For estimating average annual precipitation over a catchment, which method generally provides the most reliable spatial average when adequate contouring of isohyets is possible?

Introduction / Context:
Catchment-average precipitation is a fundamental input for hydrologic design. Accuracy improves when spatial variability is represented properly.

Given Data / Assumptions:

• Several rain gauges with known long-term records.
• Topography and storm tracks cause non-uniform rainfall fields.

Concept / Approach:
Three classical methods exist: simple average, Thiessen polygons (area weighting per gauge), and isohyetal method (area weighting of precipitation bands bordered by isohyets).

Step-by-Step Solution:
Arithmetical mean ignores spatial distribution and is least accurate for heterogeneous fields.Thiessen assigns fixed areas to gauges but still assumes uniform rain within each polygon.Isohyetal method contours precipitation to create belts of near-equal rainfall, then area-weights each belt, capturing gradients better.Therefore, isohyetal method is generally most reliable when adequate data allow contouring.

Verification / Alternative check:
When gauge density is high and orography strong, isohyets track spatial patterns more realistically than polygons.

Why Other Options Are Wrong:

• Arithmetical and Thiessen can be used but are less accurate where variability is significant.

Common Pitfalls:

• Poor isohyet drawing yields bias; expert contouring is essential.

Final Answer:
Isohyetal method