IMD standard non-recording rain gauges – collector aperture sizes According to the Indian Meteorological Department (IMD), the standard non-recording rain gauges are manufactured with collector apertures of which areas?

Introduction / Context:
Rainfall measurement is foundational for hydrologic analysis. In India, the IMD specifies standardized non-recording gauges (often called Symon’s type) with specific collector areas so that measured depths are comparable across networks and long-term records.

Given Data / Assumptions:

• Non-recording gauges require manual observation of collected rainfall depth.
• Standardization ensures uniform calibration and acceptable accuracy under field conditions.
• Two commonly used sizes are specified for different exposure and rainfall regimes.

Concept / Approach:
Collector aperture area determines the volume-to-depth conversion factor: volume collected / area = rainfall depth. IMD standardizes this area so that observers can use standard graduated measuring cylinders and comparison tables without site-specific recalibration.

Step-by-Step Solution:
Recall typical IMD specifications: standard apertures of 100 cm² and 200 cm².Match with provided options → option (c).

Verification / Alternative check:
Field manuals and hydrology textbooks summarizing IMD practices list 100 cm² and 200 cm² collectors for non-recording gauges deployed across networks.

Why Other Options Are Wrong:

• (a) and (b) include an incorrect size (50 or 150 cm²).
• (d) 250 or 500 cm² are atypically large for standard non-recording gauges.
• (e) is unnecessary because a correct pair exists.

Common Pitfalls:
Confusing non-recording (manual) gauges with recording gauges, which use tipping buckets or weighing mechanisms and are specified differently (e.g., orifice diameter, bucket volume).

Final Answer:
100 cm² or 200 cm²