Question

Read the following statements.
(A) TDR stands for Time Domain Refraction.
(B) TDR is based on the estimation of dielectric constant of water.
(C) Dielectric constant of water is 80.
(D) TDR is relatively unaffected by salinity or bulk density variations.
(E) TDR measures soil moisture suction.
Choose the correct answer from the options given below.

• A. (B) and (D) only
• B. (A), (C) and (D) only
• C. (C) and (D) only
• D. (B), (C) and (E) only

Hint:

Separate what TDR actually measures (the bulk soil's dielectric constant) from background facts like water's own dielectric constant.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
We are given five statements about Time Domain Reflectometry (TDR), a soil moisture sensing method, and must pick out only the true ones.

Step 2: Key Formula or Approach:
TDR sends a fast electrical pulse down a probe buried in soil and times how long it takes to travel and reflect back. That travel time depends on the apparent (bulk) dielectric constant of the whole soil, water and air mixture, and this bulk value is converted into volumetric water content, since water's dielectric constant (about 80) is far higher than that of dry soil solids (about 3 to 5) or air (about 1).

Step 3: Detailed Explanation:
Statement (A) is false. TDR stands for Time Domain Reflectometry, not "refraction", the technique is named after the reflected pulse, not any bending of the signal.
Statement (B) is false as worded. TDR does not estimate the dielectric constant "of water", water's dielectric constant is already a known, fixed value used in the calculation. What TDR actually measures is the apparent dielectric constant of the bulk soil, soil plus water plus air, and that measured value is what gets converted into water content.
Statement (C) is true. The dielectric constant of pure water at usual soil temperatures is close to 80, and this large, well known value is exactly why TDR is so sensitive to water content, since soil minerals and air have dielectric constants below 5.
Statement (D) is true. One of TDR's main advantages over methods like electrical resistance blocks or capacitance probes is that its reading is relatively unaffected by normal changes in soil salinity or bulk density, which is why it is widely trusted for field water content measurement.
Statement (E) is false. TDR reports volumetric water content, not soil moisture suction (tension), suction is instead measured with tensiometers or estimated indirectly from gypsum blocks.
So only (C) and (D) hold up, giving option 3.

Step 4: Final Answer:
Only statements (C) and (D) are correct, so the answer is option 3.