A line on a map connecting points of equal rainfall amount over a given period. Isohyetal maps are used to visualize the spatial distribution of precipitation across a region.
An isohyet is a contour line drawn on a map that connects all points receiving the same amount of precipitation during a specified time period. Isohyetal maps are created by interpolating between rain gauge measurements to produce a continuous representation of precipitation distribution across a region or catchment. The isohyetal method is one of the standard techniques for estimating mean areal precipitation over a watershed, and is generally considered more accurate than simpler methods such as the arithmetic mean or Thiessen polygon approaches, particularly in areas with complex topography. To apply the isohyetal method, precipitation contours are drawn, the areas between successive isohyets are measured using planimetry or GIS, and the weighted average precipitation is calculated. Isohyetal maps reveal important patterns such as orographic enhancement of rainfall on windward mountain slopes, rain shadow effects, and urban heat island influences on precipitation. Modern approaches use radar-rainfall estimates and geostatistical interpolation methods (kriging) to produce higher-resolution precipitation maps that serve a similar purpose to traditional isohyetal analysis.