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A REVIEW OF VARIOUS SOIL MOISTUREMEASUREMENT TECHNIQUES

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This paper presents review of the different eight methods for measurement of soil moisture and describes the principle used, methodology, advantages, disadvantages and comparison. Described methods are Gravimetric method, Neutron moderation (NM), Time Domain Reflectometer (TDR), Frequency Domain Reflectometer (FDR), Amplitude Domain Reflectometer (ADR), Phase Transmission (PT), Time Domain Transmission (TDT) and Tensiometer method.

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A REVIEW OF VARIOUS SOIL MOISTUREMEASUREMENT TECHNIQUES

  1. 1. PRESENTED BY Bhushan N. Patil M.E. Scholar (DE-E&TC) A REVIEW OF VARIOUS SOIL MOISTURE MEASUREMENT TECHNIQUES S.S.B.T’s COET, Bambhori, Jalgaon GUIDED BY Prof. P. H. Zope Prof. K. S. Patil
  2. 2. CONTENT 1. INTRODUCTION 2. SOIL MOISTURE MEASUREMENT TECHNIQUES i. Gravimetric Technique ii. Neutron Scattering iii. Time Domain Reflectometer (TDR) iv. Frequency Domain Reflectometer (FDR) v. Amplitude Domain Reflectometer (ADR) vi. Phase Transmission vii. Time Domain Transmission (TDT) viii. Tensio-meter Method 3. ADVANTAGES & DISADVANTAGES 4. OPERATING RANGE & ACCURACY 5. MEASUREMENT VOLUME & COST 6. CONCLUSION REFERENCES
  3. 3. 1. INTRODUCTION NEED OF SOIL MOISTURE MEASUREMENT  In agriculture & Plant science field to determine best time to Sow & plow the field.  Various physical & chemical properties of soil changes with amount of moisture present in soil.  To measure changes in infiltration, irrigation.  To study ground water recharge & Evapo-transpiration.  It is also important in the fields like Hydrology, Forestry, Agrology.  To study & determine the parameters like soil profile, surface tension related with civil & soil engineering.
  4. 4. 2. SOIL MOISTURE MEASUREMENT TECHNIQUES I. GRAVIMETRIC TECHNIQUE It is the most oldest, satisfactory & widely used soil moisture measurement technique. Used for calibration of other techniques. Principle- Based on calculations of the weighing of the soil sample before & after the drying Methodology- Measurement is carried out in following steps : 1) Collecting soil sample- Using Sampling Augers, Sampling Tubes, Core Barrels or Open-drive Sampler. 2) Weighing original soil sample 3) Drying of soil- Oven drying or Radiation drying 4) Weighing dried sample of soil 5) Calculating amount of moisture content % Moisture Measurement = * 100
  5. 5. II. NEUTRON SCATTERING Principle- High energy is lost in Neutron collision with atoms having low atomic weight like hydrogen atoms. Water content these atoms on very large scale. Methodology- 1) Uses a long & narrow cylindrical probe containing source & detector for neutrons. 2) Probe is then inserted in soil & source is then activated 3) The effect of collisions changes a fast neutron to a slow neutron. 4) The number of slow neutrons detected by a counter tube electronically 5) Measuring Gauge is calibrated in terms of moisture content . Fig-1: Schematic of Neutron Scattering [18]
  6. 6. III. TIME DOMAIN REFLECTOMETER (TDR) Principle- Dielectric constant of soil is the function of content of moisture present in soil. Traveling time of a EM wave changes as velocity of traveling wave is affected by the dielectric constant of soil. Ka = (c/v)2 = [(c × t)/(2 × L)]2 Methodology- 1) An open ended sensor transmission waveguide is designed & inserted in soil. 2) A pulse of EM is transmitted through it. A part of signal reflected back which then observed using Sampling Oscilloscope. 3) Using Time delay between transmitted & reflected signal soil moisture measurement is carried out. Fig-2: Operation of TDR measurement [1] Fig-3: TDR system [10]
  7. 7. IV. FREQUENCY DOMAIN REFLECTOMETER (FDR) Principle- The electrical capacitance of a capacitor that uses the soil as a dielectric depends on the soil water content. When connecting this capacitor (made of metal plates or rods imbedded in the soil) together with an oscillator to form an electronic circuit, changes in the circuit operating frequency detects changes in soil moisture. Methodology- 1) Probes are designed using two or more electrodes (i.e., plates, rods, or metal rings around a cylinder) inserted into the soil. 2) An electrical field is given then the soil around the electrodes serves as the dielectric of the capacitor that completing the oscillating circuit. 3) By varying the frequency soil moisture is then measured Fig-4: Capacitance probes and FDR [10]
  8. 8. V. AMPLITUDE DOMAIN REFLECTOMETRY (ADR) Principle- When an EM wave traveling along a TL reaches a section, part of the energy reflected back due to impedance mismatch. This reflected wave interacts with the original incident wave producing a voltage standing wave. Impedance of soil depends on the content of moisture that changes wave amplitude along the length of the TL. Methodology- 1) Sinusoidal EM wave is generated & applied to a coaxial TL that extends into the soil through an array of parallel metal rods. 2) It’s outer cover forms an electrical shield across the central rod. Rod assembly acts as an additional section of the TL having impedance depending on the dielectric constant of the soil between the rods. 3) Depending on the amplitude change in reflected signal moisture content is determined Fig-5: ADR sensor probe [10]
  9. 9. VI. PHASE TRANSMISSION Principle- In travelling sinusoidal wave phase is shifted relatively after traveling fixed distance with respect to initial phase at the origin. This phase shift depends on i) length of travel along the TL ii) frequency and iii) the velocity of propagation. As velocity of propagation is related to soil moisture content, so it can be determined by this phase shift at fixed frequency and length of travel. Methodology- 1) Two open concentric metal rings are used to design a waveguide probe 2) Phase is then measured at the beginning and ending of the waveguides. 3) At the end soil moisture content can be devaluated easily depending on the phase shift . [10] Fig-6: Phase transmission probe and sensor [10]
  10. 10. VII. TIME DOMAIN TRANSMISSION (TDT) Principle- Measurement of the propagation time over a known distance of the wave can be used to calculate the dielectric characteristics. To determine water content in a porous dielectric medium, an EM pulse is guided through the medium by a transmission line. Methodology- 1) It is similar to TDR, but requires an electrical connection at the beginning as well as ending of the TL. 2) The probe has a waveguide design as bent rods of metals which are inserted into the electronic block on both side. 3) Another way is the sensor consists of a long band (~3 ft), with an electronic block at both ends. 4) Soil moisture is then easily calculated depending on the travel time of EM wave. Fig-7: Time domain transmission probe [17]
  11. 11. VIII. TENSIOMETER METHOD Principle- Based on the property of the absorption i.e. Suction force of water for soil. water comes into equilibrium with the soil solution through a permeable & saturated porous material if a sealed water-filled tube is placed with the soil. Methodology- 1) The permeable ceramic cup tip is placed with the soil in the root zone of plant. 2) Water moves from the tube into the soil as the soil is normally not saturated, So that a partial vacuum(pressure) is made and evaluated by the gauge. 3) Pressure varies with amount of water potential thus gauge indicate the moisture content in the soil. Fig-8: Tensiometer example [10]
  12. 12. 3. ADVANTAGES & DISADVANTAGES Parameter (I) GM (II) NM (III) TDR (IV) FDR (V) ADR (VI) PT (VII) TDT (VIII) Tensi. 1) Requirement of Specific Calibration N N Y Y Y Y Y N 2) Affected by Soil Salinity & air gaps N Y N/Y Y Y Y Y N 3) Measurement at different depths Y Y N Y N N N N 4) Connection with Data Logger N N Y Y Y Y Y N 5) Time efficient N N Y Y Y Y Y N 6) Permanent Installation N Y Y Y Y Y Y N 7) Safety Y N Y Y Y Y Y Y 8) Automation N N Y Y Y Y Y N
  13. 13. 4. OPRATING RANGE AND ACCURACY Technique Operating Range (ft3 per ft3) Accuracy (ft3 per ft3) Neutron Moderation 0 to 0.6 ± 0.005 TDR 0.05 to saturation ± 0.01 FDR 0 to saturation ± 0.01 ADR 0 to saturation ± 0.01 to 0.05 PT 0.05 to 0.5 ± 0.01 TDT 0 to 0.7 ± 0.05 Tensiometer 0-0.80 bar ±0.01 bar
  14. 14. 5. MEASUREMENT VOLUME AND COST Technique Measurement volume Cost Neutron Moderation Sphere(radius 6-16 inches) $10,000-15,000 TDR About 1.2 inches radius around waveguide $400-23,000 FDR Sphere(radius 1.6 inches) $100-3,500 ADR Cylinder (radius 1.2 inches) $500-700 PT Cylinder $200-400 TDT Cylinder (radius 2 inches) $400-1,300 Tensiometer Sphere (Greater than 4 inch radius) $75-250
  15. 15. 6. CONCLUSION  It is concluded that each technology has some limitations, so it is said that the ideal method for measurement of soil moisture under field conditions has yet to be perfected.  Individual requirements should be indentified sufficiently before adopting a soil moisture monitoring methodology.  TDR method can be most superior for soil moisture measurement if it’s cost is reduce and if limitation over high salinity operation is removed. FUTURE SCOPE : Limitation over high salinity operation is removed by use of Polyolefin coated TDR probes.
  16. 16. REFERENCES [1] Dennis Trebbels, Alois Kern, “Miniaturized FPGA-Based High-Resolution Time-Domain Reflectometer” IEEE Transactions On Instrumentation And Measurement, Vol. 62, No. 7, July 2013. [2] A. Scheuermann and C. Huebner, “On the feasibility of pressure profile measurements with time-domain reflectometry,” IEEE Trans. Instrum. Meas., vol. 58, no. 2, pp. 467–474, Feb. 2009. [3] Alvarez-Benedí, J. and R. Muñoz-Carpena (eds). Soil-Water-Solute Process Characterization, An Integrated Approach . CRC Press LLC:Boca Raton. ISBN:.1-5667-0657-2. 2005. [4] Campbell, G.S. and W.H. Gardner, 1971. Psychrometric measurement of soil water potential: temperature and bulk density effects. Soil Sci. Soc. Am. Proc. 35: 8-12. Conference and Exposition, Feb. 2000, New Orleans, LA, pp. 861–867. [5] Gaskin, G.D. and J.D. Miller, 1996. Measurement of soil water content using simplified impedance measuring technique. Journal of Agricultural Engineering Research 63: 153-160. [6] Yoder, R.E., D.L. Johnson, J.B. Wilkerson and D.C. Yoder. 1998. Soil water sensor performance. Applied Engineering in Agriculture 14(2): 121-133. [7] Wijaya, K., T. Nishimura and K. Makoto. 2002. Estimation of bulk density of soil by using amplitude domain reflectometry (ADR) probe. 17th WCSS. Thailand. Paper no. 385. [8] A. I. JOHNSON, “Methods of Measuring Soil Moisture in the Field,” Geological Survey Water-Supply Paper 1619-U, 1992. [9] Schmugge, T.J., Jackson, T.L., and McKim, H.L., Survey of Methods for Soil Moisture Determination. Water Resources Research Vol. 16. N0.6, Pages 961-979, December 1980. [10] Rafael Munoz- Carpena, Sanjay Shukla, and Kelly Morgan, “Field Devices for Monitoring Soil Water Content”, The Southern Regional Water Program. SR-IWM-2 Pages 1-24. Available at http:// www.bae.ncsu.edutopicgo_irrigationdocsfield-devices-monitoring- .pdf&ved=OCCcQFjAA&usg=AFQjCNGMFtYwlyE7_if_NUYE4vsbAWqQpA&gigto=7Y2iZwSUOy- jFFOZA-Putw
  17. 17. [11] Painter, D. J., Moisture Near The Soil Surface. Proceedings of Soil and Plant Water Symposium: Palmerston North, 25-27 May 1976 Pages 7-12. [12] Sophie Proulx, “Evaluation of The Performance Of Soll Moisture Sensors In Laboratory-Scale Lysimeters,” Department of Biosystems Engineering University of Manitoba Winnipeg, Manitoba, August-2001 Pages 1-124. [13] Wobschall, D. “A theory of the complex dielectric permittivity of soil containing Water” IEEE Transactions on Geoscience Electronics 1977. GE-15(1): 49-58. [14] Markus Stacheder*, Franz Koeniger and Rainer Schuhmann, “New Dielectric Sensors and Sensing Techniques for Soil and Snow Moisture Measurements”, Sensors 2009, 9, 2951-2967. [15] Soil water monitoring - an information package-2nd edition (2005)Available at http://www.irrigationfutures.org.au/imagesDB/news/soilwatermonitoring2ed.pdf. [Accessed 24-August- 2014] [16] Ron Heiniger, “Sensors and Monitors for Measuring Soil Moisture” Vernon G. James Research and Extension Center, Plymouth, NC 27962, Spring 2013 PP 1-5. [17] J.M. Blonquist Jr.*, S.B. Jones, D.A. Robinson, “A time domain transmission sensor with TDR performance characteristics”, Journal of Hydrology 314 (2005) 235–245. [18] A. L. Ward, R. S. Wittman, U.S. Department of Energy under Contract DE-AC05-76RL01830, August 2009. PNNL-18539 Available at http://www.pnl.gov/main/publications/external/technical_reports/PNNL-18539.pdf [Accessed on 5 Nov- 2014 ]

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