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Field Evaluation of Conventional and Downhole TDR Soil Water Sensors for Irrigation Scheduling in a Clay Loam Soil
Gary W. Marek1,*, Steve Evett1, Thomas H. Marek2, Dana O. Porter3, Robert C. Schwartz1
Published in Applied Engineering in Agriculture 39(5): 495-507 (doi: 10.13031/aea.15574). 2023 American Society of Agricultural and Biological Engineers.
1USDA ARS, Bushland, Texas, USA.
2Texas A&M University, Amarillo, Texas, USA.
3Texas A&M AgriLife Extension Service, Lubbock, Texas, USA.
*Correspondence: gary.marek@usda.gov
The authors have paid for open access for this article. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License https://creative commons.org/licenses/by-nc-nd/4.0/
Submitted for review on 22 February 2023 as manuscript number NRES 15574; approved for publication as a Research Article by Associate Editor Dr. Daran Rudnick and Community Editor Dr. Kati Migliaccio of the Natural Resources & Environmental Systems Community of ASABE on 29 August 2023.
Mention of company or trade names is for description only and does not imply endorsement by the USDA. The USDA is an equal opportunity provider and employer.
Highlights
Abstract. A field study was performed to evaluate the efficacy of two commercially available time domain reflectometry (TDR) soil water sensors for irrigation scheduling in a clay loam soil near Bushland, Texas. SoilVUE10 (Campbell Scientific Inc., Logan, Utah) and TDR-315 (Acclima Inc., Meridian, Idaho) sensors were installed within 30 cm of neutron moisture meter (NMM) access tubes in a research field planted to corn (Zea mays L) in 2020 and irrigated by a center pivot sprinkler system. Irrigation treatments included 50%, 75%, and 100% of evapotranspiration (ET) replacement with two access tubes installed in each plot, totaling six sensor evaluation sites. Semiweekly measurements with a field-calibrated NMM were used to monitor soil water status and schedule irrigation throughout the growing season. Soil profile water content values integrated over the surface to 1.1-m depth range were derived from SoilVUE10 and vertically distributed arrays of Acclima TDR-315 sensors installed at equivalent depths and were compared with those from NMM data. Average profile soil water contents from the TDR-315 sensors trended well with those from the NMM having mean bias difference (MBD) values of -9.8, -3.1, and 8.4 mm for the 50%, 75%, and 100% treatments, respectively. In contrast, soil profile water content values from the SoilVUE10 sensors grossly underestimated those from the NMM for all irrigation treatments with MBD values of -54.4, -70.5, and -89.8 mm for the 50%, 75%, and 100% treatments, respectively. Comparisons of volumetric water content (VWC) at each of the nine depths common to both electromagnetic sensor types revealed that values from the SoilVUE10 sensors were consistently less than TDR-315 values for all irrigation treatments. Underestimation at the near surface (5 and 10 cm depths) was attributed to loss of soil to electrode contact possibly associated with clay shrinkage during periodic drying following irrigation. Although soil to electrode contact can be problematic at greater depths, the explanation for chronic underestimation of VWC was less obvious except to note that underestimation occurred immediately after installation, which indicated poor electrode-soil contact after installation despite use of manufacturer guidelines and tools. Other possible reasons include challenges for accurate estimation of soil permittivity for a measured permittivity that includes the plastic sensor body. Results from this study suggest vertically distributed arrays of TDR-315 sensors can provide profile water content values adequate for monitoring soil water status for irrigation scheduling in a clay loam soil. The chronic underestimation observed for the SoilVUE10 sensors does not support their use for water resources research and irrigation management and could lead to over irrigation. Additionally, the relatively short 1 m length is less than the rooting depth of many regional crops and thus not capable of determining percolation below the root zone.
Irrigation is used to supplement insufficient and unpredictable precipitation to sustain profitable crop yields in the Texas High Plains and other semiarid regions. Effective irrigation scheduling aims to satisfy targeted crop water requirements while optimizing crop water productivity (CWP) by limiting evaporative losses and avoiding percolation below the root zone. Although numerous irrigation scheduling approaches have been developed, most are based principally on one of two fundamental approaches: 1) replacement of estimated crop water use, also known as evapotranspiration (ET), or 2) managing soil water status/depletion in the crop rooting zone (Elliott et al., 2000; Evett, 2007, Evett et al., 2009a, 2011b, 2012b, 2012c, 2016, 2019, 2020a, 2020c; Howell et al., 1995, 1997, 1998, 2004, 2006, 2008, 2015; Marek et al., 1996, 2006, 2020, 2021). Most ET-based methods employ a daily reference ET value and a crop coefficient to estimate daily crop ET throughout the growing season. Benefits include minimal instrumentation requirements and the availability of published regional crop coefficients. Drawbacks include need for frequent assessments of crop development, assimilation of daily weather data, and coarse overall spatial scale of readily available weather data. Soil water depletion methods require periodic and accurate determination of soil profile water content. Arguably the most accurate methods for determining soil profile water content are soil coring (gravimetric soil water content determination) and neutron scattering using a neutron moisture meter (NMM) (Evett et al., 2008). However, both present considerable challenges for irrigation scheduling. Soil coring is labor and time intensive, which may limit its usefulness for irrigation scheduling. Labor requirements for regular NMM measurements are not trivial, and this cost plus large initial costs and associated regulatory burdens make use of the NMM in most crop production settings impractical. Furthermore, although soil water sensors are easily employed, multiple monitoring sites are often required to adequately assess field-scale soil profile water content even in uniform soils, with additional sites needed in fields with multiple soil textures and/or slope. In addition, few soil water sensors are adequately accurate for irrigation management (Evett et al., 2008; Evett and Parkin, 2005).
Electromagnetic soil water sensors may provide continuous automated data in field settings at relatively low costs. In recent years, a myriad of soil water sensors and associated soil water sensing systems have inundated in the commercial market. Their use is often supported through government cost sharing programs such as the USDA-NRCS Environmental Quality Incentives Program (EQIP). Data from soil water sensors and systems lend themselves to not only conventional irrigation scheduling methods but can also be integrated into many supervisory control and data acquisition (SCADA) systems designed to help producers manage water more efficiently (O’Shaughnessy et al., 2020; Evett et al., 2020b). In these cases, sensors were deployed at multiple depths so that the data could be used to calculate profile water content for a crop root zone, which is directly useful for irrigation management. However, accurate soil water content data are paramount for effective soil water-based irrigation scheduling and not all sensors are equally capable in this regard. Electromagnetic soil water sensors vary in several important characteristics including susceptibility to interference, precision, accuracy, and volume sensed (Evett and Parkin, 2005). The volume sensed by popular and inexpensive capacitance sensors can change with small-scale variations in water content and bulk density that exist in most soils, resulting in inaccurate data and high variability (Evett et al., 2006, 2009a, 2012; Mazahrih et al., 2008; Evett and Schwartz, 2010). Dielectic loss originating from bulk electrical conductivity (EC) can dominate the low frequency loss spectrum in soils, causing changes in apparent permittivity, and thus contribute to errors in estimated water content (Schwartz et al., 2013). Measurement of permittivity by capacitance-based sensors is particularly susceptible to bulk EC as they typically operate at relatively small (<100 MHz) frequencies, often leading to inaccurate estimates of soil water content as large as 0.25 m3m-3 (Baumhardt et al., 2000; Evett and Parkin, 2005; Evett et al., 2006, 2011a; Schwartz et al., 2013). The development of time domain reflectometry (TDR) in the early 1980’s (Topp et al., 1980; Dalton et al., 1984) represented a seminal example of a successful means for estimating volumetric water content (VWC) from changes in bulk dielectric permittivity. However, early TDR systems were bulky, expensive, and had large power requirements making them largely impractical for irrigation scheduling in production settings. In contrast, modern TDR sensors are relatively compact, low-power, relatively inexpensive and far more user friendly (Schwartz et al., 2016; Evett et al., 2020c). However, despite numerous applications of TDR and other commercially available instrumentation, serious difficulties in estimating accurate soil water contents under field conditions remain, especially in fine textured soils (Schwartz et al., 2009a). TDR-measured permittivities have also exhibited relatively large sensitivities to bulk EC, principally in lossy soils with large (>100 m2 g-1) specific surface areas (Evett et al., 2005; Schwartz et al., 2009a, 2009b). Therefore, many modern TDR sensor outputs include ancillary measurements of bulk EC and temperature that can be used with permittivity to develop calibrations that correct for such affects to improve estimates of soil water content (Schwartz et al., 2013b, 2020). As such, field evaluations of modern TDR-based soil water sensors are needed to assess their usefulness in irrigation management. Wilson et al. (2023) presented one such study comparing the HydraProbe, the SoilVUE™10 sensor, and the TDR-315L™ sensor, the latter two being TDR sensor types. However, in their study the HydraProbe and TDR-315L sensors were only at 10-cm depth in the soil whereas the SoilVUE10 has sensors at center depths of 5, 10, 20, 30, 40, and 50 cm, precluding direct comparison at depths other than 10 cm and precluding any comparison of soil profile water content data. Also, their soil was a coarse loam and they called for repeat studies in higher clay soils.
Thus, a field study was performed to evaluate the efficacy of factory-calibrated Acclima TDR-315 and Campbell Scientific SoilVUE10 sensors for irrigation scheduling in a clay loam soil near Bushland, Texas. Specific objectives included: 1) comparison of profile soil water content values derived from both sensors with those from a NMM under full and deficit irrigation, and 2) comparison of depth-specific values of VWC for the TDR-315 and SoilVUE10 sensors.
Materials and Methods
Study Site
The study was conducted at the Texas A&M AgriLife Research Emeny Field during the 2020 summer growing season. The field was located due east of and immediately adjacent to the USDA-ARS Conservation and Production Research Laboratory (CPRL) near Bushland, Texas (35° 11’ N, 102° 6’ W, 1170 m elevation above MSL). The field was serviced by a seven span, 372 m center pivot sprinkler irrigation system fitted with low elevation spray application (LESA) drops on 1.5 m spacing, positioned approximately 0.5 m above the ground surface. The sprinkler was nozzled for uniform water application along the lateral with application depth determined by nozzle size and angular speed of the system. Soils were classified as Pullman silty clay loam (fine, mixed, superactive, thermic Torrertic Paleustoll) having slopes of less than 0.3% (Taylor et al., 1963; Tolk et al., 1998) (table 1). The slowly permeable soil has a dense Bt horizon (0.3 to 0.5 m depth) and a caliche layer below approximately 1.4 m depth that presents a texture contrast that restricts water movement in some seasons. The mean annual precipitation measured in the rain gauge network at the laboratory over the past 20 years was 479 mm. Seasonal extremes are characterized by hot summers and cold winters. Annual pan evaporation exceeds 2400 mm (Kohler et al., 1959; Farnsworth et al., 1982).
| Table 1. Soil description by horizon for the Pullman soil (fine, mixed, superactive, thermic Torrertic Paleustoll) (Tolk et al., 1998). | ||||||
| Horizon | Depth Range (m) | Texture | Sand (%) | Silt (%) | Clay (%) | Bulk Density (Mg m-3) |
| Ap | 0.0-0.18 | cl | 26.4 | 49.3 | 30.3 | 1.35 |
| Bt1 | 0.18-0.46 | sicl | 17.2 | 15.2 | 37.6 | 1.44 |
| Bt2 | 0.46-0.74 | sicl | 18.1 | 44.7 | 37.2 | 1.50 |
| Bt3 | 0.74-1.02 | cl | 20.3 | 42.8 | 36.9 | 1.47 |
| Bt4 | 1.02-1.35 | cl | 22.9 | 41.0 | 36.1 | 1.55 |
Agronomy
Irrigation treatments in three adjacent 20° sectors were designed to satisfy 100%, 75%, and 50% of crop water requirements (ET) for a full season corn hybrid (Pioneer 1366AM) having a comparative relative maturity (CRM) value of 113 (fig. 1). Irrigations for the 100% ET treatment were scheduled to limit management allowed depletion (MAD) of plant available soil water in a 1.5 m rooting zone to less than 55%, with scaled irrigation depths for the 75% and 50% treatments, achieved by increasing the pivot speed (fig. 2). In order to establish the crop, however, all treatments received irrigations equal to that of the 100% treatment from planting (DOY 140) through the V8 plant growth stage (DOY 164). The different irrigation treatments allowed the development of a wide range of soil water contents in the field, which was the condition desired for the comparison of soil water sensing methods. All sectors received irrigation during an irrigation event with the system always traveling clockwise, beginning with the 50% treatment, and finishing with the 100%. Targeted irrigation depths for the 100% treatment were typically 25.4 mm but ranged from 19.1 to 38.1 mm depending on precipitation and scheduling accommodations with other field studies. Irrigation depth and application rate were managed to minimize runoff from the field while applying sufficiently deep irrigations to lessen the fraction of evaporative loss. Planted populations for the 100%, 75%, and 50% treatments were 83,000, 78,000, and 63,000 plants ha-1, respectively. Plants were sown on 0.762-m spacing in flat, unbedded soil. Corn was managed for high yield potential using practices common to the Texas Panhandle, including nitrogen and phosphorus fertilizer applications based on commercial soil testing.
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| Figure 1. Location and plot layout of the Emeny Center Pivot Research Field near Bushland, Texas. Neutron moisture meter (NMM) access tubes and sensor evaluation sites were located in the outer half of the sixth sprinkler span (full season maize), having two in each of the 50%, 75%, and 100% ET irrigation treatments, denoted by the yellow stars. |
Soil Water Sensors
Neutron Moisture Meter
The neutron moisture meter (NMM) [model 503DR Hydroprobe, CPN International (formerly known as Campbell Pacific Nuclear), Instrotek, Martinez, Calif.] had a probe diameter of 38.1 mm (1.5 in.) and was used with a depth control stand (Evett et al., 2003, 2008, 2022). The NMM was calibrated to accuracy of 0.01 m3 m-3 or better in the Pullman soil using multiple soil cores of 60 cm3 volume taken within the NMM sensing volume at every depth of sensing in three access tubes in a dry soil and three access tubes in a soil at field capacity as described by Evett et al. (2022). Three soil horizon-specific equations were developed for the NMM including one for a singular depth of 10 cm, one for depths ranging from 30 to 130 cm, and one for depths ranging from 150 to 230 cm, which was a soil horizon dominated by larger concentration of calcium carbonate. Evett et al. (2003) determined that depths of less than 30 cm require specific equations to account for neutron escape to the atmosphere. Results of that calibration study revealed that taking volumetric soil samples outside of the volume sensed can result in water content values that are not representative of the water content within the sensed volume. Spatial variability studies of water content have shown that variance does not go to zero at small distances, the so-called nugget effect (Gajem et al., 1981; Ceddia et al., 2009; Li et al., 2019). Further exacerbating the problems of soil sampling are the facts that small-scale water content and bulk density variabilities tend to increase as soil dries Hawley et al., 1982; (Famiglietti et al., 1999; Hupet and Vanclooster, 2002; Schmitz and Sourell, 2000) and may also be dependent on sampling depth (Hawley et al., 1982).
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| Figure 2. Seasonal soil profile water contents for 100%, 75%, and 50% irrigation treatments for maize grown at the Emeny Research Field with differential irrigations beginning after the V8 growth stage. Water contents were determined with the neutron moisture meter. Also shown are irrigation and precipitation events. |
Campbell Scientific SoilVUE10
The SoilVUE10 is marketed as a time domain reflectometer (TDR) soil water content profile sensor with individual sensors integrated into a single sensor body housing. Such sensors are commonly referred to as “downhole” sensors as soil water sensing at multiple depths is be achieved by installation of a singular sensor body into a vertical bore hole. To our knowledge, the SoilVue10 is the first commercially available TDR downhole sensor and is unique in that the wave guides are helical and formed into the outside of the threaded polymer sensor body (fig. 3). The product brochure states that threaded body configuration is designed to maximize soil contact while minimizing preferential water flow along the sensor body. The SoilVUE10 is available in two probe lengths: the 0.5- and 1.0-m version, both having six fixed measurement depths of 5, 10, 20, 30, 40, 50 mm, with the latter having three additional depths of 60, 75, and 100 cm. The 1.0 m version was used in this study. Sensor outputs for each depth include volumetric water content (VWC) (advertised as ±1.5%, for most soils), relative permittivity of the soil (advertised as ±1 unit, for values between 4 and 42), bulk electrical conductivity [advertised as ±2%, (0 to 2.5 dS/m), ±5%, (full range)], and temperature (advertised as ±0.15°C, between -30°C and 40°C). Data for all wave guide depths are transmitted to a datalogger via a single detachable coaxial cable using the serial data interface 1200 baud (SDI-12) protocol.
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| Figure 3. Illustration of the 0.5-m SoilVUE10 downhole TDR soil water sensor. Helical wave guides are formed into the threaded sensor body. The 1.0 m version was used in this study, having the six nominal measurement depths shown here with additional depths of 60, 75, and 100 cm. Depth values shown are 2.5 cm more than the vertical depth of the center of the volume sensed by the helical waveguides. |
Acclima TRU-TDR™ 315L and 315H
The TDR-315L and -315H (Acclima Inc., Meridian, Idaho) soil water sensors are TDR sensors and essentially interchangeable for VWC measurements. Advertised advantages of the more recent 315H at the time of publication include decreased power consumption and waveform capture technology using the Acclima SDI-12 sensor reader. Both models share the same form factor, featuring a low-profile sensor head and a three-element stainless steel waveguide (fig. 4). Overall sensor dimensions are relatively compact, measuring 21.2 cm (L) overall × 5.3 cm (W) × 1.5 cm (H) with exposed electrode length of ~15 cm, making it ideal for horizontal installation at shallow depths. The sensors can be installed in any orientation but are typically installed horizontally at shallow depths and vertically at greater depths using a vertical bore hole. An electrode spacer is used to ensure the 15-cm waveguide rods remain parallel during insertion. Both 315L and 315H sensors were used in this study. Sensor outputs include percent VWC [advertised as ±1%, (course and medium textured soils), ±2.5% (fine textured soils)], relative permittivity [±1%, (course and medium textured soils), ±2% (fine textured soils)], bulk electrical conductivity [advertised as ±2.5%, (1000 to 2000 µS/cm), ±5%, (2000 to 5000 µS/cm)], and temperature (advertised as ±0.25°C, between -40°C and 60°C). The Acclima TDR-315 sensors are equipped with a data cable that connects to a datalogger using the SDI-12 protocol.
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| Figure 4. Illustration and dimensions of the Acclima TDR-315L (a) and TDR-315H (b) soil water sensors. Both models feature the same dimensions and functionality for VWC measurements while the 315H features lower power requirements and waveform capture technology. | ||||
Sensor Evaluation Sites
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| Figure 5. Overhead field schematic illustrating the relative location and orientation of each of the two soil water sensor evaluation sites replicated in each of the three irrigation treatment plots. SoilVUE10 sensors were installed ~30 cm from the NMM access tubes while the vertical array of TDR 315 sensors was installed at the midpoint, ~15 cm from both the access tube and the SoilVUE10. |
Two sensor evaluation sites were established in each of the three irrigation treatment sectors for a total of six sites (fig. 1), with each pair separated by approximately 18 m within a sector. Each site consisted of a dedicated NMM access tube, a SoilVUE10, and an array of TDR-315 sensors installed such that all measurements would be of the same approximate soil volume and at the same center depth as for the SoilVUE10. The access tubes were installed between two adjacent crop rows (interrow), with ~0.15 m of the tubes exposed above the soil surface and capped with a rubber stopper. The access tubes were installed in undisturbed soil and bracketed two additional access tubes associated with a separate study. As such, the access tube sites for the 50%, 75%, and 100% irrigation treatments were numbered 1 and 4, 5, and 8, and 9 and 12, respectively. The additional access tubes were several meters away from the sensor evaluation sites. A SoilVUE10 sensor was installed adjacent to each access tube, in the same interrow, positioned approximately 0.3 m from the tube. A near-vertical array of nine TDR-315 sensors were installed at the midpoint between the access tube and the SoilVUE10 at depths equal to those of the SoilVUE10 center depths (fig. 5). The five uppermost sensors were installed horizontally at depths equal to the midpoints of the corresponding SoilVUE10 helical electrodes (2.5, 7.5, 17.5, 27.5, 37.5 cm) while the lower four were installed vertically, at depths where the midpoint of the TDR-315 electrodes equaled that of the midpoints of the SoilVUE10 electrodes (47.5, 57.5, 72.5, 97.5 cm) (fig. 6). A relatively shallow pit was excavated and sensors were installed horizontally to capture the large changes and difference in water content that occur near the surface from precipitation and irrigation. We installed the deeper sensors in auger holes to avoid excavating a deep pit and the consequent severe disturbance of the deeper soil profile. The vertically installed TDR-315 sensors were offset horizontally slightly from one another so as to avoid the sensor heads from interfering with the electrodes of the sensors immediately above. Installation of the TDR-315 sensors used tools illustrated in Caldwell et al. (2022). Installation of the SoilVUE10 sensors was performed using the manufacturer’s recommended installation kit. A bore hole was created using a 5.1 cm diameter Edelman hand auger equipped with a smooth auger extension shaft. The sensor body was then misted with water and threaded to grade using the hex head tool. The sensors at each site were connected to a CR1000 datalogger (Campbell Scientific, Logan, Utah) and cables were routed underground to prevent damage to wires by rodents.
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| Figure 6. Illustration of sensor depths and orientation at each of the six evaluation sites. The TDR-315 sensors were installed so that measurement depths would equal the midpoints of the helical electrodes of the SoilVUE10. The four vertically installed TDR-315 sensors were offset to avoid head to electrode interference of adjacent sensors. The offset is exaggerated here for clarity purposes. All measurements in cm. |
Soil Water Measurements
SoilVUE10 and TDR-315 Sensors
Data from the SoilVUE10 and TDR-315 sensors were received every 3 min and used to compute 15-min average data files for the 2020 summer growing season. Data were compiled and processed using Microsoft Excel spreadsheets for statistical and graphical analysis of data. Soil profile water content values (mm depth of water in the profile) were calculated by integrating VWC values across a one-dimensional profile depth totaling 110 cm, determined by assuming that the 97.5-cm measurement was appropriate for a depth range of 85 to 110 cm. Soil profile water content values derived from both the TDR-315 and SoilVUE10 sensors were compared to those derived from NMM measurements using the 15-min average VWC values that most closely approximated the time of NMM readings. Datalogger issues resulted in missing soil water sensor values for the final two readings of the season on DOY 237 and 241 for the 75% (tubes 5 and 8) and 100% (tubes 9 and 12) treatments. As such, soil profile water content values for all sensors were compared on DOY 234 for final comparisons. Comparisons of VWC reported by the TDR-315 and SoilVUE10 sensors were also performed at every sensor depth for each of the six evaluation sites. Such depth-wise comparisons of NMM VWC values with those of the two TDR sensors was avoided due to the very large sensed volume difference between the NMM and TDR sensors.
Neutron Moisture Meter
Soil profile water was monitored semiweekly, weather permitting, using a field calibrated NMM and a depth control stand (Evett et al., 2003, 2022) by averaging data from two access tubes located within each of the 0.68-ha treatment plots, located in the outer half of span six in each of the sectors (fig. 1).
Sensing with the NMM in each of the six access tubes was performed periodically throughout the growing season and typically occurred on days immediately prior to and following irrigation events, when possible. Sensing occurred on 23 days spanning DOY 150 to 241 using a NMM equipped with a probe data cable having depth stops positioned so as to center measurements at the midpoints of the measurement depth of the SoilVUE10. Calibration equations derived from nearby soils by Evett et al. (2022) were used to calculate VWC values. Of the three most shallow measurement depths (2.5, 7.5, and 17.5 cm), only the 7.5-cm measurement was relatively close to the 10-cm calibration equation. As such, measurements for the 2.5- and 17.5-cm depths were discarded while measurements taken at the 7.5-cm depth were adjusted for use with the 10-cm calibration. This was accomplished using a prorated correction based on the ratio of the initial VWC calculated using the 10-cm equation to the difference in water content between air dry (0.05 m3 m-3) and saturated (~0.42 m3 m-3) soil. Resulting VWC values were considered to integrate water content from the surface to 17.5-cm depth. Values for VWC at the 27-cm and deeper depths were calculated using the calibration equation for the depth range of 30 to 130 cm.
Results
Soil Profile Water Content
Sensor specific profile water content values resulted from integration of the TDR-315, SoilVUE10, and NMM data over the depth range of 0 to 1.10 m. The profile water content values for each sensor and from each of the six evaluation sites were plotted throughout the season. Figures 7 and 8 illustrate both the individual profile and average of two profile water contents of the 1.1 m profile computed from the TDR-315, SoilVUE10, and NMM data. The profile water values derived from NMM measurements from the two access tubes in each treatment plot agreed well with one another. In general, values derived from the TDR-315 sensors trended well with those from the NMM while data from the SoilVUE10 deviated considerably from NMM data. Results from each treatment plot are discussed in detail in the following.
50% Irrigation Treatment
Profile water content values derived from the NMM agreed well with one another throughout the season having an average absolute difference of 7.0 mm and differences ranging from 0.4 to 20.5 mm. Greater variation in values between the TDR-315 sensors was observed with a corresponding average absolute difference value of 28.1 mm and differences ranging from 10.3 to 63.9 mm. Variation in values for the SoilVUE10 were even greater having an average absolute difference value of 56.9 mm and differences ranging from 38.5 to 84.9 mm. Overall, the average soil profile
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| Figure 7. Soil profile water values derived from TDR-315, SoilVUE10, and NMM data for the (a) 50%, (b) 75%, and (c) 100% irrigation treatments for a 1.1 m profile. Plots include individual and average profile water totals for both evaluation sites located in each irrigation treatment sector plot. | |||
water content from the TDR-315 sensors trended fairly well with those from the NMM. The mean bias difference (MBD) between TDR-315 and NMM profile water content data was 8.4 mm, with minimum and maximum difference values of -4.8 and 22.0 mm, respectively, and with a root mean square difference (RMSD) of 10.6 mm. The additional 15 mm estimated by DOY 234 for TDR-315 sensors represented a 6.2% overestimation of soil water as compared with the NMM average value. In contrast, average profile water content values from the SoilVUE10 were markedly less than those of the NMM and divergence increased throughout the season (fig. 7a). The MBD of SoilVUE10 from the NMM values was -89.8 mm and ranged from -126.9 to -44.7 mm having an RMSD of 94.4 mm. The difference of -124.0 mm on DOY 234 represented a 51.0% underestimation of soil water as compared with the NMM. Regression analysis showed good agreement between soil water profile estimates from the TDR-315 sensors and the NMM for values ranging from slightly greater than field capacity (FC) to those approaching permanent wilting point (PWP) while SoilVUE10 data demonstrated chronic underestimation with potentially important consequences for irrigation management (fig. 8a).
75% Irrigation Treatment
Similar overall trends in soil profile water content were for observed for the 75% irrigation treatment with the TDR-315 values trending well with the NMM while those for the SoilVUE10 were again markedly smaller, albeit to a lesser degree (fig. 7b). The difference between the two NMM profile water content values was greater than that observed for the 50% treatment with an average of 11.0 mm and a range of 0.1 to 33.6 mm. Variation between the TDR-315 sensors was less than that of the 50% treatment having an average difference of 24.9 mm and differences ranging from 3.1 to 47.8 mm. The MBD between profile water content from the TDR-315 sensors and the NMM was -9.8 mm and ranged from -26.1 to 14.7 mm with an RMSD of 14.3 mm. The difference of -6.6 mm on DOY 234 represented a 2.6% underestimation as compared with the NMM average values. Variation between the soil profile water contents for the SoilVUE10 sensors was considerably less than that observed for the 50% treatment, but still relatively large with an average difference of 19.9 mm and a range of differences from 0.7 to 39.5 mm. Average profile values were again consistently less than those of the NMM and diverged more as the season progressed. The average difference from the NMM profile water content was -70.5 mm with a range of -117.2 to -35.6 mm with an RMSD of 73.9 mm. The greatest difference of -117.2 mm was observed on DOY 234, representing a 15.3% underestimation of soil water as compared with the NMM average value. Regression analysis was similar to that of the 50% treatment although the TDR-315 sensors underestimated soil profile water slightly early in the season for values approaching FC (fig. 8b).
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| Figure 8. Regression plots for TDR-315 and SoilVUE10 profile water content (over 1.1 m depth) values compared with those from NMM measurements for the (a) 50% (n=138), (b) 75% (n=126), and (c) 100% (n=126) irrigation treatments. Sensor data points represent the average profile water value of two sites in each treatment, each integrated using nine VWC values. NMM profile water values were calculated using VWC values from 5 depths. | |
100% Irrigation Treatment
Profile water content values for the TDR-315 sensors trended well with those from the NMM and while values from the SoilVUE10 sensors were considerably less, seasonal divergence was less pronounced as compared to the 50% and 75% treatments, mostly because the range of water contents in this treatment was smaller than in the 50% and 75% treatments (fig. 7c). The average difference between NMM profiles was similar to that observed in the 75% treatment at 11.8 mm and with a range of 0.1 to 27 mm. Differences between the TDR-315 profile values were smallest for the 100% treatment having a average of 15.4 mm and a range of 0.6 to 27.2 mm. The MBD between the average TDR-315 and NMM profiles was -3.1 mm and ranged from -15.4 to 13.9 mm with an RMSD of 8.0 mm. A difference of -0.1 mm on DOY 234 represented a negligible difference between the TDR-315 and NMM profile values. Interestingly, differences between the SoilVUE10 profile values were greater than those of the 75% treatment having an average of 30.6 mm and a range of 0.9 to 53.3 mm. Differences between average SoilVUE10 and NMM profile values was smallest however for the 100% treatment with an average of -54.4 mm and a range of -100.0 to 28.0 mm with an RMSD of 58.3 mm. The difference of -82.7 mm on DOY 234 represented a 24.8% underestimation of soil water as compared with the NMM average values. Regression analysis was similar to that for the 75% treatment with the TDR-315 sensors slightly underestimating values near FC and the SoilVUE10 demonstrating chronic underestimation as compared with NMM values (fig. 8c).
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| Figure 9. Graphical comparisons of VWC values for TDR-315 and SoilVUE10 sensors at each of the nine measurement depths for the 50%, 75%, and 100% irrigation treatments. |
Comparison of TDR-315 and SoilVUE10 Measurements
The 15-min average VWC values used to calculate soil profile water content values for the TDR-315 and SoilVUE10 sensors were compared for each of the nine nominal measurement depths for each evaluation site. As expected from plots of the profile water contents, the vast majority of VWC values for the SoilVUE10 sensors were less than those of the corresponding TDR-315 sensors (fig. 9). This was particularly evident at the 5-cm depth (fig. 9a) where the SoilVUE10 sensors reported several zero or near zero (0.001) values. Values of zero and 0.001 were recorded for sites 1 and 4 (50% irrigation), respectively on the morning of DOY 185 following an extended drying period immediately preceding irrigation later that day. Identical values were recorded on DOY 220 while zero values for both sites 1 and 4 were recorded on DOY 227, 230, and 241. Sensing on DOY 220, 227, and 241 occurred following a period of drying and immediately prior to an irrigation event. Sensing on DOY 230 occurred two days after an irrigation event on DOY 228. A similar pattern was observed for the 75% irrigation with zero values recorded on DOY 185, 204, 213, 220, 227, 230, and 234 for site 5 and DOY 234 for site 8. Several relatively small values (0.006-0.019) for VWC were also recorded for site 5 at the 5 cm depth (fig. 9a). Values of 0.001 were recorded for site 8 on DOY 185 and 227. Zero values were also recorded on DOY 185 for sites 9 and 12 (100% irrigation).
Measurements of VWC by the SoilVUE10 were greater than those of the TDR315 sensors in some instances and typically occurred at measured water contents at or greater than FC (~0.33 m3 m-3) at shallow depths. Such values were observed for sites 9 and 12 from the 100% irrigation treatment at the 5, 10, and 20 cm depths (figs. 9a, 9b, 9c). Several VWC values for site 4 (50% irrigation) at the 10 cm depth were greater for the SoilVUE10 (fig. 9b). Most notably, several SoilVUE10 values from sites 5 and 8 (75% irrigation) were consistently greater than those of the TDR-315 at the 20 to 60 cm depths and the 100 cm depths (figs. 9c-9g, 9i). Inspection of the slope of regression lines for all sites at all depths suggested that differences between SoilVUE10 and TDR-315 VWC values were greatest for smaller water contents and decreased for greater water content conditions (table 2). Coefficient of determination values in table 2 also indicated a large range of variability across all sites and depths.
| Table 2. Linear regression equations and coefficient of determination (R2) values for TDR-315 and SoilVUE10 VWC measurements by depth and irrigation treatment. | |||||||||||
| Depth (cm) | 50% Irrigation | 75% Irrigation | 100% Irrigation | ||||||||
| Site 1 | Site 4 | Site 5 | Site 8 | Site 9 | Site 12 | ||||||
| 5 | y = 1.0191x - 0.122R2 = 0.8285 | y = 0.8448x - 0.0927R2 = 0.8127 | y = -0.0198x + 0.0513R2 = 0.0006 | y = 1.5744x - 0.1738R2 = 0.8385 | y = 1.7005x - 0.241R2 = 0.8789 | y = 1.9471x - 0.3372R2 = 0.9165 | |||||
| 10 | y = 0.9304x - 0.1277R2 = 0.8197 | y = 2.0995x - 0.2192R2 = 0.8523 | y = 0.7008x - 0.0723R2 = 0.7022 | y = 1.2939x - 0.1253R2 = 0.7 | y = 1.9507x - 0.3334R2 = 0.8076 | y = 3.406x - 0.8642R2 = 0.9058 | |||||
| 20 | y = 1.7463x - 0.2724R2 = 0.9291 | y = 1.9053x - 0.3629R2 = 0.8349 | y = 0.6324x + 0.003R2 = 0.1634 | y = 2.3355x - 0.4628R2 = 0.7767 | y = 2.1652x - 0.4605R2 = 0.6107 | y = 2.5789x - 0.6143R2 = 0.8058 | |||||
| 30 | y = 2.4747x - 0.6149R2 = 0.9575 | y = 2.3351x - 0.5096R2 = 0.9357 | y = 0.8575x - 0.1166R2 = 0.4292 | y = 3.015x - 0.7124R2 = 0.7963 | y = 2.7509x - 0.7395R2 = 0.8375 | y = 3.1822x - 0.8527R2 = 0.9307 | |||||
| 40 | y = 2.1328x - 0.5181R2 = 0.9019 | y = 2.3636x - 0.5515R2 = 0.9379 | y = 1.9357x - 0.4389R2 = 0.7746 | y = 2.5334x - 0.5643R2 = 0.7288 | y = 2.5035x - 0.627R2 = 0.9372 | y = 2.7534x - 0.6854R2 = 0.858 | |||||
| 50 | y = 1.6723x - 0.4171R2 = 0.8952 | y = 1.7056x - 0.2961R2 = 0.9648 | y = 1.7101x - 0.1915R2 = 0.8166 | y = 2.6655x - 0.5992R2 = 0.5582 | y = 1.5175x - 0.1989R2 = 0.8798 | y = 0.9512x + 0.0097R2 = 0.6303 | |||||
| 60 | y = 2.0709x - 0.4504R2 = 0.8292 | y = 1.1354x - 0.1665R2 = 0.9303 | y = 1.4706x - 0.0944R2 = 0.9027 | y = 1.8842x - 0.4119R2 = 0.5484 | y = 1.0142x - 0.0005R2 = 0.877 | y = 1.5799x - 0.2917R2 = 0.2825 | |||||
| 75 | y = 1.7291x - 0.3349R2 = 0.9238 | y = 0.9223x - 0.0773R2 = 0.8737 | y = 1.424x - 0.2496R2 = 0.9438 | y = 1.5749x - 0.3116R2 = 0.5534 | y = 1.1177x - 0.1603R2 = 0.5945 | y = 1.6221x - 0.2987R2 = 0.1111 | |||||
| 100 | y = 0.9361x - 0.0565R2 = 0.6119 | y = 1.5469x - 0.2847R2 = 0.9324 | y = 1.0991x + 0.0097R2 = 0.986 | y = 1.2689x - 0.0246R2 = 0.8556 | y = 2.6599x - 0.631R2 = 0.8095 | y = 3.4743x - 0.9608R2 = 0.3583 | |||||
Discussion
The threaded sensor body of the SoilVUE10 was designed to facilitate good electrode contact with the soil and provide the benefits of a singular downhole sensor body. While the threaded installation method avoids the use of a slurry that can bias measurements due to the relatively small sensing volume of EM sensors, there are intrinsic deficiencies associated with the SoilVUE10 design. The nominal diameter of the bore hole is designed to maximize sensor to soil contact while allowing for ease of installation (using no more than 54 Nm of torque). This is analogous to thread depth percentage values for different drill sizes in machine tapping operations. In essence, a bore hole too large in diameter can result in excessive gapping while too small can preclude installation to depth and damage the sensor. The SoilVUE10 installation auger diameter is likely a compromise for use in multiple soil textures and conditions. However, even largely homogenous soils such as the Pullman can have differences in soil peds with depth due to clay and soil water content gradients that contribute to lack of electrode contact. Furthermore, natural or other voids created by burrowing animals can result in rather large soil voids. As such, it is nearly impossible to have 100% electrode contact for any soil water sensor. But the circumstances are particularly different for the threaded downhole sensor design of the SoilVUE compared with sensors designed to have their electrodes inserted into the soil such as the TDR315. The installation method itself can result in minor deviations in the bore hole diameter as the action of removing and inserting the auger can cause sluffing off the sidewalls resulting in small voids that result in poor electrode contact. Such voids may also allow for cascading effects where soil water may flow preferentially through voids along the sensor body threading and saturate the soil in an area below having good electrode to soil contact.
The incidence of zero and near zero values at shallow depths were likely due to poor contact between the SoilVUE10 electrodes and the soil. The pattern of periodic reporting of zero values that coincided with irrigation and/or precipitation events followed by drying periods could be due to shrink-swell action of the clay soil. However, the TDR-315 sensors at shallow depths reported no zero values during periods of drying. This suggests that large soil voids may have been formed near the surface due to increased soil disturbance caused by the entire length of the SoilVUE10 passing through the soil grooves during installation, resulting in poor soil to electrode contact. Although the zero and very small values were not representative of the bulk VWC at shallow depths, they did not fully account for the small soil profile water values due to their relatively small percentage of the integrated depth. Wilson et al. (2023) reported systematically smaller water content values from the SoilVUE10 than from both TDR-315 sensors and gravimetric samples at shallow depths. Values for VWC at greater depths and water contents were consistently smaller than those determined by the Acclima TDR-315 sensors and the field-calibrated NMM. This systematic underestimation of VWC may be due to poor sensor-soil contact or to one or more fundamental issues associated with TDR sensing of soil water content. Sensors that include some dielectric material backing the electrodes sense permittivity that is a combination of the soil permittivity and the permittivity of the sensor body, making it difficult to translate such values into a soil water content (Casanova et al., 2012). Conventional TDR sensors use straight electrodes, and although it is unclear how the helical shape of those of the SoilVUE10 affected TDR waveforms and their analysis, curved TDR electrodes have been used in other studies with no untoward effects when completely surrounded by soil.
Conclusions
Soil profile water content values derived from Campbell Scientific SoilVUE10 and Acclima TDR-315 TDR sensors installed in a Pullman clay loam soil near Bushland, Texas, were compared with those from NMM measurement under full and limited irrigation treatments. Overall, profile water content values from the TDR-315 sensors trended well with NMM values resulting in marginal underestimation for the 75% and 100% irrigation treatments having MBD values of -9.8 and -3.1 mm, respectively. Values for the 50% treatment resulted in slight overestimation of profile water with a MBD of 8.4 mm. Corresponding values for the SoilVUE10 sensors grossly underestimated profile water content compared with data from the NMM for all irrigation treatments with end of season deficits being large (at least -80 mm) in all cases but greatest for the 50% treatment (-124.0 mm). Values of MBD for profile water content values from the SoilVUE10 sensors the NMM were -54.4, -70.5, and -89.8 mm for the 100%, 75%, and 50% irrigation treatments, respectively. Comparisons of VWC at each of the nine TDR sensor depths revealed that SoilVUE10 values were generally less than TDR-315 values for all irrigation treatments. Underestimation at the near surface (5 and 10 cm depths) was easily attributed to loss of soil to electrode contact possibly associated with clay shrinkage during periodic drying events following irrigation. Although soil to electrode contact can be problematic at greater depths, the explanation for chronic underestimation of VWC is less obvious. Likely reasons include not only poor electrode-soil contact, but also challenges for accurate determination of soil permittivity from a measured permittivity that includes a plastic sensor body. Results from this study suggest vertically distributed arrays of TDR-315 sensors can provide profile water content values adequate for monitoring soil water status for irrigation scheduling in a clay loam soil. This conclusion is supported by work of several others who have used vertical arrays of TDR-315 sensors for irrigation scheduling in soils of textures ranging from sandy to clayey and clay types ranging from smectitic to kaolinitic (Evett et al., 2020a; O’Shaughnessy et al., 2020; Stone et al., 2020; Vories et al., 2020). The chronic underestimation observed for the SoilVUE10 sensors does not support its use for irrigation scheduling in a clay loam soil and could lead to over irrigation. The large variation in sensor response compared to the TDR-315 sensors was likely caused by differences in electrode contact due to voids created during installation. This essentially precludes the use of calibrations to correct the sensor output. The relatively short 1 m length is less than rooting depth of many regional crops (e.g., alfalfa, cotton, corn, sorghum, sunflower) and is not long enough to determine percolation below the root zone.
Acknowledgments
This research was supported in part by the Ogallala Aquifer Program, a consortium between USDA-Agricultural Research Service, Kansas State University, Texas A&M AgriLife Research, Texas A&M AgriLife Extension Service, Texas Tech University, and West Texas A&M University. Supplemental funding was provided via a Research and Demonstration Grant awarded by the High Plains Underground Water Conservation District No. 1., Lubbock, Texas.
Disclaimer
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