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a USDA-ARS, Conservation and Production Research Lab., P.O. Drawer 10, Bushland, TX 79012-0010 USA
b Texas Agricultural Experiment Station, Route 3, Box 219, Lubbock, TX 79401-9757 USA
lbaumhar{at}ag.gov
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONNOTESMaterials and methodsResults and discussionSummary and conclusionsREFERENCES |
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v, m3 m-3) measurements. Their v measurements are more convenient than gravimetric, and don't carry the nuclear regulatory burdens. Previous studies noted potential salinity and temperature effects on MCAP v determinations. Our objectives were to calibrate and verify MCAP v measurement accuracy in two soil materials, two water salinities (1.3 and 11.3 dS m-1), and with diurnal temperature fluctuations. The surface and calcic horizons of an Olton soil (fine, mixed, superactive, thermic Aridic Paleustoll) were packed into triplicate, 0.5-m-tall, 100-L columns and wetted. We compared v determined by volumetric measurements, time domain reflectometry (TDR), and MCAPs. The TDR v were within ±0.01 m3 m-3 of volumetric determinations for air-dry and saturated soil. The factory supplied universal MCAP calibration provided accurate v estimates for air dry (±0.01 m3 m-3) surface and calcic soil materials but not after wetting (
-0.05 m3 m-3). Also, imprecise MCAP sensor positioning during water frequency parameter determination was problematic and biased initial v measurements. After calibration against TDR, the MCAP v varied ±0.01 m3 m-3 from measured v for air-dry and saturated conditions for both soil materials, which were then pooled to obtain one calibration. Column resaturation with saline water affected permittivity and elevated MCAP v 0.25 m3 m-3 above the available pore space. Cyclical soil temperature fluctuations of 15°C induced similar fluctuations in indicated v throughout the column (0.04 m3 m-3 for MCAP and 0.02 m3 m-3 for TDR), which was attributed to variations in permittivity.
Abbreviations: DOY, day of year • EC, electrical conductivity • MCAP, multisensor capacitance probe • TDR, time domain reflectometry • Fs, Fa, and Fw, sensor frequency in soil, air, and water • RMSE, root mean square error • SD, standard deviation • SF, scaled sensor frequency • v, volumetric water content
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONNOTESMaterials and methodsResults and discussionSummary and conclusionsREFERENCES |
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A multisensor capacitance probe (MCAP) marketed as EnviroSCAN (Sentek Pty. Ltd., South Australia; SENTEK, 1995) determines soil water content by measuring the frequency change induced by the changing permittivity of the soil permeated by the fringing fields of the capacitor sensor.1
Paltineanu and Starr (1997) described the EnviroSCAN system components and function as part of their instrument calibration experiment. Briefly, the capacitance sensor output frequency varies proportionally with the soil permittivity and increasing soil water. The sensor frequency in soil, Fs, is scaled, SF, using the equation:
 | (1) |
v) can be calculated using the factory supplied calibration equation
 | (2) |
(SENTEK, 1995) or a soil specific calibration. Paltineanu and Starr (1997) developed a power function relationship between SF and v. They concluded that the shapes of the soil water–SF relationships developed from independent data sets were "essentially the same", and that soil temperature effects on sensor-estimated v were minimal. These results support a universal calibration independent of soil temperature. In contrast, significant salinity and temperature effects on soil water measurements have been reported while using this equipment (Mead et al., 1995, 1996).
Further tests are needed to establish whether soil water content can be estimated accurately with a universal calibration equation supplied either by the factory (SENTEK, 1995) or reported in the literature. Furthermore, soil temperature and salinity affects on MCAP measured soil water content need to be clearly established and characterized or eliminated from consideration. Therefore, the objectives of our study were (i) to test the accuracy of MCAP soil water measurements using the factory calibration and other published calibrations, (ii) to develop specific laboratory calibrations for two soil materials, and (iii) to characterize soil temperature and water salinity effects on MCAP and TDR soil water measurements under laboratory conditions.
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Materials and methods |
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TOPABSTRACTINTRODUCTIONNOTESMaterials and methodsResults and discussionSummary and conclusionsREFERENCES |
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Soil Column Preparation
Soil column containers were made from 1.02 m tall by 0.5 m diameter 120-L plastic barrels by removing the tapered upper 0.26 m and installing a 12-mm-o.d. hose barb in the bottom for water addition. By wetting the soil columns from the bottom, air entrapment and uneven water distribution were avoided. Washed and air-dried coarse gravel with a mean diameter >12 mm was placed into the tapered bottom to a depth of 0.26 m at the same time that MCAP access tubes were positioned near the soil column center. The gravel was covered by a 0.5-m-diam. nylon window screen disk and cheesecloth to reduce soil sifting into and mixing with gravel during soil column preparation. Plastic barrels, nylon window screen, and cheesecloth materials were used to avoid potential interference with electrical soil water content sensors. The barrel above the gravel was a nearly cylindrical column 0.5 m tall and 0.5 m in diameter (Fig. 1)
with a volume of 93 L (92.75 L as measured using water). All six soil columns were packed in 0.10-m increments above the gravel to an overall average density of 1.40 Mg m-3 with a standard deviation (SD) of ±0.02 Mg m-3 for the three surface soil columns and 1.39 ± 0.04 Mg m-3 for the three calcic soil columns.
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v was installed during soil column preparation. The MCAP access tubes were positioned near the center of the soil column, while TDR sensors were installed equidistant from the soil column wall and MCAP access tube (Fig. 1, TOP). Sensors of the MCAP array (EnviroScan) were positioned at depths of 0.1, 0.2, 0.3, and 0.4 m; and 0.2-m trifilar TDR probes (TR-100, Dynamax, Houston, TX) were placed horizontally at depths of 0.05, 0.15, 0.25, 0.35, and 0.45 m (Fig. 1). Vertical and horizontal separation between MCAP and TDR sensors was 5 cm to avoid potential interference. In each soil column, temperatures at 0.05-, 0.1-, and 0.3-m depths were measured on the side opposite of the TDR probes and 0.1 m from the soil column wall using copper-constantan thermocouples mounted on 1-cm-square 0.45-m-long wood stakes.
Measurements
Measurements included soil temperature, v, and the amount of water added to each soil column. Soil temperature was measured every 15 s with sealed thermocouples made from twisted and soldered 22-ga Cu-Constantan wire (Omega Engineering, Stamford, CT) averaged and electronically recorded every 10 min using a CR-10 data logger (Campbell Scientific, Logan, UT). Using automated data collection systems, the soil water content was determined from Fs measurements recorded every 2 min during initial water additions for calibration data, and then every 10 min after wetting. The TDR probes were connected to a cable tester (model 1502C, Tektronix, Beaverton, OR) through multiplexers (model TR-200, Dynamax), which were controlled by a laptop computer running the TACQ software (Evett, 1998). The equation of Topp et al. (1980) was used to relate apparent permittivity to water content. Values for Fa and Fw in Eq. [1] were determined for the MCAP sensors according to the recommended factory procedures in air and in a water bath using local tap water (EC of 1.30 dS m-1). Water added to soil columns from the constant head Marriott system was measured to obtain a mean v for the soil column using an electronically recorded balance (Mettler PM34, Mettler-Toledo, Greifensee, Switzerland).
Experimental
Our experiment proceeded in four steps that included measurement of soil water content gravimetrically and by the TDR or MCAP systems with collateral measurement of soil temperature. First, the volumetric water content of the air-dry soil was determined from the gravimetric water content of the soils used for packing and the mean bulk densities of the packed soil columns. The soil columns were sealed with 0.0254-mm plastic bags, and measurements of soil temperature and air-dry water content using TDR and MCAP sensors were made during day of year (DOY) 89 to 125 in 1997. Second, for calibration, v with depth was measured by TDR and MCAP sensors from DOY 126 to 136 while adding water (EC of 1.30 dS m-1) from a constant head supply (0.5 m above the soil surface) through the bottom inlet of the column. Soil water content measured using TDR was regressed on the calculated scaled frequency, Eq. [1], using nonlinear fitting methods (SAS Institute, 1988). Third, the near-saturated v (MCAP and TDR) and soil temperature were measured for resealed soil columns (sealed surface and inlet), from DOY 136 to 160. Fourth, from DOY 161 to 168, v with depth was measured while adding saline water (EC of 11.30 dS m-1) from a constant head (2.5 cm above the soil surface) supply to the unsealed column surface. Effluent from the columns was measured with 1-L graduated cylinders and an aliquot taken for EC determination. The EC of the column effluent indicated the breakthrough of saline water in the column (EC elevated more than 2.0 dS m-1 above the initial value), at which time infiltration was terminated and drainage allowed. After the completion of the experiment (DOY 170), soil columns were destructively sampled to verify all sensor positions. Comparisons of different water measurement methods and water quality effects on indicated water contents were based on univariate and unpaired t-test statistics.
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Results and discussion |
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TOPABSTRACTINTRODUCTIONNOTESMaterials and methodsResults and discussionSummary and conclusionsREFERENCES |
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v, using the gravimetric water content and column bulk density data, averaged 0.02 m3 m-3 with a SD of ±0.01 m3 m-3 for the air-dry surface soil and 0.03 ± 0.01 m3 m-3 for calcic soil. The air-dry surface and calcic soil v measured using TDR and MCAP sensors, shown in Table 1
, agreed with the calculated values to within ±0.01 m3 m-3 or the level of precision for our test. Postsaturation v calculated from the added water volume was 0.40 ± 0.01 m3 m-3 for the surface soil and 0.39 ± 0.01 m3 m-3 for the calcic soil. The corresponding TDR measured v of 0.41 ± 0.02 m3 m-3 for the surface soil and 0.40 ± 0.01 m3 m-3 for the calcic soil were not statistically different from gravimetric measurements. The initial saturated calcic and surface soil v determined using the MCAP and factory calibration, however, were about 50% larger than the gravimetric measurements.
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v calculated from SF using Eq. [1] and measured Fs depends on accurate Fa and Fw sensor values and an appropriate calibration equation. The mean sensor Fa values of
were consistent with the factory-suggested typical 35000 to 37000 range (P. Buss, 1999, personal communication) and Fa values of
and
determined for two MCAP systems in 1995 by J.L. Starr (1999, personal communication). Measurements of Fw made in a factory-supplied portable water container averaged
, which exceeded the factory-suggested range of 23500 to 25500 for fresh and saline water and Starr's Fw values of
. Consequently, the MCAP returned unrealistically high SF values (>1.0) for near-saturated soil conditions. We, therefore, abandoned the suspicious Fw and repeated the Fw and Fa determination for tap water and air using a 0.30 m diam. by 1.70 m tall polyvinyl chloride plastic column that contained the entire MCAP and access tube unit. The resulting Fa of 35950 ± 290 remained practically unchanged, but the repeated Fw measurement averaged
, was consistent with the factory specified typical range, and produced SF values that never exceeded 1.0.
Imprecise sensor positioning while determining the Fw value within the portable containers may have permitted the sensors to integrate air effects resulting in an upward bias. When using the new mean Fa of 35950 and Fw of 23695 to obtain SF, the air-dry v calculated from the factory calibration for the soil and calcic horizons was not different from either volumetric or TDR measured soil water contents (Table 1). The MCAP-measured v was, however, less than either the volumetric or TDR-measured values for near-saturated conditions. Because the differences in measured v and the factory-calibrated MCAP varied as a fraction of the actual water content, the resulting MCAP indicated relative changes in v would not duplicate actual changes in soil water content. This illustrates the need to calibrate the MCAP instrument.
The MCAP was subsequently calibrated by regressing the indicated SF on the corresponding TDR-measured v values obtained while wetting air-dry surface and calcic soil materials to near saturation. The 5-cm vertical separation between TDR and MCAP sensors limited calibration comparisons to air-dry conditions and to water contents >0.20 m3 m-3. The v of the calcic and surface soil materials, shown in Fig. 2A
, were similar across the range of SF measured; therefore, regression analyses were performed for individual and pooled data sets. Soil temperatures corresponding with these v data averaged 29.6 ± 5.3°C.
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v as a function of SF by Paltineanu and Starr (1997) used an equation of the form
 | (3) |
v and SF measurements from the pooled surface and calcic soil materials resulted in values of
and
(Table 2) . While the coefficient of determination, r2, was 0.987, the resulting fit appeared undesirable (Fig. 2A) with a root mean square error (RMSE) of 0.147 m3 m-3. Because log transformation of data imparts multiplicative type error in a least squares regression that assumes additive error, the fit is biased in favor of points occurring near the extremes (Myers, 1986). As recommended by Myers (1986), we attempted a nonlinear fit of the untransformed data to Eq. [3]. The r2 was reduced from 0.987 to 0.967 (Table 2), but the RMSE was improved to 0.028 m3 m-3 or 20% of the RMSE obtained when regressing the ln-transformed data. This relationship did underestimate air-dry
when SF was 0.30 ± 0.05 because the equation asymptotically approaches
.
By including a value for a non-zero residual water content, c, in Eq. [3], the resulting equation
 | (4) |
v values that approach a residual water content value asymptotically at the low SF, air-dry, conditions. Both surface and calcic soil materials were individually fitted to Eq. [4], resulting in similar parameter estimates of 0.798 and 0.752 for a, 4.25 and 4.26 for b, and 0.020 and 0.017 for c, respectively (Table 2). Because of the similarity in the relationship between v and SF for these two different soil materials (Fig. 2A), the data were pooled and fitted using Eq. [4] to obtain parameter values of
. These nonlinear fits had r2 > 0.95 and RMSE < 0.032 m3 m-3 without biasing the additive error.
The v calculated using our calibration and the calibrations supplied by the factory or presented by Paltineanu and Starr (1997) are plotted in Fig. 2B. Our calibration of the MCAP system produced little difference in the calculated air-dry v compared with the factory calibration for the calcic and surface soil (see also, Table 1). Compared with the other calibration equations, however, our calibration significantly improved the accuracy of v estimated by the MCAP system in the 0.20 to 0.45 m3 m-3 range (Fig. 2B). Our results show that the MCAP instrument yielded soil specific v measurements and must be calibrated for individual soils. Paltineanu and Starr (1997) concluded that differences in soil mineralogy, especially 2:1 clays, could also affect MCAP instrument calibration. That is, the increased surface area of 2:1 clays affect the bound water and corresponding bulk permittivity (Bridge et al., 1996; Wraith and Or, 1999). The Olton soil used in this test has mixed mineralogy that includes the 2:1 clay, montmorillonite, as the dominant species (USDA-NRCS, 1999).
Temperature
The MCAP instrument was calibrated in a greenhouse, which had diurnal air temperature fluctuations as much as 20 to 25°C. Examples of the cyclic air and soil temperatures at depths of 0.1 and 0.3 m for DOY 100 to 106 in 1997 are shown (Fig. 3A)
. Maximum air temperature occurred around mid day, while peak soil temperatures occurred later; lagging more with increasing soil depth. Beginning on DOY 102, both maximum and minimum soil and air temperatures increased gradually, with greenhouse heaters maintaining a minimum 15°C air temperature after DOY 103.
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v in sealed columns varied cyclically, but less than one SD, with the diurnal soil temperature variations (Fig. 3B). The gradually increasing v corresponded with increasing soil temperature after DOY 102. Maximum indicated v at 0.1-m depth corresponded with the maximum temperature for that depth; however, at 0.3-m depth the maximum v lagged 4 to 6 h after the maximum temperature. Because the MCAP sensors centered in the soil column were further from the warming surface than the thermocouples, except at the 0.1-m depth, the MCAP measured water content lagged behind the soil temperature. The diurnal MCAP-measured v fluctuation for air-dry soil
but no corroborating TDR v data were available after a nearby lightning strike and electrical surge damaged (hard disk crash) the lap-top system used to monitor TDR equipment.
The diurnal air and soil temperature fluctuations, after wetting the columns to near saturation, are shown for a representative period, DOY 150 to 155, in Fig. 4A
. Air temperature ranged from a minimum of 15 to >35°C, with warmer minimum temperatures for the soil. As in the case of the air-dry soil, the lag between peak soil and air temperatures increased with increasing sensor depth. The corresponding MCAP-measured v for the sealed soil columns fluctuated 0.02 to 0.04 m3 m-3 diurnally in close synchronization with soil temperature (Fig. 4B) at both 0.1- and 0.3-m depths. We attributed this, primarily, to more rapid heat conductance under saturated conditions. Similarly, diurnal fluctuations in v of 0.01 to 0.02 m3 m-3 were indicated by TDR measurements, especially, after DOY 152 (Fig. 4C).
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v with increasing soil temperature. Using vapor transport, Parlange et al. (1998) calculated diurnal field v fluctuations; however, the calculated moisture content change was lowest at mid day due to water losses with warmer temperatures. Using gravimetric methods, Jackson (1973) also measured diurnally fluctuating surface v that peaked at sunrise and declined until mid afternoon because of evaporation. We consider water transport, whether as vapor or otherwise, within nearly saturated sealed soil columns along an increasing temperature gradient an unlikely explanation for the diurnal fluctuations in our v observations.
Recently, it was theoretically established that changes in temperature of the mineral soil fraction had little impact on soil permittivity (Yu et al., 1999). However, Wraith and Or (1999) demonstrated experimentally that temperature affects the dielectric properties of water bound near the surface of 2:1 clay minerals, thus changing soil permittivity. They calculated the effect of soil temperature changes on the apparent v as measured using permittivity detection methods (Or and Wraith, 1999). Diurnal v fluctuations indicated by both TDR and MCAP systems for our sealed soil columns were attributed, therefore, to temperature dependent fluctuations in the soil permittivity.
A basis for correcting temperature effects on MCAP sensor water content estimates could be implemented with a calibration relationship to concurrently measured soil temperature. However, an effective system for doing this is complicated by two factors. First, the temperature effect is probably soil (and soil horizon) specific, that is, greater with 2:1 clays and negligible in sandy soils, as was shown for TDR by Wraith and Or (1999). Second, integrating temperature measurements with each MCAP sensor must address temperature effects on soil permittivity and not the sensor. The MCAP uses a type of capacitance sensor that is unaffected by temperature changes (<0.005 m3 m-3 for a 30°C temperature variance, Dean et al., 1987). Therefore, an integrated temperature and capacitance sensor used within an access tube would have to detect temperature changes in the surrounding bulk soil that are synchronous and of equal magnitude to achieve dependable temperature correction.
Salinity
The effects of water salinity on MCAP- and TDR-measured v were determined by adding saline water (EC of 11.30 dS m-1) during a period of 10 h. The amount of saline water required to achieve saline water breakthrough in surface and calcic soil materials was 0.92 ± 0.05 and 0.86 ± 0.03 pore volumes, respectively, as indicated by increased EC of drainage water. About 0.70 pore volume of water was drained from the near-saturated column as saline water was added.
Soil water content is independent of solution electrolyte concentration; therefore, the gravimetrically calculated saturated water content remains unchanged from that listed in Table 1. When the wetting front of the saline water passed TDR wave-guides, these sensors failed to generate an interpretable waveform, resulting in no v measurement under the conditions of our test. In contrast, the MCAP system continued to measure v under these saline conditions. However, the MCAP indicated mean v for near-saturated conditions using the factory calibration increased 0.10 m3 m-3 more than the corresponding tap water (Table 1), thus illustrating MCAP sensitivity to soil salinity conditions. The mean v using our custom calibration was 0.20 m3 m-3 greater than the gravimetrically determined v (Table 1). Neither the factory nor the custom calibrations diminished MCAP sensitivity to saline conditions or improved the accuracy of the indicated saturated v.
Typical increases in v measured using the MCAP system with our custom calibration during the addition of saline water are illustrated in Fig. 5
. Also shown are practically simultaneous diurnal variations in measured water content with depth that are attributed to corresponding soil temperature variations. Before saline water was added, v at 0.3- and 0.4-m depths were 0.40 m3 m-3, nearly saturated, but the upper soil column was not saturated (0.1 and 0.2 m v of 0.32 and 0.35 m3 m-3). The addition of saline water on DOY 164 caused MCAP sensors to show an immediate increase in v. While some increase in v was expected because of column resaturation, the v increase measured by the MCAP to 0.60 m3 m-3 exceeded the available pore space (0.40 m3 m-3). We attribute this overestimation of soil water content to salinity effects on permittivity. A gradual increase in soil salinity due to irrigation, which is typical for many areas, may result in a subtle increase in the MCAP-indicated water content that does not occur and could be misleading.
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Summary and conclusions |
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TOPABSTRACTINTRODUCTIONNOTESMaterials and methodsResults and discussionSummary and conclusionsREFERENCES |
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v in controlled column experiments for the purpose of testing the MCAP factory calibration, developing a calibration specific for our soils, and characterizing instrument response to soil temperature and salinity conditions. Accuracy of the MCAP-estimated v depended on accurate Fw and Fa sensor values and an appropriate calibration equation. Imprecise sensor positioning within the factory-supplied portable water container allowed the integration of air effects on Fw and biased initial measurements. The factory-supplied universal MCAP calibration equation accurately estimated v from SF for both soil materials when air dry but not when nearly saturated, thus illustrating the need for a soil specific calibration. Without calibration, v measured using the TDR system was within ±0.01 m3 m-3 of volumetrically determined air-dry and saturated values for both soil materials. A custom calibration of the MCAP instrument obtained using a nonlinear fit of untransformed TDR-measured v regressed on the MCAP SF improved v determinations across the entire water content range.
The MCAP-indicated v was sensitive to both soil temperature and soil water salinity, which were attributed to related soil permittivity changes. For example, diurnal soil temperature fluctuations resulted in synchronized v fluctuations that increased in amplitude with increasing real soil water content. We conclude that variations in temperature must be considered when interpreting MCAP-determined v under variable soil temperature conditions. Concurrent soil temperature measurements during calibration and use of MCAP sensors would be a basis for recalculating the correct soil water content. The addition of saline water caused uninterpretable TDR wave forms and an overestimation of v by MCAPs. Because of MCAP system's sensitivity to soil salinity, we conclude that variations in soil salinity resulting from irrigation should also be considered when interpreting the MCAP v data. Soil specific calibration of MCAP v for various soil salinities could be a basis to recalculate corrected water content.1999
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ACKNOWLEDGMENTS |
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NOTES |
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TOPABSTRACTINTRODUCTIONNOTESMaterials and methodsResults and discussionSummary and conclusionsREFERENCES |
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Received for publication November 29, 1999.
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TOPABSTRACTINTRODUCTIONNOTESMaterials and methodsResults and discussionSummary and conclusionsREFERENCES |
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