Isohyets

An isohyet is a meteorological map line joining locations that receive the same amount of precipitation over a defined period, such as a month, season, or year. The term combines the Greek words for equal (isos) and rain (hyetos). Isohyets are used to visualize spatial patterns of rainfall and to delineate regions with similar rainfall amounts, aiding hydrological planning, agriculture, and climate analysis. Maps may employ common intervals like 10, 20, or 50 millimeters, though the spacing depends on data density and regional rainfall variability.

Constructing isohyet maps requires rainfall observations from gauge networks. Because gauges are unevenly distributed, estimates of rainfall between stations are produced using interpolation or areal averaging methods such as Thiessen polygons, kriging, or inverse distance weighting. The resulting lines reflect estimated, not exact, rainfall boundaries and are sensitive to data quality and density. Topography often shapes isohyets, with mountains producing sharp gradients due to orographic effects.

Isohyets help identify rainfall belts, regional drought or flood risk, and water resource availability. They are used in watershed management, irrigation planning, agricultural zoning, and climate impact assessments. While conceptually related to lines like isotherms (equal temperature) or isobars (equal atmospheric pressure), isohyets specifically map precipitation distribution. In modern practice, digital climate datasets and geographic information systems enable high-resolution isohyet maps derived from gridded models and satellite observations, complementing ground-based measurements.