Rainfall terminology and indices (hydrology): Pick the correct statement(s) about annual rainfall, normal rainfall and index of wetness.

Introduction / Context:
Basic rainfall definitions are foundational for hydrologic design and water resources planning. Examinations frequently test precise terminology such as annual rainfall, normal rainfall, and indices used to describe inter-annual variability.

Given Data / Assumptions:

• Station rainfall records exist over multiple years.
• Normal (long-term mean) is computed over a suitably long reference period (often several decades).
• Index of wetness compares a year’s rainfall with the normal value.

Concept / Approach:
Annual rainfall is the total depth accumulated over the defined year. Normal annual rainfall is a long-term mean over many years (commonly a few decades). Index of wetness (or related indices) indicates relative wetness/dryness compared to the normal, aiding quick classification of years.

Step-by-Step Clarification:
Define the annual total at a station by summing daily/monthly totals across the year.Compute the normal by averaging annual totals over a baseline period (e.g., a few decades, often cited around 30–35 years in many curricula).Compute an index (e.g., ratio to normal or percentage departure) to judge wetness of the year.

Verification / Alternative check:
Hydrologic services commonly publish normals and departures. A year whose total is close to the normal is classified as an “average” year, supporting option (d).

Why Other Options Are Wrong (if taken individually):

• Each individual statement (a–d) is correct in standard textbook usage; hence the composite best answer is “All the above”.

Common Pitfalls:

• Confusing “normal” with median or a short-term mean.
• Using inconsistent reference periods across stations, leading to biased comparisons.

Final Answer:
All the above