The World Meteorological Organization (WMO) and the U.S. Climate Reference Network (USCRN) have published general guidelines to classify sites for precipitation gauges such that they are “Regionally and Spatially” representative. This detailed information is available in their online publications WMO-No8 CIMO Guide, 2010 and U.S. Climate Reference Network Site Information Handbook, 2002. The following summarizes the essence of their guidelines:
This post is an excerpt from the Belfort Instrument Engineering Guide to Siting Precipitation Gauges. Click the button below to download the free guide now (PDF).
Class 1 Precipitation Measurement Site
- Flat horizontal ground with a slope less than 1/3 (an elevation change of 1 unit over a horizontal distance of 3 units) or 19 degrees with no impact from surrounding terrain features (such as mountains, etc.)
- Surrounding trees, shrubbery or other obstacles of a uniform height at a distance equal to or greater than 4 times the height of the precipitation gauge inlet
- Uniformity for the above purpose means that there is less than a factor of 2 difference in lowest and highest obstacle and an obstacle is defined as material that subtends over at least a 10 degree arc as viewed from the gauge
Class 2 Precipitation Measurement Site (Site induced measurement error up to 5%)
- Same as Class 1 site except the obstacle is located a distance of at least 2 times the height of the precipitation gauge inlet
Class 3 Precipitation Measurement Site (Site induced measurement error of 10% to 20%)
- Open land surrounding gauge with a slope less than 1/2 (30 degrees)
- Obstacles are located at a distance of at least 1 times the height of the precipitation
Class 4 Precipitation Measurement Site (Site induced measurement error over 20%)
- Land surrounding gauge with a slope greater than 1/2 (30 degrees)
- Obstacles are located at a distance of less than 1 times the height of the precipitation
Class 5 Precipitation Measurement Site (Site induced measurement error over 50%)
- Obstacles overhanging the gauge, such as trees, roof edges, etc.
Precipitation gauge siting considerations can lead to significantly reduced errors in the measurement of representative samples of area precipitation with a minimum number of gauges. Knowing that any single precipitation event can be spatially non-uniform introduces a requirement for multiple measurement sites over a wide geographic area. Knowing that cities, mountains, river valleys, and lakes systematically modify the Mesoscale level of precipitation introduces a requirement to site gauges in a manner that compensates for these known systematic spatial non-uniformities in order to gather more meaningful area wide data. Knowing that wind modification at the Microscale level of precipitation introduces a requirement to locate gauges where wind is reduced to a minimum without blocking blowing precipitation. Proper Mesoscale and Microscale precipitation gauge siting considerations can reduce systematic errors in representation of large area precipitation data by over 50%.