The isohyetal method estimates average rainfall over an area by drawing lines of equal rainfall and weighting the rainfall between them by area.
The isohyetal method estimates average rainfall over a catchment from rain gauge readings. It is generally considered the most accurate of the three classic methods, the others being the arithmetic mean and Thiessen polygons, when the gauges are dense enough and the analyst understands the terrain. The steps are: plot the gauge readings on a map, draw isohyets (lines of equal rainfall) by interpolating between gauges, measure the area between each pair of neighbouring lines, multiply each area by the mean of its two bounding isohyets, add the results and divide by the total area. For example, take a catchment with 40 km² between the 50 and 60 mm lines, and 60 km² between the 40 and 50 mm lines. The average is (40 × 55 + 60 × 45) / 100 = 49 mm. Its strength is that the analyst can take account of mountains, wind direction and storm shape when drawing the lines. That is especially useful for orographic rainfall. Its weakness is that results depend on judgement, and the lines must be redrawn for each storm. Today isohyets are usually drawn in GIS software with interpolation methods such as kriging or inverse distance weighting.