Precipitation MCQ with answer and solution

Concept

Isohyetal method advantages in mountainous terrain

Formula

P = Σ (A_i × P_i) / Σ A_i

Step-by-step solution

1. The isohyetal method computes mean areal rainfall by drawing contours of equal precipitation (isohyets). 2. The area between adjacent isohyets is measured, and weighted rainfall is calculated: P = Σ(A_i × P_i) / Σ A_i. 3. In mountainous areas, orographic lifting creates steep rainfall gradients that are well-captured by isohyetal contouring. 4. Unlike arithmetic or Thiessen methods, it accounts for topography and elevation, making it the most accurate method. Answer: C

Explanation

The isohyetal method determines average basin precipitation by drawing contours of equal rainfall depth (isohyets) and planimetering the areas between successive contours. It is regarded as the most accurate method in rugged, mountainous topography because it explicitly incorporates elevation changes, storm tracks, and orographic uplift into contour drawing.

Why the other options are wrong

A) Arithmetic mean assumes uniform station distribution and flat terrain, introducing large errors in mountains. B) Thiessen polygon method considers station spacing but ignores topography and orographic precipitation. D) Normal ratio method is used to estimate missing precipitation records, not mean areal rainfall.

Shortcut

Mountainous / rugged terrain = Isohyetal method (accounts for orographic effects and topography).

Common mistake

Selecting Thiessen polygon method, which relies strictly on geometric spacing and completely ignores elevation changes.