Climatology and hydrology: the “average annual rainfall” at a rain-gauge station is computed as the average of the annual totals over how many consecutive years for a reliable normal?

Introduction / Context:
A rainfall “normal” smooths year-to-year variability to represent a station’s typical climate. Hydrologic design often relies on multi-decade normals to benchmark water availability, drought severity, and flood potential.

Given Data / Assumptions:

• We seek the conventional length of record used for computing the average annual rainfall at a station.

Concept / Approach:
Classical hydrology texts and many exam standards define the average annual rainfall as the mean of annual totals over a long, continuous record. Although modern climatology often uses 30-year normals, many traditional problems specify 35 years as the representative averaging period.

Step-by-Step Solution:
Identify the standard long-term averaging window used in conventional problems. Select the next higher decadal multiple close to modern 30-year practice that appears in classical references: 35 years. Confirm that shorter records (7–28 years) are more vulnerable to sampling bias.

Verification / Alternative check:
Longer windows reduce variance of the mean and better capture interannual oscillations (e.g., ENSO), supporting the choice of a multi-decadal period like 35 years in older design literature.

Why Other Options Are Wrong:

• 7, 14, 21, 28 years: Too short for a robust “normal”; high sensitivity to wet/dry cycles.

Common Pitfalls:

• Confusing contemporary 30-year WMO normals with older civil-engineering conventions; exam keys often expect 35 years.

Final Answer:
35 years.