Question

Given below are two statements, one is labeled as Assertion (A), and the other one is labeled as Reason (R).
Assertion (A): Equilibrium water content in soil at a certain matric potential is higher when the soil is under the drying process (desorption) compared to when the soil is under the wetting process (sorption).
Reason (R): The contact angle between water and the soil solid phase is greater during the imbibition of water than during drainage.
In light of the above statements, choose the most appropriate answer from the options given below:

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A).
• B. Both (A) and (R) are correct but (R) is NOT the correct explanation of (A).
• C. (A) is correct but (R) is not correct.
• D. (A) is not correct but (R) is correct.

Hint:

Other causes of hysteresis include the ink-bottle effect (varying pore radii), entrapped air during wetting, and soil structure changes such as swelling and shrinking.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Soil water hysteresis is the phenomenon where the relationship between soil matric potential and water content is non-unique and depends on the wetting or drying history of the soil.

Step 2: Detailed Explanation:
- Assertion (A) is correct: A soil always retains more water at a given matric suction during drying (desorption) than during wetting (sorption).
- Reason (R) is correct: During imbibition (wetting), the advancing contact angle is larger ($\alpha_a > \alpha_r$).
Since the capillary suction head is related to the contact angle by:
\[ h = \frac{2\gamma \cos\alpha}{\rho_w g r} \] A larger contact angle during wetting ($\alpha_a$) leads to a smaller $\cos\alpha$, reducing capillary suction at a given water content compared to drying.
Thus, the contact angle effect is a key thermodynamic cause of hysteresis, explaining why wetting curves lie below drying curves.

Step 3: Final Answer:
Both (A) and (R) are correct, and (R) is the correct explanation of (A).