Purchasing Considerations for Field Soil Water Content Profile Probes

Field measurements of soil water content are essential for irrigation managers, consultants, and environmental researchers. Changes in soil water content can infer wetting-front, infiltration rate, water-table depth, and root water-extraction zones.

Figure 1 -  Neutron Hydroprobe on an access tube in the field.

For decades, the Neutron Hydroprobe from Campbell Pacific Nuclear (InstroTek, Raleigh, NC) has been considered the industry standard for its accuracy and reliability in making nondestructive, in situ measurements of moisture content along soil profiles (Figure 1). More recently, time domain reflectometry (TDR) and capacitance sensors have been developed to make profile measurements. The Diviner 2000® and EnviroSCAN (Sentek Technologies, Stepney, Adelaide, South Australia) capacitance probes are widely used by many researchers and managers. Similar to the Neutron Hydroprobe, the Diviner 2000 (Figure 2) is a portable field instrument with an LCD digital meter, whereas the EnviroSCAN (Figure 3) is designed to be left in the field for continuous in situ soil water content monitoring and requires an RT6 (Sentek Technologies), CR200 (Campbell Scientific, Logan, UT), or some other datalogging device.

Water content probes

The Neutron Hydroprobe allows for fairly rapid measurements at discrete depths along a 2.44-m profile (minimum, longer cables may be purchased), but the Diviner 2000’s automatic depth and orientation sensing of the capacitance sensor (up to 1.6 m) allows swipe-and-go capabilities that significantly decrease sampling time (Table 1). Both probes permit data to be stored by the LCD meter, downloaded, or printed from the unit with an RS232 serial cable, but the Diviner 2000’s handheld meter enables users to view data graphically in the field.

Figure 2 - Diviner 2000 with meter, screwcap, and padded cases.

The EnviroSCAN is the only probe available that is capable of continuous soil water content and salinity measurements from 0 to 40 m deep. With up to 16 capacitance sensors spaced along the field-adjustable probe, repeatability and accuracy are ensured by applying soil-specific calibrations to the sensors individually and using built-in depth and orientation settings. The EnviroSCAN permits real-time continuous monitoring of field sites, and can be set up as part of an environmental monitoring station where the data are accessed remotely, or by downloading them directly from an individual datalogging device (Table 1).

Calibrating these instruments requires obtaining soil samples of varying water contents with a core sampler, calculating the soil’s volumetric water content, and fitting a linear function to the data to estimate calibration coefficients. Recalibration is necessary to check for drift in coefficients at long-term monitoring and research sites. Additionally, calibration curves will differ between field sites and with sampling tube thickness. The Neutron Hydroprobe, Diviner 2000, and EnviroSCAN all require soil-specific calibrations, but that does not mean that they are all well suited for each soil type and field setting. The physics must be considered when choosing a probe, because different types of measurements are governed by different principles.

Figure 3 - EnviroSCAN.

Neutron emitting probes

The Neutron Hydroprobe contains a 50-millicurie (mCi) radioactive Americium- 241/Beryllium source that emits fast neutrons, which collide with hydrogen in soil–water and return low-energy (thermalized) neutrons to the detector. Slow neutrons react with the nuclei of 3He gas in the detector, yielding electrical impulses that can be counted. The count of thermalized neutrons is proportional to the concentration of hydrogen within the “sphere of influence” of the radioactive emitter. Since hydrogen is mainly present in soil–water, this becomes a function of water content. However, organic material in soils tends to be acidic and associated with high levels of hydrogen; thus the detector will register hydrogen from both sources, and the counts will be subsequently higher. Heterogeneity of organic material in the soil profile will be reflected in the detector counts and may be explained by considering soil classification and plowing depths. Other chemical constituents, such as boron and chloride, are “neutron absorbers” and thus affect Hydroprobe readings when concentrations are high. However, readings are not affected by salinity.1

Table 1 - Instrument specifications and ranges for Neutron Hydroprobe, Diviner 2000, and EnviroSCAN

The radioactive neutron emitting source forces users to acquire an Atomic Energy Commission (AEC) permit, which requires that all users be trained in nuclear gauge safety, possess radiation-detecting film badges, and periodically submit the badges to the AEC for radiation monitoring. Costs of registering users on the AEC permits and decommissioning old probes have been increasing; therefore, using the Hydroprobe has become less feasible for many farm and environmental managers.

Capacitance probes

The Diviner 2000 and EnviroSCAN rely on capacitance sensors that use frequency domain reflectometry. Since the electrical capacitance of the soil is considered part of the sensor’s circuit, the dielectric constant of the soil is determined from the charge time of the capacitor. As soil water content increases, the dielectric constant of the soil increases, which increases charge time along the conductors. The dielectric constants of air (1) and soil (3–5) are much lower than that of water (80) due to its polarity; therefore, changes in the dielectric constant can be primarily attributed to the changes in water content. Since the soil’s electrical capacitance changes with soil type, limitations occur in gravelly and coarse stony soils where air spaces are more abundant and the overall capacitance of the soil is low.2

Sphere of influence and site selection

The sphere of influence, or region of soil measured by the probes, is much smaller for capacitance sensors (10 cm) than for the neutron-emitting source (15–25 cm) (Table 1). However, the Neutron Hydroprobe’s sphere of influence increases as soil–moisture decreases because neutrons have to travel farther before colliding with hydrogen atoms. A large sphere of influence provides a higher level of sensitivity and accuracy, and a better representation of soil water content, but a smaller sphere size with increased water contents changes the sample size and decreases accuracy in saturated soils. Limitations may be reached in heavy clayey soils with high moisture contents.1

Concessions need to be made when taking shallow measurements because neutrons escaping to the atmosphere from cracks and the soil surface can skew data. Similarly, large cracks read by the capacitance sensor will yield “dryer” water content readings. Variability in the field is larger for capacitance probes because the sensors are more sensitive to variations in soil macroporosity and soil disturbance during access tube installation due to having a smaller sphere of influence.

The sphere of influence also affects site selection. For capacitance probes to effectively measure plant–water consumption for irrigation management, the probes must be placed close to the plants or nestled in a row of crops. If placed on the edge of a field, the sphere of influence may be too small to detect root water–extraction.

Installation

Proper access tube installation and site location are extremely important to ensure accuracy of the measurements. Improper installation leads to air gaps and preferential flow down the tubes, which has a significant effect on the capacitance probes. The least amount of soil should be disturbed so that the region being measured is still representative of the surrounding soil. Access tubes may be installed by the standard (i.e., “tight-fit”) or slurry method. Standard installations are recommended, and provide the highest level of reliability because the soil is left intact by using an auger that is slightly smaller than the access tube and securely fitting the tube. The slurry method requires auguring or drilling a larger hole and positioning the tube in a kaolinite and cement slurry. This method is recommended for stony and gravelly soils only because they are difficult to augur, and the risk for air pocket formation is high; moreover, the slurry method is recommended for installations at depths greater than 2.5 m.3 Though it disrupts the surrounding soil that is being measured by the sphere of influence, the moisture content of the slurry equilibrates with that of the surrounding soil. This will create larger problems for capacitance sensors than the Hydroprobe because a greater proportion of the measurement region will be the slurry.

The thinner-walled access tubes from Sentek help to compensate for the small sphere of influence, and can be used interchangeably for the Diviner 2000 and EnviroSCAN. Additionally, the screw-caps that seal the probe into the tube prevent water or particles from getting into the tubes, which maintains accuracy and reliability. This is especially important for the orientation and depth sensing of the Diviner 2000 and EnviroSCAN probes. Neutron Hydroprobe access tubes can be made or purchased from durable 2-in. schedule 40 PVC or aluminum pipe. The increased durability of the material allows the tube to remain in the soil at deep depths for a longer period of time before needing to be replaced.

Conclusion

When choosing which probe to purchase, probe capabilities and limitations need to be considered with respect to soil type, location of access tubes, and whether or not continuous monitoring is necessary. The Diviner 2000 and EnviroSCAN are less expensive and are not subject to the regulations and permit requirements imposed on the Hydroprobe. However, the Hydroprobe is extremely durable, reliable, and accurate, which is necessary for many research projects.

References

1. Kramer, J.H.; Cullen, S.J. et al. Vadose zone monitoring with the neutron moisture probe. Ground Water Monitoring Review  1992, 12(3), 177–87.

2. Hanson, B.R.; Peters, D.W. Soil type affects accuracy of dielectric moisture sensors. California Agriculture  2000, 54(3), 43–7.

3. Sentek Sensor Technologies. Access tube installation guide for EnviroSCAN, EnviroSMART and Diviner 2000, 2003, Version 1. http://www.campbellsci.com/documents/ manuals/sentek_guidev1.pdf.

Emily S. Tozzi, M.Sc., is a Research Biologist with SynTech Research, 17915 East Annadale Ave., Sanger, CA 93657, U.S.A., e-mail: estozzi@gmail.com.

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