Question

Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R
Assertion A: The effective rainfall erosion index of a given area is linearly proportional to the percent of ground that is not covered by vegetation
Reason R: Rain erosion index includes both the kinetic energy of rain and the maximum 30-min rain intensity
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:

Rainfall erosivity is determined by the $EI_{30}$ index (Kinetic Energy $\times$ Max 30-min Intensity).
Vegetation protects the soil by intercepting this energy, meaning the effective erosion index is directly proportional to the percentage of bare soil.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Soil erosion by water is driven by rainfall erosivity, soil erodibility, topography, and surface cover.
The Universal Soil Loss Equation (USLE) uses a rainfall erosivity factor ($R$) to quantify the capacity of rainfall to cause soil detachment and transport.

Step 2: Detailed Explanation:
Let us analyze the correctness of both statements:
1. Reason R: The rainfall erosion index ($EI_{30}$) is calculated as the product of the total kinetic energy of a rainstorm ($E$) and its maximum continuous 30-minute intensity ($I_{30}$).
This index quantifies the combined effect of raindrop impact and runoff rate during the storm, making Reason R correct.
2. Assertion A: The effective rainfall erosion index represents the net erosive force of rainfall that reaches the soil surface after passing through any vegetation canopy or crop residue.
Vegetation and mulch absorb the kinetic energy of falling raindrops, protecting the soil.
The effective erosion index is linearly proportional to the percentage of ground left bare (not covered by vegetation), as bare soil is directly exposed to raindrop impact. Thus, Assertion A is correct.
While both statements are correct, the definition of the rain erosion index in terms of kinetic energy and 30-minute intensity (R) does not explain the biological shielding mechanism by which vegetation cover reduces effective erosivity (A).
Therefore, Reason R is not the correct explanation of Assertion A.

Step 3: Final Answer:
Both Assertion A and Reason R are correct, but Reason R is not the correct explanation of Assertion A, which corresponds to Option (B).