The Thiessen polygon method for estimating mean areal rainfall over a catchment uses:
Choose the correct answer
Arithmetic average of all gauge readings
Weighted average where each gauge is assigned a weight proportional to the area of its Thiessen polygon (the polygon of perpendicular bisectors between adjacent gauges)
Isohyetal method with interpolated contours
Only the highest reading gauge
Correct Answer
B. Weighted average where each gauge is assigned a weight proportional to the area of its Thiessen polygon (the polygon of perpendicular bisectors between adjacent gauges)
Why is this the answer?
Thiessen polygons: draw perpendicular bisectors between all adjacent rain gauge pairs, forming polygons around each gauge. Area of each polygon within catchment = A_i. Mean areal rainfall P = sum(P_i x A_i) / A_total. More accurate than arithmetic mean when gauges are unevenly distributed. Does not account for orographic effects (isohyetal method better for hilly terrain).
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