Question

Which of the following methods does not directly account for orographic influences on rainfall ?

• A. Arithmetic mean method
• B. Reciprocal distance squared method
• C. Thiessen method
• D. Isohyetal method

Hint:

The Isohyetal method is the only standard average-precipitation method that directly incorporates orographic and topographic effects because the curves can be physically adjusted based on terrain features.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
To compute the average precipitation over a catchment basin, hydrologists use three primary methods: the Arithmetic Mean method, the Thiessen Polygon method, and the Isohyetal method.
These methods vary in how they account for spatial distribution and topographic (orographic) variations.
Detailed Explanation:
Let us analyze how each method handles spatial and topographic factors:
- Arithmetic Mean Method: Simply averages the rainfall recorded at all stations. While extremely simple, it is highly inaccurate for mountainous regions.
- Thiessen Polygon Method: Assigns a weight to each rain gauge based on its representative area (drawn via perpendicular bisectors of lines connecting adjacent stations). This accounts for the non-uniform spatial distribution of gauges. However, because the polygons are constructed purely geometrically, it completely ignores the elevation and orographic features of the basin. Thus, it does not directly account for orographic influences.
- Isohyetal Method: Involves drawing lines of equal rainfall (isohyets) on a map. When drawing these lines, hydrologists can directly adjust them based on topographic and orographic factors, making it the most accurate method for mountainous areas.

Step 2: Final Answer:
The Thiessen method does not account for orographic influences, matching Option (C).