Rainfall statistics and indices: choose the correct set of statements.

Introduction / Context:
Hydrometeorological terminology must be used precisely. This question checks understanding of daily/annual rainfall definitions, the concept of a climatic normal period, and the index of wetness used to compare a year’s rainfall with the long-term normal.

Given Data / Assumptions:

• (a) Daily rainfall = amount in any 24-hour day.
• (b) Annual rainfall = total rainfall in a calendar (or hydrologic) year.
• (c) Rain cycle period in India is 35 years (proposed statement).
• (d) Index of wetness = actual annual rainfall / normal annual rainfall.

Concept / Approach:
World Meteorological Organization (WMO) climatic normals are taken over 30-year periods. In Indian practice, the “normal” commonly uses a 30-year base period (e.g., 1901–1930, 1931–1960, etc.). Statements about 35 years are not standard and are generally incorrect in this context.

Step-by-Step Solution:

Verification / Alternative check:
National meteorological agencies publish normals based on 30-year periods; hydrology texts echo this standard convention.

Why Other Options Are Wrong:

• “a and b only” ignores (d) which is correct.
• “a, b, c, and d” wrongly includes (c).
• “b and d only” omits (a), which is correct.
• “c only” accepts the incorrect 35-year claim.

Common Pitfalls:
Confusing local data archive lengths with the official normal period; mixing hydrologic year with calendar year without clarity; forgetting the ratio form of the index of wetness.

Final Answer:
a, b, and d only are correct