INSTALLATION
Site Selection - The two main criteria for selecting a site for the rain gage are 1) the exposure of the gage to rain and 2) the convienence of the gage for the reader while minimizing attraction to vandals.
1) The best location is where the gage is uniformly protected from the wind in all directions, such as in an opening in a grove of trees. The height of the protection should not exceed twice its distance from the gage. As a general rule, the windier the gage location is, the greater the precipitation error will be. Thus, gages should not be located in wide-open spaces or on tops of buildings, because of wind and turbulence. Gages should also not be located close to isolated obstructions, such as trees and buildings, which may deflect precipitation due to turbulence.
2) The gage should be located in a convenient location for the observer, such as along the route to the stream gage. This will encourage daily observations taken at the same time as the river stage readings. However, the gage should not be in a prominent location that will attract vandalism.
Installation - Mount the gage on a sturdy post, such as a 4” x 4” or two 2” x 4” boards nailed together. The gage should be mounted so that the top on the gage is level and is above the top of the post.
The 4” diameter gage has a funnel at the top that directs the rain into a smaller center tube; this magnifies the rain such that 1” of rain will fill the center tube. If it rains more than an inch, the excess will flow into the outer cylinder. Readings are taken as follows:
No rain in outer cylinder - The rainfall amount of 1 inch or less can be read directly from the markings on the center measuring tube. Note that each mark is 0.01 inch and each 0.10 inch is labeled. After taking the reading and writing it down, empty the center tube completely and replace tube and funnel into position.
Rain collected in outer cylinder - Remove the funnel and read the center measuring tube to check that it is full at the 1.00 level; mark down this reading. Remove the center tube and empty it either to the ground or into another container if you wish to remeasure the total rain. Remove the outer cylinder from the bracket and carefully pour the excess rain into the measuring tube using the funnel, taking care not to overfill the measuring tube beyond 1.00 inch. Take a reading, write it down, and repeat until the outer cylinder has been emptied. Total all the readings to get the total rainfall.
