Areal Reduction Factor (ARF) Helper

Converts point rainfall depth or intensity at a gauge to an approximate areal value over a catchment, using a simple generic ARF curve that depends on area, duration and storm type. Note: this tool is for preliminary / educational use – replace ARF with local official curves where available.

1. Inputs Area, Duration & Storm Type

Provide basin area and storm duration, choose a storm‑type family, and optionally enter a point rainfall depth or intensity. The tool returns an approximate ARF and areal rainfall, plus a small ARF‑vs‑duration table.
Only affects the labels; ARF itself is dimensionless.
Use km² (SI) or mi² (US). ARF decreases as area grows, especially for short‑duration convective storms.
Duration corresponding to your point depth / intensity (e.g. 1‑hr, 3‑hr, 6‑hr IDF).
Convective storms usually have stronger areal reduction; widespread systems have gentler reduction.
ARF scales either total depth or average intensity – the factor is the same.
Point depth or intensity at the gauge. Leave blank if you only want ARF values (no areal rainfall).
Appears in the summary and exports only.
Generic ARF curve Area & duration dependent Areal depth / intensity Small ARF vs duration table