Accuracy of soil moisture measurement by tdr technique (original) (raw)
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Journal of Soil Science, 1992
The paper is addressed to soil scientists who use Time-Domain Reflectometry (TDR) technology to measure soil moisture. The practical aspects of the measurement calibration are discussed, and an empirical approach to establishing the existence of a universal calibration function is presented. Samples of 11 mineral soil horizons and seven organic soil horizons with different chemical and physical properties (including magnetic properties) were selected with the aim of determining their dielectric constant-volumetric water content relationship as calibration functions for TDR soil moisture measurements. These samples were supplemented by other, soil-like, capillary-porous reference materials (montmorillonite, glass beads, washed sand and a sand from a C horizon). The study showed that a unique calibration function for mineral soils and another distinct calibration function for organic soils can be established.
Calibration of TDR probe for the estimation of moisture in typical red Ferrallitic soils
2018
ABSTRACT. The dielectric method to estimate soil moisture content is based in the existent closed relationship between dielectric permittivity and water content. The time-domain reflectometry (TDR) is an indirect technique to estimate the soil water content based on the mentioned relationship given by polynomials of different ranges inserted in the equipment. However, the absence of generalization of these models to transform the data of the sensors into readings of moisture content of different soils makes necessary the obtaining of the place-specific calibration curves. The objective of the present work consists on the calibration of a probe TDR 300 model 6430FS for its use in typical red Ferrallitic soils of Ciego de Ávila, Cuba.
Calibration of a TDR probe for low soil water content measurements
Sensors and Actuators A-physical, 2008
Time domain reflectometry (TDR) probes are increasingly used for field and laboratory estimation of soil water content. Usual calibration of TDR probes for the determination of soil water content uses two media: air for low and water for high values of dielectric permittivity, although the measured range of dielectric permittivity in soil is much smaller as compared to the range implied by the calibration media. The use of air for calibration of short TDR probes gives calibration errors due to overlapping incident and reflected pulses in the reflectogram, which result in their relative shift in time. This phenomenon, named the convolution effect, can be avoided by the application of selected calibration media. The presented approach minimizes of dielectric permittivity measurement errors by choosing the calibration media with dielectric permittivity values close to the limits of the measurement range and the possibility to use TDR probes of various lengths. The comparison of errors of TDR apparent dielectric permittivity measurement in three sandy soils, based on the probe calibrations in various media, is also presented.
Measuring Soil Water Content With Time Domain Reflectometry
Soil Science, 2010
Time domain reflectometry has been used extensively for measuring soil water content (5 v ) based on the relationship between 5 v and dielectric constant (K). The objective of this study was to develop an improved 5 v -K equation by including soil bulk density (Q b ). Laboratorymeasured 5 v , Q b , and K data on 12 soils of different textures were used to establish the calibration equation. The performance of the new equation was then evaluated on independent soils under field conditions. The results showed that the Topp equation (Topp, G. C., J. L. Davis, and A. P. Annan. 1980. Electromagnetic determination of soil water content: Measurements in coaxial transmission lines. Water Resour. Res. 16:574Y582) worked well on soils of different types and textures, in the intermediate Q b range (1.20Y1.40 g cm j3 ) but produced large discrepancy on soil with low Q b values. On the other hand, the errors of 5 v estimated from the new equation were generally within T0.02 cm 3 cm j3 . In addition, the new equation performed better than the previous calibration equations (Roth, K., R. Shulin, H. Flühler, and W. Attinger. 1990. Calibration of time domain reflectometry for water content measurement using a composite dielectric approach. Water Resour. Res. 26:2267Y2273; Malicki, M. A., R. Plagge, and C. H. Roth. 1996. Improving the calibration of dielectric TDR soil moisture determination taking into account the solid soil. Eur. J. Soil Sci. 47:357Y366.) that had considered soil bulk density.
Comparison of laboratory TDR soil water measurements
2005
Reliable soil moisture sensors are essential for agricultural application. Time Domain Reflectometry (TDR) is a useful method for nondestructive, continuous measurements of soil water content. Laboratory measurements of soil volumetric water content by the TDR 100 Time Domain Reflectometer were compared to gravimetric measurements in three soils, Clay Loam, Silt Loam and Sand soil. Comparison between original and homemade 10
Calibration of Time Domain Reflectometry (TDR) Soil Moisture Point Probe for Two Soils
Journal of Applied and Emerging Sciences, 2011
Maintaining adequate soil moisture in the root zone is crucial in achieving good plant growth. Accurate measurement of soil moisture is essential to keep the right level of soil moisture. Many studies have reported the successful application of Time Domain Reflectometry (TDR) for soil moisture measurement. This study was initiated to obtain calibration curves for soil water content determinations by TDR for two soil types. Measurements were taken in the laboratory for a silt loam and a sandy loam soils, using TDR Soil Moisture Measurement Instrument, Moisture Point TM Model MP-917, and Moisture Point Probe type-K. TDR probe calibration was performed for two soil types contained in wooden boxes (100 cm x 100 cm x 80 cm). The calibration was accomplished by comparing the volumetric moisture content (q TDR) and time delay (t TDR) response of TDR probe to that of the gravimetric volumetric moisture content (q grav). The TDR measurements were taken, in triplicates, at four depths (0-15 cm, 15-30 cm, 30-45 cm, and 45-60 cm) for 38 days after wetting the soil. Soil samples for the gravimetric moisture content measurements were collected from the same locations from where TDR readings were taken. The study has demonstrated that the TDR technique is a reliable alternative method for measuring soil moisture content. The moisture content measurements obtained with TDR were comparable to that of the gravimetric method and showed a good relationship to gravimetric determinations (r 2 =0.85 for silt loam and 0.89 for sandy loam).
Measurement of volumetric water content by TDR in saline soils
European Journal of Soil Science, 1997
Time-domain reflectometry (TDR) evaluates the bulk dielectric constant, K , of the soil by measuring the travel time of an electromagnetic pulse through a sensor, and through it estimates the volumetric water content. We show that for saline soils the effects of conductivity and frequency on the travel time cannot be neglected and that, as a result, TDR systematically overestimates the water content in saline soils. Simultaneously the bulk electrical conductivity of soils can be estimated by TDR. The equivalent impedance after multiple reflections is related to the bulk electrical conductivity, o. This relation differs from sensor to sensor and requires calibration for each individual sensor. A method is proposed for correcting the volumetric water content in saline soils. First, the bulk electrical conductivity, o, is estimated from the equivalent impedance at a specific equivalent distance of cable, several times the actual length of the sensor. The zero-salinity dielectric constant, K O , of this soil is obtained by correcting the apparent K as a function of the measured bulk electrical conductivity. The volumetric water content is estimated from KO. The correction of K is a function of the equivalent frequency of the electromagnetic pulse. The imaginary part of the dielectric constant is primarily due to ohmic losses. The model, which calculates the velocity of propagation of the electromagnetic pulse and which takes into consideration the imaginary part, performs reasonably well. An empirical approach based on calibration gave slightly better results.
Soil moisture calibration of TDR multilevel probes
Scientia Agricola, 2000
Time domain reflectometry (TDR) probes are increasingly used for field estimation of soil water content. The objective of this study was to evaluate the accuracy of the multilevel TDR probe under field conditions. For this purpose, eight such TDR probes were installed in small plots that were seeded with beans and sorghum. Data collection from the probes was such that soil moisture readings were automated and logged using a standalone field unit. Neutron probe measurements were used to calibrate the TDR probes. Soil-probe contact and soil compaction were critical to the accuracy of the TDR, especially when a number of TDR probes are combined for a single calibration curve. If each probe is calibrated individually, approximate measurement errors were between 0.005 and 0.015 m 3 m-3. However, measurement errors doubled to approximately 0.025 to 0.03 m 3 m-3 , when TDR probes were combined to yield a single calibration curve.
1998
The methodology developed by Siddiqui and Drnevich (1995) for measuring soil water content and density using Time Domain Reflectometry (TDR) was extended for routine use in the quality control testing of compacted soils. The objectives of the study were to develop computer software to automate data interpretation and data reduction, develop prototype equipment for field use, compare the results of the TDR method with the sand cone and nuclear density methods on actual construction sites and develop draft specifications for ASTM and AASHTO.