MM</td>
			<td>Two bottom conduits are required for above and below ground instruments</td>
		</tr>
		<tr>
			<td>Mounting pole (47 inch) with pedestal legs</td>
			<td>Campbell Scientific&#160;</td>
			<td>CM305-PL</td>
			<td>Smaller footprint, not tall enough for weather sensors</td>
		</tr>
		<tr>
			<td>Rain Gage with 8 in. Orifice, 20 ft of cable</td>
			<td>Campbell Scientific&#160;</td>
			<td>TE525WS-L20-PT</td>
			<td>Recommend installing rain gage on a separate vertical pole some distance from the instrument stand</td>
		</tr>
		<tr>
			<td>Sensors, 12 cm water content reflectometer, 17ft cable, -VS SDI-12 address varies</td>
			<td>Campbell Scientific&#160;</td>
			<td>CS655-17-PT-VS</td>
			<td>See Supplement Table 1 for more options&#160;</td>
		</tr>
		<tr>
			<td>Solar panel, 20 W</td>
			<td>Campbell Scientific&#160;</td>
			<td>SP20</td>
			<td>Use higher wattage panels for northern sites and lower for southern sites with higher exposre</td>
		</tr>
		<tr>
			<td>System components, optional</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Cellular Antenna, 2 dB multiband omnidirectional&#160;</td>
			<td>Campbell Scientific&#160;</td>
			<td>32262</td>
			<td>Directional antennas can improve signal, if the tower location is known.&#160;</td>
		</tr>
		<tr>
			<td>Cellular modem for Verizon/ATT</td>
			<td>Campbell Scientific&#160;</td>
			<td>CELL210/205</td>
			<td>Provider is site-dependent</td>
		</tr>
		<tr>
			<td>Crossarm mount, 4 feet</td>
			<td>Campbell Scientific&#160;</td>
			<td>CM204</td>
			<td>Ideal for mounting 2 m sensors&#160;</td>
		</tr>
		<tr>
			<td>Data aquistion software, advanced</td>
			<td>Campbell Scientific&#160;</td>
			<td>Loggernet</td>
			<td>More advanced commercial sofware that includes remote communications options and advanced programming</td>
		</tr>
		<tr>
			<td>DIN Rail Perforated Steel</td>
			<td>Phoenix Contact</td>
			<td>1207639</td>
			<td>Used to mount terminal blocks inside enclosure</td>
		</tr>
		<tr>
			<td>Galvanized steel water pipe, 1.5 or 2 inch diameter, 10 ft in length</td>
			<td>Any home supply store</td>
			<td></td>
			<td>The most economical option for an instrument mast. Can be cut to length. Replaces the 47 inch mounting pole with legs</td>
		</tr>
		<tr>
			<td>Instrument tripod, 10 foot stainless-Steel with grounding kit</td>
			<td>Campbell Scientific&#160;</td>
			<td>CM110</td>
			<td>Taller instrument stand for 2 m meteorologic sensors</td>
		</tr>
		<tr>
			<td>Lever nut connectors, five ports (Figure 5)</td>
			<td>Digi-Key</td>
			<td>222-415/VE00/1000</td>
			<td>Connect one SDI-12 wire to 4 sensor wires. Alternative to DIN rail.</td>
		</tr>
		<tr>
			<td>Null modem cable&#160;</td>
			<td>Campbell Scientific&#160;</td>
			<td>18663</td>
			<td>Inteface cable between DCP with modem. Not required for integrated cellular modems</td>
		</tr>
		<tr>
			<td>Plug-in bridge - FBS 3-5</td>
			<td>Phoenix Contact</td>
			<td>3030174</td>
			<td>Used to connect the curcuit of multiple terminal blocks. Available at mouser.com</td>
		</tr>
		<tr>
			<td>Secure Set Foam, 10 Post Kit (2 gallon)</td>
			<td>Any home supply store</td>
			<td></td>
			<td>Altnerative to concrete when using a steel pipe mast or for precipation gage pole. Two part foam mixture</td>
		</tr>
		<tr>
			<td>Sensor, air temperature and relative humidity, 10 ft cable&#160;</td>
			<td>Campbell Scientific&#160;</td>
			<td>HygroVUE10-10-PT</td>
			<td>Lower accuracy and pression option. Replacable chips are the fastest means to meet annual calibration cycles.&#160;</td>
		</tr>
		<tr>
			<td>Sensor, solar radiation pyranometer, digital thermopile&#160;</td>
			<td>Campbell Scientific&#160;</td>
			<td>CS320</td>
			<td>Most inexpensive, ISO class C (second class). Better options are available but much more expensive</td>
		</tr>
		<tr>
			<td>Sensor, wind speed anemometer, 10 ft cable</td>
			<td>Met One</td>
			<td>014A-10</td>
			<td>More expensive options include wind direction, or sonic sensors with no moving parts</td>
		</tr>
		<tr>
			<td>Solar shield for air temperature and relative humidity sensor</td>
			<td>Campbell Scientific&#160;</td>
			<td>RAD10E</td>
			<td>All air temperature sensors require sheilded from the sun</td>
		</tr>
		<tr>
			<td>Terminal blocks (Figure 5), feed-through&#160;</td>
			<td>Phoenix Contact</td>
			<td>3064085</td>
			<td>The most secure method to connect multiple SDI-12 sensor wires. Available at mouser.com</td>
		</tr>
		<tr>
			<td>Field tools, essential</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Freezer bags: quart and gallon sized</td>
			<td>Any grocery store</td>
			<td></td>
			<td>Storage for soil samples collected for characterization</td>
		</tr>
		<tr>
			<td>Miscellaneous digging tools including hand trowl, flat spade, and pointed spade</td>
			<td>Any home supply store</td>
			<td></td>
			<td>Backup tools to aid excavation&#39;</td>
		</tr>
		<tr>
			<td>Shovel (Sharpshooter) 16 in. D-handle drain spade</td>
			<td>Razorback</td>
			<td></td>
			<td>Manual tool for excavating soil pit. Any narrow pointed spade will work.&#160;</td>
		</tr>
		<tr>
			<td>Shovel, trenching, 4 in wide steel blade&#160;</td>
			<td>Any home supply store</td>
			<td></td>
			<td>Ideal trenching tool for burying cable or conduit</td>
		</tr>
		<tr>
			<td>Soil auger (&#60;4 in diameter) with T-handle or and extension bar as needed for r test holes</td>
			<td>AMS Samplers</td>
			<td>400.06</td>
			<td>Recommended for test holes. The auger type should match soil, but &#39;regular&#39; performs well in most soils</td>
		</tr>
		<tr>
			<td>Tarp (plastic) or plywood sheet</td>
			<td>Any home supply store</td>
			<td></td>
			<td>Soil management during excavation and trenching</td>
		</tr>
		<tr>
			<td>Field tools, optional</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>2,000 lb Mini Excavator</td>
			<td>Sunbelt Rentals</td>
			<td>350110</td>
			<td>Rental equipment for mechanical excavation</td>
		</tr>
		<tr>
			<td>Breaker or digging bar</td>
			<td>Any home supply store</td>
			<td></td>
			<td>Useful to break rocks and cut roots during excavation</td>
		</tr>
		<tr>
			<td>Galvanized Cattle Fence Panel, 16 ft x 50 in</td>
			<td>Tractor Supply Co.&#160;</td>
			<td>350207799</td>
			<td>Recommend cutting fencing panels into 8&#39; sections</td>
		</tr>
		<tr>
			<td>Pick mattock or pulaski&#160;</td>
			<td>Any home supply store</td>
			<td></td>
			<td>Great for loosening in hard or rocky soils&#160;</td>
		</tr>
		<tr>
			<td>Post Hole Auger Hydraulic Tow Behind with 18&#34; diameter auger</td>
			<td>Sunbelt Rentals</td>
			<td>700033</td>
			<td>Rental equipment for mechanical excavation</td>
		</tr>
		<tr>
			<td>Post hole digger, 48 in handle</td>
			<td>Any home supply store</td>
			<td></td>
			<td>Useful to clear soil in bottom of pit, or for test holes</td>
		</tr>
		<tr>
			<td>Steel fence T-posts, 6 feet tall and fence post driver, ~14 lb.</td>
			<td>Any home supply store</td>
			<td></td>
			<td>Fencing support and installation</td>
		</tr>
		<tr>
			<td>Steel rake</td>
			<td>Any home supply store</td>
			<td></td>
			<td>Ideal for smoothing disturbed soil at field area</td>
		</tr>
		<tr>
			<td>Every Day Carry (EDC), recommendations for any field technician&#39;s toolbag</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Adjustable wrench with insulated handle&#160;</td>
			<td>Any home supply store</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Assorted UV-resistant zip ties</td>
			<td>Any home supply store</td>
			<td></td>
			<td>Critical for neat wiring&#160;</td>
		</tr>
		<tr>
			<td>Diagonal cutting pliers</td>
			<td>Any home supply store</td>
			<td></td>
			<td>Efficient way to cut light and heavy wires and snip zip ties</td>
		</tr>
		<tr>
			<td>Digital camera, GPS, and compass</td>
			<td>Misc.&#160;</td>
			<td></td>
			<td>Ideally, these are all on your smartphone</td>
		</tr>
		<tr>
			<td>Digital multimeter</td>
			<td>Any home supply store</td>
			<td></td>
			<td>Key tool for troubleshooting power and connectivity issues in electrical systems</td>
		</tr>
		<tr>
			<td>Electrical tape&#160;</td>
			<td>Any home supply store</td>
			<td></td>
			<td>Non-black tape can be used for labeling&#160;</td>
		</tr>
		<tr>
			<td>Electrician&#39;s Puddy for filling entrance holes of enclosures</td>
			<td>Any home supply store</td>
			<td></td>
			<td>Needed to close and seal all conduit ports in the enclosure</td>
		</tr>
		<tr>
			<td>Hex key sets in both standard and metric sizes</td>
			<td>Any home supply store</td>
			<td></td>
			<td>Required for many sensor mounts</td>
		</tr>
		<tr>
			<td>Magnetic torpedo level (8 to 12&#34;)</td>
			<td>Any home supply store</td>
			<td></td>
			<td>Needed to get instrument stand vertical and leveling any meteorlogical sensors</td>
		</tr>
		<tr>
			<td>Metric tape measure&#160;</td>
			<td>Any home supply store</td>
			<td></td>
			<td>Critical for inserting probes and sampling soils - both use metric depths.&#160;</td>
		</tr>
		<tr>
			<td>Pliers: needle nose, lineman&#39;s, and channel-lock</td>
			<td>Any home supply store</td>
			<td></td>
			<td>Lineman&#39;s pliers are essential for bailing wire fences.&#160;</td>
		</tr>
		<tr>
			<td>Portable drill, bits, nut drivers</td>
			<td>Any home supply store</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Ratchet wrench and appropriate socket sizes&#160;</td>
			<td>Any home supply store</td>
			<td></td>
			<td>Ratch wrenches can get into tight spaces around sensor mounts where standard box wrenches do not work</td>
		</tr>
		<tr>
			<td>Safety: first aid kit, water (5 gallons),&#160; trash bags, gloves, sunscreen, insect repellent</td>
			<td>Any home supply store</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Screw drivers: small and large size with insulated handles&#160;</td>
			<td>Any home supply store</td>
			<td></td>
			<td>Screws on DCP and terminal blocks are very small. Small flat and phillips heads are required. Larger tools will also come in handy</td>
		</tr>
		<tr>
			<td>Sharpies, pencils, and notebook</td>
			<td>Forestry Supplier</td>
			<td></td>
			<td>Basic record keeping is essential for metadata</td>
		</tr>
		<tr>
			<td>Step ladder, 6 ft</td>
			<td>Any home supply store</td>
			<td></td>
			<td>Hard to install 2m sensors without a ladder</td>
		</tr>
		<tr>
			<td>Utility knife and box cutter</td>
			<td>Any home supply store</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Vegetation control: hand loppers, weed whacker, saw</td>
			<td>Any home supply store</td>
			<td></td>
			<td>Depending on the environment, vegetation can quickly overwhelm a fenced off areas.</td>
		</tr>
		<tr>
			<td>Wire strippers (8-20 gage)</td>
			<td>Any home supply store</td>
			<td></td>
			<td>Essential tool for preparing wires for insertion into DCP or terminal blocks. Self-adjusting strippers are the latest rage</td>
		</tr>
		<tr>
			<td>Annual Maintenance Supplies</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Battery cleaner (baking soda) and brush</td>
			<td>Any grocery store</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Cleaning:compressed air, isopropyl alcohol, tooth brush, pipe cleaners, paper towels&#160;</td>
			<td>Any grocery store</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Desiccant, silica gel bags</td>
			<td>Clariant</td>
			<td>Desi Pak</td>
			<td>Reusable after oven drying at 105 &#176;C for over 24 h. Swap out annually.&#160;</td>
		</tr>
		<tr>
			<td>Field calibration device for rain gage</td>
			<td>R.M. Young</td>
			<td>52260</td>
			<td>Device that drips water into a rain gage at varying intensity&#160;</td>
		</tr>
		<tr>
			<td>Handheld Weather Meter</td>
			<td>Kestrel Instruments</td>
			<td>0830</td>
			<td>Direct measurement of air temperature, relative humidity, and wind speed for field verification</td>
		</tr>
		<tr>
			<td>One quart and one gallon freezer bags</td>
			<td>Any grocery store</td>
			<td></td>
			<td>Storage for any gravimetric soil samples</td>
		</tr>
		<tr>
			<td>Portable soil moisture sensor&#160;</td>
			<td>Delta-T Devics</td>
			<td>SM150T</td>
			<td>A variety of sensors exist. See evaluation at https://acsess.onlinelibrary.wiley.com/doi/pdf/10.1002/vzj2.20033</td>
		</tr>
		<tr>
			<td>Soil core sampler, 2-1/4 in. Diameter</td>
			<td>Soilmoisture Equipment Corp.</td>
			<td>0200</td>
			<td>Gravimetric soil moisture and bulk density sampler&#160;</td>
		</tr>
	</tbody>
</table>

in situ soil moisture sensors in undisturbed soils

Soil moisture directly affects operational hydrology, food security, ecosystem services, and the climate system. However, the adoption of soil moisture data has been slow due to inconsistent data collection, poor standardization, and typically short record duration. Soil moisture, or quantitatively volumetric soil water content (SWC), is measured using buried, in situ sensors that infer SWC from an electromagnetic response. This signal can vary considerably with local site conditions such as clay content and mineralogy, soil salinity or bulk electrical conductivity, and soil temperature; each of these can have varying impacts depending on the sensor technology.
Furthermore, poor soil contact and sensor degradation can affect the quality of these readings over time. Unlike more traditional environmental sensors, there are no accepted standards, maintenance practices, or quality controls for SWC data. As such, SWC is a challenging measurement for many environmental monitoring networks to implement. Here, we attempt to establish a community-based standard of practice for in situ SWC sensors so that future research and applications have consistent guidance on site selection, sensor installation, data interpretation, and long-term maintenance of monitoring stations.
The videography focuses on a multi-agency consensus of best-practices and recommendations for the installation of in situ SWC sensors. This paper presents an overview of this protocol along with the various steps essential for high-quality and long-term SWC data collection. This protocol will be of use to scientists and engineers hoping to deploy a single station or an entire network.

Watch this Scientific Journal Video about In Situ Soil Moisture Sensors in Undisturbed Soils at JoVE.com

In Situ Soil Moisture Sensors in Undisturbed Soils

The determination of soil water content is a critical mission requirement for many state and federal agencies. This protocol synthesizes multi-agency efforts to measure soil water content using buried in situ sensors.

In Situ Soil Moisture Sensors in Undisturbed Soils

Soil moisture directly affects operational hydrology, food security, ecosystem services, and the climate system. However, the adoption of soil moisture ...

Research

JoVE Journal

Environment

Design and Operation of a Continuous 13C and 15N Labeling Chamber for Uniform or Differential, Metabolic and Structural, Plant Isotope Labeling

Design and Operation of a Continuous 13C and 15N Labeling Chamber for Uniform or Differential, Metabolic and Structural, Plant Isotope Labeling

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Integrated Field Lysimetry and Porewater Sampling for Evaluation of Chemical Mobility in Soils and Established Vegetation

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Measurement of Greenhouse Gas Flux from Agricultural Soils Using Static Chambers

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A Bending Test for Determining the Atterberg Plastic Limit in Soils

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Extraction and Analysis of Microbial Phospholipid Fatty Acids in Soils

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Assessment of Labile Organic Carbon in Soil Using Sequential Fumigation Incubation Procedures

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Methods of Soil Resampling to Monitor Changes in the Chemical Concentrations of Forest Soils

Recent soils research has shown that important chemical soil characteristics can change in less than a decade, often the result of broad environmental ...

Protocols for Quantifying Transferable Pesticide Residues in Turfgrass Systems

Plant canopies in established turfgrass systems can intercept an appreciable amount of sprayed pesticides, which can be transferred through various routes ...

The Calibration and Use of Capacitance Sensors to Monitor Stem Water Content in Trees

Water transport and storage through the soil-plant-atmosphere continuum is critical to the terrestrial water cycle, and has become a major research focus ...