Soil moisture - impedance method (Theta Probe)

ML2 probe with hand held HH2

ML3 probe

Parameter to be measured:

• old ML2: Soil moisture, permitivity for a volume of 300mm³ (60mm long x 26.5mm

diameter).

• new ML3: soil moisture and temperature

Method:

Frequency Domain Sensor working with the so called impedance-method at 100 MHz. Usable in saline soils - up to 2000 mS/m.

Equipment:

• Delta-T Device: Theta-Probe (ML2x) with 4 rods of 600 mm length, HH2 display for manual reading or a data logger
• Reqiuerments to loggers: Requires 5 - 15V DC at 20mA. Provides 0 - 1V DC output.

Advantages:

• easy and fast measurement of punctual soil moisture
• manual as well as continuously logged measurements possible in that case long cable runs possible.

Disadvantages:

• Each sensor has to be calibrated
• careful application of probe in stony and dry soils, because rods break easy
• during field application cable breakes often occures at the crossing between probe head and cable
• Underestimating of permitivity in media with low permitivity (ε > 40, clay and organic soils)
• unaccuarate values in saturated media

What to watch out for:

• Good contact between probe and medium has to be proved

Price:

• 1270 € for ML2 probe and HH2 (2008)

Links

Projects that used the above equipment:

• The ECHO project
• COMTESS

Other related web sites:

• John Miller Theta Probe: Theta Probe ML2X Principles of operation and application
• Delta-T Devices:
  • User Manual of the old ML2 probe
  • User Manual of the new ML3 probe

References

Blonquist, JM; Jones, SB; Robinson, DA. 2005. Standardizing characterization of electromagnetic water content sensors: Part 2. Evaluation of seven sensing systems. VADOSE ZONE JOURNAL 4(4), 1059-1069.

Gaskin, GJ; Miller, JD. 1996. Measurement of soil water content using a simplified impedance measuring technique. JOURNAL OF AGRICULTURAL ENGINEERING RESEARCH 63(2), 153-159.

Miller, JD; Gaskin, GJ; Anderson, HA. 1997. From drought to flood: Catchment responses revealed using novel soil water probes. HYDROLOGICAL PROCESSES 11(5), 533-541.

Robinson, D.A., Gardner, C.M.K., Cooper, J.D. 1999. Measurement of relative permittivity in sandy soils using TDR, capacitance and theta probes: comparison, including the effects of bulk soil electrical conductivity. Jounral of Hydrology 223, 198-211.

Seyfried, MS; Murdock, MD. 2004. Measurement of soil water content with a 50-MHz soil dielectric sensor. SOIL SCIENCE SOCIETY OF AMERICA JOURNAL 68(2), 394-403.