Published online 1 January 2007
Published in Soil Sci Soc Am J 71:51-52 (2007)
DOI: 10.2136/sssaj2006.0090N
© 2007 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
SOIL PHYSICS NOTES
Soil Water Measurement Comparisons Between Semi-Permanent and Portable Capacitance Probes
J. L. Starr and
R. Rowland*
USDA-ARS Hydrology and Remote Sensing Lab., Beltsville, MD 20705
* Corresponding author (rowlandr{at}ba.ars.usda.gov).
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ABSTRACT
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The objectives of this study were to compare and characterize the soil water measurement responses using the EnviroSCAN semi permanent multisensor capacitance probes (MCP) and the Diviner 2000 (D2k) portable capacitance probe.Both probes are manufactured by Sentek Pty. Ltd., Stepney, South Australia. Comparative soil water measurements were made using the portable D2k and the MCP probes at three field sites where 48 polyvinylchloride (PVC) access tubes were previously installed and being used to near-continuously monitor soil water content at four or more depths. The D2k and MCP soil water sensing data were highly correlated values (R2 > 0.98), but the D2k water contents averaged about 5% more than the corresponding MCP values. As a result of this high correlation we were able to transform the D2k readings so that the two probes gave essentially the same water content values when measured as the same time and position.
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INTRODUCTION
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THERE HAS BEEN CONSIDERABLE RESEARCH comparing various technologies used in in situ soil water content measurement (Evett et al., 2002; Chanzy et al., 1998; Starr and Paltineanu, 1998; Fares and Alva, 2000a). There is, however, no clearly preferred soil water sensing/monitoring method, since each has its advantages and disadvantages depending on the desired information and application. Recently, several researchers (Starr and Paltineanu, 1998; Chanzy et al., 1998; Baumhardt et al., 2000; Morgan et al., 1999; Kelleners et al., 2004) have used capacitance probe methods as a viable alternative to neutron probes and other more conventional soil water monitoring methods.
The EnviroSCAN multisensor capacitance probes (MCPs) and soil water monitoring system have been extensively used as an irrigation management tool since 1991 (Buss, 1993), and more recently in soil-water research investigations over large areas (Starr and Paltineanu, 1998; Paltineanu and Starr, 2000; Fares and Alva, 2000b; Starr and Timlin, 2004). Detailed description of the MCPs and comparisons with other measuring devices is shown elsewhere (Paltineanu and Starr, 1997; Starr and Paltineanu, 2002). Briefly, the capacitance sensors consist of one or more pairs of cylindrical metal electrodes mounted on a support rod that is inserted into a previously installed polyvinylchloride (PVC) access pipe. Accurate soil water content measurement for these capacitance and all electromagnetic based sensors requires careful installation procedures to prevent formation of air gaps along the sensors or alteration of soil properties within the sensor's zone of influence (Starr and Paltineanu, 2002), and site specific calibration equations. The zone of major influence of the sensors represents a cylinder of soil, approximately 10 cm in length along the axis of the probe, with a 10-cm ring around its 5-cm diameter PVC access tube. Multiple capacitance sensors can be placed on a single probe, with the number and spacing of sensors set by the user. Sensor reading-time intervals are also flexible and can be set by the user, and can vary from 1 min to 6.9 d.
Sensor readings are related to volumetric soil water content (
v) through a nonlinear relationship with SF, often of the form
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The SF represents the ratio of frequencies measured by each sensor (from inside the PVC pipe) in the surrounding soil (Fs), in relation to prior sensor readings in the air (Fa) and in nonsaline water (Fw) as,
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The use of SF minimizes sensor specific electronic differences, so that the same calibration curve can be used for all the capacitance sensors for a given soil.
Sentek's D2k is a single sensor portable version of the MCP. The D2k uses essentially the same cylindrical metal electrode as the MCP, but it is encased in form-fitting plastic and mounted on a portable rod. When attached to the top of the access tube, the D2k automatically takes sensor readings every 10-cm in a single swipe down and up the access tube. The two probes may give somewhat different SF values for sensor readings at the same space-time position due, in part, to the plastic housing added to the D2k sensor. In addition to calibration equations for the D2k for a wide range of soil textures (Groves and Rose, 2004), comparative results for the MCP and the D2k are needed, especially by those who may use both soil water sensing probes simultaneously, or want to make use of the published MCP calibration equations when using the portable D2k. The objective of this study was to assess and compare the relationships between D2k and MCP soil water sensing probes, and if needed to develop a correction factor that could be used to normalize reading outputs the two methods.
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Materials and Methods
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Scaled frequency measurements were made using Sentek's D2k and MCP water sensing probes at three field sites at the Beltsville Agricultural Research Center, Beltsville, MD. Site 1 is a large two-sided field lysimeter, one side with a sandy loam and the other side a silt loam; Site 2 is in one of four subwatersheds that is being used as a long-term precision agriculture site, that has a sandy soil (Fine-loamy, siliceous, semiactive, mesic Typic Hapludults); and Site 3 is a research site for assessing long-term no-tillage corn, that has a Mattapex silt loam soil (fine-silty, mixed, active, mesic Aquic Hapludult). A total of 48 MCP access tubes were previously installed at the three field sites, 16 MCP at each site, following standard recommendations to ensure good soil to PVC access tube contact. Before MCP installation and D2k readings, capacitance sensor frequency readings in air and water were taken so that the frequency readings in soil could be converted to SF values using Eq. [2] as described above (Starr and Paltineanu, 2002).
Four or more capacitance sensors were placed on each MCP rod, with depths ranging from 10- to 100-cm. Note that the varying depths should not influence the comparisons of the two probes, since capacitance readings were conducted in the same access tube, at the same depths, and at approximately the same times with both probes. Between D2k readings, each MCP near-continuously read all sensor depths, logging data every 10 min. The D2k readings were made on selected days at which time each MCP was temporarily removed from its access tube and the portable D2k probe was inserted and soil water content immediately read at the same MCP sensor depths. The exact time of the manual reading was also recorded so that the MCP reading for that specific time could be correlated with the D2k reading. The MCP was then reinserted in its own PVC access tube.
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Results and Discussion
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A total of 242 D2k and MCP sensor-paired reading sets were obtained from three field sites. In general, the water content comparisons (v values) with the two probes were linearly correlated (R2 = 0.982), when using the same calibration equation (Eq. [1]) coefficients for the two probes. On average, however, the D2k water content values were about 4.8% greater than the MCP values. A rather quick correction for this discrepancy might be to use different calibration equations for the two probes. However, this approach does not lend itself to converting one set of readings to the other, especially when calibration equations are not available for both probes. The source of the differences in soil water contents using the two probes is most likely due to differences in SF values for the two probes, since all sensor readings with both probes were performed in the same access tubes and at the same depths and times.
The SF relationship for the two probes is shown in Fig. 1
. Data show the SF being highly correlated both for within the field site locations and across all sites. Linear regression provided the most significant correlation fit (R2 = 0.985), with the slope and intercept 0.883 and 0.1332, respectively. Interestingly, separate linear correlations for each field site were nearly identical to the composite data correlation line shown in this figure (separate fitted lines not being shown here).
The D2k SF values were adjusted to the MCP SF by using the linear correlation equation shown in Fig. 1. The new D2k water contents based on this adjusted D2k SF values resulted in a nearly 1:1 correlation of the water contents of the D2k and MCP probes, as shown in Fig. 2
. Considering the small data scatter around the regression line (SE = 0.67), this adjusted SF procedure for these soil conditions shows that the portable D2k gives essentially the same water content values as the MCP. Thus, when using this adjusted SF method, the expanding library of EnviroSCAN calibration curves should work equally well with the D2k.
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Fig. 2. Diviner 2000 vs. EnviroSCAN volumetric water content using modified scaled frequencies for Diviner 2000.
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NOTES
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Abbreviations: D2k, Diviner 2000 portable capacitance probe; MCP, EnviroSCAN semi permanent multisensor capacitance probe; PVC, polyvinylchloride; SF, scaling frequency.
Trade names are used in this publication to provide specific information. Mention of a trade name does not constitute a guarantee or warranty of the product or equipment by the USDA or an endorsement over other similar products.
Received for publication February 27, 2006.
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REFERENCES
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- Buss, P. 1993. The use of capacitance based measurements of real time soil water profile dynamics for irrigation scheduling. In Under pressure. Irrig. 93. Proc. Natl. Conf. Irrig and Drainage, Launceston, Tasmania. 17–19 May 1993. Irrig. Assoc. of Australia, Homebush, NSW.
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- Paltineanu, I.C., and J.L. Starr. 1997. Real-time soil water dynamics using multisensor capacitance probes: Laboratory calibration. Soil Sci. Soc. Am. J. 61:1576–1585.[ISI]
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- Starr, J.L., and D.J. Timlin. 2004 Using high-resolution soil moisture data to assess soil water dynamics in the vadose zone. Vadose Zone J. 3:926–935.[Abstract/Free Full Text]
TOP
ABSTRACT
INTRODUCTION
