Rainfall measurement

Rainfall is the basic input to most hydrological calculations, so it must be measured carefully. A rain gauge collects rain falling on a known area. Non-recording gauges, such as the standard Symons type with a 127 mm diameter collector, are read once a day by measuring the collected water. Recording gauges, using a tipping bucket, weighing mechanism or float, give a continuous record of rainfall intensity, needed for flood studies and urban drainage. Automatic weather stations transmit data in real time. Weather radar and satellites estimate rainfall over large areas and are calibrated against gauges.

Siting matters: gauges should be on level ground, away from trees and buildings by at least twice their height, with the rim at a standard height and the site fenced. A network must be dense enough to capture spatial variation, especially in hilly areas where rainfall changes rapidly with elevation.

Point rainfall is converted to average catchment rainfall by the arithmetic mean (for flat areas with uniform gauges), Thiessen polygons (weights by the area nearest each gauge) or isohyets (contours of equal rainfall drawn using judgement). Missing records are estimated from neighbouring stations by the normal ratio method. Consistency of long records is checked by double-mass curves. Intensity-duration-frequency (IDF) curves, built from recording gauge data, give the design intensity for a chosen return period and storm duration, the key input to drain and culvert design.